Tolerability of Otic Solutions Containing Different Enrofloxacin Concentrations in Dogs with Healthy Ears
ABSTRACT
Otitis externa (OE) is a common disease in dogs, and topical medications are the preferred treatment. Compounded solutions of enrofloxacin are commonly used in practice to treat bacterial OE; however, the tolerability of different concentrations of this antibiotic in the ear canals of dogs has not been evaluated. The objective of this study is to determine if a higher concentration of enrofloxacin applied to the external ear canal is clinically tolerated in dogs with healthy ears. Sixteen client-owned dogs with bilateral healthy ears and no previous history of OE were enrolled. Injectable enrofloxacin 2.27% diluted with sterile sodium chloride in 1:1 (11.35 mg/mL) and 2:1 (15 mg/mL) ratios were applied into the dogs’ right and left ears, respectively, q 12 hr for 14 days. Based on video otoscopic examination, clinical score for canine OE (OTIS3) results before application were ≤1 for all dogs. During the study and at the conclusion, all scores remained ≤2, which is considered normal. No cytologic inflammatory cells were seen in any of the dogs’ ears throughout the study. Different concentrations of enrofloxacin solution applied topically were well tolerated by dogs with healthy ears and can be considered for the treatment of dogs with bacterial OE.
Introduction
Otitis externa (OE) is a routinely seen condition in veterinary practice worldwide. Underlying allergic disease is a common cause of OE in dogs, frequently diagnosed with concurrent secondary bacterial infection as a complicating perpetuating factor.1,2
Topical treatments are the preferred and most effective form of therapy for OE, and most commercially available otic products contain a combination of antibiotic, antifungal, and anti-inflammatory medications.3 To target bacteria and inflammation, otic solutions containing antibiotics can be compounded and formulated for a patient’s requirements. A topical otic antibiotic has direct contact with bacterial isolates and therefore will achieve higher concentrations compared with oral antibiotic therapies.4 It is theorized that higher concentrations of antibiotics like enrofloxacin will be more effective against resistant bacterial isolates because enrofloxacin is bactericidal and concentration dependent.4 Compounded enrofloxacin solution for the treatment of bacterial OE is often prescribed in clinical settings. These solutions commonly contain injectable enrofloxacin (22.7 mg/mL) diluted in saline or Tris-EDTA (ethylenediaminetetraacetic acid) or injectable dexamethasone sodium phosphate (4 mg/mL) at a 1:1–1:4 ratio (1 part of enrofloxacin to 1–4 parts of the diluent).1,5,6 Anecdotal concerns about the use of otic compounded solutions with enrofloxacin in equal or higher concentration than the diluent have been raised; however, no tolerability studies have been performed.
The goal of this study was to evaluate the tolerability of a 14-day course of twice daily application of two compounded solutions containing enrofloxacin and saline at a 1:1 and 2:1 ratio in the external ear canals of healthy dogs. We hypothesized that both otic solutions would be well tolerated as assessed by (i) frequent video otoscopic examination and clinical evaluation; (ii) a lack of development of OE; and (iii) a lack of otic cytology changes.
Materials and Methods
Ethics
This study was approved by the Institutional Animal Care and Use Committee (protocol 21211), and informed owner consent was obtained before enrollment of each dog.
Animals
Clinically healthy dogs with normal ear canals confirmed by video otoscopic examination were eligible for inclusion in the study. Concomitant otic treatments and systemic antimicrobial or glucocorticoid medications were not allowed before or during the study. Dogs with cytologic overgrowth of micro-organisms or inflammatory cells in the ear or with historical episodes of OE were not enrolled.
Analytical Procedures
Otoscopy, Clinical, and Cytology Evaluation
Video otoscopic examinationa of the ear canal was performed by two blinded investigators (a boarded certified dermatologist and a dermatology intern) on Days (D) 0, D3, D7, D10, D14. Ears were also assessed using a clinical score for canine otitis externa (OTIS3)7 and by cytologic examination at all visits. Otic samples for cytology were collected after the second investigator completed the examination. Slides were stained with a modified Wright-Giemsa stain (Diff-Quik) and evaluated under the microscope at 100× for the presence of inflammatory cells, bacteria, and yeast at all study days. A semiquantitative scale was applied to assess mean organism counts (1–2 micro-organism(s)/slide: rare; 1 micro-organism/every other field: occasional; <10 micro-organisms/field: 1+; ≥10 micro-organisms/field: 2+; 30–50 organisms/field: 3+; and ≥50 micro-organisms/field: 4+). Criteria of a contact reaction included the development of cytologic inflammatory cells or the observation of erythema, erosions, or ulcerations of the ear canal in conjunction with discomfort or pain.7
Otic Application
Enrofloxacin 2.27% injectableb was used in combination with saline (0.9% sodium chloride for injection) for two compounded otic solutions at 1:1 ratio (1 part enrofloxacin and 1 part sterile saline, 11.35 mg/mL of enrofloxacin) and 2:1 ratio (2 parts enrofloxacin and 1 part sterile saline, 15.13 mg/mL of enrofloxacin). All enrolled dogs were owned by veterinarians or veterinary students, and owners were responsible for the application of the 1:1 ratio solution into the right ear and the 2:1 ratio into the left ear q 12 hr for 14 days. Two different bottles for administration were dispensed, and owners were blinded regarding the solution concentration. Each bottle was labeled by a color indicator (yellow for the right ear and green for the left ear). A dose-marked syringe was supplied for each ear. The volume of otic solution applied was determined by the body weight of the individual dog. Small-size dogs weighing <10 kg received 0.3 mL, medium-size dogs weighing 10–<25 kg received 0.5 mL, and large-size dogs weighing 25–<40 kg received 0.8 mL.8
Sample Size Determination
Primary outcome measures were the number of ear canals with increased OTIS3 scores compared with the baseline and the number of ear canals with evidence of cytologic inflammation. Determining change from baseline was considered most important for the purpose of establishing tolerability in healthy ears. A difference from baseline OTIS3 score of ≥4 was considered clinically important in the study design because this is defined as meeting the criteria for diagnosis of OE,9 with an anticipated standard deviation of 4. To detect this change with a power of 80%, 16 ears were required in each treatment group.
Results
Sixteen dogs were enrolled in the study for bilateral ear assessment, and each ear was evaluated separately. Eleven dogs were castrated males, and 5 were spayed females with a mean age of 5 yr (range 1–13 yr) and a mean weight of 20 kg (range 2.9–31.2 kg). Breeds included 12 mixed-breed dogs (three small, seven medium, and two large breed size), two galgo, one Chihuahua, and one old English sheepdog. Results of the ears’ assessment using the clinical score are described in Table 1. Inclement weather prevented evaluations at one time point for three dogs during the study.
OTIS3 scores on D0 before application were ≤ 1 for all dogs. During the study, all OTIS3 scores remained ≤2 for all ears evaluated. Five (31.2%) dogs received consistent OTIS3 scores of 0 in both ears for all evaluations; 10 (62.5%) dogs received an OTIS3 score of 1 in either the right or left ear at various time points in the study by either one or both investigators. One (6.2%) dog received an OTIS3 score of 2 in both right and left ears at various time points in the study by either one or both investigators. Because no dogs had clinically important increases in OTIS3 scores (OTIS3 score >4), statistical analysis was not considered necessary to evaluate for significant changes from baseline. OTIS3 results for all ears at all visits are reported (Table 1).
No inflammatory cells were seen cytologically in any dogs’ ears throughout the study. In the right ears (1:1 enrofloxacin:saline), cytology at various time points showed rare, occasional, or 1+ yeast and rare cocci. In the left ears (2:1 enrofloxacin:saline), cytology at various time points showed rare yeast and rare rods and cocci. Cytology results from ears at all visits are reported (Table 2).
Discussion
The tolerability of different concentrations of enrofloxacin otic compounded solutions have not been previously evaluated. In this study, otic application of compounded enrofloxacin in equal and higher concentrations than the diluent was well tolerated by dogs with healthy ear canals.
Topical therapy is most commonly used to treat canine otitis externa. Systemic antibiotics do not achieve sufficient tissue concentrations in the external ear canals, and exposure to subinhibitory concentrations of antibiotic favor the development of bacterial resistance.10,11 Topical antibiotics can reach high concentrations at the site of infection, about 100–1000× higher compared to systemically delivered antibiotics, and high concentrations of topical antibiotics are expected to overcome even resistant bacteria.10,12
The only currently available commercial otic product containing enrofloxacinc contains enrofloxacin (5 mg/mL) and silver sulfadiazine (10 mg/mL) active ingredients. Combining injectable enrofloxacin with different diluents allows for the ability to make variable solutions to tailor therapy according to a patient’s needs. Combinations with Tris-EDTA, dexamethasone sodium phosphate, and saline are used anecdotally by veterinary practitioners, typically with enrofloxacin in a lower concentration compared to other ingredients. In our study, enrofloxacin was diluted with saline and applied either as a 1:1 ratio (11.35 mg/mL enrofloxacin) or a 2:1 ratio (15 mg/mL enrofloxacin) into dogs’ ears. Both concentrations of enrofloxacin were well tolerated, and no adverse side effects were observed. A saline diluent was chosen because of its inert action on the ear canal. Comparatively, dexamethasone and Tris-EDTA were not elected because of anti-inflammatory and antimicrobial effects, respectively, which could interfere with the clinical evaluation, especially in the circumstance of a reaction.3,5,13 Twice daily application of enrofloxacin otic solution, as well as the volume applied according to the dog’s size, were used following previous literature recommendations for the treatment of OE.1,5,14
Video otoscopic examination confirmed intact tympanic membranes before application of the otic solution, and no changes were noted during the study. Fluoroquinolones, particularly enrofloxacin, are safe for the middle ear15 and can be applied in cases in which the tympanic membrane is not intact or cannot be visualized, which is one of the reasons why this antibiotic is widely used to treat OE in dogs. Additionally, enrofloxacin has good activity against Pseudomonas spp.,1,4,6Staphylococcus spp.,6 and Corynebacterium spp.,16 which are frequent isolates in cases of bacterial OE in dogs.4 A recent publication documented 28 days of in vitro bactericidal efficacy and chemical stability of enrofloxacin (10 mg/mL) when combined with saline and dexamethasone against Staphylococcus pseudintermedius isolates obtained from dogs with OE, supporting the use of compounded solutions for the treatment of ear infections.17
Clinically normal dogs with no previous history of otitis externa were used in this study to avoid the possibility that an underlying disease might cause confounding inflammation during the study period. No evidence of a contact reaction was observed in any of the enrolled dogs. All OTIS3 score results were ≤2 throughout the study. An OTIS3 score ≥4 is consistent with OE,7 and therefore all dogs remained within the normal ear score criteria. Features and causes of otic topical contact reactions in human literature have been described, but characterization of otic contact reactions in dogs in veterinary literature is limited. An otic contact reaction is broadly accepted as the presence of neutrophils without micro-organisms in cytologic examination of the ear canals.5,18 Concern for a contact reaction following topical otic applications have been described as an increase in discomfort or pain along with visualization of erythema, erosions, or ulcerations of dogs’ ear canals.19 In one clinical photograph, erythema of the concave surface of the pinna and ear canal entrance were described as clinical features of a contact reaction to otic neomycin application in a dog.6
Six dogs (seven ears) had rare or occasional yeast on cytology at the beginning of the study, but ear canals were clinically normal by video otoscopy and OTIS3 score. In one of these dogs (dog 7), an increased number of Malassezia (1+) was observed at D14 of treatment with enrofloxacin, which correlates with previous observations that an opportunistic overgrowth of Malassezia can occur in dogs with OE being treated with topical antibiotics without the combination of an antifungal medication.20
Each investigator examined the dogs’ ears and reported scores independently. Most dogs were tolerant of two video otoscopic examinations, but a few dogs were less tolerant and allowed only one examination. When this occurred, the investigators examined the ears concurrently. To limit bias, no verbal communication occurred during examination, and scores were still reported independently. Cytology samples were always collected after the second investigator completed the evaluation to eliminate the likelihood of interference from any minor trauma or change in the appearance of the ear canal from sampling.
The limitation of this study included reliance on appropriate and consistent application of the otic solutions by the owner; however, all owners were veterinarians or veterinary students, which is expected to improve compliance. A checklist was provided to remind owners of the twice daily application and to verify that all doses were administered as directed. Color coding of the bottles and marked syringes were provided to ensure the correct concentrated solution was applied into the appropriate ear.
Conclusion
A 14-day course of twice daily application of two compounded solutions containing enrofloxacin and saline at a 1:1 and 2:1 ratio in the external ear canals was well tolerated in the ears of healthy dogs. Further study to determine the efficacy of the higher concentrated enrofloxacin otic solution in dogs with resistant bacterial OE is currently under investigation.
Contributor Notes
