Brucellosis in Cattle (Contagious abortion, Bang’s disease)
BRUCELLA ABORTUS is a major cause of abortion worldwide but has been controlled in most parts of the world. In countries with a high incidence of bovine brucellosis B. MELITENSIS is sometimes isolated from cattle. Most abortions occur at 6-8 months of gestation. Can rarely cause hygromas of the limbs or supraspinous bursa in cattle or infect the reproductive tract of bulls. Arthritis has also been reported.
Abortion is the most obvious manifestation. Infections may also cause stillborn or weak calves, retained placentas, and reduced milk yield. Usually, general health is not impaired in uncomplicated abortions. Seminal vesicles, ampullae, testicles, and epididymides may be infected in bulls; therefore, organisms are present in the semen. Agglutinins may be demonstrated in seminal plasma from infected bulls. Testicular abscesses may occur. Longstanding infections may result in arthritic joints in some cattle.
Diagnosis is based on bacteriology or serology. B abortus can be recovered from the placenta but more conveniently in pure culture from the stomach and lungs of an aborted fetus. Most cows cease shedding organisms from the genital tract when uterine involution is complete. Foci of infection remain in some parts of the reticuloendothelial system, especially supramammary lymph nodes, and in the udder. B abortus can frequently be isolated from secretions of nonlactating udders.
Serum agglutination tests have been the standard diagnostic method. Agglutination tests may also detect antibodies in milk, whey, semen, and plasma. An ELISA has been developed to detect antibodies in milk and serum. When the standard plate or tube serum agglutination test is used, complete agglutination at dilutions of 1:100 or more in serum samples of nonvaccinated animals, and of 1:200 of animals vaccinated between 4 and 12 mo of age, are considered positive, and the animals are classified as reactors. Other tests that may be used are complement fixation, rivanol precipitation, and acidified antigen procedures.
- Brucella milk ring test (BRT):
In official control and eradication programs on an area basis, the BRT has been effective in locating infected dairy herds, but there is a high percentage of false positive tests. The brucellosis status of dairy herds in any area can be monitored by implementing the BRT at 3- to 4-mo intervals. Milk samples from individual herds are collected at the farm or milk processing plant. Cows in herds with a positive BRT are individually blood tested, and reactors are slaughtered.
- Market cattle testing:
Nondairy and dairy herds in an area may also be screened for brucellosis by testing serum samples collected from cattle destined for slaughter or replacements through intermediate and terminal markets, or at abattoirs. Reactors are traced to the herd of origin, and the herd is tested. The cost of identifying reactors by this method is minimal compared with that of testing all cattle in all herds. Screening tests, including the brucellosis card (or rose bengal) test and plate test, may be used in markets and laboratories to identify presumptively infected animals, thus reducing the number of more expensive and laborious diagnostic tests.
Brucellosis-free areas can be achieved and maintained, effectively and economically, by using the BRT on dairy herds and through market cattle testing.
Supplemental tests using sensitive screening methods may be used in cattle in which the brucellosis status is unclear. Use of a battery of these tests improves the probability of detecting infected cattle that have remained in some herds as possible reservoirs of infection. Supplemental tests are also used to clarify the results of plate or card tests, especially in serum samples from vaccinated cattle. These tests, which include complement fixation and rivanol precipitation, are designed to detect primarily the antibodies specifically associated with Brucella infection. Another supplemental diagnostic procedure is testing milk samples from individual udder quarters by serial dilution BRT, which can be used to detect chronic infection in udders of cows that may have equivocal serum test reactions.