VIVO Infectious Diseases Bacterial Diseases of Man and Animals

Brucellosis in Ruminants

Ruminants are without doubt the most important hosts for the major species of Brucella. Not only does brucellosis in ruminants result in substantial losses to animal agriculture, but infected ruminants are clearly the most important source of brucellosis in humans.

As described below, abortion is the hallmark of infection of ruminants with virulent species of Brucella. Brucellae clearly have a tropism for phagocytic cells, but also for trophoblast cells of the developing fetus, and infection of trophoblast cells is what makes them abortogenic. One caution, however, is that a large number of other pathogens (Coxiella, Campylobacter, Chlamydia, Toxoplasma to name a few) also cause abortion in small ruminants that are clinically indistinguishable from brucellosis. Clearly, diagnostics are required to identify the causative pathogen in such cases.

Cattle, Buffalo, Bison, and Yaks

Bovine brucellosis is most commonly the result of infection with Brucella abortus, but these animals can also be infected by B. melitensis and B. suis. The disease is sometimes called Bang's disease in recognition of Bernhard Bang, a Danish veterinarian who first isolated B. abortus in 1897.

The most prominent feature of bovine brucellosis is abortion, usually in late gestation, but birth of weak calves, retained placentae, and poor milk production after calving are also seen as signs of infection. Other than those signs, infection is typically not associated with overt clinical illness. Interestingly, most cows that abort from brucellosis do so only once.

What about males? Bulls can also be infected and suffer infection of reproductive organs. Semen from infected bulls often contains Brucella organisms, but venereal transmission to cows appears to be uncommon.

Transmission of Brucella in cattle and other bovids is most commonly by inhalation or ingestion of contaminated material, especically from abortions. When a cow aborts due to brucellosis, the organisms is present in the fetus, fetal membranes, placenta and fetal fluids. Consider this: it has been estimated that up to 1014 (100 trillion) B. abortus organisms are shed during an abortion, which is a huge number and is probably enough to infect about 1 millon other cattle! Considering the gregarious nature of cattle, this figure makes it easy to understand how one case of abortion in previously infected herd can result in widespread transmission to other cows.

Infected cows shed Brucella bacteria in milk, which is exceptionally important for transmission of brucellosis to humans. Shedding in milk is commonly seen as life-long sequela to infection, even in the absense of additional abortions.

Small Ruminants: Goats and Sheep

Brucellosis in goats and sheep is most commonly due to infection by B. melitensis, but they can also be infected with B. abortus and possibly B. suis. B. melitensis is widely considered the most pathogenic species of Brucella for humans. This disease remains endemic is large geographic regions, especically Africa, Asia, the Mediterranian, parts of Central and South America, and the Middle East.

As with cattle, the most common sign of brucellosis in small ruminants is late term abortion, stillbirth and birth of weak offspring. Except for those manifestations, infection in adults is essentially subclinical. Transmission is also similar to what is seen in catte: inhalation, ingestion and exposure to environments contaminated with the products of Brucella-induced abortions. Males can also be infected and may develop reproductive tract lesions and excreted bacteria in semen, but do not seems to be of importance in transmission dynamics.

Another important feature of small ruminant brucellosis is shedding of the organism in milk. As with cattle, milk and milk products from infected goats and sheep pose a clear danger to humans that ingest them. Boiling or pasteurization of such milk is an important public health practice.

There is another species of Brucella that infects and causes disease in sheep: Brucella ovis. Infection with this organism is distinctly different than what is seen with bruellosis due to B. melitensis. First, the main feature of this disease is reproductive lesions in male sheep, more specifically epididymitis. It does also cause abortions in pregnant ewes, but this is not a common problem. B. ovis does not seem to be pathogenic to humans.

Camels

Camels are susceptible to both B. abortus and B. melitensis. The species of Brucella that infects specific groups of camels seems to depend mostly on the whether they are in contact with cattle or small ruminants; infection of camels with B. melitensis is by most frequent. Brucellosis is common, sometimes alarmingly so, among dromedaries in all major camel producing areas except Australia; infection of Bactrian camels appears to be less common. Signs of brucellosis in camels is similar to what is seen in other ruminants: mild signs are observed in males and non-pregnant females, while infection of pregnant females often results in fetal death, mummification, abortion and retained placenta.

Camel milk and meat are important food sources for people in camel producing regions and for cultural and religious reasons, camel milk is rarely boiled. Thus, Brucella infection camels is a significant source of brucellosis in humans and high seroprevalence among camel producers has been reported.

New world camelids - llamas and alpacas - are also susceptible to brucellosis, but does not appear to be a major problem.

Cervids (Deer, Elk, Raindeer, Caribou)

Brucellosis has been well studied in red deer in Europe and elk in the United States, and it is likely that all cervid species are susceptible to infection with Brucella. In the U.S., B. abortus infection of elk and bison in the region of Yellowstone Park remains an unsolved problem and, most troublesome, there is occasional spillover of B. abortus from these wildlife reservoirs to local populations of cattle.

Serologic surveys and clinical observations indicate that brucellosis is not uncommon in arctic cervids, especially wild caribou and domestic raindeer in both North America and Siberia; this infection has also been reported in moose and musk oxen. The bacteria responsible for these cold-climate infections is Brucella suis. Clinical signs, especially in caribou, include abortions, enlarged joints and a swollen scrotum in males. This type of brucella does pose a threat to humans that butcher such animals, especially populations of subsistence hunters in the arctic.

Vaccines for Brucellosis in Ruminants

Enormous resources have been spent in attempts to control brucellosis. Several countries have virtually eliminated bovine brucellosis through a combination of rigid requirements for testing and slaughter of cattle that are serologically positive. In conjunction with this, elimination of human cases of brucellosis has been achieved not only be control of the disease in animals but by widespread adoption of milk pasturization.

In addition to test-and-slaughter programs, control of brucellosis has been aided by vaccination. It is important to recognize that scientists have been trying to develop effective brucellosis vaccines for many, many years and, to date, there are no vaccines that are both highly effective and totally safe. The search goes on and developing improved vaccines for controlling brucellosis in ruminants is a very high priority goal.

The two vaccines most widely used to control brucellosis in cattle are:

Strain 19 and RB51 vaccines have undoubtedly been useful in brucellosis eradication programs, but neither is ideal for several reasons. First, neither is hightly efficacious and both vaccines induce abortion when administered pregnant cattle, albeit at low rates. Second, both strains of vaccine are mildly or moderately pathogenic to humans. Finally both Strain 19 and RB51 strains are shed in milk of immunized cows and humans that consume such can become infected; another justification for pasteurizing raw milk!

RB51 vaccine has also been tested for efficacy in red deer, bison and elk. Aside from the logistic challenges of immunizing wild ungulates, these trials have not been encouraging as a method for control of brucellosis in such animals.

The most effective vaccine for control of B. melitensis in small ruminants is called Rev 1. RB51 has also been tested, but Rev 1 is considerably more effective. Again, however, there are some major problems with use of Rev 1: it induces abortion in a large fraction of pregnant goats and sheep and is also significantly virulent in humans.

References

Aguilar XF, Mavrot F, Surujballi O, Leclerc LM, et al. Brucellosis emergence in the Canadian Arctic. One Health. 2024; 18:100712.

Carvalho TF, Haddad JP, Paixão TA, Santos RL. Meta-Analysis and Advancement of Brucellosis Vaccinology. PLoS One. 2016; 11:e0166582.

Dorneles EMS, Sriranganathan N, Lage AP. Recent advances in Brucella abortus vaccines. Vet Res 2015; 46:76-85.

Gwida M, El-Gohary A, Melzer F, Khan I, Rösler U, Neubauer H. Brucellosis in camels. Res Vet Sci. 2012; 92:351-5.

Khurana SK, Sehrawat A, Tiwari R, Prasad M, Gulati B, et al. Bovine brucellosis - a comprehensive review. Vet Q. 2021; 41:61-88.

Rossetti CA, Maurizio E, Rossi UA. Comparative Review of Brucellosis in Small Domestic Ruminants. Front Vet Sci. 2022; 9:887671.

Brucellosis in Humans Brucellosis in Swine

Updated February 2026. Send comments to Richard Bowen: rabowendvm@gmail.com