Bovine Tuberculosis (bTB)
complex of diseases of livestock results from infection of animals with
mycobacterial pathogens, primarily M.
bovis and Mycobacterium avium
subspecies avium (MAA).
organisms can cause disease in multiple livestock and wild animal species and
can be readily transmitted to humans. M. bovis, whose disease and infections is
our primary focus, is closely related to the organism that causes human
a disease of antiquity that has resulted in considerable economic loss to
animal agriculture and, as a zoonotic disease, contributed greatly to human
suffering prior to the widespread requirement for milk pasteurization. In fact,
at the turn of the 20th century, M. bovis
was considered to be the cause of greater economic losses to livestock
production than all other infectious diseases combined.
implementation of rigorous control and disease eradication programs, including
test and slaughter or test and segregate programs, have
reduced or eliminated tuberculosis in cattle in the US and most developed
countries. However, reservoirs in
wildlife have precluded complete eradication. TB continues to be a significant
recurring concern in many countries, including Ireland, the United Kingdom (UK)
and New Zealand. In addition, both
bovine tuberculosis and M. bovis infections in humans remain common in less developed
countries, resulting in considerable economic losses due to disease and
While TB incidence in the US remains low, there is considerable
concern that we may be experiencing a resurgence of this disease in livestock
species, primarily cattle. In 1994, a white-tailed deer (WT deer) from northeastern
Michigan was found to be infected with M.
bovis. This led to wide-scale
testing of cattle and deer with subsequent identification of M. bovis in both populations within this
area. The spread of M. bovis in Michigan was slowed by a strict policy of total herd
depopulation upon identification of positive cattle, as well as large-scale hunter
education programs and a massive testing initiative in WT deer. Still, in Michigan, over 650 cases of M. bovis infection in WT deer and 49
positive cattle herds have been identified to date.
M. bovis has now spread to
other states. M. bovis was recently detected in 27 WT deer and 12 cattle herds in
Minnesota and has been confirmed in cattle from Colorado, Nebraska, Indiana,
Kentucky, North Dakota, South Dakota, New Mexico, and California. Detection of M. bovis infection has lead to quarantine and depopulation of
nearly all affected herds. Clearly, this
disease is continuing its resurgence throughout the US, particularly where
cattle and WT deer commingle.
second major source of M. bovis
infected cattle in the US is imported animals from other countries where the
disease is endemic, particularly Mexico. Indeed, molecular epidemiology studies
have demonstrated that M. bovis cases
in all states other than Michigan are likely of Mexican origin.
USDA regulations stipulate that imported cattle must be tested within 60 days
of import, the low sensitivity of most approved M. bovis diagnostic tests suggests that some infected animals will
cattle are only held at the border for 48 to 72 hours, there is little time to
conduct additional testing at the point of entry. In addition, the lack of mandatory animal
identification in the US limits the ability to track cattle after introduction
into the country. Clearly, it is crucial
to have rapid diagnostics with improved sensitivity that could be deployed at
points of entry. It is equally important
to improve information on cattle movements to control importation of M. bovis infected cattle.
While M. bovis does not cause the consistent
losses characterized by other diseases, such as JD, it is of significant
concern to government agencies and cattle industries due to associated
economic, social and potential public health problems.