Brucellosis of cattle, also known as "contagious abortion" and "Bangs disease", is caused by infection with the bacterium  Brucella abortus, which can also cause a disease of humans known as "undulant fever".  Brucellosis infection of cattle causes abortion or premature calving of recently infected animals, most often between the fifth and eight month of pregancy.  Although federal and state regulations have helped to control this disease, there is still a threat.  Infected cows frequently suffer from retained afterbirth, are difficult to get rebred and sometimes become sterile.

Etiology
The disease in cattle, water buffalo, and bison is caused almost exclusively by Brucella abortus ; however, Brucella suis or Brucella melitensis is occasionally implicated in some cattle herds. Brucella suis does not appear to be contagious from cow to cow. 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 resist infection and a small percentage of infected cows 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 decrease of fluids following 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.

Transmission   
Bovine brucellosis is almost always transmitted from herd to herd through the movement of infected cattle. Cows shed large numbers of organisms when they abort. Bacteria are also excreted intermittently in milk throughout the lactation.
Animals become infected through ingestion of contaminated feed or water, or after licking an infected placenta, foetus or genitalia of another cow, after it has aborted.
Most cows remain chronically infected. Urine, faeces and hygroma fluids are also sources of bacteria. Infected bulls may excrete the organism in their semen. Congenital transmission may occur through in utero infection.
Humans are infected through handling infected cows or their tissues, or through drinking infected milk. Pasteurisation will prevent the latter.

Clinical signs
The dominant feature of the disease in cattle is abortion.
Cattle

• Usually occurs at about 5-7 months. Full-term calves may die soon after birth
• Abortion rates in herds vary. In fully susceptible herds rates may vary from 30% to 80%, although in some cases, abortions may be more insidious
• Retained placenta and secondary metritis is common and may lead to permanent sterility
• In bulls acute or chronic infections of the reproductive tract may occur (orchitis, epididymitis, seminal vesiculitis · hygromas, particularly of the carpal joints, occur in some animals in chronically affected herds

Horses

• Chronic bursal enlargements of the neck and withers with chronic draining sinuses
• ‘Poll evil’ localisation of infection between the nuchal ligament, atlas and axis
• ‘Fistulous withers’ inflammation of bursa between the nuchal ligament and dorsal spines of thoracic vertebrae

Humans

• ‘Undulant fever’ (fluctuating, irregular fever)
• Chills, depression, weakness
• Headache, joint pains, generalised aches
• May be complications affecting cardiovascular or central nervous systems

 

Post-mortem findings
In cattle there is considerable variability in uterine lesions after abortion

• Mild to severe endometritis
• Placenta is thickened, oedematous, yellow-grey and may have exudate on surface
• Mammary gland lesions
• Inflamed regional lymph nodes

While some aborted foetuses will appear normal, others may show:

• Varying degrees of sub-cutaneous oedema
• Blood stained fluid in body cavities
• Enlarged orange-brown discoloured liver
• Fibrous pleuritis and focal pneumonia

Diagnosis:
Diagnosis is based on bacteriology or serology. Brucella 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. Brucella 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.
Screening Tests:

1. 1) 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.
2. 2) 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.

Differential diagnosis
Other cause of abortion and reproductive failure in cattle include:

• Trichomoniasis
• Vibriosis
• Leptospirosis
• Listeriosis
• Infectious bovine rhinotracheitis
• Mycoses

Brucellosis should always be suspected when there are multiple late-term abortions in a herd.

Control:
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 2 or 3 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, which may confuse diagnostic test results. To minimize this problem, calves in the USA are vaccinated with a vaccine that contains 3-10 billion viable B abortus Strain 19 organisms per 2 mL dose. Strain RB51 has largely replaced Strain 19. 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 is necessary. The low prevalence of brucellosis in cattle in the USA has resulted in reduced use of vaccines and current emphasis on depopulation of infected herds.
Brucellosis is endemic in some nondomesticated bison and elk herds in the USA. Transmission of B abortus to domestic cattle herds is rare. Nevertheless, many controversies have developed concerning possible control methods. Vaccination of some elk populations has been practiced.