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Tuberculosis (TB) is a serious disease caused when bacteria attack the respiratory system. There are three types of TB - human, avian, and bovine. Human TB is rarely transmitted to non-humans, avian TB is typically restricted to birds (pigs and occasionally other animals have been found to be susceptible), and bovine TB - or cattle TB - is the most infectious, capable of infecting most mammals. Bovine TB is caused by the bacterium Mycobacterium bovis (M. bovis) which is part of the Mycobacterium tuberculosis complex.
In 1994, a hunter in southwestern Alpena County shot a 4-year old male white-tailed deer infected with bovine TB. The only other time TB had been found in a wild deer in Michigan was in 1975, when a hunter killed a 9-year old bovine TB infected female white-tailed deer in Alcona County.
Starting in 1995 hunter harvested, road killed, and other dead deer were examined for bovine TB infection. White-tailed deer in Michigan have since been tested year round for bovine TB. Testing revealed that most of the TB positive animals were located in a core area in the northeastern part of Michigan's Lower Peninsula. The core area is located around the four corners where the counties of Montmorency, Alpena, Oscoda and Alcona meet. Antrim, Crawford, Emmet, Iosco, Mecosta, Osceola, Otsego, Presque Isle, and Roscommon Counties have also had animals test positive for bovine TB.
In Michigan bovine TB has been found in white-tailed deer, elk, black bear, bobcat, coyote, opossum, raccoon, and red fox.
Bovine TB is a chronic disease and it can take years to develop. M. Bovis grows very slowly and only replicates every 12-20 hours. The lymph nodes in the animal's head usually show infection first and as the disease progresses lesions will begin to develop on the surface of the lungs and chest cavity. In severely infected deer, lesions can usually be found throughout the animal's entire body. Non-cervid animals on the other hand do not develop the disease as extensively and lesions are usually not found in lungs or other tissues.
Bovine TB infected deer not showing lesions in the chest cavity can be diagnosed by performing a visual inspection of the lymph nodes in the deer's head. Affected lymph nodes, when cut, will contain one or more necrotic nodules. These nodules may vary in size and be filled with yellow-green or tan pus.
Tuberculosis is a chronic, progressive disease that can cause gradual debilitation, emaciation, depression, and intolerance to exercise. Coughing, nasal discharge, and difficulty breathing can result in cases where the lungs become severely affected. In some instances, superficial lymph nodes in the neck will develop large abscesses that may rupture and drain through the skin.
The rest of the sample is transferred to culture (growth) media which will allow any acid-fast bacilli which are present to multiply. Over the next two months, culture media is examined regularly looking for growth typical of M. bovis. This normally appears in 10-14 days. The growth is tested using genetic probes to determine whether the culture contains M. tuberculosis complex, of which M. bovis is a member. Additional biochemical testing, which requires three to five weeks, will confirm the final identification.
Humans can be skin-tested to determine if they have been exposed to TB. These tests can be done at either the local health department or a private physician's office. A positive skin test, however, does not identify the source of the infection. Remember, most people get the infection from other people.
At risk are Michigan's deer herd and other wildlife species with their many social, ecological, and economic values as well as Michigan's livestock industry. By continuing to eliminate TB-infected animals from wild and domestic animal populations, paying close attention to the meat inspection and pasteurization processes, using proper food handling, and good management practices, the chance of bovine TB transmission from animals to humans is virtually eliminated.
For more information, visit www.michigan.gov/emergingdiseases
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.
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