Infection with Brucella abortus can cause abortion, birth of stillborn or weak calves, retained placenta, and reduced milk production. Infected cattle usually abort only once but can still shed the organism in amniotic fluid and milk at subsequent calvings. This is important because the organism is highly infectious to humans. No practical treatment is available; therefore, efforts are directed at detection of infected herds and preventing infection from infiltrating uninfected herds. Young cattle within a herd often abort within a relatively short timeframe, referred to as an "abortion storm."
Etiology and Epidemiology of Brucellosis in Cattle
Brucellosis in cattle, water buffalo, and bison is caused almost exclusively by Brucella abortus. B suis is occasionally isolated from seropositive cows but does not appear to cause clinical signs and is not transmitted between cows. In some countries, B melitensis, which is not present in the US, can produce a similar syndrome in cattle to that caused by B abortus.
Infection spreads rapidly and causes many abortions in a herd of 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; however, organisms can still be shed in the placenta, amniotic fluid, and milk.
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; however, it may be delayed in ~15% of infected 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 and amniotic fluids, 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. Vaginal shedding largely ceases 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 udder secretions of nonlactating cows.
Natural transmission occurs via ingestion of organisms, which are present in large numbers in aborted fetuses, fetal membranes, and uterine and amniotic fluids. Cattle may ingest contaminated feed or water or may lick contaminated genitals of other animals. Venereal transmission by infected bulls to susceptible cows appears to be rare. Transmission may occur via artificial insemination when Brucella-contaminated semen is deposited in the uterus; however, apparently, not when deposited in the midcervix. Brucellae may enter the body through mucous membranes, conjunctivae, wounds, or intact skin in humans and animals.
Brucellae have been recovered from fetuses and from feces that have remained in a cool environment for > 2 months. Exposure to direct sunlight kills the organisms within a few hours.
Clinical Findings for Brucellosis in Cattle
Abortion is the most obvious sign. Infections may also cause stillborn or weak calves, retained placentas, and decreased milk production. Usually, overall health is not impaired in uncomplicated abortions.
Seminal vesicles, ampullae, testicles, and epididymides may be infected in bulls. Consequently, organisms are present in the semen, and agglutinins may be seen in seminal plasma samples from infected bulls. Testicular abscesses may occur. Chronic infections may result in arthritic joints in some cattle.
Diagnosis of Brucellosis in Cattle
Based on clinical signs, results of serologic testing, microbial culture
Samples for culture include placenta, udder secretions from live animals; tissue samples (eg, stomach contents, lung) from aborted fetuses
B abortus can be cultured from the placenta; however, the more typical and convenient samples are stomach contents and lungs from 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 the udder. These tissues can be cultured to confirm infection in cows at necropsy, whereas udder secretions are the preferred specimens for culture from a live cow.
Serologic tests have been the standard diagnostic method and can detect antibodies in blood, milk, whey, and semen samples. When the standard plate or tube serum agglutination test is used, complete agglutination at dilutions of 1:100 or greater in serum samples obtained from nonvaccinated animals, and of 1:200 or greater in animals vaccinated at 4–12 months of age, are considered positive, with animals are classified as reactors. Other serologic tests include complement fixation, ELISA (which detects antibodies in milk and serum), rivanol precipitation plate agglutination testing, and acidified antigen procedures.
In official eradication programs for dairy herds, the Brucella milk ring test (BRT) is typically used as a screening test at 3- to 6-month intervals to identify infected herds. Milk samples from individual herds are collected at the farm or milk processing plant. However, there are many false-positive tests; therefore, all cows in herds with a positive BRT are individually blood tested. Depending upon the stage of the program in an area or country, seropositive cows are often culled to more accurately determine a herd's status.
Non dairy and dairy herds in an area may also be screened for brucellosis by means of testing serum samples collected from cattle destined for slaughter or replacement through intermediate and terminal markets, or at abattoirs. Reactor animals are traced to the herd of origin, and the herd is tested. The cost of identifying reactor animals 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 rivanol precipitation plate test, may be used in markets and laboratories to identify presumptively infected animals, thus reducing overall testing costs.
Brucellosis-free areas can be achieved and maintained, effectively and economically, by use of the BRT on dairy herds and by market cattle testing. Samples from adult cattle are collected at the time of slaughter.
Supplemental tests using more sensitive screening methods may be used in cattle in which the brucellosis status is unclear. Use of various 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.
Control of Brucellosis in Cattle
No practical treatment is available. Control programs must rely on detection of infected herds and preventing infection infiltrating "clean" herds.
Eventual eradication depends on repeated testing and culling of all positive animals.
Brucellosis has been eradicated from many individual herds by repeat testing and culling of all reactors. Herds must be tested at regular intervals until two or three successive tests are negative.
The greatest danger for the introduction of infection into noninfected ("clean") herds is via replacement animals. Additions (replacements) 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. Replacement animals 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 against Strain 19 that may persist for years and can cause misleading diagnostic test results. To minimize this problem, calves in the US are typically vaccinated with a vaccine of Strain RB51. It is a rough attenuated strain and does not cause production of the antibodies 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 US 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 many countries, including the US, where all states are considered free of brucellosis. Vaccination of young animals is ceased near the end stages of eradication campaigns to reduce the frequency of false-positive serologic test reactions.
Brucellosis is endemic in some nondomesticated bison and elk herds in the US. 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.
Brucellosis causes contagious abortion.
Infection can be controlled through vaccination of young cows before they are bred.
Eradication from herds and areas requires regular serologic testing and elimination of reactors ("test and slaughter" programs).
Infection in humans follow exposure to aborted fetuses, placentae, and amniotic/uterine fluids or ingestion of contaminated milk.