March 18, 2008

Since 1994, the state of Michigan has recognized a problem with Mycobacterium bovis in wild white-tailed deer from a fourteen county area in northeastern Lower Michigan.  In 2007, surveillance activities for M. bovis continued statewide, with an emphasis on the northern half of Lower Peninsula. In white-tailed deer, 27 animals cultured positive from 8,308 deer submitted for testing.

Since the index cases were first identified, over 162,000 free-ranging deer have been tested for bovine tuberculosis; 594 infected animals have been found.  Increasingly, the spatial epidemiology of the disease is revealing a highly focal, clustered pattern.  Approximately 97% percent of all positive deer identified to date originated from a five county area.  Moreover, within that area, the vast majority of positive deer were from Deer Management Unit (DMU) 452.  Even within DMU 452, the spatial arrangement of cases is highly clustered, in spite of the fact that sampling effort has been relatively uniform geographically.

White-tailed deer are the maintenance host and primary reservoir for TB in the Michigan outbreak. If eradication is to be achieved, control strategies must focus on the disease in deer. Strategies for eradication of TB from Michigan wildlife continue to focus on 1) reducing deer population densities to biological carrying capacity and 2) reducing artificial congregation of deer by restriction or elimination of baiting and feeding.  These strategies have been implemented through provisions of a late firearm antlerless deer season, sufficient antlerless deer licenses to reduce the deer population, and by prohibition of deer baiting and feeding.

Population estimates based on reconstruction techniques similar to the sex?age?kill method described by Creed et al. (1984)¹ suggest that the deer population in the five county area has declined approximately 27% since 1995. The achievement of this substantial population reduction highlights the critical role that hunters have played in the control of TB in Michigan. Nonetheless, persistent focal areas of high density on private land remain problematic.  Baiting and feeding have been prohibited in the seven counties from which 98% of all TB positive deer have originated.  Policy makers have committed to keeping these regulations consistent for a minimum of five years starting June 2002, in order to improve compliance and enforcement.  The overall scope of feeding has declined dramatically since 1997, with large scale feeding largely a thing of the past.  While some illegal baiting and feeding continues to occur, the size of these sites is substantially reduced, and it is hoped that heightened enforcement is expected to reduce the practice further over the next several years.

While much work remains, substantial progress has been made towards eradication of TB from Michigan wildlife.  Apparent prevalence in the core area of the outbreak DMU 452 was 1.4% in 2007.  Trend analysis of prevalence data from 1995 to 2007 indicates a statistically significant decreasing trend. 

Michigan's TB intervention strategies are working; however, it is too early to claim victory in eradicating the disease.  The need to stay the course is important, but will be difficult, due to ever increasing pressure from a variety of sources to lessen these intervention strategies.

The intervention strategies have been successful in bringing down the average prevalence in DMU 452; however, there are clusters of disease that will be more difficult to manage. With that in mind, the State of Michigan is evaluating a new intervention strategy that may be more acceptable to many hunters and landowners.  The new strategy is based on live-trapping and TB-testing of wild deer, and removal of positive animals. And if a safe and effective TB vaccine could be developed, then captured deer that tested negative for TB could be vaccinated before release.  This strategy is not intended to replace initial strategies, but may assist them in eliminating TB from the deer herd in focal areas.

The Michigan Department of Natural Resources (MDNR) initiated the new strategy in a township with relatively high TB prevalence within DMU 452 during the winter of 2003.  Link to report on the new strategy  The results of the pilot are cause for optimism on a number of fronts.  The project was well received and supported by the public.  Appreciable numbers of deer were captured with reasonable efficiency and low mortality.  Tracking and removal techniques worked well.  The one facet of the project that failed was the blood test. 

An effort to develop a more accurate blood testing procedure was the focus during the 2004 and 2005 hunting seasons.  Hunters were asked to collect blood from deer harvested in DMU 452, and to submit the blood and the deer head to a deer check station.  The lymph nodes from the deer heads were cultured for TB and culture results compared with results from seven TB blood tests.  One blood test, the Rapid Test (RT) that can be done in 20 minutes in the field with whole blood looks promising for field use.

During the winters of 2007 and 2008, the MDNR and the United States Department of Agriculture -Wildlife Services (USDA-WS) pilot-trialed the capture, test and cull strategy that the MDNR had been working on since 2003, in a relatively high TB prevalence area in DMU 452. Almost 800 deer were captured and tested for bovine TB using the RT. Eight deer tested positive on the Rapid Test. The pilot project showed that a substantial number of deer can be captured and quickly tested for TB in a field situation. Most of the deer that were both culture positive and Rapid Test positive had extensive TB lesions in the chest cavity indicating advanced infection. It appears that the RT works best at detecting highly infected animals when numerous TB lesions are present.

The DNR is working with USDA researchers in Ames, Iowa to develop a TB vaccine for use in wild deer. Preliminary results are encouraging, and the vaccine appears to give some protection from disease. Vaccinated groups of deer given the vaccine orally or subcutaneously had statistically significantly fewer visible TB lesions and less severe TB lesions than unvaccinated deer.

In summary, Michigan is showing progress in eradicating bovine TB from its wild deer population. However, this success is fragile and we need to be diligent in maintaining our control strategies.

¹Creed, W. A., F. Haberland, B. E. Kohn, and K. R. McCaffery. 1984. Harvest management: the Wisconsin experience. Pages 243?260 in L. K. Halls, editor. White-tailed deer ecology and management. Stackpole, Harrisburg, Pennsylvania, USA.