Brucella Abortus Biology in Yellowstone Bison


Brucellosis is a bacterium found worldwide that infects mammals. The most significant symptom in bison is abortion of the first calf. This was discovered after cattle herds in New England were suffering from contagious abortions. The Bureau of Animal Industries began research and control of the disease in 1911. Brucellosis was discovered in Yellowstone National Park in 1917, most probably from contact with settler's cattle. The disease is transmittable to humans, and is known as undulant fever.


Hosts in North America of brucella organisms include:

cattle: B. abortus
goats: B. melitensis
swine: B suis
sheep: B. ovis

bison: B. abortus
elk: B. abortus
caribou: B suis
reindeer: B. suis
feral swine: B. suis
canine: B. canis
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The bacteria is remarkable in that it is an intracellular organism. Most bacterium inhabit the space between the host cells, whereas brucella enters the cell membrane in a viral fashion.  The hosts immune system isolates the diseasesed cells  in the lymphatic system, which may experience swelling. Other symptoms include abortion in the third trimester, swollen testes, swollen joints, decreased mammary milk supply, and other birth complications such as placental retention. In bison, cows abort only their first calf and can reproduce successfully thereafter. The birth material and milk are still contagious.


Transmission of the disease is by ingestion or contact with birth products or aborted fetuses. Contagious individuals are cows. Bulls may carry the disease and spread it venereally, but they do not give birth and release the organism into the environment. Scavengers eating the flesh of infected animals may acquire brucellosis.

The major threat to the cattle industry from brucellosis is the transmission from bison. The remaining herds are managed with the difficult goal of eradication of the disease. Bison can transmit the disease back to cattle but this has not occurred in the wild outside of experimental conditions.

Elk give birth away from the herd and there is less chance of transmission immediately after birth. The presence of brucella in elk may be continued by reinfection from the bison herds. The bison give birth within their group and other individuals are attracted to the birth products. Cattle give birth in similar circumstances and are drawn by a natural curiosity to birth products.


Current testing methods test for antibodies in milk and blood. A single positive result declares the entire herd to be affected.  Animals incubating the bacteria but not diseased yet may test negative for antibodies yet develop them shortly after. Naturally resistant animals will test positive for antibodies, but blood and tissue samples will prove negative for presence of the bacteria. Culturing the organism can be difficult depending on where it is isolated in the host's body.  There are usually more positively tested animals than there are contagious animals, and there are usually more infected individuals than  positive results.
For more information on testing, see wildlife page


Eradication of Brucellosis is being pursued through a combination of test and slaughter, and vaccination.  The difficulties in this goal are compunded by political issues, available resources, and the failings of available testing methods.
Two vaccines have been developed: Brucellosis Vaccine Strain 19, and BV-RB 51. Two other vaccines have been developed:  Rev 1 and Strain 2 for b. melitensis and b suis, respectively.
Strain 19 is more effective on cattle than on bison. 65-70% of vaccinated cattle develop resistance. Pregnant bison develop the disease and abort when vaccinated.  Strain 19 appears to be as effective on elk as on cattle.  Humans must be careful when using it as it can cause disease.
A further complication of BVS19 is that the disease organisms cannot be distinguised from the vaccine organisms by the most commonly used tests.

RB51 is effective at building resistance in cattle but causes some pregnant bison to abort their fetuses.  During testing, this vaccine can be distinguised from the disease. 

Sources of Information
GYIBC sources:

Brucellosis in the Greater Yellowstone Area: Board on Agriculture