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).