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Brucellosis in Sheep

By Paul Nicoletti, DVM, MS, DACVPM (Deceased), College of Veterinary Medicine, University of Florida

Brucella melitensis infection in certain breeds of sheep causes clinical disease similar to that in goats (see Brucellosis in Goats). However, B ovis produces a disease unique to sheep, in which epididymitis and orchitis impair fertility—the principal economic effect. Occasionally, placentitis and abortion are seen, and there may be perinatal mortality. The disease was first described in New Zealand and Australia and has since been reported from many sheep-raising areas of the world. B ovis infection among sheep in the USA is rare.

There is no evidence that the disease is present in any other animal species. Rare natural and experimental infections in farmed red deer stags have been reported in New Zealand.

Rams as young as 8 wk have been infected experimentally by various nonvenereal routes. The disease can be transmitted among rams by direct contact. Active infection in ewes is unusual but has developed after mating with naturally infected rams. Contaminated pastures do not appear to be important in spread of the disease. Infection frequently persists in rams, and a high percentage shed B ovis intermittently for several years.

Primary manifestations are lesions of the epididymis, tunica, and testis in rams; placentitis and abortion in ewes; and occasionally perinatal death in lambs. Lesions may develop rapidly. In rams, the first detectable abnormality may be a marked deterioration in semen quality associated with the presence of inflammatory cells and organisms. An acute systemic phase is rarely seen in naturally occurring infections. After regression of the acute phase—which may be so mild as to go unobserved—lesions may be palpated in the epididymis and scrotal tunics. Epididymal enlargement may be unilateral or bilateral. The tail of the epididymis is involved more frequently than the head or body, and the most prominent lesion is spermatoceles of variable size containing partially inspissated spermatic fluid. The tunics frequently become thickened and fibrous, and extensive adhesions develop between them. The testes may show fibrous atrophy; these lesions are usually permanent. In a few cases, palpable lesions are transient, while in others, organisms may be present in semen over long periods without clinically detectable lesions.

Because not all infected rams show palpable abnormalities of scrotal tissues (and not all cases of epididymitis are due to brucellosis), the remaining rams must be examined further. Rams shedding organisms, but having no lesions, must be identified by culture of semen. Repeated examinations may be necessary to identify intermittent shedders. Microscopic examination of stained semen smears may also be helpful; fluorescent antibody examination is a highly specific diagnostic aid. Serologic tests used for eradication of disease and certification of animals include indirect ELISA, complement fixation, hemagglutination inhibition, indirect agglutination, and gel diffusion.

Incidence and spread of the disease may be reduced by regular examination of rams before the breeding season and culling of those with obvious genital abnormalities. Because susceptibility in rams increases markedly with age, it is advantageous to keep a young ram flock and isolate noninfected rams from older, possibly infected rams.

Immunization of weaner rams with attenuated (Rev. 1) B melitensis has been recommended in some countries. Because infection in ewes apparently originates almost exclusively from service by infected rams, lamb losses through infection of ewes may be controlled economically by restricting vaccination to rams. There is no recommended vaccination in the USA.

Chlortetracycline and streptomycin used concurrently have effected bacteriologic cures. However, treatment is not economic except in especially valuable rams, and even if infection is eliminated, fertility may remain impaired.