Brucella Abortus Biology in Yellowstone
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
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
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
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
in the Greater Yellowstone Area: Board on Agriculture