A Significant Economic and Health Risk

In cattle, TB is caused by infection with the Mycobacterium bovis bacterium (M. bovis), a close relative of the human pathogen, Mycobacterium tuberculosis. Bovine TB causes substantial economic loss worldwide, and can be transmitted from livestock to humans and other animals. No other organism has as great a host range as TB, which can infect all warm blooded animals.

TB spreads through the air by infectious droplets contacting eyes or mucus membranes, or by contact with infected carcasses. Animals transmit TB infections when they congregate at managed feeding sites. TB has been recorded in wild populations of white-tailed deer, badgers, raccoons and opossums.

General surveillance of cattle is based on inspection at slaughter and histopathological investigation of granulomatous lesions found in carcasses. In herds suspected of harboring TB, screening is carried out by administering the tuberculin test via caudal fold (tail) or cervical (neck) injections. Animals are confined for several days until an expert can interpretate the reactions, and then animals that retest positive on subsequent ancillary tests are culled.

TB testing by injection of tuberculin under the skin requires substantial expertise to administer and must be followed up by expert visual interpretation of results days later. These methods are often limited by the cost and availability of experts. Follow-on diagnostic methods such as growth in culture and antibiotic susceptibility are hampered by slow microbial growth rates.

Tuberculin testing in cattle has high specificity (96-99%) in herds with widespread infection, but its moderate sensitivity of 72% allows 28% of infected animals to go undetected. Those misdiagnosed animals may be released back into herds, spreading the infection to other cattle.