Chapter 52 Tuberculosis in Michigan Deer
Tuberculosis (TB) caused by Mycobacterium bovis is endemic in the free-ranging white-tailed deer (Odocoileus virginianus) population in northeastern lower Michigan. The first documented case was shot by a hunter during the 1975 hunting season, an older doe estimated age 9.5 years. It had caseous abscesses in the lungs and chest cavity that cultured positive for M. bovis. At the time, Michigan had not yet attained “tuberculosis accredited-free” status and was in the second year of the required 5 years of no new TB cases in the state’s cattle population.
The Department of Natural Resources (DNR) and Department of Agriculture discussed the significance of this case of bovine TB in a free-ranging wild deer. It was thought that the deer must have been an orphaned fawn found and raised on raw milk by a local dairy farmer with recent M. bovis in cattle. Tuberculosis in free-ranging white-tailed deer was considered unlikely because the disease was believed to be associated with crowded and stressed animals in confinement. Therefore, the case was dismissed as an isolated incident, and no testing or surveillance efforts were made on wild deer in the area.
The state of Michigan achieved “tuberculosis accredited-free” status in 1979 for its cattle population. No further cases were noted until fall 1994, when another hunter shot a deer that was diagnosed with TB in the same general area where the positive deer was found in 1975. This animal was a 4.5-year-old buck. This case was still considered an incidental finding, but this time, surveillance efforts were implemented, with testing of cattle and examination of wild deer in the area.
It was hypothesized that the deer must have contracted bovine TB from infected cattle because it was again considered unlikely that deer could sustain TB in a free-ranging population. Cattle were no longer tested because Michigan was accredited as TB free, so regulatory officials were reasonably sure they would find the source of the disease in the cattle population.
Tuberculosis testing of all test-eligible livestock (cattle, captive deer, bison, goats) within a 5-mile (∼16-km) radius of the initial 1994 deer case failed to identify a source of the infection in domestic livestock. The location of the deer with TB found in 1975 was within this test area, so it now seemed likely that TB was present in the deer and may have been for some time.
Hunter-harvested deer were tested in the initial 16-km radius during the 1995 hunting season, with 18 more positive deer found, plus a few cases in the general area around the official test area. To facilitate surveillance and management of the problem, the DNR created a new Deer Management Unit (DMU) 452 in 1996.
The history and development of this unique situation in Michigan have been analyzed and chronicled by many authors. There is a significant body of knowledge in regard to the disease caused by M. bovis in cervids.2 One may only speculate on the origin of the M. bovis. However, an analysis of how the problem developed may help to explain what went wrong and to prevent the development of future problems, as well as explore methods to manage this disease in cervids.14
The new DMU 52 encompassed many “hunt clubs,” large tracts of privately owned land that had been bought by wealthy businessmen in the late 1800s and early 1900s to provide areas with members-only hunting privileges. These large tracts were available because the land had been cleared of trees and farmed with rather poor success; the soils were light and did not sustain agricultural crops well. Often these farms failed, and land reverted back to the state when taxes were not paid, allowing the land to be bought inexpensively by clubs.
Public lands were managed by DNR to increase the carrying capacity of the habitat for white-tailed deer with the Deer Range Improvement Program (DRIP). Deer hunting is a tremendously important contribution to Michigan’s economy, so much effort and many state and federal funds go into managing the deer herd. Rules and regulations on how deer are hunted and shot (e.g., from tree stands, over bait, type of weapon, length of season) are promulgated and enforced, with zones (DMUs) and quotas determined by biologists in an effort to keep the population healthy. When a firearms deer license is purchased in Michigan, it allows the hunter to shoot an antlered buck anywhere, but antlerless deer (does and fawns) may only be harvested with an additional permit for designated DMUs. Issuance of these limited permits is based on hunting pressure and deer productivity in an effort by biologists to regulate the population within the carrying capacity of its habitat, using hunting as a management tool.
This “biologic carrying capacity” is determined by study of established wildlife management values, such as available food, cover, productivity, and winter severity. In contrast, often the hunters and private land owners apparently base decisions on the “social carrying capacity” of an area. In my view, this determination is based more on political values, such as stakeholders’ attitudes, and is a more difficult figure to calculate. It is a common practice for some hunters to secure antlerless permits with no intention of taking a doe or fawn because they believe the deer population is insufficient in the area. The DNR has an even greater challenge when using hunting and other traditional management tools, such as controlled timber harvest or habitat manipulations on private lands, especially the large tracts with “limited hunting pressure” controlled by clubs. Although they did not fence their lands, the clubs controlled land use and regulated or limited hunting pressure much more than on public lands.
Additionally, many clubs baited during the fall to attract and keep deer on their land for an enhanced success rate, with less effort for their members, and to protect or keep “their” deer from being shot on adjacent public lands. Some clubs fed deer year-round in an effort to provide more and larger deer for their members. Unfortunately, more deer translates into smaller, less healthy deer when food availability is limited in quantity and quality.
It is known that adequate nutritious food is essential in the early fall to build up fat reserves to be used during the winter, when natural food and movement are limited by cold and snow. If white-tailed deer do not have adequate fat reserves to draw on, they may not be able to eat enough during the winter to survive, even if supplements are provided and are nutritionally complete.17
Congregating artificially high numbers of socially and nutritionally stressed deer over artificial food sources also created a greatly enhanced risk of disease transmission. As the population of deer grew, some clubs attained seasonal local deer densities of almost 200 deer per square mile. In an effort to increase the deer harvest, managers made the already-widespread practice of “shooting over bait” legal. Many hunters did so even if they were philosophically opposed to the concept of baiting, because deer stayed around these large bait sites on someone else’s land if they did not bait on the land they could hunt.
Baiting became big business, with the demand for cull crops such as apples, potatoes, and carrots exceeding supply. Farmers raising crops for deer bait were complaining that deer were eating their crops! They applied for and received crop damage, or “depredation,” permits from the DNR to shoot deer legally in the early fall before the regular gun season. They could let hunters fill these permits and charge for the opportunity to hunt on their land as a way to recoup lost revenue because of the damaged crops.
In one case, a farmer was issued a depredation permit to shoot 30 deer on his land, which he legally did by charging hunters to help reach the quota. Of the 30 deer shot, 29 were large bucks. This did remove some more deer from an already high population and compensate the farmer for some of his losses, but it did not remove the reproductive segment of the population (does), as intended by the wildlife biologists issuing the permits. Furthermore, hunters on adjacent lands during the regular season were less successful at harvesting a trophy buck and increased their baiting efforts to attract more deer to improve their chances of success.
Much of the baiting was being done during the critical fall period, when fat would normally be accumulated by eating more nutritious foods such as acorns and forage dispersed over large areas. These bait and feeding stations sometimes kept deer from their natural or traditional movement to winter yards, and the artificially high densities over artificial food supplies have helped create the current problem with M. bovis. It is likely that TB persisted at low levels in the deer herd when the population was within the carrying capacity of the habitat but had not spread due to lower densities.
Tuberculosis may transmit vertically in smaller deer family groups and may have spread horizontally in areas (e.g., some clubs) where cattle were fed along with deer. Many of these clubs had a history of raising cattle in the 1950s, when bovine TB was known to be common in Michigan cattle.13 Cattle were rounded up, tested, and slaughtered, but the free-ranging deer likely perpetuated the disease after the cattle source was removed. Previously, infected livestock were considered necessary to spread TB to cervids, and it was thought that infection would not be maintained in a properly managed free-ranging population.4
This might have been true, but management of the white-tailed deer population in Michigan clearly was not achieving the goals of having a healthy, sustainable population kept within the carrying capacity of the habitat.
Studies show that disease may be maintained at low, nonpathogenic levels when a population is below the carrying capacity of the available habitat. As the number of animals increases to or exceeds the carrying capacity of the habitat, the percentage of animals infected and the severity of infection increase.5 The consequences of mismanaging populations of white-tailed deer, with the potential for increased disease and parasites, have been documented.3
As the deer population increased on public lands as a result of successful management efforts, biologists increased the numbers of antlerless permits and allowed baiting in an effort to increase the harvest. Large-scale baiting increased the physical contact and stresses, but also escalated as hunters baited on public lands to compete. The clubs and private landowners put out even bigger piles to attract and keep deer around their lands. This facilitated increased transmission, so more animals were infected, with disease lesions showing up even earlier. Tuberculosis could easily have gone undetected in a population within the carrying capacity of the area where only bucks are harvested, because the vast majority of antlered bucks taken are 1.5 to 2.5 years old in the fall hunt. Animals with advanced lesions are able to infect other deer directly when in close contact, as at a bait or feeding station. Chewing and slobbering on large food items such as sugar beets could leave mycobacteria for the next deer to consume. A startled deer snorts and whistles, which may aerosolize organisms into the environment, allowing inhalation and contamination of feed, water, and soil, again in these high-density areas. Studies have shown that M. bovis may be cultured from bait and feed as well as soil and water samples and may persist as a potential source of infection for at least 1 month under natural conditions.6