Cultural isolation and antibiotic sensitivity in cases of otitis externa in dogs

Introduction

Otitis externa is defined as inflammation of the external auditory canal and is one of the most common ear diseases in dogs [1,2] . It can present as acute or chronic and is often associated with bacterial and fungal pathogens. The canine ear canal is long, narrow, and L-shaped, providing a warm and moist environment favorable for microbial colonization [3] . The etiology of otitis externa is multifactorial, and it is often explained using the "primary, secondary, predisposing, and perpetuating" factors model. Primary factors include parasites (Otodectes cynotis), foreign bodies, allergic skin diseases, and endocrinopathies. Secondary factors are bacterial and fungal overgrowth, while predisposing factors include breed predisposition, pendulous ear pinnae, hair in the ear canal, and excessive cerumen. Perpetuating factors include hyperplasia, stenosis, and chronic inflammation that maintain the disease state [4,5] . Among bacteria, Staphylococcus pseudintermedius is the most frequently isolated, followed by Pseudomonas aeruginosa, Proteus spp., and Escherichia coli. Fungal involvement is usually due to Malassezia pachydermatis, a commensal yeast that proliferates under favourable conditions such as allergy or antibiotic overuse [3,6] . The disease is not only of clinical importance due to pain, pruritus, and reduced quality of life in affected dogs, but also due to its economic impact on pet owners. Chronic otitis can progress to otitis media or interna, leading to neurological signs, deafness, and in severe cases, intracranial complications [7,13] . Antimicrobial resistance has emerged as a major challenge in veterinary medicine. Methicillinresistant Staphylococcus pseudintermedius (MRSP) and multidrug-resistant Pseudomonas aeruginosa are now frequently reported worldwide [7,8] . Resistance is driven by indiscriminate use of broad-spectrum antibiotics, over-the-counter availability, and lack of culture-guided therapy.

Several studies from India have highlighted the predominance of Staphylococcus spp. and Pseudomonas spp. in canine otitis externa [9][10][11][12][13] . However, resistance profiles differ regionally. For example, isolates from Jabalpur showed higher resistance to fluoroquinolones compared to those from Himachal Pradesh [6,9] . International studies also report similar patterns [2, 10, 11] . Given this scenario, the present study was undertaken to determine the prevalence of bacterial and fungal isolates in cases of canine otitis externa and to evaluate their antimicrobial susceptibility profiles. The study also aimed to compare the findings with published reports from India and abroad to better understand trends in antimicrobial resistance.

Materials and Methods

Study Population

Fifty dogs of different breeds, ages, and sexes were included. Inclusion criteria were clinical signs of otitis externa such as head shaking, scratching, pain on palpation of the ear, erythema, malodorous discharge, or ceruminous debris. Dogs under antibiotic or antifungal treatment in the preceding 7 days were excluded.

Clinical Examination

Cases were classified into mild, moderate, and severe otitis externa based on clinical grading. Mild cases showed erythema and mild pruritus; moderate cases had pain, discharge, and erythema; severe cases exhibited chronic thickening, ulceration, and purulent discharge.

Sample Collection

Sterile cotton swabs were used to collect ear canal samples prior to cleaning or treatment. Two swabs per case were collected: One for bacterial culture and another for fungal culture. Samples were transported immediately to the laboratory in transport medium under refrigeration.

Cultural Isolation and Identification

Bacterial swabs were streaked onto Blood agar, MacConkey agar, and Nutrient agar. Fungal swabs were inoculated onto Sabouraud's dextrose agar with chloramphenicol. Bacterial cultures were incubated aerobically at 37 °C for 24-48 hours, while fungal plates were incubated at 25-28 °C for up to 7 days. Isolates were identified based on colony morphology, hemolysis, pigmentation, Gram's staining, and biochemical tests (oxidase, catalase, indole, citrate utilization, triple sugar iron, urease). Staphylococcus spp. were further identified using coagulase and mannitol fermentation tests. Pseudomonas was confirmed by oxidase positivity, pigment production, and growth on cetrimide agar. Malassezia pachydermatis was identified microscopically (bottle-shaped yeast cells) and confirmed by germ tube test.

Antibiotic Sensitivity Testing

Antibiotic susceptibility was tested using the Kirby-Bauer disc diffusion method on Mueller-Hinton agar following CLSI guidelines. The antibiotics tested were:  Amoxicillin-clavulanic acid (30 µg)  Cefotaxime (30 µg)  Gentamicin (10 µg)  Enrofloxacin (5 µg)  Ciprofloxacin (5 µg)  Amikacin (30 µg)  Tetracycline (30 µg)  Chloramphenicol (30 µg)  Penicillin G (10 units)  Ampicillin (10 µg) For fungal isolates, antifungal sensitivity was tested using ketoconazole and itraconazole discs. Zones of inhibition were measured, and isolates were categorized as Sensitive, Intermediate, or Resistant.

Data Analysis

Microbial prevalence was calculated in percentages. Statistical associations between age, sex, breed, and occurrence of otitis externa were evaluated using Chi-square test (p<0.05). Antibiotic sensitivity was summarized descriptively.

Results

Overall Prevalence

Out of 50 samples, 42 (84%) were positive for microbial growth. A total of 47 isolates were obtained (5 cases showed mixed infections). Malassezia pachydermatis 11.9

Breed, Age, and Sex Distribution

Breeds most affected: Labrador Retrievers (28%), German Shepherds (18%), Pomeranians (12%), non-descript dogs (22%), and others (20%). Age group: Dogs between 1-5 years accounted for the highest prevalence (46%), followed by >5 years (36%) and < 1 year (18%). Sex: Males (60%) were more affected than females (40%).

Antibiotic Sensitivity

Staphylococcus spp. were highly sensitive to enrofloxacin (85.7%), gentamicin (81.0%), and amoxicillin-clavulanic acid (76.2%), while resistance was highest to penicillin G (23.8%) and ampicillin (28.6%). Pseudomonas spp. showed highest sensitivity to imipenem (88.0%) and amikacin (72.0%) but were resistant to tetracycline (16.0%) and chloramphenicol (20.0%).

Discussion

This study demonstrated that Staphylococcus spp. (40.4%) and Pseudomonas spp. (26.1%) were the predominant isolates in canine otitis externa. These findings are in line with earlier Indian reports [5,6,9] and also comparable to international studies [2,10] . The high prevalence of Staphylococcus reflects its dual role as both a commensal and opportunistic pathogen of the canine skin and mucosa. In contrast, Pseudomonas infections were often associated with chronic, purulent otitis. The ability of Pseudomonas to form biofilms and produce enzymes contributes to persistence and resistance [7,10] . Breed predisposition observed in Labradors and German Shepherds supports earlier reports that dogs with pendulous ears, hairy ear canals, or heavy cerumen production are more prone to otitis [4,9] . Males were slightly more affected, consistent with Singh et al. [8] . Antibiotic sensitivity results showed that Staphylococcus spp. were highly sensitive to fluoroquinolones and aminoglycosides but resistant to β-lactams. This is consistent with reports of methicillin-resistant strains emerging worldwide [11] . Pseudomonas spp. showed multidrug resistance, retaining susceptibility to imipenem and amikacin, similar to Korbelik et al. [10] . Fungal isolates of Malassezia pachydermatis were universally sensitive to azoles, consistent with findings from India [12] and Europe [3] . Culture-guided therapy resulted in faster recovery (10-14 days) compared to empirical therapy (> 21 days). Similar observations have been made by Sharma et al. [6] and Hariharan et al. [1] . These findings emphasize the need for rational antibiotic use and routine susceptibility testing to combat antimicrobial resistance. Preventive strategies such as regular ear cleaning, control of underlying conditions (allergy, ectoparasites), and owner education are also crucial.

Conclusion

Otitis externa in dogs is primarily caused by Staphylococcus spp. and Pseudomonas spp. Antibiotic resistance, particularly in Pseudomonas spp., presents a significant therapeutic challenge. Culture and sensitivity testing should be adopted as routine practice to ensure rational therapy. Malassezia pachydermatis remains sensitive to azoles. Long-term surveillance of resistance trends is essential for guiding therapy and supporting One Health antimicrobial stewardship efforts.

Conflict of Interest Not available

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References

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