2011 AAHA Canine Vaccination Guidelines*†

Vaccine Stability

Because the antigenic bacteria or virus used in noninfectious (killed) vaccine is incapable of replicating, killed vaccines are prepared and sold as an aqueous (liquid) product that can be directly administered to the patient. During storage, noninfectious vaccines are highly stable. Although refrigeration is recommended, noninfectious vaccines are significantly less susceptible to heat inactivation than infectious vaccines. Noninfectious vaccines can, however, be denatured. If exposed to chemicals (e.g., rewashed, reused syringes), a noninfectious vaccine could become ineffective. Therefore, sterile, unused syringes should be used when administering vaccines. Noninfectious vaccines should be administered before the expiration date printed on the vial.

Both infectious and noninfectious vaccines are important and required in every canine vaccination program. However, it is important not to mix noninfectious vaccines with infectious vaccines in the same syringe, unless specified by the manufacturer.^14,19–24

Multiple Dose Vials

Multiple dose, also called “tank” vials, of killed rabies vaccine are available. Typically prepared in 10 mL (10 dose) vials, these products should only be used for high volume vaccination clinics or by shelter immunization programs where large numbers of dogs are vaccinated over a short period of time (same day). Multiple dose vials require multiple needle penetrations over time, thereby increasing the risk for contamination. Tank vials should be shaken frequently to ensure the concentration of antigen/adjuvant is consistent among doses withdrawn from a single vial. Single-dose vials are available and are strongly recommended for use in general veterinary practice.

Routes of Administration

Because noninfectious canine vaccines cannot infect or replicate, they must be administered parenterally (subcutaneously [SQ] or intramuscularly [IM]); noninfectious vaccine should not be administered directly onto mucosal surfaces (e.g., intranasal [IN] administration). Noninfectious canine vaccines stimulate primarily systemic humoral immunity (immunoglobulin-M [IgM] and -G [IgG]) with limited or no cell mediated immunity, depending on the antigen and the adjuvant.^11,24

The canine oral melanoma vaccine is a noninfectious recombinant (DNA) vaccine licensed for needle-free transdermal administration only. It is currently the only vaccine licensed for transdermal administration in dogs.

Initial Vaccination

Most noninfectious vaccines require at least two initial doses to immunize, regardless of the dog's age.^1,14,25,26 The first dose of a noninfectious vaccine generally primes the immune response and the second dose, which should be administered 2–6 wk later, provides the protective immune response. Immunity typically develops approximately 7 days after the second dose. Therefore, the minimum time for onset of immunity is approximately 3 wk after administration of the first dose of a noninfectious vaccine.

When the interval between the initial two doses of a noninfectious vaccine exceeds 6 wk, it is recommended the dog be revaccinated, administering two doses, 2–6 wk apart, to ensure protective immunity has developed.

Rabies vaccine is the obvious exception. Rabies vaccine antigen is highly immunogenic. Throughout the US and Canada, a single dose, administered at ≥12 wk of age, is considered to induce protective immunity. It should be noted that the onset of immunity after administration of the initial rabies vaccine may be defined by applicable legal requirements.

Minimum Age at the Time of Initial Vaccination

Administration of a noninfectious vaccine to a dog <12 wk of age may be blocked by maternally derived antibody (MDA). A second dose, even if given after 12 wk of age, would not be expected to immunize the patient (rabies being the exception). To ensure that puppies are effectively immunized, it is recommended that the first vaccine dose in the initial series of most noninfectious (inactivated, killed) vaccines be administered not earlier than 12 wk of age. Among orphans or those puppies that are known not to have received colostrum, the first dose of a noninfectious vaccine may be administered as early as 6 wk of age.

Immunization in the Presence of Maternally Derived Antibody

The mechanism whereby MDA interferes with noninfectious vaccine is different than that for infectious vaccine. Through a mechanism known as “antigen masking,” MDA covers, or “masks,” antigenic epitopes on the vaccine virus or bacteria that are necessary to elicit a protective immune response. In an effort to overcome MDA-induced interference with noninfectious vaccines, vaccine manufacturers can use a variety of methods. including the addition of adjuvant as well as increasing the antigen concentration in each dose of vaccine.

Because high titers of MDA specific for protective epitopes are generally required to cause “antigen masking,” MDA interference to most bacterins is uncommon after 6–9 wk of age. However, as noted previously, two doses of a noninfectious vaccine are required to induce a protective immune response. If sufficient MDA is present to interfere with the first dose, the second dose will not immunize. Therefore, it is recommended that the earliest age for administering the first dose of a noninfectious vaccine be 12 wk. Also, it is recommended that the noninfectious bacterins (e.g., Leptospira or Lyme) be given at ≥12 wk because the immune system is more mature. Thus, it is more likely that a protective immune response, rather than hypersensitive response, will develop.¹

Onset of Immunity

After initial vaccination, the onset of protective immunity requires more time to develop with noninfectious vaccines than with infectious vaccines. With most noninfectious vaccines, the minimum time from administration of the first dose in the initial vaccination series to development of protective immunity in a naïve dog is 3 wk (2 wk minimum interval between doses plus 1 wk for antibody production, for a minimum of 3 wk).¹⁴

The immune (antibody) response after administration of a single dose of a noninfectious vaccine in adult dogs that have been vaccinated within the previous year is considered to be rapid (hours to days) and protective.

The legally defined onset of immunity after administration of the first dose of a rabies vaccine is usually stipulated by state, local, or provincial requirements. Because the defined interval between the rabies vaccination and rabies immunization may vary among states and within states, veterinarians are encouraged to contact appropriate authorities regarding a specified onset of immunity interval for rabies.

Missed Dose—Initial Series

When administering a noninfectious vaccine for the first time in the life of a dog, at least two doses, administered 2–6 wk apart, is recommended. If the interval between the first two doses exceeds 6 wk, it is recommended that two additional doses be administered at an interval of 2–6 wk, thereby insuring that both immune priming and immunization occur.

Missed Dose—Adult Booster

Because noninfectious vaccines generally have a duration of immunity (DOI) that is shorter than infectious vaccines, annual revaccination (“booster”) is commonly recommended. A dog that failed to receive a noninfectious vaccine at the recommended interval of 12 mo is unlikely to maintain protective immunity for the same length of time (years) that occurs after administration of infectious viral (core) vaccines. At some point beyond 12 mo, administration of a single dose of a noninfectious vaccine may fail to induce a protective immune response (due to loss of immunologic “memory”); in such cases, administration of two doses, 2–6 wk apart, may be required to immunize.

However, intervals defining when two doses versus one dose would be required to immunize have not been established. Specific intervals will vary, depending on: (1) the vaccine, (2) the patient's (intrinsic) immune response, (3) time elapsed since administration of the last dose, and (4) total lifetime doses the dog received. The decision to revaccinate a dog with two doses versus one dose is left to the discretion of the veterinarian.

The following general guidance is offered for dogs that are overdue for a noninfectious vaccine and are considered to be at risk for exposure.

• Leptospirosis: limited studies have been conducted to assess immune response to a single dose of vaccine in dogs that have not received a booster vaccination in >12 mo. Among dogs with a high risk of exposure, it is reasonable to consider administering two doses of vaccine, 2–6 wk apart, if the interval between doses exceeds 24 mo.¹⁴

• Lyme disease: only limited (unpublished) studies have been performed to evaluate the immune response to a single dose of vaccine in dogs that have not received a booster vaccination in >12 mo. Although a single dose of Lyme vaccine given years after the initial doses can raise antibody levels, the protective quality of these antibodies has not been confirmed by challenge. Among dogs with a high risk of exposure, it is reasonable to consider administering two doses of vaccine, 2–6 wk apart, if the interval between doses exceeds 24 mo.²⁷

• CIV: studies have not been performed to evaluate the immune response to a single dose of vaccine in dogs that have not received a booster vaccination in >12 mo. Among dogs having a high risk of exposure, it is reasonable to consider administering two doses of vaccine, 2–6 wk apart, if the interval between doses exceeds 36 mo.

• Rabies: revaccination with killed rabies vaccine in dogs that exceeded the stipulated interval, 1 yr (initial two doses) or 3 yr (revaccination), is defined by applicable legal requirements. In most states, a dog that exceeded the defined interval for rabies vaccination may receive a single dose of a 3 yr vaccine regardless of the time elapsed since administration of the last dose; that dose will be considered protective for up to 3 yr.

Duration of Immunity and Booster Recommendations

Several noninfectious vaccines are routinely administered to dogs in the US and Canada. Although DOI studies are limited, it is reasonable to recommend annual boosters with most noninfectious vaccine in dogs considered to be at reasonable risk of exposure to the infectious agent.^14,24,28,29

RV antigen (glycoprotein G) is highly immunogenic, especially in the presence of adjuvant. Therefore, the DOI in dogs vaccinated with two initial doses, 12 mo apart, is expected to be 3 yr (when using a 3 yr rabies vaccine) in dogs that are ≥1 yr of age.

Vaccine Stability

Because antigenic virus/bacteria in infectious vaccines is live, these products often inherently lack thermostability.³³ To extend the stability of infectious vaccines during shipment and storage and to sustain vaccine efficacy, manufacturers typically prepare and sell infectious vaccines in a lyophilized (freeze-dried) state. Dehydrating the product into a “cake” significantly extends the shelf-life of perishable infectious vaccine antigens. Once diluent is added to the lyophilized product, the vaccine antigens quickly regain instability and may lose efficacy over time. Stability after reconstitution can vary among the various vaccine antigens in combination (multivalent) products (e.g., modified live virus [MLV] CDV + CPV-2 + CAV-2). It is recommended that infectious vaccines, after reconstitution, be administered within 1 hr. Reconstituted vaccine that is not administered within 1 hr should be discarded.

Once rehydrated, infectious vaccines are highly susceptible to chemical inactivation. For this reason, it is generally not recommended to cleanse the skin with alcohol before inoculation. Furthermore, syringes should never be washed and reused. Chemical residues in the syringe can easily inactivate the infectious vaccines. Infectious vaccines should be administered before the expiration date printed on the vial, as infectivity is lost over time.

It is important not to mix noninfectious vaccines with infectious vaccines in the same syringe, unless specified by the manufacturer, and even then, there may be advantages to administering a noninfectious vaccine in a different site on the animal from the infectious vaccine's administration site.^1,14,25

Multiple Dose Vials

Infectious vaccines licensed for use in dogs are not commonly sold in multiple dose (also called “tank”) vials. For the same reasons outlined previously for noninfectious vaccines, use of multiple dose vials of infectious (parvovirus) vaccine is not generally recommended.

Routes of Administration

Infectious vaccines contain avirulent live virus or bacteria that are capable of infecting cells in much the same manner as the virulent virus or bacteria does during natural infection. Therefore, infectious vaccines may be administered by the IN route (e.g., Bb + canine parainfluenza virus [CPiV]) as well as by the parenteral route (SQ or IM). Vaccines intended for IN administration must never be administered parenterally. Furthermore, IN vaccines administered orally are quickly inactivated and will not immunize.

Initial Vaccination

One dose of infectious vaccine will prime, immunize, and boost the immune response, provided the MDA does not interfere with the vaccine antigen (virus or bacteria). Because it is not practical to establish the level of maternal antibody in every puppy presented for initial vaccination, it is recommended that puppies receive doses of infectious vaccine (e.g., CDV + CPV-2 + CAV-2) every 3–4 wk between 8 and 16 wk of age. The final dose administered at 14–16 wk of age should insure the puppy will receive at least one dose of vaccine at an age when the level of MDA is insufficient to prevent active (vaccine-induced) immunity. Administration of infectious vaccine to dogs <6 wk of age, even in the absence of MDA, is not recommended.^1,14,24

Because dogs older than 14–16 wk of age are not likely to have interfering levels of MDA, administration of a single initial dose of an infectious vaccine to an adult dog can be expected to induce a protective immune response. The administration of a single, initial dose of infectious vaccine to dogs >16 wk of age is considered protective and acceptable (Table 1). It is common practice, however, in the US and Canada, to administer two initial doses, 2 to 4 weeks apart, to adult dogs without a history of prior vaccination.

Minimum Age at the Time of Initial Vaccination

In practice, predicting the exact age at which a puppy will first respond to administration of an infectious vaccine is difficult. MDA is the most common reason early vaccination fails to immunize. Puppies that received colostrum from an immunized dam might not respond to vaccination until 12 wk of age. In contrast, orphan puppies and puppies that were denied colostrum might respond to initial vaccination much earlier. The minimum age recommended for initial vaccination with an infectious (core) vaccine is 6 wk. Even in the absence of MDA, administration of an infectious vaccine to any dog <6 wk of age may result in a suboptimal immune response due to age-related immunologic incompetency.

In contrast, administration of an infectious vaccine labeled for IN administration (e.g., IN Bb + parainfluenza virus) may induce a protective, local (mucosal) immune response as early as 3–4 wk of age. MDA does not interfere with local immunity.

Immunization in the Presence of Maternally Derived Antibody

In general, MDA is more effective at interfering with infectious vaccines than noninfectious vaccines. Various mechanisms have been suggested, including rapid neutralization of infectious vaccine virus by maternal antibodies, prevention of replication, and insufficient antigen to prime B cells.^1,14,25

Different vaccine manufacturing methods have been successful in developing infectious vaccines that are able to overcome MDA in puppies at an earlier age. Such methods include increasing the virus titers within the product (e.g., “high titer” CPV-2 vaccine), using a more infectious virus (which often means more virulent), or administering the infectious vaccine via the IN route where the MDA is either limited or not present.

Like the heterotypic measles virus (MV) vaccine, the rCDV canarypox vectored vaccine has been shown to immunize puppies 2–4 wk earlier than MLV CDV vaccines.^46,47 However, neither of these vaccines can immunize puppies that have very high levels of MDA because of antigen masking. Thus, with all the methods used to avoid blocking by MDA, it may be possible to immunize earlier (days or weeks), but not to immunize all puppies at any age.^{19,22,30,37,45–49}

Onset of Immunity

The onset of immunity after administration of a single dose of infectious core vaccine is approximately 4±3 days in the absence of MDA. Variability among individual dogs and among different vaccines may alter these times slightly, with CDV providing the earliest protection within 1–2 days, CPV-2 providing protection in about 3 days, and CAV-2 providing protection in 5–7 days.^38,50,51 However, a small percentage of dogs are genetically incapable of developing an immune response to CPV-2 vaccines (estimated 1/1,000 dogs) or to CDV vaccines (estimated 1/5,000 dogs). These dogs are described as “nonresponders.” Immunologic unresponsiveness to vaccination is determined by genetic factors.

Because the number of nonresponders and low responders within the canine population is considered low, and nonresponder status is difficult to confirm, unique breed-specific vaccination recommendations for dogs are not stipulated in the Guidelines, but they may be recommended by some breed organizations.

Missed Dose—Initial Series

When administering an infectious vaccine for the first time in the life of a dog that is ≥6 wk of age, a single dose, in the absence of MDA, will immunize. If a puppy exceeds the recommended interval between doses of the initial vaccination series, it is left to discretion of the veterinarian whether to administer one or two additional doses.

If a puppy receives the first dose in the initial series of core vaccines between 6 and 8 wk of age but fails to return until 12 or 14 wk of age, administration of two doses, at least 2 wk apart, is recommended. In contrast, if the same puppy is >14 wk of age when returning to the veterinarian, administration of a single dose of an infectious vaccine is expected to immunize.

Missed Dose—Adult Booster

The DOI conferred by infectious core vaccines is known to last for many years. Even if serum antibody levels are determined to be below “protective” levels, immunologic memory (T- and B-lymphocytes) is likely to be sustained. Therefore, a single dose of infectious vaccine administered to an adult dog is considered protective regardless of the time since a previous vaccine was administered.^{20,31,43,52–54}

Duration of Immunity and Booster Recommendations

In general, DOI to infectious viral and bacterial vaccines is longer than to noninfectious viral and bacterial vaccines, and immunity conferred is generally much longer to viral vaccines than to bacterial vaccines. DOI is often related to the immunologic mechanisms of killing or control of the pathogens, and also to the complexity of the disease and the disease agent.

Infectious core vaccines are not only highly effective, they also provide the longest DOI, extending from 5 yr up to the life of the dog. A ≥3 yr interval is currently recommended for revaccinating adult dogs with infectious viral core vaccines. In contrast, revaccination of dogs with infectious bacterial vaccines (specifically IN Bb vaccine) is recommended annually. The ≥3 yr recommendation for core vaccines is made on the basis of minimum DOI studies over the past 30 yr for canine vaccines. These studies were done by all of the major vaccine companies, as well as by independent researchers. The results of the studies conducted by the major manufacturers for canine core vaccine demonstrated that a minimum DOI for their core vaccines (CDV, CPV-2, CAV) was ≥3 yr, based on challenge and/or serologic studies. Similar minimum DOI studies were conducted for the 3 yr rabies vaccines using challenge studies only.^{14,20,30,52–68}

Box 1 summarizes key immunologic features of noninfectious and infectious vaccines.

Box 1 Key Immunologic Features of Noninfectious and Infectious Vaccines

Bb, Bordetella bronchiseptica; CAV-1, canine adenovirus, type 1 (cause of canine viral hepatitis)—protection from CAV-1 infection is provided by parenterally administered CAV-2 vaccine; CAV-2, canine adenovirus, type 2; CCoV, canine coronavirus cause of enteric coronavirus infection (antigenically distinct from the canine respiratory coronavirus [CRCoV]); CDV, canine distemper virus; CIV, canine influenza virus—H3N8; CPiV, canine parainfluenza virus; CPV-2, canine parvovirus, type 2; DOI, duration of immunity; IgG, immunoglobulin G—a class of humoral antibody; most common type associated with immune response to parenteral vaccine; also the most common class of antibody measured as serum titers; IgM immunoglobulin M—a class of antibody, generally short lived and associated with early infection and initial vaccination; IM, intramuscular (route of administration); IMHA, immune-mediated hemolytic anemia; IMTP, immune-mediated thrombocytopenia; IN, intranasl; MLV, modified live virus—attenuated virus vaccine; MV, measles virus; OspA, outer surface protein A (antigen) of Borrelia burgdorferi; RA, rheumatoid arthritis, RV, rabies virus; sIgA, secretory immunoglobulin A—a class of antibody, most commonly associated with a local (mucosal) immune response after IN vaccination; SQ, subcutaneous.

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Box 1 (continued)

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Requirements

In the US, the Animal and Plant Health Inspection Service (APHIS), a multifaceted agency of the USDA, is responsible for regulating veterinary biologics (vaccines, bacterins, antisera, diagnostic kits, and other products of biologic origin) intended for the diagnosis, prevention, or treatment of animal diseases. For domestic manufacture, a facility license is required, along with a license for each product to be distributed. Imported products are issued a permit for sale and distribution. This work is done by APHIS's Center for Veterinary Biologics (CVBs).

Before the issuance of a license or permit, the manufacturer of a vaccine intended for sale and distribution within the US must demonstrate, to the satisfaction of the USDA CVB, that the proposed product is pure, safe, potent, and efficacious. The facility in which the product is prepared must meet USDA standards and pass inspection by the CVB. After licensure, each batch of vaccine is subject to random premarketing testing by the CVB to verify the manufacturer's quality assurance and quality control.

Purity assures the final product is free of extraneous microorganisms and extraneous material (organic or inorganic).

Safety is defined as freedom from properties causing undue local or systemic reactions when the vaccine is used as labeled. As part of the prelicense process, attenuated (live, whole agent) vaccines are evaluated in dogs to assess the potential of the vaccine organism to revert to virulence and the potential for dogs to shed the vaccine virus and/or bacteria. In addition, field safety studies are performed in a large group of dogs (typically at least 600), a substantial proportion of which must be at the minimum age indicated for administration. Postmarketing surveillance, including investigation of consumer complaints, is intended to identify relatively rare or uncommon safety issues that might not be detected in a prelicense field safety study. It should be noted safety studies are not a guarantee that a vaccine, once released for sale, will be entirely free of risk.

Efficacy is the ability or capacity of the product to effect the result for which it is offered when the product is used according to its label. Vaccine efficacy is conventionally determined through defined vaccination-challenge studies conducted by the manufacturer. Although challenge methods and criteria for evaluating protection will vary with the immunizing agent, tests are generally conducted under controlled conditions using seronegative dogs of the youngest age recommended on the label.

Potency is the relative strength of a biologic product as determined by test methods approved by the CVB. Potency testing is intended to assure that each serial (batch) of vaccine marketed is equal to, or more potent than, a defined reference serial of known efficacy.

DOI is noted here due to its interest to practitioners. However, the definition of the term is often interpreted differently in different contexts. The CVB views DOI as confirming, typically by a vaccination-challenge study, that the immunity conferred by the product lasts at least as long as indicated on the label. Practitioners may view these studies as confirming efficacy at a specified point, rather than a demonstration of the maximum reasonable duration of immunologic protection conferred to patients. Traditionally, vaccine challenge models were intended to demonstrate the onset of immunity in younger dogs using products titrated to the minimum protective dose. These products typically carried the historically based label recommendation for annual revaccination. Therefore, for most of the canine vaccines licensed in veterinary medicine, the CVB has not required manufacturers to conduct DOI studies, unless making a specific claim differing from 1 yr. Current CVB policy requires manufacturers to conduct DOI studies for all rabies vaccines and all new (novel) antigens, regardless of the revaccination interval.

Conditional Licensure

The time to market for a new vaccine can require several years. The USDA utilizes a pathway called conditional licensure to speed the availability to veterinarians of vaccines that address unmet needs, emergencies, or other special circumstances. In this process, a manufacturer is required to demonstrate that the product is safe, pure, has a reasonable expectation of efficacy, and that it is manufactured in compliance with standard USDA regulations. The USDA typically places time limits on such a license, during which the manufacturer must provide data to fully demonstrate efficacy or appropriate progress toward so doing. The USDA requires distinctive labeling to differentiate those products marketed under a conditional license, and the label must state that the product is conditionally licensed. The DOI of a conditionally licensed vaccine has not been confirmed by a vaccination-challenge study at the time the product is released for sale in the US.

(Conditional canine vaccines at this writing are: Crotalus atrox toxoid [Western Diamondback rattlesnake vaccine].)

Interpreting Results of Serologic Tests

Despite the confusion and controversy surrounding antibody testing, these serologic tests are useful for monitoring immunity to CDV, CPV-2, CAV-1, and RV. Because of this, many practitioners perform large numbers of tests for antibodies on a routine basis at state diagnostic and commercial laboratories or the tests are done with in-house diagnostics. The tests are also medically useful to ensure that a dog responds to a specific core virus vaccine and/or to determine if immunity is present in a previously vaccinated dog. Those tests are also used to demonstrate protective immunity as well as DOI.^56–69

Antibody assays for CDV and CPV-2—the two tests performed most often—are the tests of greatest benefit in monitoring immunity, especially after the puppy vaccination series. The serologic test considered the “gold standard” for CDV is virus neutralization (VN). VN and hemagglutination inhibition (HI) are the gold standard tests for antibodies to CPV-2.^1,14,67 Although most state diagnostic laboratories use the gold standard tests, most commercial laboratories use other methods, such as immunofluorescence assays or enzyme immunoassays. During the past 5 yr, most, if not all, laboratories have qualified and standardized their methodologies with samples that were tested by the gold standard methods. Also, standardization of these tests was done with samples collected from dogs protected from challenge with virulent virus.⁵⁴

Notwithstanding this development, titer results may vary among tests and between laboratories. Most state diagnostic laboratories report classic titers, in which two-fold dilutions of serum are made and the highest dilution that neutralizes the virus (CDV, CPV-2, CAV-1, RV), inhibits hemagglutination by the virus (CPV-2), or binds to viral antigen and is detected with a fluorescent or enzyme probe (CDV, CPV-2, RV) is reported. Using the standard two-fold dilution technique, the amount of error is approximately a four-fold dilution. The titer of a single serum sample would be in the range of one doubling dilution below the reported value and one doubling dilution above the reported value. For example, a CDV virus neutralization titer reported at 128 in reality is between 64 and 256; similarly, a CPV-2 HI titer of 1,280 is between 640 and 2,560. Some laboratories simply report results of >5 as positive and <5 as negative, and other tests are simply positive (antibody is present) or negative (no antibody was detected).

There are currently two in-hospital tests that provide a positive or negative result that have been approved by the USDA. A positive CDV result on these tests indicates that a serum sample has an antibody titer that is >8 on the VN test. A positive result for CPV-2 indicates the serum sample has an antibody titer that is >20 with the HI test. A negative test indicates that the dog has a titer less than these values or that it has no antibody. Obviously, some dogs with a negative result on this test are immune, but most of these dogs would benefit from revaccination by developing a higher titer. After performing and comparing many serologic tests for thousands of dogs, researchers found that approximately 15±5% of dogs will have low (≤32 VN) or no antibody to CDV. A similar percentage but different dogs will have low or no antibody to CPV-2 (≤80 HI) on the test. With CDV and/or CPV-2 tests, dogs with a negative result, regardless of the test used, should be considered as having no antibody and may be susceptible to infection with CDV and/or CPV-2; thus, these dogs should be revaccinated to ensure there is immunity. In contrast, any dog with a positive result, regardless of the test performed, should be considered immune and does not need to be revaccinated.^42,54

Applications of Serologic Testing

On completion of the puppy core vaccination series with the last dose given at 14–16 wk of age, a dog can be expected to have an antibody titer or positive test result, regardless of the serologic test performed, provided the serum sample is collected ≥2 wk after the last dose of vaccine. If the dog does not have antibody, it should be revaccinated, perhaps using a different product, and then retested ≥2 wk later. If the antibody test is again negative, the individual dog should be considered a low responder or a nonresponder (see Part I, Types of Vaccines) and possibly incapable of developing a protective antibody response.

Challenge with virulent virus or serologic testing is the only practical way to ensure a puppy develops an immune response after vaccination. The serologic test is the only acceptable way to ensure a client-owned dog develops an immune response. Young dogs are at greatest risk of infection from CDV and CPV-2, and these infections lead to severe disease and death in ≥50% of susceptible puppies. Antibody tests are useful as a medical procedure to ensure the dog develops an immune response to CDV and CPV-2 vaccines after the primary series of vaccinations. Vaccines can fail for various reasons.²⁵ However, the following are the three main reasons for vaccination failure: (1) the puppy has a sufficient amount of MDA to block the vaccine; (2) the vaccine is not immunogenic (e.g., if the vaccine was improperly stored); or (3) the dog is a poor or nonresponder (i.e., the immune system fails to recognize the antigenic determinants of the specific vaccine).

The most common reason for vaccination failure in young dogs is that MDA blocked the vaccine response. During the initial puppy vaccination series, the last dose of CDV and CPV should be administered at 14–16 wk of age. At this age, MDA should be at a level that will not block active immunization in most puppies (>98%) when a combination MLV vaccine is administered.^1,24,25 When the puppy fails to produce antibody ≥2 wks after a dose of vaccine administered at 14–16 wk, the practitioner must consider the other two explanations for vaccine failure. If, after one or more attempts at revaccination with a product different than the one originally used, the dog fails to develop an antibody response to CDV or CPV-2 by VN or HI test, the dog should be considered a transient or permanent nonresponder.

Because immunologic nonresponsiveness is genetically controlled, certain breeds or families of dogs may be suspected to have a higher prevalence of low or nonresponders than the general canine population. It is believed by some (but not proven) that the increased susceptibility to CPV-2 recognized in certain rottweilers and Doberman pinschers during the early and mid-1980s (regardless of their vaccination history) was due to an increased prevalence of nonresponders; it was also demonstrated that some early vaccination failures were attributable to the poor quality vaccines available at that time. Today, these two breeds appear to have no greater numbers of low or nonresponders than other breeds.^52,68

A high titer of antibody to CDV and/or CPV-2 as a result of active immunization from vaccination or from natural exposure protects from infection; therefore, no detectable virus replication occurs. Although a virus may be capable of replicating in a dog whose antibody titers have decreased, memory B and T cells should provide an anamnestic (secondary) humoral- and cell-mediated immune response that limits virus replication and prevents disease. Immune responses to modified live vaccines like CDV, CPV-2, and CAV-2, because of their complexity, always stimulate both humoral- and cell-mediated immunity. Although antibody is a product of humoral immunity, cellular immunity is always required for antibody production, as T-helper cells must be activated by the virus to produce a B-cell response. Therefore, although rarely considered, the presence of antibody in the dog to specific viruses demonstrates not only humoral immunity but also that cell-mediated immunity was stimulated as well. It is also incorrectly assumed that antibody to MLV vaccines containing CDV, CPV-2, and CAV-2 often disappears after relatively short periods of time (e.g., months or a few years). It was shown in many studies that antibody to those viruses persisted for many years, even in the absence of the viruses or revaccination.

The persistence of antibody to these viruses is from a population of long lived plasma cells that has been referred to as “memory effector B cells.” This is a population of cells that continues to produce the antibody they were programmed to produce (e.g., CDV) long after vaccination. Too much emphasis has been placed on the antibody titer (dilution of antibody that is positive). It was found repeatedly in controlled challenge studies with CDV, CPV-2, and CAV-1 that actively immune dogs (vaccinated at 14–16 wk of age or younger dogs without MDA) with actively produced antibody, regardless of titer or test used to detect the antibody, were resistant to challenge. Therefore, it is not necessary, as some have suggested, to have an antibody titer of ≥32 with the serum neutralization test for CDV or a titer of ≥80 on the HI test for CPV-2 for the vaccinated dog to be completely protected when challenged. Thus, most of the concerns expressed about the variability in titers among serologic tests have little or no validity when applied to protection from CDV, CPV-2, CAV-1, and RV. Furthermore, with the development of some of the in-hospital tests, serum dilutions are not performed and titers are not the end point; instead, the test is considered positive or negative.^43,54

Application of Serology to Evaluate Duration of Immunity

Antibody tests can also be used to demonstrate the DOI to vaccines or from natural immunization. As discussed previously, dogs were shown to maintain antibody titers to the core viruses CDV, CPV-2, and CAV-1 in viral-free environments for many years. In a study reported in 1997, dogs vaccinated with a product containing CDV and then placed in an environment without CDV maintained antibody titers for at least 10 yr.⁶¹ In a more recent controlled study of puppies without MDA vaccinated at 7 and 10 wk of age (and housed with nonvaccinated dogs to ensure CDV, CPV-2, and CAV-1 were not present), it was shown that vaccinated dogs maintained antibody titers for >4 yr.^61,54,69 These and other studies clearly demonstrated that antibody correlated with protection from infection and/or protection from disease because the vaccinated antibody-positive dogs remained healthy after experimental challenge with virulent strains of the viruses. These and other studies also clearly demonstrated that antibodies to the core vaccine viruses might persist in the absence of revaccination for many years. All of the major vaccine manufacturers have products that were shown to provide a minimum DOI of 3 yr. In addition, it was demonstrated that antibody correlated with protection from infection and/or protection from disease because the vaccinated antibody-positive dogs remained healthy after experimental challenge with virulent strains of the viruses.^55–57,69 In contrast, vaccinated dogs that did not develop antibody to CDV, as well as unvaccinated control dogs that were antibody negative, became infected. Many dogs develop disease and die when challenged. When antibody is absent (irrespective of the serologic test used to determine this fact), it should be assumed the dog is susceptible to infection and may develop disease. Therefore, antibody negative dogs should be revaccinated. Similarly, dogs that have been actively immunized by vaccination or naturally by infection that have antibodies to CDV, CPV-2, or CAV-1 do not need to be revaccinated. Some clients are now having titers performed for CDV and CPV-2 in lieu of revaccinating.

Antibody titers to additional vaccine antigens are sometimes determined to diagnose susceptibility to disease, but the best correlations between antibody and protective immunity are as stated previously for CDV, CPV-2, CAV-1, and RV. Very sensitive and well-documented titers to RV are done by a small number of approved laboratories. Although most widely used when shipping dogs to rabies-free countries, rabies titers are sometimes performed in dogs that developed an adverse reaction to the vaccine.^70–72 However, RV titers cannot currently be used in place of revaccination, which is required on an annual or triennial basis depending upon governing law. Medical exemption laws exist in certain areas where a dog with a known medical condition can be exempted from rabies vaccine. However, a titer cannot be used in place of vaccination. When RV vaccination is not current, the dog must be considered unvaccinated, and if it bites someone, it must be quarantined.

Antibody titers to vaccines other than CDV, CPV-2, CAV-1, and RV have limited or no value because the antibody may persist for a short time (e.g., Leptospira products), or there is no known correlation between serum antibody test routinely performed and protection (e.g., CPiV, Lyme, Leptospira). However, researchers are attempting to find serologic correlates of protective immunity for diseases other than the four core viruses (CDV, CPV-2, CAV, and RV).

What Constitutes a Vaccine Adverse Event?

A vaccine AE is generally defined as any undesirable side effect or unintended effect (including lack of desired result) associated with the administration of a licensed biologic product (vaccine). For vaccines administered to dogs, AEs are those involving the health of the treated dog and include the apparent failure to protect against a disease. An AE event includes any injury, toxicity, or sensitivity reaction associated with the use of a vaccine, whether the event can be directly attributed to the vaccine. In other words, it is appropriate to report any known or suspected negative event associated with vaccination.

Although the incidence of vaccine AEs is unknown and causality cannot always be confirmed, the list that follows includes categories of adverse reactions that have been attributed to vaccine administration. The list of categories is not considered comprehensive; other, undocumented adverse reactions associated with vaccine administration could occur. Furthermore, causality has not been definitively established for each of the categories listed:

• Injection-site reactions: lumps (abscess, granuloma, seroma), pain, swelling, hair loss associated with ischemic vasculitis

• Transient postvaccinal nonspecific illness: lethargy, anorexia, fever, regional lymphadenomegaly, soreness, abortion, encephalitis, polyneuritis, arthritis, seizures, behavioral changes, hair loss or color change at the injection site, respiratory disease

• Allergic (hypersensitivity) and immune-mediated reactions:

  • –Type 1 (acute anaphylaxis): angioedema (especially the head), anaphylaxis (shock), and death

  • –Type 2 (cytolytic): immune-mediated hemolytic anemia, immune-mediated thrombocytopenia (suspected only; causality has not been confirmed)

  • –Type 3 (immune-complex): cutaneous ischemic vasculopathy associated with rabies vaccine, corneal edema (‘blue-eye’) associated with CAV-1 vaccine, immune-mediated disease

• Failure to immunize: maternal antibody interference with vaccination is considered the most common cause; administration of vaccine at a volume and/or dose less than that prescribed by the manufacturer; “nonresponder” (genetic predisposition?); inactivation of vaccine antigen (e.g., allowing reconstituted infectious [attenuated, avirulent, modified live, recombinant viral vectored] vaccine to stand at room temperature for >2 hr), mixing of incompatible vaccines in the same syringe

• Tumorigenesis: vaccine-associated sarcoma or other tumors

• Multisystemic infectious/inflammatory disorder of young Weimaraner dogs: may be genetically linked to both a poorly characterized immunodeficiency and to autoimmune disorders (e.g., hypothyroidism and hypertrophic osteodystrophy [HOD] that are detected shortly after vaccination

• Vaccine-induced immunosuppression: associated with first or second dose of combination MLV vaccines containing CDV and CAV-1 or CAV-2 with or without other vaccines (e.g., CPV-2, CPI). Immunosuppression begins 3 days after vaccination and persists for 7–10 days. The suppression may be associated with increased susceptibility to other diseases.¹⁷

• Reactions caused by the incorrect or inappropriate administration of vaccine: fatalities have been reported after subcutaneous administration of an avirulent-live Bb bacterin (intended for IN administration); inadvertent or intentional administration of vaccine by the intravenous route

• Reactions associated with residual virulence attenuated vaccine: postvaccinal sneezing associated with IN administration of attenuated vaccine (e.g., Bb + parainfluenza virus)

• Vaccine-induced interference with diagnostic tests: false-positive polymerase chaine reaction (PCR) test results for parvovirus antigen in feces in dogs recently receiving a MLV parvovirus vaccine. Not an adverse reaction.

• Reversion of vaccine virus to a virulent pathogen: generally considered rare to nonexistent among currently licensed canine vaccines when vaccines are used in the species for which they were licensed. This can become a significant problem when vaccine is used in the wild and/or exotic animals.^{8,9,12,13,16–18}

How to Report a Known or Suspected Vaccine Adverse Event

Veterinarians are encouraged to participate in the vaccine AE reporting process by reporting suspected and known AEs to one of the following:

• Vaccine Manufacturer: Companies that manufacture vaccines maintain a technical services section that will accept and address AE reports from veterinarians who use their product(s). Veterinarians are encouraged to report AEs to the manufacturer(s) before contacting the appropriate regulatory agency. Manufacturers are required to maintain files of any reported vaccine AE. However, manufacturers are under no obligation to compensate the owner or the veterinarian for diagnostic or treatment services related to a known or suspected AE.

• CVB: Subsequent to reporting a known or suspected vaccine AE to the manufacturer, veterinarians practicing within the US may contact the USDA, APHIS CVB in one of the following ways:

Once an adverse event has been reported to the manufacturer, the CVB may be contacted:

• Online: https://web01.aphis.usda.gov/CVB/adverseeventreport.nsf/Adverse%20Event%20Report%20Form?OpenForm

• By fax or mail: download the PDF form at http://www.aphis.usda.gov/animal_health/vet_biologics/publications/adverseeventreportform.pdf and FAX to (515) 337–6120 or by mail to the CVB.

• By telephone: AEs may also be reported by calling the CVB at (800) 752–6255.

Canadian Food Inspection Agency

In Canada, CFIA is responsible for licensing veterinary biologics, including veterinary vaccines, manufactured and/or used in Canada. The licensing program operates under the Health of Animals Act and Regulations, and is administered by the Canadian Centre for Veterinary Biologics.

The Canadian Health of Animals Regulations require all holders of product licenses and import permits to report all “serious expected” or “serious unexpected” suspected AEs, including lack of efficacy, to the Canadian Centre for Veterinary Biologics of the CFIA within 15 days of receiving notice of the event from a veterinarian or animal owner. This can be done by notifying Canadian Centre for Veterinary Biologics directly or through the licensed vaccine manufacturer or importer.

In Canada, Form CFIA/ACIA 2205 “Notification of Suspected Adverse Events to Veterinary Biologics” can be used to report suspected AEs: http://inspection.gc.ca/english/for/pdf/c2205e.pdf

The Canadian “Veterinary Biologics Guideline 3.15E: Guideline for Reporting Suspected Adverse Events Related to Veterinary Biologics” (available: http://www.inspection.gc.ca/english/anima/vetbio/info/vb315e.shtml) provides guidelines for defining a suspected AE related to veterinary biologics as one of the following: AE, SAE, unexpected AE, and lack of efficacy. The definitions for AE, SAE, and unexpected AE are found in Section V of this guideline and are consistent with the International Cooperation on Harmonization of Technical Requirements for Registration of Veterinary Medicinal Products (VICH)'s Guideline 24: Pharmacovigilance of veterinary medicinal products: management of adverse event reports (AERs). A causality assessment should also be assigned to each SAE. Each case should be classified as probable, possible, unlikely, or unknown.

Managing Adverse Event Risk in Individual Patients

Specific recommendations for mitigating the risk of a vaccine AE in dogs have not been validated. Efforts to manage risk are highly varied and largely unsubstantiated. It is not possible to completely avoid a vaccine AE in any patient. Vaccine risk management should focus on dogs having a known or suspected history of a vaccine reaction and in small breeds. Recommendations are outlined in the following.

The reduction of vaccine volume to mitigate risk of an AE is not recommended. Doing so may result in suboptimal immunization or no immune response without reducing risk of an AE. Vaccine dose is not based on size (body mass); therefore, small dogs require the same dose of vaccines as large dogs.

Patients with a Known or Suspected Vaccine Adverse Event History

Acute hypersensitivity (nonsystemic) and injection-site reactions are among the most common vaccine AEs reported because they occur within hours or a few days after vaccination. The decision to administer vaccine to any patient with a history of having experienced an acute-onset (minutes to 1–2 days postvaccination) reaction is left to the discretion of the veterinarian. History of an acute-onset AE is not predictive of future risk.

Administration of an antihistamine or NSAID before vaccination to prevent transient postvaccinal nonspecific illness has not been studied adequately in dogs to make specific recommendations on their use or benefit. However, it is common practice to administer an antihistamine (diphenhydramine, 2–4 mg/kg orally or 1 mg/kg parenterally) to patients with a history of an acute adverse reaction. A single dose is generally administered 15–30 min before administering vaccine. In such cases, it is recommended that the patient remain at the practice and be monitored for at least 30 min postvaccination.

In an attempt to mitigate the risk associated with administering vaccine to any patient with acute-onset vaccine AE, veterinarians may also elect to administer the same vaccine type but one produced by a different manufacturer.

The decision to administer pretreatment and/or a vaccine produced by a different manufacturer to any patient with a history of having a known or suspected vaccine AE does not guarantee that an AE will be prevented.

It is reasonable to avoid administration of any vaccine to patients with a history of systemic disease suspected to be associated with previous vaccination (e.g., immune-mediated hemolytic anemia, immune-mediated thrombocytopenia) or known to be caused by vaccine (vaccination-site cutaneous ischemic vasculitis after administration of rabies vaccine). In lieu of annual or triennial revaccination, assessment of antibody titers can be determined (CDV and CPV) (see “Serologic Testing”). Dogs with a “positive” titer are considered protected. These patients can be considered to have sufficient immune memory to mount a protective humoral immune response for several years and may not require vaccination. Dogs with a “negative” antibody titer may be susceptible to infection. Whether to administer vaccine to dogs with a negative antibody titer is left to the discretion of the veterinarian. However, a negative antibody test for CDV and/or CPV-2 may indicate the dog is susceptible to either of these significant diseases.

The decision not to administer rabies vaccine for health reasons is problematic in locations that require rabies vaccinations yet do not grant rabies exemption authority to veterinarians. Some states and/or provinces do grant rabies vaccination exemption authority to veterinarians who have examined a patient and determined, for health reasons, vaccine should not be administered. Such waivers generally remain in effect until the patient is deemed sufficiently healthy to receive the vaccine. Veterinarians are urged to contact state, provincial, and/or local authorities to determine whether such exemption authority exists.

Small Breed Dogs

One study addressed vaccine AEs in >1.2 million dogs that received >3.4 million doses of vaccine.⁷³ This study provided important insight on risk associated with administration of multiple vaccine doses to small breed dogs at the same appointment. History of a vaccine AE in a small breed dog is not predictive of future risk. Any dog, regardless of size, breed, gender, or age, can experience a vaccine AE.

Mitigating risk in small dogs (puppies and small breeds) by reducing the volume of vaccine is not recommended. Doing so may result in a suboptimal response to the vaccine and may not eliminate risk associated with hypersensitivity to one or more vaccine constituents. As with all dogs, small breed dogs should be assessed for risk of exposure to infectious pathogens and only those vaccines considered essential should be administered. Furthermore, prioritizing administration of core vaccines (CDV, CPV-2, CAV-2, and rabies) to all dogs at the appropriate age (see Table 1) is recommended.

The decision to administer one or more noncore vaccines to a dog should be based on reasonable knowledge of exposure risk in the individual patient. It should also be noted that most of the noncore vaccines listed within the Guidelines are inactivated (killed) vaccines and that these vaccines may be associated with a higher incidence of AEs when administered at the same time as other vaccines, particularly in small breed dogs. Therefore, veterinarians may wish to delay administration of inactivated noncore vaccines to small breed dogs until after completion of the initial core vaccine series.

Do Veterinarians Have Professional Discretion in the Use of Vaccines in Their Practice?

Yes, with a few limitations. The recommendations contained in the Guidelines may differ in places from statements on product labels. However, veterinarians in small animal practice in the US have considerable discretion in exercising their judgment relative to the use of veterinary biologic products licensed by the USDA within their professional practice.^a The same is true for veterinarians in Canada using biologic products approved under the Canadian Food and Drug Act.^b As such, practitioners have the ability to incorporate use of the Guidelines into their practices.

The USDA CVB regulates the licensure and preparation of most veterinary biologics, including all material on their labeling. CVB does not regulate the practice of veterinary medicine. Although CVB does have the statutory authority to stop the sale, barter, or exchange of “any worthless, contaminated, dangerous, or harmful virus, serum, toxin, or analogous product,” they would only take action against a small animal practitioner under extraordinary circumstances. Before initiating such action, CVB would most likely contact the veterinarian and/or undertake a profession-wide educational initiative.^c

Vaccines licensed by the USDA and prepared in establishments licensed by the USDA are not directly subject to the Animal Medicinal Drug Use Clarification Act (AMDUCA) or Food and Drug Administration's (FDA) implementing regulations. However, it is possible for the FDA's Center for Veterinary Medicine to regulate some products that most practitioners would consider biologicals. Products that are approved by the FDA are subject to AMDUCA's and FDA's established specific rules for “extra-label” drug use. Products regulated by the USDA may be identified by the “USDA Establishment Number” that appears on labeling.

States may also regulate the discretionary use of biologic products by veterinarians. This can be confusing, as the state and federal terminology may be similar but applied differently. The state's definition of “drug” may include biologic products, and the state may use the term “extra-label” differently than the federal application.^d Veterinarians should be aware of any state-specific restrictions in their state's veterinary practice act or implementing regulations. However, it is the authors' belief that such restrictions are sufficiently general that they should not interfere with the ability to use these Guidelines. In Canada, the provinces have the legal authority to regulate the veterinary profession but no authority whatever relating to trade in drugs, medications, and biologics. In this context, the provincial veterinary legislation may, for instance, require that a veterinarian obtain the informed consent of the client before using a substance in a manner that differs from its labeled indications.

Rabies vaccine represents a unique class of products due to the public health concern. The USDA places restrictions on the licenses for rabies products, such that their distribution in each state is limited to authorized recipients as designated by proper state officials (e.g., the state veterinarian) and under such additional conditions as these authorities may require. Each state, in turn, has its own rabies control program. The substance of this law varies among jurisdictions and can encompass state, provincial, and/or local requirements. A common theme to this regulation is compulsory vaccination, irrespective of the label statements that the products are for use in healthy animals. Sometimes veterinarians and/or clients desire to forego rabies vaccination, believing it to be contraindicated due to the health or age of the dog. Veterinarians must be very careful in such circumstances. Although some states have procedures for addressing this situation, it is not addressed in most states.^e Veterinarians must not assume they have the discretion to recommend against vaccination in the face of mandatory state vaccination laws. Therefore, it is imperative that veterinarians investigate, understand, and follow the legal requirements for rabies vaccination in the areas in which they practice. The same approach is prudent in Canada.

Potential for Liability Associated with Vaccine Administration

Potential liability for medical decision making is a fact of life for any health care provider, including veterinarians. This potential professional liability encompasses all aspects of veterinary practice, including the selection and use of vaccines and other biologic products. Most lawsuits against practitioners are grounded in negligence, although the range of possible legal liability theories is broad and limited only by the creativity of the plaintiff's attorney. There is no reason to believe a veterinarian's use of vaccines would be treated differently or carry any greater risk than other areas of small animal practice.

Medical Negligence

Legal actions against a veterinarian alleging professional negligence are commonly called “malpractice” or “medical malpractice” cases. The body of law for professional medical negligence has evolved in the context of human medicine. Most jurisdictions apply many of the legal concepts developed in the litigation of physician malpractice cases to veterinary malpractice cases, particularly the requirement for expert testimony. The traditional elements of a medical malpractice lawsuit are the duty to conform to a certain standard, a failure to conform to the required standard, actual injury or damage, and a legally sufficient causal connection between the conduct and the injury.^f

Medical Negligence as It Applies to Vaccination Decisions

The basic scenarios that could potentially give rise to a claim or lawsuit are where (1) a patient that is not vaccinated contracts the disease for which vaccination was forgone; or (2) a patient experiences an AE attributed to a vaccination later considered unnecessary by the client. In either case, the plaintiff would be required to have expert testimony that the defendant's professional judgment under the specific circumstances was a departure from the standard of care and the cause of the injury to the dog. Although such claims do occur, the risk of a lawsuit is considered low and can be mitigated through effective, documented communication with the client.

Consent Versus Informed Consent

Consent is the giving of permission, approval or agreement. Consent can be expressed or implied, written or verbal, documented or not. A veterinarian should understand regulations relative to obtaining or documenting consent in states where they practice, as a state's practice act or regulations may address necessary documentation of client consent.^g

Informed consent is consent based upon the disclosure of the material risks of a proposed treatment or procedure and potential alternatives, including the risk of no treatment.^h The legal doctrine of informed consent developed as human medicine evolved from a paternalistic profession to one that recognizes the importance of a patient's self-determination. It is based upon the theory that a competent human being has the right to determine what is done with their body. To date, most states and provinces have not formally addressed the question of applying informed consent law to veterinarians. There are, however, a few states and provinces with reported court decisions addressing the application of the doctrine of informed consent to veterinary practice in some fashion.ⁱ Additionally, there are a few states and provinces where the veterinary practice act and/or implementing regulations incorporate either the doctrine of informed consent or elements of it, and the American Association of Veterinary State Boards has developed a model practice act that recommends to states the incorporation of the requirement to obtain informed consent by board regulation.^j However, within the US there remains ongoing debate about whether informed consent law should be applied to veterinary practice. This is not the case with Canada, where the incorporation into veterinary practice is readily accepted, either by regulation or convention. Some within the veterinary community advocate forgoing use of the term “informed consent” for other terms while incorporating risk communication elements in an analogous manner. The intent here is not to advocate for or against the doctrine of informed consent or its particulars. Rather, it is to acknowledge that allegations of a failure to obtain consent or informed consent, historically common in physician medical malpractice litigation, are not uncommon in complaints against veterinarians as well. Therefore, it is prudent to understand the issue and to understand that one of the best deterrents to an informed consent lawsuit (or other legal action for that matter) is effective communication with clients.

Documentation of Consent

Documentation of consent discussions is always helpful if there is ever need to defend a veterinarian's actions. Such documentation could include a note in the chart that such a discussion took place (with or without co-signature by the client); a note in the chart that in addition to discussion, a specific client handout was given^k; or use of a consent form signed by the client. Although defense lawyers like more documentation, the task for practitioners is to determine the method that best suits their practice and level of risk tolerance.

Where consent forms are used, the more general the language used, the less helpful the documentation may prove in court; conversely, the more specific the language, the more helpful to the defense of a case. However, the practitioner should have a medically or scientifically defensible basis for making any representations in a consent document. If precise numbers cannot be justified, then more general statements are preferable.

Medical Record Documentation (AAHA Accreditation Standards)

At the time of vaccine administration, the following information should be recorded in the patient's permanent medical record:

• Vaccines recommended for this patient

• Date of vaccine administration

• Identity (name, initials, or code) of the person administering the vaccine

• Vaccine name, lot or serial number, expiration date, and manufacturer of vaccines actually administered

• Site and route of vaccine administration

• Any concurrent medications/therapy

• Future recommended vaccinations

AEs should be recorded in a manner that will alert all staff members during future visits. Consent should be documented in the medical record to demonstrate that relevant information was provided to the client and that the client authorized the procedure.

Special Considerations of a Shelter Vaccination Program

The relatively high likelihood of disease exposure in most shelters and the potentially devastating consequences of infection necessitate a clearly defined shelter vaccination program with exacting requirements. It is necessary to define not only what vaccines are appropriate, but also when vaccines should be administered with respect to shelter entry, which animals are candidates for vaccination, and how and by whom vaccines should be administered, including record keeping and documentation of AEs. For vaccines that offer significant protection against common and severe infectious diseases, the appropriate vaccination program may be one that is more aggressive than is generally indicated in private practice. Such a program may include, for example, vaccinating dogs at the short end of the suggested intervals or at a relatively early age.

With the use of vaccines at shorter intervals or in an expanded population, it is also important to minimize the vaccines given to those that are clearly indicated by the immediate and significant disease risks. Vaccines are often administered to stray dogs not legally belonging to the shelter and may be given by lay staff under indirect veterinary supervision. These considerations make it even more crucial to develop a vaccine program that minimizes the risk of vaccine-induced adverse reactions. Furthermore, cost differences that are trivial for one individual become significant when multiplied by thousands of doses. Therefore, only those vaccines that demonstrate a clear benefit against common and significant shelter diseases should be used. Adopters should be encouraged to discuss an individually tailored vaccination program with their own veterinarian after adoption.

Core Vaccines for Shelter-Housed Dogs

Vaccines for shelter use are categorized for pet dogs, as core and noncore (optional) (Table 2). A number of other vaccines discussed in the following are not recommended. Although the Task Force acknowledges that variable shelter circumstances make it impractical to provide universally applicable recommendations, those vaccines categorized as core are essential vaccines that should be administered to all dogs at the time of entry (CDV, CPV-2, CAV-2, IN Bb + CPiV) or at the time of release (RV).^51,74–77

TABLE 2 2011 Canine Vaccination Guidelines for Shelter-Housed Dogs

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TABLE 2 2011 Canine Vaccination Guidelines for Shelter-Housed Dogs

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Contact Information for Biologics Manufacturers

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It is recommended that all dogs be vaccinated for rabies before release from a shelter. If a long-term stay is anticipated or for shelters where virtually all dogs will be adopted, rabies vaccine should be administered on intake with the other core vaccines. The earliest age at which rabies vaccine should be given is 12 wk, and it is recommended that it be given at a site on the body different than where the CDV, CPV-2, CAV-2 vaccines are administered. At open-intake shelters, rabies vaccine should be administered at the time of release. Although ideally vaccines should be given at least 2 wk apart to avoid vaccine interference, the public health benefit of ensuring rabies vaccination before release is considered to outweigh the small risk of interference in this case. If state or local requirements prevent issuance of a rabies certificate for vaccines administered at the shelter (e.g., due to lack of veterinary supervision), vaccination for the purpose of legal recognition and licensing should be repeated at the owner's veterinarian 2–4 wk later. Unless a certificate documenting previous rabies vaccination is available, it should be assumed that previous vaccination has not been received, and revaccination 1 yr later will be required.⁶⁹

Noncore Vaccines for Shelter-Housed Dogs

The CIV vaccine may be recommended (noncore) in selected shelters located within endemic communities or in shelters that transport dogs to or from communities considered to be endemic for canine influenza. This is a killed vaccine that requires two doses be given at least 2 wk apart. Immunity is expected 1 wk after the second dose. Therefore, even in shelters located within endemic communities, the benefit of this vaccine will be limited if exposure cannot be prevented before onset of protection or in dogs unlikely to stay long enough to receive the full series of vaccines.^78,79

Vaccines Not Recommended for Use in the Shelter Environment

The vaccines listed in the not recommended category are for diseases that do not represent a significant threat to the population of dogs residing in shelters, would not provide protection because there is inadequate time for immunity to develop, or that have limited efficacy against clinical disease. Among the various canine vaccines licensed for use within the US, the following vaccines are not recommended for routine use in shelter-housed dogs: leptospirosis; canine coronavirus; canine Lyme borreliosis (Lyme disease); Crotalus atrox (rattlesnake) vaccine; parenterally administered Bb (see Table 2 for exception); and parenterally administered CPiV. Because most of these vaccines are killed (inactivated) and, therefore, require two doses at least 2 wk apart, use of these vaccines is viewed as impractical and unnecessary in most shelter-housed dogs.

Dogs with a Documented Vaccination History at Time of Admission

There is no compelling reason to administer vaccines to an individual dog at the time of admission to a shelter if clear documentation confirms current vaccination administered after the age of 16 wk is provided. The following is the minimum information acceptable as documenting proof that a valid vaccination has been administered:

• Proprietary name of product

• Manufacturer name

• Serial/lot number

• Date vaccine was administered (at least month and year)

• Expiration date of vaccine administered

• Signature of a licensed veterinarian

This information should be associated with a medical record that clearly describes the dog in question. If any of this information is not available at the time of admission or cannot be associated with a formal record for the dog, then immediate vaccination is indicated.

Long-Term Shelter-Housed Dogs

It is recommended that all dogs entering a long-term care facility (or any dog entering a shelter for which a long-term stay is anticipated) be inoculated with all core vaccines, including rabies vaccine, at the time of admission to the facility. If a dog is routinely exposed to the outdoors, then noncore (optional) vaccines should be considered (as for pet dogs), depending on the dog's risk profile.

Because it can be difficult or impossible to determine whether young dogs (<4 mo of age) have received any vaccines at all, implementation of an initial series (CDV, CPV-2, CAV-2 [IM, SQ], Bb, and CPiV [IN]), beginning as early as 4 wk of age (as early as 3–4 wk of age for IN administered vaccines), may be indicated. Parenterally administered core vaccines should not be administered before 6 wk of age. When it is the decision of the facility to initiate the series (i.e., “puppy shots”) to an individual dog, then the recommended vaccines should be administered at 2 wk (rather than 3 or 4 wk) intervals until the dog reaches ≥16 wk of age.

In the event that an individual dog resides in the facility long enough to justify booster vaccination, it is recommended that the revaccination schedule recommended for individual pets be followed (Table 1).^{29,51,74,76,80–82}

Vaccination of Pregnant Dogs in the Shelter Environment

Shelter personnel may be faced with the dilemma of whether to vaccinate a pregnant dog upon admission to a facility. Historically, vaccination during pregnancy has not been recommended in small animal medicine. This is due in part to the paucity of data concerning vaccine safety and efficacy during gestation and the expectation that, in nonimmune pregnant bitches, MLV vaccine can cause fetal damage or death.^22,30,49 When the immunity of the dog is unknown, however, the risk of maternal, fetal, and neonatal infection with field strain virus must be weighed against the risk of vaccination. If nonimmune pregnant dogs are likely to be exposed to field strain infection with pathogens such as parvovirus or distemper, serious illness or death of both bitch and fetuses may result. Unless facilities are available to completely isolate them from other dogs, pregnant bitches should either be vaccinated or not remain in the shelter.

Vaccination of Sick Dogs in the Shelter Environment

As with pregnant dogs, veterinary medicine has advised against vaccination during illness, due to concerns about suboptimal protection, or worse, vaccine-induced illness. The decision to administer or delay vaccination because of a current illness depends on the severity of disease and its etiology.

The shelter environment does not usually permit the luxury of isolating dogs and delaying their vaccination until concurrent illness is resolved. Therefore, vaccination is advised upon admission for dogs with minor illness (e.g., otitis, dermatitis, upper respiratory tract infection with or without fever) or injuries. Vaccination of dogs with severe signs of disease ideally should be delayed whenever feasible. However, unvaccinated shelter dogs may develop more severe disease if left unvaccinated, and thus would be at greater risk of dying. In the high-risk shelter environment, vaccination of sick dogs with core vaccines should be the rule with very few exceptions.⁵¹