July 19, 2001

Tiger mosquito spreads West Nile virus

By Vaishali Honawar THE WASHINGTON TIMES

An exotic mosquito species that potentially could carry several deadly viral diseases, including the West Nile virus, is multiplying rapidly in Prince George's and Montgomery counties, according to Maryland's chief of mosquito control.

The Asian tiger mosquito (Aedes albopictus), first discovered in Baltimore in 1986, has spread to 16 counties in Maryland, particularly along the Eastern Shore. Tiger mosquito populations also have been growing rapidly in the District and Virginia.

While the species has not been known to spread any major diseases, laboratory tests have confirmed it is an effective carrier of encephalitis and the West Nile virus, said Cyrus Lesser, chief of the mosquito control division of the Maryland Department of Agriculture.

Before the arrival of the tiger mosquito, the only parts of Prince George's County that had high mosquito infestation were areas near the flood plains, Mr. Lesser said. "Now most residential areas of the county have a mosquito problem," he said.

Yesterday, the Maryland Department of Health and Mental Hygiene reported finding the first mosquito pool to test positive for the West Nile virus. The virus was found in a pool of six Culex salinarius mosquitoes collected on July 11. The infected mosquitoes bring the state one step closer to the likelihood of a human infection because mosquitoes carry the virus from birds to humans. Some reports say the tiger mosquito also is the most aggressive species in attacking humans.

In Maryland, "we are concerned [the Asian tiger mosquito] could be involved in the West Nile virus cycle, and we are sampling and testing this species," Mr. Lesser said. In Asia, the tiger mosquito has been known to carry dengue and Japanese encephalitis. Closer to home, in Florida, the mosquito has been found to carry the Eastern equine encephalitis virus, which affects humans and horses, and usually occurs in Maryland in summer.

Concerns about mosquito populations have increased since the spread of the West Nile virus.

It is hard to control the growth of the tiger mosquito, Mr. Lesser said, because it can breed just about anywhere, including inside artificial containers, while other species rely on heavy rainfall.

"We have 63 species of mosquitoes in Maryland, and almost all prefer ditches, swamps and marshes. The Asian tiger mosquito, however, prefers small containers, like bamboo shoots, buckets, rain gutters, and flower pots," he said.

It was through artificial containers -- used rubber tires -- that the mosquito is believed to first have entered the United States. Rubber tires imported from Japan and other Asian countries could have served as breeding grounds for the mosquitoes and entered the United States at ports, including Baltimore, Mr. Lesser said.

Baltimore City, which has had most of the positive bird infections of West Nile Virus, also has a large population of the tiger mosquito, so named because of the pattern of white stripes on its black body.

Although no human cases of West Nile virus have been found in the District, Maryland and Virginia, health officials in the region told The Washington Times last month that they expect to see a human infection this year, given the rapid spread of the virus among birds.

Since the year began, Baltimore has found 25 birds infected with the potentially deadly virus, with one infected crow reported yesterday. The District also yesterday reported finding its first infected crow in Northeast. Last year, five infected crows were reported in the District, but so far no infected mosquitoes have been found.

Mosquitoes, particularly those of the Culex pipiens variety, are known carriers of the virus. But the population of Culex pipiens has been remarkably low this year, Mr. Lesser said.

Populations of other species of mosquito also have fluctuated widely in the area. While the heavy rainfall in May and June saw a sharp increase in their numbers, their populations declined this month, Mr. Lesser said. The tiger mosquito and the West Nile virus are found predominantly in urban and suburban areas.

Conventional mosquito control efforts did not work with the tiger mosquito, he said. "It is beyond the scope of mosquito control agencies," he said, adding that there has to be a community effort to remove possible breeding grounds for the mosquitoes.

Meanwhile, the crow that tested positive in the District was found on July 10 between the 1000 and 1500 block of Quincy Street in Northeast, D.C. health officials said yesterday.

Dr. Ivan C. A. Walks, chief health officer for the District, said there is a need for integrated pest management to control different varieties of mosquitoes found in the city. But Dr. Walks said he did not expect to see a human case this year because the possibility of a human infection is very low, and because of the District's efforts to control the spread of the virus.

"The District started early with larviciding protocol and educated the public on not maintaining standing water around their houses" among other things, he said. Dr. Walks said the District has no plans to spray for mosquitoes in the area where the dead bird was found.

Residents who spot a dead bird are advised not to touch the carcass but report it to the West Nile Virus hot line at (202)442-9239 for pickup.

-- Anonymous, July 19, 2001