Factors associated with Malassezia colonization in the ear canals of healthy dogs

Abstract

This study analysed the relationships between animal characteristics, clinical signs and Malassezia populations in the ear canals of healthy dogs. Forty‐two healthy dogs of any breed, age and sex were selected. The presence of cerumen readily visible at the opening of the ear canal was the inclusion criterion. Exclusion criteria were signs of otitis externa (severe ear canal irritation, exudate, pruritus, pain), any other ear disease, and treatment in the previous week with antimicrobial or anti‐inflammatory agents. Veterinarians performed visual examination of the pinnae, olfactory inspection of the canal openings, and otoscopic investigation of the canals. Quantity of cerumen in the ears, malodour and erythema were graded on a 5‐point severity scale. Cerumen consistency (soft or dry) and colour (yellowish or brown) were also noted. Malassezia populations were recorded by cytology from ear swabs. Yeast counts were performed on 10 contiguous high‐power fields (hpf ×100 oil immersion objective) from representative areas on slides (Diff‐Quik® staining). Spearman correlation coefficients (r) were calculated to examine the relationships between Malassezia counts and possible quantitative explanatory variables. Differences in the number of Malassezia between subgroups of dogs were tested by Kruskal–Wallis analysis of variance. Dogs included in the study represented a large variety of breeds (4–40 kg body weight) and ages (4 months to 15 years). The sex ratio was balanced (18 females:24 males). Strong correlation was found for all clinical signs and Malassezia counts between left and right ears (0.75 ≤ r ≤ 0.93). Consequently, only the average scores and counts of both ears were considered for each individual. No Malassezia could be detected in the cerumen samples taken from 12 dogs (28.6%). Mean yeast numbers in the range of 0.1–1.3 and 1.5–3.5 organisms/hpf were determined for 21 (50%) and seven dogs (16.7%), respectively. Two dogs (4.7%) yielded mean yeast counts in the range of four to five organisms per hpf. Breeds documented to be at higher risk for ear disease (Basset Artésien, Bichon frise, poodles, cocker and other spaniels, Labrador retrievers, Labrit, Lhasa Apso, setters and German shepherd dogs) did not exhibit significantly higher Malassezia counts as compared to other breeds represented. Dogs with pendulous ears were not more prone to yield higher yeast counts as compared to dogs that carry their ears erect. Dogs’ age and sex, cerumen consistency and quantity, as well as aural malodour, were not found to be significantly associated with Malassezia counts in ears. However, dogs’ weight, cerumen colour, and intensity of associated aural erythema correlated positively with yeast counts (P < 0.05). Higher yeast populations in cerumen samples (mean = 1.3 Malassezia/hpf) were detected in dogs weighing more than 15 kg and presenting with aural erythema as compared to their erythema‐free lighter counterparts (mean = 0.6 Malassezia/hpf). Cerumen was brown in the vast majority of cases (72.2%) for the heavier dogs presenting with erythema. This study suggests that dog size, cerumen colour and aural erythema are factors most frequently associated with higher basal Malassezia counts in the ear canal of healthy dogs. Funding: Virbac SA.