Corynebacterium spp. in Dogs and Cats with Otitis Externa and/or Media: A Retrospective Study

Introduction

Corynebacterium spp. are gram-positive, rod- or club-shaped, nonspore forming, aerobic bacteria.¹ They are present in the environment, including soil, water, and sediments.² Although various Corynebacterium spp. have been associated with specific diseases in both humans and animals, their pathogenicity has been questioned.^3–10

Corynebacterium spp. have been cultured from normal canine ear canals and middle ears as well as the ear canals and middle ears of dogs with otitis externa and/or otitis media.^3,10–14 Despite their association with otitis in dogs, Corynebacterium spp. pathogenicity and appropriate antimicrobial therapy are unclear and controversial.^3,10 There are no reports of Corynebacterium spp. in normal feline ear canals or middle ears, but Corynebacterium spp. have been documented in feline otitis externa.^15–18 Corynebacterium spp. in feline otitis media have not been reported.

The pathogenicity of Corynebacterium spp. has been questioned in canine otitis because Corynebacterium spp. are part of the normal flora of canine ear canals and because Corynebacterium spp. have always been reported in combination with other microbes in cases with otitis externa and/or media.^3,10,19 In human studies, criteria suggested to support a direct association between Corynebacterium spp. and disease include finding a pure culture at the site of disease and isolating the bacteria from sites that are usually sterile.¹

Anecdotally, antimicrobial treatment of canine and feline otitis associated with Corynebacterium spp. is empirical. This is largely because otitis externa in dogs and cats is most frequently treated empirically with products that contain broad-spectrum antibiotics, such as gentamicin, neomycin, polymyxin B, or enrofloxacin. The suitability of those therapies has not been assessed for Corynebacterium spp. because of the relative lack of reported sensitivity data for Corynebacterium spp. and due to failure to document response to antimicrobial therapy. Having access to reference sensitivity data for Corynebacterium spp. is desirable because even when Corynebacterium spp. are cultured, some laboratories fail to routinely provide sensitivity data. This is most likely because the pathogenicity of Corynebacterium spp. remains unclear. The objectives of this retrospective study were to (1) better establish the pathogenicity of Corynebacterium spp. in otitis externa/media utilizing reported criteria and by assessing clinical response to antibiotic therapy and (2) to determine the antimicrobial susceptibility patterns of Corynebacterium spp. isolated from the ears of dogs and cats with otitis externa and/or media.

Materials and Methods

This was a retrospective study evaluating canine and feline cases of otitis externa and/or media with cultures positive for Corynebacterium spp. A computer search for ear cultures showing growth of Corynebacterium spp. was performed by the Colorado State University Veterinary Diagnostic Laboratory. All positive cultures used in the study came from the Colorado State University Veterinary Teaching Hospital case samples submitted between Jan 1, 1998 and Dec 31, 2008. The medical records from those cases were reviewed. Signalment and clinical, microbiologic, cytologic, and therapeutic data were recorded (when available). Signalment included age and breed. Clinical data included notation as to whether the otitis was restricted to the external ear canal and/or the middle ear as well as otoscopic examination findings. Microbiologic data included the notation of the number of cases in which Corynebacterium spp. were single isolates and those in which Corynebacterium spp. were part of multiple isolates. When multiple isolates were present, the identity of all other cultured bacteria and yeast were noted. The antibiotic sensitivities for all Corynebacterium spp. were tabulated. Antibiotic sensitivity was determined by the Kirby-Bauer disc diffusion using breakpoints based on methods of antimicrobial dilution and disk susceptibility testing of infrequent or fastidious bacteria defined by the Clinical and Laboratory Standard Institute.²⁰ Antibiotic sensitivity testing was based on a routine antibiotic panel used by the Colorado State University Veterinary Diagnostic Laboratory (Table 1). An extended sensitivity panel, including one or more additional antibiotics, was tested when requested by the clinician (Table 1).

TABLE 1 Antimicrobial Sensitivity Pattern of Isolated Corynebacterium spp.

*

Antimicrobials that were added to sensitivity panel in some cases

Data are presented as number and percent. AMC, amoxicillin trihydrate/clavulanate potassium; AMK, amikacin; AMP, ampicillin; CEF, cefoxitin; CEPH, cephalothin; CHL, chloramphenicol; ENR: enrofloxacin, ERY, erythromycin; GEN, gentamicin; I, intermediate; MAR, marbofloxacin; OXA, oxacillin; PMB, polymyxin B; R, resistant; S, susceptible; TET, tetracycline; TIC, ticarcillin; TMS, trimethoprim-sulfamethoxazole.

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Cytologic data included the notation of rod-shaped bacteria, cocci, yeast, and/or neutrophils on cytologic preparations (stained with Diff-Quick) of ear swabs taken on the day the culture was performed and, for treated ears, at each recheck visit until either rod-shaped bacteria were no longer evident on cytologic examination or ear cultures were negative for Corynebacterium spp. The relative numbers of rod-shaped bacteria, cocci, and yeast were noted at each visit and recorded on a scale from 0 to 4+ based on a rough estimate of organisms/ high-power field (HPF). The number 0 indicated 0 organisms per HPF, the 1+ represented 1–5 organisms or cells/HPF, 2+ represented 6–10/HPF, 3+ represented 11–15/HPF, and 4+ represented >15 organisms or cells/HPF. Each 1+ increment was assigned a value of one point. Improvement was judged as any decrease in points utilizing this scale.

Therapeutic data included the topical and/or systemic antibiotics used prior to culture and to treat the otitis, as well as the number of ears that showed that Corynebacterium spp. were either sensitive or resistant to the used oral or topical antibiotics. Clinical response to therapy was documented when available.

Results

Microbiologic Data

Seventy-nine of the 81 cultures had other bacteria isolated along with Corynebacterium spp. In addition, Malassezia was documented in three cases. The most common bacteria cultured concurrently with Corynebacterium spp. were Staphylococcus pseudintermedius (previously documented as S. intermedius, but renamed during study period) in 56.8% (46/81) of the samples, β-hemolytic streptococci in 39.51% (32/81), and Enterococcus faecalis in 27.16% (22/81).²¹ Specific data regarding all of the bacteria cultured is provided in Table 2.

TABLE 2 Number and Percentage of Isolates Concurrently Cultured with Corynebacterium spp.

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Pure cultures of Corynebacterium spp. were obtained for 2/81 cultures. Each of those pure cultures came from one ear of two dogs with otitis externa.

Antimicrobial sensitivity data for Corynebacterium spp. was available for 54/81 cultures positive for Corynebacterium spp., including 48 dogs and 3 cats. The remaining 27 samples were positive for Corynebacterium spp., but a sensitivity panel was either not performed or the organism did not grow during sensitivity testing. The five antibiotics that most Corynebacterium spp. were sensitive to were chloramphenicol (98%), amikacin (93%), tetracycline (93%), gentamicin (85%), and potentiated sulfonamides (83%). The three antibiotics that Corynebacterium spp. showed least sensitivity to marbofloxacin (15%), ticarcillin (33%), and oxacillin (37%). Specific data for all antibiotics is provided in Table 1.

Discussion

Corynebacterium spp. are reportedly part of the normal flora in 25.8% of normal canine external ear canals and 5% of normal middle ears.^11,19 There are no reports of Corynebacterium spp. in the external ear canal and middle ears of normal cats, but study numbers have been small.^15,16 Corynebacterium spp. are reported in dogs with otitis externa and/or otitis media. For example, Corynebacterium spp. were isolated in 15.8% of ear canals and 13.2% of middle ears in 23 dogs with otitis media and in 16% of 55 dogs with otitis externa.^3,12 Corynebacterium spp. were also isolated from two cats with otitis externa and from 54% of ear canals (24 cats) with otitis externa due to Otodectes cynotis.^17,18 To date, there are no reports of Corynebacterium spp. associated with feline otitis media.^22,23 Corynebacterium spp. cultured from dogs with otitis in the current study were from breeds that were previously noted to be prone to allergy, the most common underlying cause of canine otitis externa.²⁴

The pathogenicity of Corynebacterium spp. as it relates to canine otitis has been questioned because these bacteria may be part of the normal flora of the ear and skin of the dog and, more importantly, because in all otitis cases reported to date, Corynebacterium spp. have always been found with other bacteria and/or yeast.^{3,10,11,13,14} The potential for pathogenicity is clinically significant because when Corynebacterium spp. are cultured, a decision has to be made as to whether the antibiotic selection process should involve consideration of Corynebacterium spp. sensitivities. This is especially true if the Corynebacterium spp. and other bacteria do not share antibiotic sensitivities. If Corynebacterium spp. are only opportunists, treating concurrent infections and/or simply reducing inflammation within an ear may result in spontaneous Corynebacterium spp. resolution.

The guidelines that have been suggested by Funke et al. (1997) to establish a direct association between Corynebacterium spp. and disease include the finding of a pure culture at the site of disease and the isolation of the bacteria from the sites that are normally sterile.¹ Utilizing the first criterion, it can be argued that Corynebacterium spp. are opportunists as only 2/81 cultures were solely Corynebacterium spp. in this study. In all previously reported studies of canine otitis, Corynebacterium spp. have invariably been found concurrently with other bacteria and/or yeast.^3,10,12 The most common bacteria cultured concurrently with Corynebacterium spp. were S. pseudintermedius, β-hemolytic streptococci, and E. faecalis. Although Malassezia was cultured from only three ears, cytology performed in 61 of the ears showed Malassezia spp. in 20. The incidence and types of concurrent bacteria and yeast found with Corynebacterium spp. were similar to what has been noted in other studies.^3,10,12

A further assessment of pathogenicity may be suggested by evaluating the clinical response to removal of Corynebacterium spp. through appropriate antimicrobial therapy. Twenty-five cases in the current study received topical and/or systemic antibiotic therapy and had adequate cytologic and/or culture follow-up until rod-shape bacteria were either no longer present or ears culture negative for Corynebacterium spp. Although all ears improved clinically, it was impossible to link this improvement to the removal of the Corynebacterium spp. alone because concurrent infections were present in 97.5% of cases, and those concurrent infections also resolved or improved. Also, all topical therapies contained a glucocorticoid that could have nonspecifically reduced inflammation.

In 7/19 ears for which antibiotic sensitivity data were available, the cultured Corynebacterium spp. were resistant to topical only, topical and systemic, or systemic only antibiotics. The disappearance of Corynebacterium spp. in the face of antibiotic resistance would suggest that resolution of other infection and/or a reduction of inflammation may have resulted in the spontaneous removal of the Corynebacterium spp. However, even in those seven cases, the data may be misleading. Corynebacterium spp. may indeed have responded to the antibiotic in the topically treated ears. Even when resistance to an antibiotic is documented, the minimal inhibitory concentration for a given Corynebacterium spp. may actually be exceeded by topical antimicrobial therapy, making the antibiotic effective.²⁵ In addition, four of the seven treated ears also received topical ear cleanser that included ingredients with potential antimicrobial effects, which could have eliminated the Corynebacterium spp.

This study did generate some data that might support Corynebacterium spp. as occasionally playing a role in the pathogenesis of otitis. Two of the 81 cultures were pure cultures of Corynebacterium spp. Each came from one ear of two dogs whose ear disease was restricted to the external ear canals. Both ear canals showed a high amount of waxy (nonpurulent) exudate on examination as well as mild or moderate erythema (each case, respectively) and severe stenosis (both ears). Both ears showed only rod-shaped bacteria on initial cytologic examination. In both ears, previous cultures identified Corynebacterium spp. in combination with other bacteria. Corynebacterium spp. and clinical signs persisted in both ears despite treatment with a topical antibiotic/glucocorticoid (one dog) and systemic and topical antibiotic/glucocorticoid and ear cleanser (one dog). Clinical signs did not improve until the Corynebacterium spp. were eliminated. An underlying allergy contributing to the persistence of clinical signs could not be ruled out in those two cases. Both ears were treated with only a topical enrofloxacin/dexamethasone combination after culture. In one case, Corynebacterium spp. was resistant to enrofloxacin, and there was no sensitivity data for the other. It is not known if those Corynebacterium spp. were C. auriscanis (most commonly found in the ear of dogs with otitis) or if this could represent a different, more unique species with a greater potential for pathogenicity.^3,10 These observations would suggest that, on occasion, Corynebacterium spp. may indeed be disease-causing in otitis and require specific therapy.

Another criterion listed by Funke et al. (1997) for assessing Corynebacterium spp. pathogenicity is the finding of those bacteria in an area that is usually sterile.¹ Although the canals and middle ear of both cat and dog may be sterile at a given time, they also have been noted to have a normal flora.^15,16 This observation would appear to preclude the use of this criterion in an assessment of Corynebacterium spp. pathogenicity in the ear.

This study did include the finding of Corynebacterium spp. in otitis externa/media in cats. To the authors’ knowledge, there are no previous reports of Corynebacterium spp. in feline otitis media. In four ears from four cats, one cat had only otitis externa. Two cats were diagnosed with an aural polyp involving both the middle ear and external ear canal. The cultures from these two cats came from the middle ears. It is possible that the Corynebacterium spp. were isolates from the external ear canal as an extension of the otitis externa. One cat had only otitis media and an intact tympanum. Corynebacterium spp. cultured from that cat could have originated from either the posterior pharynx or oral cavity via the eustachian tube. All cultures were mixed bacterial cultures. As in the canine cases, a role for Corynebacterium spp. pathogenicity could not be made.

Antibiotic sensitivities of Corynebacterium spp. isolated from otitis externa and/or media in dogs have only been reported for a relatively small number of isolates in the literature (6, 16, 21, 34 respectively).^3,12–14 Eighty-one Corynebacterium spp. isolates were evaluated in the current study. Good susceptibility of Corynebacterium spp. to antibiotics commonly used in the topical treatment of otitis externa was noted. Susceptibility to gentamicin was seen in 85% (46 of the 54 tested isolates, 85.2%), which is higher than in a study performed by Guedeja-Marrón et al. (1998) where 60% of six Corynebacterium spp. strains were sensitive to gentamicin, but similar to a study performed by Graham-Mize and Rosser (2004) where 95% of the 21 isolates were sensitive to gentamicin.^13,14 Susceptibility to enrofloxacin was lower in the current study (59%) compared with the 67% and 86% seen in the aforementioned studies, respectively. In both previous studies, a relatively small number of isolates were tested, which makes comparison with the current study that had a larger sample size difficult. It is also possible that there were variations in susceptibility testing between studies. In this study, it was not possible to compare procedures used to establish susceptibility data because the referenced studies did not outline the guidelines used for establishing sensitivity breakpoints. In addition, the reason for the differences in sensitivity could be associated with the more frequent contemporary use of topical enrofloxacin products for treating otitis and changes in resistance patterns of organisms over the years. Pretreatment with antibiotics may also have had an influence on the resistance pattern of the Corynebacterium spp. cultured. This is supported by a study performed by Guedeja-Marrón et al. (1998) showing a significantly higher resistance of C. urealyticum to several antibiotics if the patient was pretreated with antibiotics.¹³ The results of the current study suggest that both gentamicin and enrofloxacin may be reasonable choices for empirical use in treating otitis externa when antimicrobial sensitivity is not available for Corynebacterium spp. Both antibiotics are commonly found in veterinary otic products. No sensitivity data were available for neomycin in this study. Thus, neomycin should be evaluated in future studies because it is a common component in veterinary otic products. It is important to note that the value of antibiotic sensitivity test data are controversial in the management of canine otitis because of problems with reproducibility of data.²⁶ Even when in vitro susceptibility data shows resistance to a certain antibiotic, the high antibiotic concentrations found in otic products may overcome in vitro breakpoints, defining resistance.²⁵ Bacteria reported as resistant to a given antibiotic may therefore respond to therapy with that antibiotic.

Conclusion

Corynebacterium spp. are almost invariably found with other microbes in cases of otitis externa and/or media, suggesting that Corynebacterium spp. proliferation requires the presence of other microorganisms. Because of this, and the observation that Corynebacterium spp. may disappear from the ear with resolution of other infections, even when the Corynebacterium spp. are resistant to the antibiotics used for therapy, makes their pathogenicity questionable. When presented with antibiotic sensitivity data from mixed bacterial infections, less emphasis can likely be placed on antibiotic selection based on Corynebacterium spp. sensitivity results. Antibiotic selection may be based on sensitivity results of other concurrent isolates known to cause otitis; however, on occasion, when Corynebacterium spp. are isolated as a single organism, they may function as pathogens, and appropriate antimicrobial therapy may be indicated. Among the topical antibiotics that are commonly used in otic therapy, Corynebacterium spp. were most commonly sensitive to gentamicin, whereas enrofloxacin was moderately effective. Future studies should include sensitivity testing of larger numbers of more commonly used topical antibiotics, specifically neomycin and polymyxin B. Future studies regarding the pathogenicity of Corynebacterium spp. in otitis should include the speciation of Corynebacterium spp. taken from the ear and the culturing of more acute and milder otitis cases to better assess the sequential development of mixed microbial infections. The clinical and microbiologic response to appropriate antibiotic therapy for Corynebacterium spp. could be better assessed by limiting treatment to a single antibiotic, without concurrent use of antimicrobial ear cleansers and anti-inflammatory medications.