A study of the bacterial and fungal flora of the canine ear: with particular reference to the condition known clinically as 'canker' (otitis externa and media)

The commonest breeds to be affected with otitis were Spaniels and other long- haired dogs with overlapping ears.

The most important of the predisposing factors to otitis were skin diseases from which 38 and 59 per cent respectively of the dogs in the unselected and chronic otitis groups were suffering. Only 13 per cent of the dogs were infested with ectoparasites.

In most cases the primary lesion of otitis was due to the extension of a skin condition to the external acoustic meatus or, occasionally, to irritation by ear mites and foreign bodies such as grit and grass awns.

Infection of the ear lesion was due either to commensal staphylococci or yeasts, or to faecal contamination with Pseudomonas, Proteus or coliform organisms.

The incidence of Pseudomonas and Proteus was greatest in dogs with chronic otitis that were also suffering from a clinical skin disease. In a number of cases infection occurred during, or shortly after, a course of treatment with antibiotics.

Pseudomonas, Proteus and coliform infections were generally associated with the copious, purulent types of discharge, whereas staphylococci and Pityrosporum predominated in the dryish, dark - coloured exudates.

The tissue changes in affected ears were more closely related to the nature of the discharge and the identity of the infectious agent than to the duration of the condition. The numerous, large, cystic diverticula of the tubular portions of the modified ceruminous, or apocrine, glands were a constant feature of the more purulent forms of otitis.

All the Pseudomonas strains from infected ears were identified as Ps. aeruginosa, a number of which were sensitive to phages of human strains.

Pr. mirabilis accounted for 95 per cent of the Proteus strains, about half of which were antigenically similar. To some extent, species identification was determined by the strain's in vitro sensitivities to antibiotics.

The commonest coliform organisms recovered from infected ears were faecal type 1 of E. coli.

Unlike Group G. haemolytic streptococci, Group M. strains were rarely isolated from otitic material, although they were frequently present in the tonsils of healthy dogs.

Canine staphylococci differed markedly from those from human and certain other animal sources. Most of the dog strains were non -pigmented and coagulated rabbit but not human plasmas. Their coagulase activity was positively correlated with the production of delta but not alpha haemolysin, while a number of coagulase positive strains formed a new haemolysin. Of the few (7 per cent) penicillin resistant dog staphylococci, most were of the alpha -delta haemolytic pattern and produced aureus pigment which which are features of human, rather than canine, strains.

Pit rosporum species were isolated from most of the healthy and infected external ears, but from no other site, and differed from other members of the genus by growing well in maltose agar without the addition of oleic acid.