Recent weather has turned mosquitoes into incubators for West Nile virus

1:35 p.m., Aug. 28, 2003--The number of animals infected with West Nile Virus in Delaware is still small, but the number of infected birds grew geometrically during the first two weeks of August, according to UD mosquito expert Jack Gingrich.

The number of infected birds in the state rose from 9 to 24 during the first two weeks of the month, he said.

Gingrich, a medical entomologist in the College of Agriculture and Natural Resources’ Department of Entomology and Applied Ecology and an expert in mosquito-borne and tick-borne illnesses, is focusing on West Nile, a virus introduced in the U.S. in 1999.

Gingrich said weather conditions are right for breeding of the mosquitoes that carry the virus. Recent warm days combined with warm nights turn mosquitoes into small incubators for the West Nile virus, and this summer’s above-normal rainfall gives mosquitoes the water they need to breed.

“The head of mosquito control in Maryland said he’s been there 27 years and he’s never seen a mosquito population this large,” Gingrich said.

At the same time, he said, there is a proliferation of incorrect and misleading information about the dangers of West Nile popping up on professional-looking web pages, and, actually, much information about mosquito breeding in the Mid-Atlantic states is just being gathered.

Gingrich and his graduate students have been working 10-hour days, researching three mosquito control projects. He said 56 species of mosquito live in Delaware, but only seven or eight of them are among the most important of the 37 species that spread West Nile. In addition, mosquitoes must be at least 10 days old to develop the virus enough that they can pass it on, but many individual mosquitoes live only 5 or 6 days and the longest-lived lasts about 20 days.

“Even in an epidemic, probably only one out of 1,000 mosquitoes are affected,” he said, emphasizing that is an oft-cited ballpark figure. “It’s an interesting numerical challenge to figure out how this is going to play out. The numbers are very high this year.”

The virus usually is noticed first in birds. More than 130 species have been found to be infected in nature.

The confirmed cases of West Nile are usually in larger, more easily spotted birds, such as cardinals, blue jays and crows, but Gingrich said some owners of caged ornamental finches and exotic zoo species report that those birds have been felled by West Nile.

Jack Gingrich, a medical entomologist in UD’s College of Agriculture and Natural Resources’ Department of Entomology and Applied Ecology, is an expert in mosquito-borne and tick-borne illnesses.

Gingrich said no true scientific figures on Delaware’s bird population are available, so there is no baseline to measure deaths among various bird species. “There is a suspicion that raptorial birds [birds of prey] are being disproportionately affected,’ he said. “It’s under study, but there is not enough information.”

One owl was felled by West Nile last year, and another this year.

August and September are the months when mosquitoes are most active in Delaware, although the danger of virus spreading will last until the first hard frost.
Based on past years, Gingrich said, if there are any human cases, they would probably surface between late August and mid-September. If attack rates are normal, he said there may be only one or two cases in Delaware.

Mosquitoes who bite infected birds can pass the virus along to humans they bite. The virus, which can lead to headaches, fever or deadly encephalitis, usually presents no symptoms at all in healthy individuals.

“The likelihood of a healthy individual getting an infection is pretty low,” Gingrich said. Symptoms usually occur mostly in elderly patients, sometimes in infants, in people who have had splenectomies, and in individuals whose immune systems are comprised, such as chemotherapy patients, he said.

“It’s a serious disease, but, for the average, healthy adult, I wouldn’t suggest changing their lifestyle to account for West Nile. I think you still have your barbecues, you still go to ballgames,” he said. “If you’re elderly, take precautions if you’re outside during feeding periods—from dusk to dawn.”

Dogs and cats normally are not sickened by West Nile, and they cannot pass the disease to their owners. The virus has a serious effect on horses, often causing death or forcing the animal to be put down, so vaccinations are recommended.

Luckily for Delaware’s economy, the virus does not sicken chickens, and humans cannot get the virus by eating chicken.

Gingrich said homeowners who see mosquitoes in their yards during daytime hours probably have a breeding ground within 50 to 100 yards. Mosquitoes that alight in the evening hours have a wider range.

Gingrich suggested homeowners be aware of the spots that can become breeding grounds for mosquitoes that may carry West Nile—clogged drain pipes that leave water in gutters, bird baths, the water-filled saucers under potted plants, tire ruts in yards, tire swings, wading pools and water that collects atop lawn furniture covers or patio candles.

He said the areas with the greatest incidence of West Nile in Delaware during 2002 were Claymont, North Wilmington, Wilmington, New Castle, Newark, Elsmere, Clayton and Lewes.

Gingrich, who has been bitten by mosquitoes thousands of times in the course of his work and who researched the more deadly mosquito-borne Japanese encephalitis and dengue fever when he was a researcher at Walter Reed Army Institute of Research, is taking precautions against mosquitoes at his Milton home this year.

Gingrich said he uses hand-held, butane-fired repellent units that emit effective insecticides to protect his deck area. He sets the units in a triangular pattern and turns them on about 30 minutes before he uses the space after dusk.

When Gingrich goes out into the field, he uses Deep Woods Off containing DEET.

“There is an anti-DEET and anti-pesticide constituency out there that I’d be concerned about,” he said. “I understand people don’t want to use DEET, but this is a different year. We’ve had something like 34 inches of rain already. It’s way off the charts.”

He said the deterrent effect of a popular skin softener some parents use on their children instead of pesticides will last about 12 minutes. He estimated an insect spray with a DEET formulation for children will be effective for one to two hours, and a strong adult DEET formulation will last about four to five hours.

The hot, wet weather is conducive to virus incubation, Gingrich said, and it also makes people more likely to be where the mosquitoes are.

“People are trying to get out whenever it’s not raining,” he said. “They’re taking those windows of opportunity, and the mosquitoes are out, too.

“I think there’s going to be higher interaction between mosquitoes and people this year. The question is: How many of those mosquitoes are infected? We don’t know, but we’ll find out over the next month or so.”

Article by Kathy Canavan
Photos by Jon Cox

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