Bovine Tuberculosis (bTB)


The TB complex of diseases of livestock results from infection of animals with mycobacterial pathogens, primarily M. bovis and Mycobacterium avium subspecies avium (MAA).

These 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 tuberculosis, Mycobacterium tuberculosis (MTB).

TB is 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.

The 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 trade restrictions.

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.

Alarmingly, 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.

A 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.

Although 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 be missed.  

Because 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.