Bovine tuberculosis in Michigan (Proceedings)

Bovine Tuberculosis (bTB) is an infectious bacterial disease that poses a risk to domestic livestock and wildlife in the United States (U.S). In 1917 the U.S. government began a comprehensive national bovine TB eradication program. The disease has been nearly eradicated from livestock in the U.S., but periodically areas of infection resurface. As part of the United States bTB eradication program, Michigan was declared free of bTB in cattle in 1979. In 1975, a 9-year-old female white-tailed deer located in Northeast lower - Michigan, was found to have lesions consistent with bovine tuberculosis (bTB). Subsequently Mycobacterium bovis was isolated. This was believed to be an isolated case and no further testing was done on the surrounding livestock or deer. Historically, bTB in wild deer has been rare in the United States. Each of the 8 cases reported prior to 1995 was found to be associated with exposure to infected cattle, bison, captive elk, or feral swine and no evidence of further transmission between white tail deer was evident.

In 1994, a hunter in Northeast Michigan, shot a 4-year-old male whitetail deer, which had lesions consistent with TB, and M. bovis was isolated. The deer was harvested approximately 10 miles from the site of the 1975 infected deer. Because of Michigan's bTB free status in cattle, it was decided to test the surrounding cattle and captive cervid herds. No evidence of bovine TB was found. In the fall of 1995, surveillance of hunter-killed deer was initiated and 27/814 deer were found to be culture-positive for bovine TB.

The Bovine TB Eradication Project was established as a multiagency partnership to investigate the issues. The project consisted of personnel from the United States Department of Agriculture (USDA); the Michigan Departments of Agriculture (MDA), Natural Resources (DNR), and Community Health (MDCH); and Michigan State University (MSU).

Since 1995, wildlife surveys have been conducted by the DNR in the surrounding area. Cervid and noncervid carcasses and/or heads are examined grossly for lesions. Suspicious lymph nodes and tissues from cervids were submitted to the National Veterinary Services Laboratories (NVSL) or MDCH for culture and to MSU and NVSL for histopathology.

As of the end of 2007, over 161,000 whitetail deer and 1,876 elk have been examined for bTB. From these animals, 593 white tailed deer and 5 elk have been identified as positive for bTB. In addition, several non-cervid species have been surveyed primarily from Northeast Michigan and include (number of positive in parentheses) badgers, black bears (7), bobcats, coyotes (18), feral cats, feral dogs, gray fox, mink, opossum (2), otters, porcupine, raccoons (8), red fox (3), skunks, snowshoe hare, and weasel. The majority of the infected wildlife has come from 12 northeastern Lower Peninsula counties in Michigan (Figure 1). In the winter of 2008, a single white tailed deer was found infected with bTB in the southern part of the lower peninsula of MI. This is only the third positive deer found outside of Northeast MI and represents the most distant positive animal from the core of the bTB outbreak. A 10 mile circle test of all cattle around this infected deer has not found any indication of infected cattle to date.

Figure 1: Location of bovine TB positive animals in Michigan, 1975-2003.

Michigan has approximately 1.3 million cattle on 17,000 farms and 200,000 goats, bison, and privately owned cervids. As of Dec 31, 2008, over 2 million cattle, bison, goats and captive cervidae have been tested for bTB over the past ~10 years. In Northeast Michigan, 32 beef herds, 10 dairy herds and 3 captive cervid farms have been found infected with bTB (Table 1). Most of the cases have been found in a 5 county area that is referred to the core TB area. The majority of these farms have been depopulated. A small percentage of the farms (mainly dairy) have or are currently under a test and removal plan to clean up the disease. Three of the farms found infected, have become reinfected (2 beef, 1 dairy)

Table 1: Summary of Bovine TB positive farms in Michigan, 1997-2006

The endemic nature of bovine TB (bTB) in the state's white-tailed deer population and the finding of bTB in the cattle population is a public health concern.

Between 1994 and 2007, thirteen cases of human bTB were diagnosed. No genetic or epidemiological link to the deer/cattle outbreak has been identified among 11 of these human M. bovis cases. However, two of the cases were found to have the same strain of bTB that is currently being found in Michigan cattle and deer. These findings confirm that the current outbreak of bTB is a public health risk. Information of the bTB in humans in Michigan is available on-line at http://www.cdc.gov/eid/content/14/4/657.htm

MDCH and the USDA National Animal Disease Center (NADC) have conducted restriction fragment length polymorphism (RFLP) analysis of bTB isolates found in Northeast Michigan. The index deer and subsequent deer, elk, carnivore, and bovine isolates have identical RFLP patterns, indicating that the same strain of M. bovis is involved in the outbreak in cattle and wildlife. The most likely source of the infection in the carnivore and omnivore population was through the consumption of bTB-infected white-tailed deer.

White-tailed deer in Michigan are now recognized as a reservoir host of bovine TB. It has been determined that the most likely cause of bovine TB infection and transmission in the deer is from congregating in artificially high numbers at feed sites. Once the disease is eliminated from the deer, it is presumed that the disease should die out in the carnivorous and omnivorous species. As long as bovine TB exists in the free-ranging deer population, there will be some risk to local wildlife species that feed on bTB-infected deer carcasses or gut piles. Current wildlife strategy consists of deer management actions and wildlife disease surveys. Deer management actions such as a ban on feeding/baiting and increased deer harvest are used to eliminate bovine TB in wildlife, while wildlife disease surveys are used to monitor the prevalence of bovine TB and the geographical spread of the disease.

On the livestock side, Michigan is currently divided into three TB zones (Figure 2). The Northeast corner of the Lower Peninsula where TB has been most commonly found is Modified Accredited. The rest of the Lower Peninsula is Modified Accredited Advanced and the Upper Peninsula is Accredited TB Free. All cattle in the MA zone are tested annually. A targeted surveillance plan is in place in the MAA and TB free zone. Movement of cattle within and out of the MA zone is restricted and requires TB testing and movement permits. As a result of bTB in Michigan, all cattle in Michigan are now required to have an radio frequency identification tag (RFID) in their ear when they leave their premise of origin.

Figure 2: Michigan Bovine Tuberculosis Split State Status

More information on the bTB epidemic in Michigan can be found at http://www.michigan.gov/emergingdiseases/.