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Published: Tuesday, 7/31/2012

Drought aids breeding of West Nile carriers

Dangerous mosquitoes thrive in heat

BY MEL FLANAGAN
BLADE STAFF WRITER

If someone wrote a book about the mosquitoes of the summer of 2012, it might be titled A Tale of Two Mosquitoes, according to a biologist at the Toledo Area Sanitary District.

Biologist Lee Mitchell said this summer's hot, dry weather has cut into the population of the common nuisance mosquito, but provides a perfect breeding environment for another, more dangerous type.

"Our normal, pesky mosquitoes are definitely below average this year," he said. "But for the mosquito [that carries] the West Nile virus, they're doing very well."

The Ohio Department of Health confirmed the state's second human case this year of the West Nile virus on Monday, involving an 85-year-old Cleveland man. The first was reported Friday in southwest Ohio's Clermont County.

The first appearance of the virus occurred about a month earlier than in past years, according to the health department.

"The number of West Nile-positive mosquito pools are up significantly across the state, and it's important that residents of Ohio take note and are diligent in protecting themselves against mosquito bites," Ted Wymyslo, health department director, said in a statement.

Through last week, the health department had tested 10,636 mosquitoes for the virus and found 394 pools that tested positive.

During the same week last year, the department had tested 26,856 mosquitoes, more than double the number tested this year, but found only 59 positive pools.

For the common nuisance mosquito, this summer's drought approaches a death sentence.

Mr. Mitchell said the lack of rainfall has led to a lack of standing water, which mosquitoes need to breed.

"All through July the population numbers of [nuisance] mosquitoes have been low, which is a good thing," he said. "People are generally happy about it."

However, the Culex mosquito that carries the West Nile virus does not respond to drought conditions the same way. Mr. Mitchell said the life cycle of Culex mosquitoes speeds up when the weather is hot and little water is available.

"They know that if the water goes completely, they're done," he said.

Therefore, the bugs mature to the adult stage rapidly, which leads to virus amplification.

The Culexes, unlike common mosquitoes, can survive on very little water, and they typically lay eggs and thrive in backyard habitats such as abandoned swimming pools or standing water in tires or canoes.

Mr. Mitchell said these mosquitoes feed primarily on birds, another factor that gives them an advantage in a drought. "Because of the lack of water, birds will congregate wherever they can get water," he said. "As birds congregate, the mosquitoes can take advantage of that and they'll just hang out wherever they can find the greatest number of birds."

The mosquitoes usually switch to feeding on humans during August, Mr. Mitchell said, which is when West Nile infections typically begin popping up.

Last year, the state health department recorded 21 cases of West Nile virus, with four of those in Lucas County during August and September.

Mr. Mitchell said the actual number of cases far exceeds those that are reported. "From various studies they've established that, for every recorded human case of West Nile, there's somewhere between 140 and 350 actual cases where someone was exposed to the virus but never went to a doctor or hospital," he said.

For the majority of individuals exposed to the virus, Mr. Mitchell said, West Nile feels like the flu. However, the virus can be much more severe -- even fatal. Lucas County has only one confirmed death from West Nile since the virus appeared in 2002.

Because of the rising numbers of mosquitoes carrying West Nile, the Toledo-Lucas County Health Department recommends taking extra caution.

"Use adequate repellent, be as covered as you can," health commissioner Dr. David Grossman said. "At times of the day when mosquitoes tend to be more prevalent, in the evening, I would minimize your outdoor exposure."

Dr. Grossman also recommended trying to minimize mosquito breeding areas by preventing accumulation of stagnant water.

Contact Mel Flanagan at: mflanagan@theblade.com or 419-724-6087.



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