Special Report: Population movement signals new threats - DVM
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Special Report: Population movement signals new threats

DVM InFocus


Human and pet migration to the South and West, with accompanying building and landscape changes, is a major factor in spreading mosquito-borne animal and human illnesses, other experts say.

"You'll see the number of pet heartworm cases continue to grow in those parts of the country," says heartworm authority Thomas Nelson, DVM, surgical director and co-medical director of the Animal Medical Centers of Northeast Alabama at Anniston, Ala., and past president of the American Heartworm Society.

"As people move west, they're transforming the desert, putting in trees and lawns, irrigating, building ponds — lots of prime habitat for mosquitoes. The knothole mosquito (Aedes Sierrensis) is the prime vector in the West. It lays eggs in the knotholes of trees, and all it needs is a little rain or water from a sprinkler system," Nelson explains. Commercial real estate development is a factor, he says, in that parking lots and large buildings retain heat and push up the already-high temperatures in desert cities and towns, and that extra heat compounds the mosquito problem.

"A lot of people moving to these areas take heartworm-positive dogs with them, and they're a reservoir for infection. Literally thousands of infected dogs moved from the hurricane areas of the Gulf Coast."

Heartworm has long been a problem in the East, especially in high-density areas along the Atlantic Coast, the Gulf and the Mississippi, Nelson says. "There are about 70 different incriminating mosquitoes, with 22 of them significant factors, and now there's a new one, Aedes albopictus or 'tiger mosquito' that came in from southeast Asia in the 1980s and has proliferated in warm climates all over the country. It's voracious," Nelson says.

"What's concerning is that we have a tremendous arsenal of products that are 100 percent effective against heartworm, yet only about 55 percent to 60 percent of dog owners use them," Nelson adds. Until recent years, heartworm was not seen as a serious threat to cats, "but that's going to change. There's going to be a lot more emphasis going forward on feline testing and treatment," he says.

What effect will drought conditions, such in the Southeast and mid-South, have on mosquito-borne illnesses like West Nile Virus and eastern and western equine encephalitis this year?

Research entomologists in California and Florida say it's too early to say precisely, but do provide some insight.

"Even with reduced rainfall in parts of California, people still water grass; underground reservoirs in urban areas stagnate and provide a breeding place for mosquitoes that cause West Nile," says research entomologist Dr. William Reisen at the University of California-Davis' Center for Vectorborne Diseases.

"A lot of people walking away from foreclosed houses don't pump out their pools, but just leave them to stagnate. The chemicals evaporate, the water turns green and these pools crank out the mosquitoes. In Bakersfield alone last summer, 3,000 families abandoned houses. There were not that many with pools, but those that did have them can produce a lot of mosquitoes and become a big problem for cities."

Wherever in the nation the upcoming summer is hot and dry, West Nile and other viruses "will do better," Reisen says. "The Midwest has been having warmer summers, and it's seen some of the highest incidence of West Nile in the country."

West Nile is cyclical, Reisen explains, which puts California on the watch to see what will happen this summer. "The Los Angeles area had a West Nile epidemic in 2004. In the two succeeding years, bird hosts died back and the disease subsided but there is herd immunity in the survivors. This year, the crows are back and finch population is up. We've already had some 100-degree days, so we'll be looking intensively at what may happen."

"It's hard to say if this will be an epidemic year in Los Angeles," agrees Aaron C. Brault, PhD, associate professor also at UC-D's Center for Vectorborne Diseases, who also noted the cyclical nature of West Nile.

"The ambient air temperature is directly proportional to mosquito activity. When it's consistently warmer, usually about late August, the virus gets to their salivary glands faster and that's when you would see the most virus transmission."


Source: DVM InFocus,
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