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
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
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.