The livestock-wildlife interface in infectious disease transmission - the bovine TB example in Michigan (Proceedings)

The issue of disease transmission between species is nothing new. Veterinarians have always been aware of the potential risk of wildlife being a source of disease transmission to livestock. A classic example is transmission of Leptospirosis species from wildlife to cattle via urine contamination of the environment. Transmission of diseases between domestic livestock species has led to management practices to reduce this risk. As a simple example, cattle and sheep are less likely to be housed together because of the risk of malignant catarrhal fever transmission from carrier sheep to highly susceptible cattle.

Transmission of infectious agents between cattle and wildlife are dependent on many factors. These include specific behaviors, management practices, physiologic events, environmental circumstances behavior and density of host species, climatic conditions, and overlap of host range with other susceptible species. Attributes of the specific infectious agent and the disease characteristics are also important. These factors include an agent's ability to infect a new host, the routes of entry and exit from the normal and new host, the course of a disease (acute -vs- chronic), time interval for shedding, and overall morbidity and mortality. Depending on these factors, a new host may be a "dead-end" host or it may become a maintenance host, able to maintain infection without continual introduction from other species.

As veterinarians, we must be aware of the risks for disease transmission from wildlife species to domestic livestock and counsel clients on these risks. As we understand the potential disease risks and how transmission occurs, risk mitigation strategies can be employed. What follows is a description of Bovine tuberculosis in Michigan as an example of disease spread from wildlife to cattle and how that spread may be managed.

Table 1: Summary of Bovine TB positive farms in Michigan, 1997-2010

Bovine Tuberculosis in Michigan

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 2⅞14 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 2009, over 184,000 whitetail deer, 2,400 elk and 60 moose have been examined for bTB. From these animals, 669 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.

Michigan has approximately 1.3 million cattle on 17,000 farms and 200,000 goats, bison, and privately owned cervids. As of July 1, 2010, over 2 million cattle, bison, goats and captive cervidae have been tested for bTB over the past ~10 years. In Northeast Michigan, 35 beef herds, 11 dairy herds and 4 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. Foour of the farms found infected, have become reinfected (3 beef, 1 dairy). A summary of all bovine TB positive animals and their location is shown in figure 1.

Figure 1: Location of bovine TB positive animals 1975-2010

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 bTB. 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 bTB zones (Figure 2). The Northeast corner of the Lower Peninsula where bTB 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 bTB free zone. Movement of cattle within and out of the MA zone is restricted and requires bTB testing and movement permits. As a result of bTB in Michigan, all cattle in Michigan are now required to have a radio frequency identification tag (RFID) in their ear when they leave their premise of origin.

Figure 2: Michigan TB Status - 2010

Transmission of bTB from deer to cattle is believed to occur primarily from contaminated feedstuffs (stored hay, water, etc). Mycobacterium bovis has been demonstrated experimentally to be capable of surviving for as long as 16 weeks in temperature ranges common to NE Michigan. Management practices that are conducive to feed contamination include such things as storing hay in remote unprotected locations (e.g. along fence lines), winter feeding in wooded pastures, and use of surface water as primary water sources. To assist producers to identify practices that may increase risk of bTB transmission from deer to their cattle, a risk mitigation tool has been developed. The Wildlife Risk*A*Syst Project identifies specific risk areas, quantifies them and allows producers to prioritize managing the risk. A copy of the Wildlife Risk*A*Syst Project can be found at http://web2.msue.msu.edu/bulletins/Bulletin/pdf/fas113.pdf.

Essentially, at the farm level risk of bTB transmission from deer to cattle is being mitigated by reducing the risk of deer coming in contact with cattle feed and water, especially during the winter months when feed and water resources become scarce for deer. Strategies to reduce this risk include storing feed in enclosed facilities (barns, fenced lots, etc), feeding cattle in enclosed areas (barns, fenced lots), and eliminating surface water as primary water sources (e.g. draining or fencing off surface water areas, creating artificial water sources that are less attractive to deer). Other strategies include reducing deer numbers thru special wildlife /deer disease hunting permits available to livestock producers.

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