By Michael Podolsky
A killer is stalking Florida's livestock
and wildlife. A microscope predator known as heartwater disease living
inside ticks has hitchhiked from Africa to the Caribbean and now is seeking
a foothold in Florida.
Along the way it has preyed on cows, sheep, goats and deer, engulfing its victims' internal organs in fluid. And if it gets established in the United States, its economic impact could be devastating.
For 14 years, the University of Florida Heartwater Research Project has guarded our shores against this killer.
"Obviously, we're dealing with something we need to keep out of this country," says project director Michael Burridge, a professor of pathobiology in UF's College of Veterinary Medicine. "For many years, this has been the top foreign animal disease worry for state and national cattle producers."
Heartwater attacks blood vessels, causing fluids to accumulate in the lungs, around the heart and in the chest and abdomen. Once infected, up to 90 percent of susceptible animals die. The disease does not affect humans, horses or household pets such as dogs and cats, although all can carry the ticks. It cannot be transmitted by eating meat or drinking milk from infected animals.
Like generals laying down mines to slow the approaching enemy while they wait for the ultimate weapon, Burridge and his colleagues have focused on stopping the ticks that carry the disease until they can perfect a vaccine to prevent it.
"Tick control is central to controlling a disease like this," says Burridge, director of UF's International Program on Ticks and Tickborne Diseases.
Since 1985, the United States Agency for International Development (USAID) has awarded the Heartwater Research Project almost $10 million to study heartwater and to develop a vaccine and other preventive measures.
Burridge directs an international team, with collaborating scientists and ongoing projects in several African nations, the Caribbean and Europe. In the United States, researchers at Washington State University in Pullman are helping to develop recombinant heartwater vaccines.
Although the project is based at UF in Gainesville, much of the research on the ticks and the disease is being done in Africa, since importation into the United States of the bacterium that causes the disease, Cowdria ruminantium, is prohibited.
A growing market for exotic animals, particularly reptiles, and an inviting climate make keeping the ticks at bay a difficult task, Burridge says.
In 1997, UF veterinarians found African tortoise ticks, which can carry heartwater, on an injured tortoise brought to the UF College of Veterinary Medicine by a Florida reptile breeder.
"Finding African ticks - almost by accident - on imported tortoises was a real wake-up call. It showed how easily pests that have the potential to transmit the disease could get into Florida and the United States," Burridge says. "Fortunately, in that case, the ticks had not spread from the original infested site in Florida, but this was just one isolated case in a state where thousands of animals are being imported every year."
Complicating matters is the discovery by European researchers working under experimental conditions that two ticks native to the United States - the Gulf Coast tick (Amblyomma maculatum) and the Cayenne tick (Amblyomma cajennense) - are able to transmit heartwater. Heartwater carried by the tropical bont tick has been confirmed in much of the eastern Caribbean.
The disease also could enter the country through infected animals imported from Africa and the Caribbean for zoos or conservation and breeding purposes.
"We must keep exotic ticks out of the United States and make sure imported animals are not infected with heartwater," Burridge says.
The Heartwater Research Project is currently in the fourth phase of its efforts to control heartwater disease.
From 1985-89, Phase I centered on establishing a UF research center in Zimbabwe, refining methods for growing Cowdria ruminantium in the laboratory and initiating research on tick pheromones.
During Phase II, from 1989-1993, the lab in Zimbabwe expanded its biotechnology capabilities, trained scientists from nine African countries in heartwater cell culture techniques and heartwater diagnosis, and applied the pheromone research to development of tick decoys.
The third phase of the project, from 1993-1996, concentrated on developing heartwater vaccines, DNA probes and diagnostic tests. The project also began a quarterly newsletter about the disease, which is mailed to scientists, ranchers and others in 79 countries.
The current phase focuses on completing development of heartwater vaccines, diagnostic tests for the disease and tick decoys, all of which have been or will be patented and marketed commercially.
As a result of the pheromone research, Burridge and his team have developed a more efficient, cost-effective way of controlling ticks on livestock.
In contrast to the costly, time-consuming and environmentally hazardous process of dipping hundreds or thousands of head of cattle in a tick-killing solution every two weeks, Burridge's team developed a tick decoy that uses a sophisticated mix of attractants and pesticides to kill the pests.
"It's truly fatal attraction," Burridge says. "We use the ticks' own chemical message system to attract them to the animal. When they get to the area of the decoy, they are killed by the pesticide."
Female ticks can lay up to 20,000 eggs, Burridge says, so using the decoys to both kill the ticks and interfere with their reproductive cycle is doubly effective.
The chemicals are impregnated in a plastic tag that can be attached to a cow's tail or around its neck like a flea collar.
"The tags slowly release the pheromones over at least three months," Burridge says. "They cost less than $1 to produce. They are indestructible, they require very little manpower and they are much more environmentally friendly."
In tests in both Zimbabwe and the Caribbean, the decoy has been up to 99 percent effective in killing the tick. The decoys also can be used on sheep and goats, two other species susceptible to heartwater.
Burridge and his colleagues also are testing a patented self-medicating applicator, called the Appli-Gator, that applies pesticide to wild animals when they brush against a food dispenser.
"If heartwater were to get into the nation's wild deer population, there would be almost no way of eradicating it," he says.
In the Caribbean, scientists believe the tick is carried from one island to the next via migratory birds, particularly the cattle egret. In 1992, an egret banded in Guadeloupe was found in the Florida Keys, so the possibility of the birds spreading infected ticks is a real one, Burridge says.
Another important discovery by UF scientists working on heartwater is that several African wild animals (including African buffalo, eland, giraffe, kudu, sable antelope and wildebeest) can become carriers of heartwater.
To check the presence of the disease in animals, particularly those being brought into the United States, UF researchers have developed a new diagnostic blood test, the PCR assay. Prior to development of this test, the only way to detect the disease was by a brain biopsy, which is not practical under field conditions.
Two new vaccines - described as breakthroughs by UF researchers - have been under development for more than 10 years. The first is a conventional inactivated vaccine that has been successfully field-tested in Africa by Suman Mahan, a UF veterinary scientist based in Harare, Zimbabwe.
"We now have the first safe, affordable vaccine that provides good protection from the disease under actual field conditions," Mahan said.
The second development is a genetically engineered vaccine being developed by Anthony Barbet, a UF molecular biologist in Gainesville. Although the DNA vaccine still requires more testing in Africa and the Caribbean, it would be much cheaper to produce on a commercial basis than the inactivated vaccine.
Chuck Woods contributed to this article.
Michael J. Burridge
Professor, Department of Pathobiology
(352) 392-4700 ext. 3131 burridgem@mail.vetmed.ufl.edu
Related Web Site:
http://www.vetmed.ufl.edu/hrp/netwkhrp.htm
Industry Fears Loom
Although UF researchers have been studying heartwater disease since 1985, the discovery of large African tortoise ticks (Amblyomma marmoreum) on imported reptiles in Florida in 1997 highlighted the immediacy of the threat to Florida's livestock industry.
State Veterinarian Dr. Leroy Coffman, of the Florida Division of Animal Industry, says that if heartwater were to become established in Florida, "it would be a disaster."
"This is a List A disease, which means it's as serious as it gets," Coffman says. "From a regulatory standpoint, it's one of the most important animal diseases in the world."
Coffman says that in addition to the direct impact of losing up to half the cattle population, other states and nations would most likely embargo Florida cattle to prevent the disease from spreading.
Wayne Godwin, vice president of the Florida Cattlemen's Association, agrees the industry would be devastated if heartwater were to get a foothold in Florida.
"We have been extremely, extremely fortunate," Godwin said. "I believe that we are far more susceptible to this than anyone out there really even wants to admit."
While Godwin is not convinced the mortality rate would be as high as 50 percent, he says even half that rate would severely damage Florida's multi-million-dollar cattle industry.
"If we were to lose even 25 percent, then we are looking at a 35- to 40-percent reduction in income, because of the loss of future calf production," Godwin says. "And that doesn't even take into account the costs of treating and fighting the disease."
Godwin says heartwater infection of the wild deer population also would impact Florida cattle ranchers' livelihood.
"There are a good number of ranchers out there who use their land for hunting," he says. "If this disease got into the wild deer population, that would be another severe economic blow."
Burridge and Coffman are collaborating with the Florida Game and Fresh Water Fish Commission, the U.S. Department of Agriculture Animal and Plant Health Inspection Services and other agencies to deal with the problem. And Florida Agriculture Commissioner Bob Crawford asked U.S. Secretary of Agriculture Dan Glickman for help in fighting the threat of heartwater disease to the state and nation.