Disease spillover woes

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Veterinarians discuss disease interface between wildlife, domestic livestock

In Michigan, white-tailed deer and cattle roam the same fields, infecting each other with tuberculosis. In the Greater Yellowstone area, bison and elk serve as reservoirs for brucellosis, posing a threat to nearby cattle. These are just two of the many diseases at the interface between domestic livestock and wildlife species.

According to Dr. John R. Fischer, a professor of pathology and director of the Southeastern Cooperative Wildlife Disease Study at the University of Georgia's College of Veterinary Medicine, interest in wildlife diseases has grown in recent years, in part because of the notoriety of diseases such as West Nile and chronic wasting disease. Dr. Fischer, along with other researchers, gathered to discuss some of these diseases at a conference at the Department of Agriculture's National Animal Disease Center in Ames, Iowa, July 17-18.

A costly problem
Researchers who have been in this field for some time welcome the increased attention. The situation in Michigan illustrates just how costly not managing this interface can be. In 1975, Michigan eliminated its last known cattle herd infected with tuberculosis. In 1994, the disease was identified in white-tailed deer in northeastern Michigan, and, in 1998, an intensive surveillance program identified a cattle herd with the disease. As of June 2003, veterinarians had identified infections in 30 cattle herds and 66 individual cattle.

Since 1998, the Michigan Department of Agriculture has spent over $24 million on activities to prevent the spread of the disease, eliminate infected livestock, and ensure trading partners that Michigan animals and products are safe, according to Dr. Michael S. Vanderklok, a food animal program manager in the department's Animal Industry Division. Experts estimate that costs in 2000 associated with lost milk production, discounts on cattle sold, and inconveniences to producers ran $23 million to $26 million.

Other states, including Texas, California, and New Mexico, have also lost their TB-free status in cattle in recent years, but Michigan's situation is unique. It's the only state where the disease persists in the wild deer population.

Dr. Steve M. Schmitt, head veterinarian at the Michigan Department of Natural Resources' Rose Lake Wildlife Diseases Laboratory, says this is caused by a few factors. "We had a lot of TB in cattle back in the 1920s," he said, noting that rates were as high as 30 percent in one of seven counties where the disease is now focused in deer.

The disease, therefore, had many chances to spill over into the abundant Michigan deer population. That, in combination with hunting clubs that have been feeding and baiting deer for years, which increases contact among animals, has allowed the disease to thrive. Researchers have even shown that Mycobacterium bovis can be indirectly transmitted through the sharing of feed.

"It's like taking kids to the day care center," Dr. Fischer said. "We see some diseases in captive wildlife species that we never observe in free-ranging animals of the same species."

Wild and domestic species are generally susceptible to the same disease agents, but because wild animals are naturally dispersed, they are less likely to maintain disease. "It is very important to recognize that the transmission of disease agents between wild and domestic animals is a two-way street," Dr. Fischer said. "We wouldn't be worried about the deer with TB in Michigan right now, and we wouldn't be worried about our cattle catching TB from deer, if TB hadn't spilled over from cattle to deer years ago."

Assessing risk
Tuberculosis is only one of the interface diseases concerning scientists. Although the eradication of brucellosis from cattle in the United States is nearly complete, Brucella abortus is prevalent in bison and elk in the Greater Yellowstone area, and these animals serve as reservoirs that could reinfect domestic livestock.

Brucellosis has nearly been eradicated in domestic swine but is endemic in feral swine in 14 states. These pigs, which are not native to the United States, are a source of potential zoonoses for hunters and workers at feral swine processing plants, and researchers have documented transmission of this disease from feral swine to domestic swine and cattle.

The decade-long eradication of pseudorabies is also threatened by a growing reservoir in infected feral swine. According to Dr. Bill C. Stoffregen, a veterinary medical officer with the USDA NADC, the number and range of feral swine is dramatically increasing because of natural population dynamics and the animal's increasing popularity as a game species in many states.

While researchers have concerns about a number of other disease agents in wildlife, not all diseases pose the same threat to domestic animals and humans. Dr. Fischer emphasizes that responsible agencies must assess a disease's risk by evaluating the epidemiology of the disease agent in wildlife, humans, and domestic animals; local situation information; and other factors. Infected, wild animals can represent a true risk factor, or they may harbor serious pathogens but pose little or no threat to other species.

In the mid-1990s, poultry producers became worried that a new organism, Mycoplasma gallisepticum, which had emerged in finches in the United States, would spread conjunctivitis in their stock, but researchers were able to allay their fears. "It was found that the finch strain differed genetically from the poultry strain," said Dr. Fischer, explaining that experimental work showed the risk of transmission from finches to poultry was very low and clinical disease did not develop in poultry following inoculation.

Managing the interface
If an interface disease poses a threat, efforts can be made to manipulate the environment, disease agent, host, or human activities, and often in combination. Many individuals emphasize that measures that prevent disease transmission from domestic animals to wildlife, or vice versa, are more efficient and less expensive than eliminating a disease once it is established in either group of animals.

Some people have had success in managing the environment to make areas unappealing to wildlife. Others have had some success in managing the disease agent, as was the case with screwworms. In the early 1950s, the USDA Agricultural Research Service raised sterile male screwworms, released them in infested areas, and gradually, through successive generations, the fly bred itself out of existence.

Dr. Fischer says that managing the host, however, is often the control strategy of choice. Some vaccine programs, such as the oral rabies vaccine program in Texas, have achieved some success. "In Southern Texas, they were picking up fair numbers of coyotes and dogs that were rabid, but within a few years of the treatment, they were having a hard time finding rabid animals anymore," Dr. Fischer said. "Oral rabies vaccination projects in carnivores, in a number of locations in the United States, are starting to show some success."

In many cases, managing the host involves the public. To control TB in Michigan, for example, extending the hunting season and issuing unlimited permits on antlerless deer have helped reduce white-tailed deer density. Michigan also passed regulations to limit, and in some areas prohibit, baiting and supplemental feeding of deer and elk. Dr. Schmitt says that the prevalence in what is known as the core area, where 75 percent of all the TB-positive deer in the state are found, has gone down from 4.4 percent in 1997 to 2.8 percent. Involving the public has the added benefit of reducing costs to government agencies, and many other states have similar hunting regulations.

Controlling human-assisted transportation of animals is another example of host management involving the public. When people moved raccoons from raccoon rabies-endemic areas to the mid-Atlantic region, in an attempt to increase hunting stocks, it sparked an epizootic of rabies in raccoons. People have also moved captive elk around the country, and this has resulted in the spread of chronic wasting disease within the industry. Many states now stringently regulate animal movement and monitor diseases to prevent the introduction of diseases such as CWD into new areas.

Changing laws to manage human activity and control the host, however, can be hard. Dr. Fischer points out that wildlife-associated recreation is big business in the United States and one with a large public constituency. A survey in 2001 found that 82 million Americans are participating in some sort of wildlife-associated recreation, spending $109 billion per year.

While implementing and enforcing regulations may be hard, these efforts are bound to increase, as long as scientists are expected to manage diseases at the interface of livestock and domestic species.

"You are going to see more money going into the management of diseases at this wildlife/livestock interface, and into research of the disease agents in wildlife species and epidemiology of these agents," said Dr. Fischer, who notes that his organization has been around since 1957. "We were the first center set up to look at just wildlife health issues, research, and service ... and nobody has ever seen this kind of attention paid to wildlife health issues."