Etiology and Epidemiology:
The disease in cattle, water buffalo, and bison is caused almost exclusively by Brucella abortus; however, B suis occasionally is isolated from seropositive cows but does not appear to cause clinical signs and is not contagious from cow to cow. In some countries, the disease in cattle is caused by B melitensis. The syndrome is similar to that caused by B abortus. B melitensis is not present in the USA.
Infection spreads rapidly and causes many abortions in unvaccinated cattle. In a herd in which disease is endemic, an infected cow typically aborts only once after exposure; subsequent gestations and lactations appear normal. After exposure, cattle become bacteremic for a short period and develop agglutinins and other antibodies; some cattle resist infection, and a small percentage of infected cows spontaneously recover. A positive serum agglutination test usually precedes an abortion or a normal parturition but may be delayed in ~15% of cows. The incubation period may be variable and is inversely related to stage of gestation at time of exposure. Organisms are shed in milk and uterine discharges, and the cow may become temporarily infertile. Bacteria may be found in the uterus during pregnancy, uterine involution, and infrequently, for a prolonged time in the nongravid uterus. Shedding from the vagina largely disappears with the cessation of fluids after parturition. Some infected cows that previously aborted shed brucellae from the uterus at subsequent normal parturitions. Organisms are shed in milk for a variable length of time—in most cattle for life. B abortus can frequently be isolated from secretions of nonlactating udders.
Natural transmission occurs by ingestion of organisms, which are present in large numbers in aborted fetuses, fetal membranes, and uterine discharges. Cattle may ingest contaminated feed and water or may lick contaminated genitals of other animals. Venereal transmission by infected bulls to susceptible cows appears to be rare. Transmission may occur by artificial insemination when Brucella-contaminated semen is deposited in the uterus but, reportedly, not when deposited in the midcervix. Brucellae may enter the body through mucous membranes, conjunctivae, wounds, or intact skin in both people and animals.
Brucellae have been recovered from fetuses and from manure that has remained in a cool environment for >2 mo. Exposure to direct sunlight kills the organisms within a few hours.
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. Udder secretions are the preferred specimens for culture from a live cow.
Serum agglutination tests have been the standard diagnostic method. Agglutination tests may also detect antibodies in milk, whey, and semen. 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 at 4–12 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.
In official eradication programs on an area basis, the Brucella milk ring test (BRT) has effectively located infected dairy herds, but there are many 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 seropositive cows are slaughtered to determine herd status.
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 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. Adult cattle are sampled at the time of slaughter.
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 to test 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.
Efforts are directed at detection and prevention, because no practical treatment is available. Eventual eradication depends on testing and eliminating reactors. The disease has been eradicated from many individual herds and areas by this method. Herds must be tested at regular intervals until two or three successive tests are negative.
Noninfected herds must be protected. The greatest danger is from replacement animals. Additions should be vaccinated calves or nonpregnant heifers. If pregnant or fresh cows are added, they should originate from brucellosis-free areas or herds and be seronegative. Replacements should be isolated for ~30 days and retested before being added to the herd.
Vaccination of calves with B abortus Strain 19 or RB51 increases resistance to infection. Resistance may not be complete, and some vaccinated calves may become infected, depending on severity of exposure. A small percentage of vaccinated calves develop antibodies to Strain 19 that may persist for years and can confuse diagnostic test results. To minimize this problem, calves in the USA are mostly vaccinated with a vaccine of Strain RB51. It is a rough attenuated strain and does not cause production of antibodies, which are detected by most serologic tests.
Whole-herd adult cattle vaccination using Strain 19 or RB51 has been practiced in certain high-incidence areas and selected herds in the USA with much success.
Vaccination as the sole means of disease control has been effective. Reduction in the number of reactors in a herd is directly related to the percentage of vaccinated animals. However, when proceeding from a control to an eradication program, a test and slaughter program becomes necessary. B abortus has been eradicated from cattle herds in the USA, and all states are considered free of brucellosis.
Brucellosis is endemic in some nondomesticated bison and elk herds in the USA. Transmission of B abortus to domestic cattle herds is rare but has occurred in several cattle herds commingling with infected elk in the greater Yellowstone Park area.