Rainfall awakens buzz for blood

Hordes of mosquitoes hatch

state prepares spraying campaign

September 12, 1999|By Frank D. Roylance and Scott Shane | Frank D. Roylance and Scott Shane,SUN STAFF

It turns out there is a dark side to drought relief. Recent cloudbursts have given birth to clouds of hungry mosquitoes, and Marylanders in their path are yelling for help.

Complaints are piling up at state mosquito control offices, and authorities are cranking up a late-season spraying campaign to put down the pests and ward off bug-borne diseases.

"They're back in force," said Cy Lesser, chief of the Mosquito Control Section at the Maryland Department of Agriculture. "Normally, most of our mosquito problems are over by now.

"This year, they are literally just beginning. This drought threw a monkey wrench into everything."

Lesser's field inspectors, whose job includes holding out their arms in the woods and wetlands to count how many mosquitoes land on them, are reporting as many as 100 landings a minute in some areas.

"If we didn't do something to reduce their populations, they would persist in high numbers for the next month to two months," Lesser said.

"Whenever mosquitoes live that long, other than the annoyance, the greater is the risk they're going to transmit diseases to humans and domestic animals. We don't want to be in the situation New York City is in right now."

In New York, health authorities on foot and in helicopters have begun spraying for mosquitoes after an outbreak of St. Louis encephalitis, an inflammation of the brain. Three people have died, six others have been infected, and at least 56 possible cases are being investigated.

Maryland has seen outbreaks of Eastern equine encephalitis, another mosquito-borne disease that can kill horses, birds, poultry and, rarely, people. The last Marylander stricken died of the disease in 1989, said Dr. Diane M. Dwyer, the state epidemiologist at the Department of Health and Mental Hygiene.

St. Louis encephalitis is so rare in Maryland that the state no longer monitors for it. No cases have been reported since 1975, when nine people were infected; the most recent case before that was in 1956, Dwyer said.

She said that in light of the outbreak in New York City, which does not have a mosquito control program, patients and doctors should keep in mind the possibility of encephalitis.

Doctors concerned about a patient with headache, fever and nausea can order tests that would diagnose encephalitis, she said.

"There's a lot of travel between here and New York," Dwyer said. "So clinicians need to be aware that this is happening."

Maryland has several kinds of mosquito pests. One group includes the floodwater mosquitoes that breed on mud flats in temporary wet areas of woods and wetlands. They're the ones whose numbers have soared recently.

A more constant presence is the Culex mosquitoes. They prefer the standing, often polluted water that collects in old tires, buckets and other debris around human habitations. It is Culex pipiens that has transmitted the St. Louis encephalitis in New York City.

A more recent arrival is the Asian tiger mosquito, distinguished by black and white striped legs, a white stripe down its back and a willingness to bite during the day. It also prefers the domestic breeding spots.

Tiger mosquitoes arrived in the United States from Asia in shipments of used tires in the 1980s. They spread east and north from Texas and in recent years have become a common pest in Maryland.

Residents are being asked to clean up their property and remove or cover any old tires, buckets, boats or trash -- anything that could provide even a small pool of stagnant water as a breeding place for mosquitoes.

Pet dishes should be rinsed daily and children's pools drained once a week.

Lesser said this year's drought helped keep populations of floodwater mosquitoes down for much of the summer by drying up many of their wetland breeding spots. Eggs laid on the widening expanses of mud at the bottom of stagnant pools of water became desiccated and went dormant waiting for rain. They can survive that way for months.

But when heavy rains came late last month, the embryos in the accumulated eggs revived and hatched within hours. In a week, the larvae were adults, and hordes of females went searching for the mammal blood they need for reproduction.

The drought made things worse by cutting populations of the small fish, dragonfly nymphs and other creatures that eat mosquito eggs and larvae.

In some places close to wetlands in Anne Arundel County, Southern Maryland and on the Eastern Shore, Lesser said, "populations of mosquitoes are higher than they've been in about 10 years, since 1989."

Things are likely to get worse before cold weather.

"The full impact of the rains from tropical storm Dennis a week ago won't be felt probably until early this coming week," Lesser said. That's when all the mosquitoes hatched by the Labor Day weekend rains will emerge as adults and go looking for dinner.

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