Bovine brucellosis

What it is

Bovine brucellosis is caused by Brucella abortus, a facultative intracellular Gram-negative coccobacillus. Cattle are infected mainly by ingestion or by mucosal contact (mouth, conjunctiva) with aborted fetuses, placentas, uterine discharges and pasture or water contaminated by them; venereal transmission is of little importance, except through artificial insemination with infected semen. The bacterium multiplies in the regional lymph nodes, enters the blood and has a strong affinity for the pregnant uterus, the udder and the male genital tract. In the placenta it multiplies in the trophoblasts and causes a placentitis that leads to abortion or the birth of weak calves.

After the first abortion most cows carry subsequent pregnancies to term, but they remain infected and can shed the bacterium at every calving and in milk for years. Some heifers infected in utero or at birth develop a latent infection, often seronegative, that becomes apparent in their first pregnancy.

It is an important zoonosis. People are infected by consuming raw milk and unpasteurised fresh cheese and by contact with aborted fetuses, placentas and discharges; it is an occupational disease of farmers, veterinarians, abattoir and laboratory workers. Accidental self-injection of live vaccines can also infect people. In humans it causes undulant fever: recurrent fever, sweating, joint and muscle pain and, in some cases, osteoarticular complications, orchitis or endocarditis. Worldwide, most human cases are due to B. melitensis from sheep and goats, but B. abortus from cattle is also a common cause. Control in animals and pasteurisation of milk are the basis of prevention in people (a One Health approach).

Who it affects

Sexually mature cattle, above all unvaccinated pregnant females; young animals are more resistant. Herds become infected by buying infected animals or through contact with neighbouring herds. Several countries are officially free, while in other regions it is endemic. Buffalo, bison and some wild ruminants can act as reservoirs.

Anatomy involved

• Uterus: the bacterium colonises the placenta and causes a necrotising placentitis: the cotyledons are necrotic and the intercotyledonary placenta is thickened, oedematous and covered with exudate. It causes abortion in the last third of gestation, usually from the fifth month onwards, weak calves, retained placenta and metritis. The fetus, placenta and lochia contain huge numbers of bacteria and are the main source of infection.
• Udder: B. abortus persists in the mammary gland and the supramammary lymph nodes, often for the animal's whole life. It causes an interstitial mastitis, almost always without visible changes in the milk, and the bacterium is shed in milk intermittently. This is why raw milk is a risk for people and why bulk-tank milk is used for herd surveillance.

Clinical signs

• Abortion in the last third of gestation, often as the only sign, and "abortion storms" in newly infected herds.
• Retained placenta, metritis and reduced fertility.
• Birth of weak or dead calves.
• In bulls, orchitis and epididymitis, sometimes seminal vesiculitis, with infertility.
• In some regions, hygromas, especially of the carpus, and arthritis.
• General condition is usually good and many infected animals show no signs.

Diagnosis

• Serology, the basis of official programmes: the Rose Bengal test for screening, complement fixation for confirmation, and indirect or competitive ELISA and the fluorescence polarisation assay. The competitive ELISA helps distinguish antibodies induced by the S19 vaccine. False-positive reactions can occur because of Yersinia enterocolitica O:9.
• Milk: the milk ring test and ELISA on bulk-tank milk are used for surveillance of dairy herds.
• Direct diagnosis: bacterial culture or PCR of abomasal contents and lung of the aborted fetus, the placenta, uterine discharges or milk. Samples must be handled with biosafety precautions.
• Any bovine abortion in free areas or in eradication programmes must be investigated according to official regulations.

Treatment and control

Cattle are not treated: treatment does not reliably clear the infection and in many countries it is prohibited. The disease is notifiable in many countries and control follows official programmes:

• Test and slaughter of positive animals, with movement restrictions on affected herds and pre-movement testing.
• Calving hygiene: isolating cows about to calve or that abort, and safely destroying fetuses and placentas.
• Vaccination in endemic areas, mainly of young heifer calves. S19 can induce antibodies that interfere with serology, especially when adult animals are vaccinated. RB51 is a rough strain that does not induce the antibodies detected by standard serological tests, so it does not interfere with them. Both are live vaccines that can infect people, and RB51 is resistant to rifampicin, one of the antibiotics used to treat humans. In free areas vaccination is usually prohibited.
• Pasteurisation of milk and protection of exposed workers.

Key studies

• Andrade et al. (2024), systematic review of 71 studies and meta-analysis of 65, with 178 assays of 11 serological tests for bovine brucellosis. In studies without a case-control design, the highest sensitivity was for the indirect ELISA with bacterial suspension antigen (96.5%; 95% CI 94.1–97.9%) and the lowest for the 2-mercaptoethanol test (85.0%; 79.6–89.1%); the highest specificity was for the fluorescence polarisation assay (99.7%; 99.5–99.8%), which had the best overall accuracy. Including case-control studies overestimated the sensitivity and specificity of all tests.
• de Oliveira et al. (2022), systematic review and meta-analysis of controlled experimental challenge trials of the S19 and RB51 vaccines. Against abortion (15 trials), S19 at its best dose had an RR of 0.25 (95% CI 0.12–0.52; efficacy 75.09%) and RB51 at its best dose an RR of 0.31 (0.16–0.61; efficacy 69.25%). Against infection (23 trials), only those two subgroups gave significant protection, with efficacies of 72.03% and 57.05%. Protection depended strongly on dose and was measured in experimental challenges.
• Serpa Gonçalves et al. (2025), systematic review of 33 field trials in 17 articles (1976–2016) with S19 and RB51. 20 of the 33 trials (60.61%) showed a significant effect of vaccination, with less infection or lower prevalence in vaccinated animals. Heterogeneity precluded meta-analysis, most were cross-sectional studies and 19 of 33 applied other measures at the same time, such as slaughter of positive animals, so the effect of the vaccine alone cannot be separated.
• Dean et al. (2012), WHO-commissioned systematic review and meta-analysis of 57 studies on the clinical manifestations of human brucellosis. 78% of patients had fever, and joint (65%), muscle (47%) and back pain (45%) affected around half. One in 10 men had epididymo-orchitis, and for every 100 patients there was 1 case of endocarditis and 4 with neurological involvement. Delays in diagnosis and treatment were common.
• Laine et al. (2023), estimate of the global incidence of human brucellosis using international public health data and three statistical models. The conservative estimate was 2.1 million cases per year, much higher than previously assumed, with most of the risk in Africa and Asia. It is a model based on incomplete surveillance data, with underdiagnosis, and does not distinguish between Brucella species.
• Pereira et al. (2020), systematic review of 63 studies on brucellosis as an occupational disease. The most exposed groups were rural workers, abattoir workers, veterinarians and their assistants, laboratory staff and hunters; the main risks were direct contact with animal fluids, not using protective equipment and accidental exposure to live vaccines. In the meta-analysis of case-control studies, animal breeders, laboratory and abattoir workers had an OR of infection of 3.47 (95% CI 1.47–8.19).