As a U.S. government laboratory continued to test crows from Maryland for West Nile virus, a state official said yesterday that the virus could probably survive the winter in hibernating mosquitoes and dormant eggs and reemerge next spring.
Cyrus R. Lesser, chief of the state Department of Agriculture's mosquito control section, said the virus could survive indoors in adult mosquitoes that spend the winter in basements, and outdoors in eggs that are dormant until spring.
State officials said Thursday that the virus had been confirmed in the carcass of a crow that was found Oct. 14 near Baltimore's Inner Harbor, marking the first time the illness has been found outside the greater New York City area, where it surfaced in August.
The disease, which can cause fatal encephalitis--swelling of the brain--has killed about a half-dozen people and sickened about 40 in the New York area since the summer. It has also killed numerous birds, mostly crows, and has infected other birds and some horses.
It is spread by the bite of a mosquito and usually does not become serious in humans. Most of those killed or made seriously ill have been elderly.
Experts have been concerned about whether the illness might travel beyond New York with the fall bird migration and whether it would survive the winter and return next spring.
Lesser said that the virus appears to have been spread chiefly by the Culex pipiens mosquito but that other kinds of mosquitoes probably spread it, too. He said that though officials cannot be certain, because the virus has not been seen in the United States before, it will likely "overwinter" in those mosquitoes successfully.
"The virus can survive from mother through eggs to the next generation of mosquitoes," he said in an interview yesterday. "This is not unusual for [a] virus to be able to do this in the mosquito population."
He added: "They'll be fertilized eggs but in a dormant stage. They'll just be in place in the soil down in the wetlands, and ice will freeze and they'll be fine."
Come spring, he said, there could be a crop of mosquitoes "basically born with the virus" in their bodies.
In addition, many mosquitoes survive the winter as adults if they have sufficient humidity and temperatures that do not fall below the mid-20s Fahrenheit. "Fertilized females congregate in fairly large numbers, dozens or hundreds or thousands, in basements of buildings, crawl spaces and in abandoned buildings," Lesser said.
The mosquitoes go into a kind of hibernation. "Next spring, they'll fly out and they'll begin the cycle all over again."
The virus will "probably be here in the future, and we're going to have to learn how to deal with it," he said.
Authorities are also continuing to focus on birds, which also can become infected through mosquito bites and can carry the virus hundreds or thousands of miles. "There are probably birds carrying the virus all up and down the East Coast right now during the migratory period," Lesser said.
But birds that survive West Nile appear to purge the virus from their systems relatively quickly, said Linda Fraser, of the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis., where the preliminary virus tests are performed on the crows from Maryland.
Officials said yesterday that cell culture tests are underway on 18 of the 19 crows from Maryland that were sent to the lab for examination and that more birds were being sent for examination over the weekend.
Tissue from only one crow, the one from Baltimore, reacted positively to the tests and was sent to a federal Centers for Disease Control and Prevention facility in Fort Collins, Colo., where the presence of the virus was confirmed.
But tests on the others, in which crow brain and spleen tissue is planted in neutral cells to see whether the virus erupts, will take a week or more to complete, Fraser said in an interview yesterday.
She said the center has received dead crows from Montgomery, Prince George's, Talbot, St. Mary's, Howard, Anne Arundel, Worcester and Baltimore counties and Baltimore City.
She said 13 dead starlings from Maryland also have been sent for testing, but the center is concentrating on crows.
The state set up a hot line for reporting dead crows: 1-888-584-3110.