Infective bulbar necrosis (heel-abscess) of sheep, a mixed infection with Fusiformis necrophorus and Corynebacterium pyogenes

Abstract

Cases of “heel-abscess” from 45 flocks of sheep in south-eastern Australia have been studied. “Infective bulbar necrosis” (I.B.N.) is suggested as an appropriate name for the disease, which is distinct from lamellar suppuration (“toe-abscess”). I.B.N. occurred most often in the hind feet of ewes lambing under cool wet conditions. It arose, in many cases at least, as an extension of infection to the digital cushion from lesions of ovine interdigital dermatitis (O.I.D.). Fusiformis necrophorus penetrated into the interdigital dermis and the digital cushion while Corynebacterium pyogenes proliferated in the resulting necrotic mass. The other diphtheroid bacteria demonstrated in the lesions of O.I.D. were not detectable in the I.B.N. lesions. Experiments in which F. necrophorus and C. pyogenes were injected separately or together into the skin of sheep and guinea-pigs indicated that I.B.N. is probably a true mixed infection in which the proliferation of each organism is increased by the growth of the other. Typical I.B.N. was reproduced by injecting the mixed inoculum into the digital cushions of sheep. Clinical and histological evidence suggests that penetration of the tissues by F. necrophorus may also depend on changes in the blood supply. The purely necrotic nature of developing I.B.N. lesions is associated with an inhibition of cellular exudation in the vicinity of the invading F. necrophorus. The onset of suppuration often seen in older cases coincides with cessation of the necrotizing bacterial invasion and the replacement of F. necrophorus by C. pyogenes at the periphery of the necrotic region. In contrast with lamellar suppuration, the lesions of I.B.N. were not relieved by providing drainage.