This is the organism responsible for classic bovine tuberculosis,
but it was also of great concern in humans as a zoonotic infection
prior to the onset of pasteurization. It is still a concern in
underdeveloped regions of the world where pasteurization is not
practiced.
M. bovis infection in cows:
As with M. tuberculosis in people, M. bovis is
spread from cow-to-cow by inhalation of the organism in aerosolized
droplets. The pathology in cows is also similar to the pathology
of M. tuberculosis in humans, with pulmonary TB leading
to chronic debilitation and coughing, and the potential for systemic
spread to other organs. In addition, 1 to 2% of M. bovis
infected cows develop mycobacterial mastitis with shedding of
the organism in the milk.
- Mycobacterium bovis has been eradicated from almost
all of the U.S. However, two important foci remain. In the El
Paso, Texas "milk shed," a geographic area that contains
20,000+ dairy cows, over 250 infected animals have been identified
since 1985. And in the northeastern corner of the lower peninsula
of Michigan, hundreds of dairy and beef cattle have been euthanized
since 1995 to control an outbreak linked to M. bovis infection
in white-tailed deer. Infections in white-tailed deer in this
area were first recognized in 1974, presumably originating from
domestic cattle at that time. More recently, M. bovis
infections have been diagnosed among cattle in NM and CA.
M. bovis infection in other animals:
- Goats: Disease is similar to that in cattle.
- Pigs: Infection is usually due to ingestion
of contaminated milk or contaminated cattle feed, leading to
GI tract and associated lymph node pathology.
- Sheep, horses, dogs and cats: Infection in
these species is rare; TB lesions in lymph nodes or the GI tract.
- Exotic species: M. bovis has infected
a large percentage of the Cape Buffalo in the Kruger National
Park in South Africa, and thereby spread to lions that prey on
the buffalo. In addition, elk in Riding Mountain National Park
in Manitoba, Canada are also infected.
Zoonotic M. bovis:
- When people are infected by ingestion of M. bovis
in milk, the organism most commonly localizes to the GI tract
and related lymph nodes.
- This was the most common form of M. bovis TB in people
during the pre-pasteurization age.
- When people are infected by inhalation of the organism shed
from the airways of infected cattle, they can develop classic
pulmonary TB similar to M. tuberculosis infection.
- People can then shed the organism from their airways back
to other cattle. However, person-to-person transmission of M.
bovis is considered to be quite rare.
- As with M. tuberculosis, multi-drug resistance is
beginning to be detected in M. bovis strains, and this
is a significant concern for HIV patients in developing countries
that are still exposed to M. bovis through unpasteurized
milk.
Control of bovine TB:
Bovine tuberculosis (M. bovis), like brucellosis, is
controlled in the U.S. through a national eradication program
administered through the United States Department of Agriculture.
The fundamental aspects of this program are:
- Slaughter check for TB pathology with trace back and
individual animal testing of cattle on the farm of origin of
any cow with lesions at slaughter suggestive of TB.
- Caudal fold tuberculin test
- This is a test for a hypersensitivity immune response to
intradermally administered purified tuberculin, similar to the
skin test used in humans.
- In earlier days of bovine TB control in the U.S., individual
cattle were tested routinely every 3 years. Today, testing is
used only in cases of transport/sale certification or during
slaughter-suspect trace back investigations.
- Importantly, a positive response on this test can be due
to infection of a cow with either mammalian (M. bovis
or M. tuberculosis) or avian (M. avium-intracellulare)
TB organisms. Differentiating these possibilities is critical,
since infection with mammalian TB organisms poses a much greater
risk to both the cattle industry and the human population than
does infection with M. avium-intracellulare, which rarely
spreads from cow-to-cow or cow-to-human.
- Comparative cervical test
- This is a test in which cattle are intradermally injected
with both mammalian tuberculin and avian tuberculin and the relative
response to each is assessed. Therefore, this test differentiates
infection with mammalian TB versus avian TB organisms.
- This test is conducted only by regulatory state or federal
veterinarians.
- Positive cattle (i.e., those with a positive test to mammalian
tuberculin, but a negative test to avian tuberculin) will be
slaughtered.
The problem of cervids:
- Cervids are becoming a real concern for control of bovine
TB. Mycobacterium bovis is being increasingly detected
in various wild cervids- an infected deer is postulated to have
been the ultimate source of bovine TB cases in WI and Michigan
since 1995.
- A 1996 survey of deer in Michigan found 1.2-2.4% infection
rates, with spillover into 10 cattle herds in MI to date. This
spill-over caused the USDA to suspend Michigan's "TB-free"
status in 2000.
- Since 1997, Federal control ("Uniform Methods")
of TB in cervids has required that all farm-raised cervids must
be TB tested.
Badgers in the U.K. and opossums in New Zealand
have also been implicated as wildlife reservoirs for infection
of cattle. The badger infections in England resulted in infection
of 15.8% of cattle herds in Devon, England in 2003.
