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West Nile Virus remains top vector-borne threat this year

Article

Experts predict the severity of vector-borne diseases, fleas and ticks will be a mixed bag in 2002 with the continuing spread of West Nile Virus still the biggest threat.

Experts predict the severity of vector-borne diseases, fleas and ticks will be a mixed bag in 2002 with the continuing spread of West Nile Virus still the biggest threat.

Mare reproductive loss syndrome (MRLS), which decimated foals in Kentucky last year, may only have a 1 in 20 chance of returning to Kentucky, and the tent caterpillar may not be to blame.

Lyme disease, while still a threat, is at least under control.

How's the weather?

Dr. James Becnel, research entomologist, Mosquito and Fly Research Unit, USDA-ARS, Gainesville, Fla., says populations of pests have been pretty low because of the extreme drought of many parts of the state.

Whether populations will remain low depends on how wet the rest of the season is, Becnel says. Predictions are in for a dry season until summertime.

Dr. Charles Stoltenow

On the flip side, the strength of the winter doesn't matter too much to our parasites in the North, because they're pretty good at surviving winter and the cold, says Dr. Charlie Stoltenow, extension veterinarian, North Dakota State University.

"The drier weather is what really helps to break the cycle, because it dries out the eggs," he says.

West Nile biggest threat

"(In 2002) the only disease that I can definitely say will have a strong impact on animals and perhaps humans is West Nile Virus," says Dr. Richard L. Berry, Ph.D., BCE, entomologist, Ohio Department of Health, Vector-Borne Disease Program.

"Of the vector-borne diseases, West Nile Virus...is really burning its way through wildlife, especially with birds."

Dr. Richard Berry

He supports predictions the virus could reach California in two to four years and spread south into Mexico and Central and South America.

"I wouldn't be surprised to see it enter Mexico next year," says Berry. "The area of greatest risk is really much of the lower 49 United States. The farther south, the greater the risk, because of the longer mosquito seasons. Farther north, it seems less likely to be a threat, but we'll see."

In the meantime, researchers in Florida as well as the Connecticut Agriculture Experiment Station are testing several pathogens of mosquitoes. Already Becnel of USDA-ARS says they have located a virus that kills mosquitoes.

Although no field testing has been conducted, researchers hope to someday have an application, Becnel says. "This virus is specific for the mosquitoes that vector West Nile and other encephalitises."

However, in Berry's eyes, there's simply no stopping the progression of the West Nile Virus.

"Can't be done. Migratory birds are spreading it and there is no way to stop them. Right now everyone is overwhelmed with West Nile Virus."

Caterpillar questions

Will MRLS make a return appearance to Kentucky this year? According to Dr. Bruce Webb, a University of Kentucky research entomologist, it's "anybody's guess." The last time it occurred was 20 years ago (between 1980 and 1981).

"You can say the chances of it happening again are 5 percent."

Webb, whose role it was to help disprove the possibility that cyanide was delivered to horses via caterpillars, continues to collaborate with researchers to focus on a mycotoxin hypothesis.

"When I look at the potential for caterpillars to be involved, I see it more as an indirect link. The data that supports it is epidemiological and not hard scientific study," he says. "I don't think we've identified a cause.

"If caterpillars are involved, my impression is that they are going to be controlled, at least in this area," Webb says.

The goal is to attempt to manipulate caterpillar populations to recreate the syndrome in a controlled experiment, he says. "If we can reproduce the syndrome, then we can study it."

Pickin' on the tick

Dr. Ed Breitschwerdt, professor of medicine and infectious diseases at North Carolina State University, admits he doesn't own a good crystal ball, but does confess "the diseases transmitted by ticks and fleas are much more important today than they have been in any other point in history... ."

Veterinarians' awareness of infectious diseases has increased substantially in recent years, according to Breitschwerdt, who operates the Tick-Transmitted Disease Laboratory at NCSU. "I think we can credit Lyme disease to a large extent with starting the national awareness in human and veterinary medicine for tick-transmitted diseases."

Dr. Ed Breitschwerdt

Breitschwerdt's lab works on three groups of infectious organisms that are transmitted by fleas or ticks: Rickettsia (Rocky Mountain Spotted Fever), Ehrlichia, and Bartonella (organisms recognized to be tick-transmitted in the last three to four years.)

"One of the surprises to us is the number of dogs that we see that are concurrently infected with multiple organisms that ticks and/or fleas transmit," Breitschwerdt says.

In the cases of dogs that have extensive exposure to ticks and fleas, Breitschwerdt says the diagnosis is now based not only on just testing for antibodies, which suggest that the dog has been exposed, but on PCR testing which is a way of detecting DNA for Rickettsia, Ehrlichia, Babesia or Bartonella.

"What we have been able to do literally in the last three years is bring in molecular biology to the science of tick-transmitted infectious disease and use molecular approach to provide a unique fingerprint for these bacteria," he explains. "So when a veterinarian pulls a blood sample in their patient and sends it to us for testing, we can actually say there is DNA to that organism present, meaning there's no question whether the dog is actively infected."

Heartworm scaleback

Dr. Ron D. Smith, professor of epidemiology at the University of Illinois at Urbana-Champaign, has conducted five-year interval (1996-2000) controlled testing for heartworm disease in dogs, which shows heartworm is on the decline. This can largely be attributed to drugs on the market, he notes.

"We've gone from the daily dosing to monthly dosing to every six-months injection," he says. "Because the dog is the main reservoir for heartworm, if more and more dogs are treated, then there's going to be less and less transmission."

In 1996, 25 percent of the samples were positive for heartworm. In 2000, that percentage registered just 3.3 percent.

He plans to compare his data with Veterinary Medical Database data compiled from the teaching hospitals in the last decade to see if he can corroborate the data from other practitioners.

Screwworm radar

If Stoltenow, North Dakota State University extension veterinarian, had to rank parasites in order of threat to animals and humans of late, one dons a red flag.

"The parasite you have to watch out for is the screwworm," he says.

"It used to be in the U.S.," says Stoltenow. "We've eradicated it all the way to the Isthmus of Panama. Due to the whole bioterrorism thing, that would be a really neat one to bring into the country. It would cause so many problems because you wouldn't know where it came from."

For now USDA-APHIS monitors any attempt to re-introduce the screwworm. "Surveillance is only as good as the veterinarians out there. If they don't catch it, we won't catch it until it's too late," says Stoltenow.

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