Don’t Drown in a Disease Reservoir

Growing wildlife populations are making some diseases more difficult to
eradicate, increasing the chances of a spillover into the cattle population.

By Kristen Tribe

In close proximity, cattle and wildlife can equal a wildfire of disease.

"There are lots of diseases that can be passed between wildlife and livestock, but usually [animal] numbers and the space between them eliminates any problems," says Dr. Burke Healey, Oklahoma state veterinarian.

Artificially maintained wildlife populations are thought to have contributed to the ongoing brucellosis infection in bison and elk at Yellowstone National Park and the tuberculosis infection in white-tailed deer in Michigan. Disease reservoirs, such as these, increase chances of the infection spilling over into the cattle population and make eradication more complicated.

"As we get closer to eradicating diseases, interaction between wildlife and livestock is becoming more important," says Diana Whipple, acting research leader, Bacterial Diseases of Livestock Research Unit, USDA, ARS, National Animal Disease Center, Ames, Iowa. "We didn’t even know that tuberculosis was in white-tailed deer until this was discovered in Michigan, so we’re still trying to understand the disease in this species."

Although the United States began tuberculosis eradication in 1917, a Michigan deer killed in 1975 tested positive for TB. Since the last known case of cattle TB in Michigan was in 1974, it was assumed an isolated incident. Michigan was accredited free in 1979 and no one looked into the matter any further.

Then in 1994 a deer from the same part of the state, northeastern Michigan, tested positive for tuberculosis, and further investigation revealed a widespread infection.

The outbreak was originally thought to cover only a 600-square-mile region, but there are now 12 counties with TB in white-tailed deer, which covers almost a quarter of the state. And since 1998, 17 infected cattle herds have been detected.

Due to feeding, the deer population was twice the carrying capacity of the land. They’ve become a reservoir for the disease, which makes it nearly impossible to eradicate it from domestic livestock.

Feeding deer had become a multi-million dollar industry, and Healey says it was unlike anything he had seen in Oklahoma or Texas.

"We feed deer down here, but we’re used to seeing 55-gallon drum feeders," Healey says. "They would bring in a semi-load of corn and just dump it in a pile. These deer have become more like feedyard animals."

Whipple says aerial photos looked like a spider web with the corn pile in the center and deer trails leading to it from all directions. She actually heard of one hunting club that spent $40,000 in deer feed.

Besides deer, TB has also been found in a number of other animals including coyote, fox, black bear, opossum, raccoon and now elk, which could also become a reservoir for the disease.

"The diseases wouldn’t be as severe of a threat and possibly not a threat at all to livestock, if these populations weren’t artificially maintained," Healey says. Feeding deer is now illegal in Michigan and hunting has been increased. Testing of cattle and depopulating herds is now taking place. Unfortunately, Healey says many of those dairies and farms can’t be repopulated as long as TB is still infecting the wildlife population.

Texas’ El Paso and Hudspeth Counties, along with Michigan, were the only areas in the United States not free of tuberculosis in November 2000. Ten dairies along the Rio Grande have suffered low levels of TB infection for the past 15 years. Due to this on-going infection, Texas was given a split-state status in order to declare the rest of the state disease free.

But that status was threatened in September when a beef cattle herd in South Texas was also found to be infected with TB. At that time, neighboring herds had tested negative; all animals sold from the herd were being tracked down and efforts were being made to determine the source of infection. At that point, Texas hadn’t faced an infection of its wildlife population.

Although cattle first infected the bison and elk in Yellowstone with brucellosis, there are no infected cattle herds at this time. Healey says the rate of infection is high enough in wildlife that there is a threat of transmitting it back to them.

They have been testing cattle to ensure that it doesn’t spill over, and a vaccination program has been developed for further protection.

According to an article in Agricultural Research, "RB51 was approved by the USDA as the official vaccine to protect U.S. cattle against brucellosis . . . It replaced strain 19, a vaccine that is essentially no longer used."

Whipple says scientists are studying its use in bison and there’s a number of ongoing studies looking at its safety and efficacy. They have discovered it does not work in elk.

But even if a vaccine is discovered that will protect wildlife, there’s still the issue of how to administer it. One method that is being explored is the biobullet, which would be like delivering a dart with a vaccine in it. Whipple says they still need to find out how well it would stimulate an immune response.

Healey says the real problem in Yellowstone is jurisdiction because there are numerous state and federal agencies involved and joint jurisdictions. He says it wouldn’t be that hard to clean up the disease, if everyone could agree on the method.

After going through the court system, a Bison Management Plan has been developed but there still isn’t a brucellosis eradication plan.

As of January 2001, there were no cattle herds under quarantine for brucellosis in Texas for the first time in the 50-year battle against the disease. Texas, along with Florida and Missouri, is in the final stages of eradication. Although Texas didn’t have any infected herds at the first of the year, the Texas Animal Health Commission was actively testing and trying to find any remaining herds still infected.

"We are so close to eradication," says Dr. Terry Conger, state epidemiologist, TAHC. "If we become complacent now and leave even one infected herd out there, we could lose all this progress – and compromise the sacrifices producers have made through the years."

Although these cases in Michigan and Yellowstone make up the most widespread infections and greatest threat to livestock, there are other diseases that flare up from time to time. Animal health officials are also working diligently to learn more about emerging diseases and to protect the U.S. livestock and wildlife populations from these relatively "new" afflictions.

Anthrax is one of those culprits that’s always around, and it’s transmissible between wildlife and livestock. Endemic to Texas, spores are always present, just dormant, and an anthrax outbreak is most likely after periods of wet, cool weather, followed by several weeks of hot and dry conditions.

Some ranchers vaccinate for anthrax, if it’s a recurring problem in their area. This past summer, South Texas ranchers battled another bout of it. Wildlife can actually become a reservoir for the disease, so it was important to not only burn carcasses of animals that died from the disease, but also their bedding, manure and surrounding soil.

"If you see wildlife dying off or aborted or dead fawns, No. 1, don’t touch them and No. 2, call your local vet or the county agent. Depending on what they think it is, they may contact the Animal Health Commission," says Dr. James Johnson, diplomate of American Zoological Medicine, Texas A&M University.

Johnson says another disease of serious concern is a disease of ruminants in the Caribbean called heartwater; it’s spread by ticks.

"The species of the tick that transmits this disease in Africa and the Caribbean is not present here, but we have the genus," explains Johnson. "It's possible it could carry it as well, but if the known tick that transmits this disease was infected and loose in the country, it could be devastating."

Tortoises imported from South Africa, for the reptile pet trade, had the tick on them, and even though the ticks weren’t infected, their importation is now banned. This is just one of many precautions that animal health officials have taken in this situation to help eliminate the threat of this disease.

Healey says he feels like the government has responded quickly to arising disease issues, and in turn, the United States has avoided any related problems with international trade.

Obviously, BSE has caused trade problems for the United Kingdom, and although the United States has not had any cases, researchers are studying chronic wasting disease in an effort to determine how prion diseases cross species.

CWD is endemic to a region from Fort Collins, Colo., to Cheyenne, Wyo., to a corner of Nebraska at the North Platte Basin. It’s found in deer and elk and not thought to be transmissible to cattle; but by learning more about it, researchers hope their studies will help ensure the United States remains BSE-free.

Healey says there have been trials done where they’ve tried to infect cattle and haven’t been able to cause any chronic wasting disease. In the wild, there is a 7 percent infection rate in elk herds, but rate of infection is generally much higher in captive herds – 30 percent or 40 percent. Again, a concentrated population on limited acreage is thought to encourage such a high infection rate.

Many of these diseases are not only a threat to animals, but are transmissible to humans, too. Tuberculosis and brucellosis posed health problems in the early 1900s, but technology and modern medicine have almost eliminated these diseases and others like them.

For example, many people got tuberculosis from drinking raw milk from infected cows, but pasteurization has eliminated this threat. Healey says foodborne illnesses are really more of a problem now than these diseases.

"E. coli, campylobacter and salmonellas, those are all going to be more likely for [producers] to get because we may be working cattle and stop and eat a sandwich on the tailgate," Healey explains. "This might be more of an issue to farmers and ranchers than those people in town, but on the other hand, if you have a higher exposure rate, you might have a higher immune system than someone that hasn’t had exposure to E. coli."

Healey says we actually have a society of very naive immune systems because we’ve developed such a safe food and water supply.

"We’ve had the luxury of the safest food supply in the world," says Healey. "As we’ve purified these systems, we’ve set ourselves up to be more susceptible to these situations. You never hear of E. coli down in Mexico, but take a look at their sanitation systems."

Although these diseases don’t affect humans as much as in the past, they are obviously thriving in some of the wildlife and livestock populations. Healey says disease concerns have always been around, but the difference is that today we have emerging diseases, such as BSE, that we didn’t know about until about 15 years ago.

The bottom line is that producers need to be aware of the condition of the wildlife in their area and report any sick or dead animals that they find. While cattle are easily treated, wildlife magnify the problem, making eradication nearly impossible.

"The main thing producers need to realize," says Healey, "is that if they are artificially supporting a group of wildlife then they need to begin to consider disease as a management issue."