Summary of bovine tuberculosis in Michigan's wild deer and information on early occurrences.
Bovine TB is spread primarily through the air when an infected animal is in close contact with other animals. Bacteria released into the air through coughing and sneezing can spread the disease. Current research suggests that bovine TB can also be contracted from contaminated feed. Close contact between deer at feeding stations has been determined to be the likely point of transmission.
Bovine TB is a chronic disease in deer and it can take years for lesions to develop in the lungs. Only 40% percent of the TB-positive deer in the surveillance surveys had lesions in the chest cavity or lungs that would be recognized as unusual by most deer hunters. These deer had tan or yellow lumps on the inside surface of the rib cage and in the lung tissue.
Approximately 50 years ago, Michigan led the U.S.A. in the number of cattle testing positive for bovine tuberculosis with 30% of the total infected animals in the nation. A bovine TB eradication program was initiated in 1917 and Michigan was accredited as bovine TB-free in it's cattle by the U.S. Dept. of Agriculture (USDA) in 1979. However, a hunter killed free ranging white-tailed deer taken in Alcona county, Michigan in November 1975 had lesions which were confirmed by bacterial culture to be bovine TB (Myobacterium bovis). This was believed to be a isolated case and no further testing of the surrounding deer or livestock was done. Then a second deer, taken by a hunter in 1994, was confirmed to also be infected with the disease. This animal was harvested approximately 10 miles from the site of the 1975 infected deer.
Occurrence of bovine TB (Mycobacterium bovis) in wild deer has been rare in the U.S. Each of the eight cases reported before 1995 were found to be associated with exposure to infected cattle, bison, captive elk, or feral swine. Based on this historical data, it was thought that the 1994 deer might have been associated with tuberculous livestock. In spring, 1995, all livestock were tested within the immediate area were the 1994 positive deer was located. No evidence of M. bovis was found in the livestock. Nor was the disease found in 23 deer tested from two captive herds within the surveillance area. Surveillance of hunter-killed deer was initiated and examination of 354 deer from the same area revealed 18 more positive cases in 1995. This triggered a major concern among the State Departments of Agriculture, Natural Resources and Community Health.