The federal Centers for Disease Control has asked Maryland mosquito control officials to help with a search for Asian tiger mosquitoes, an unwelcome import from the Far East recently found in a Florida study to be carrying eastern equine encephalitis.
The CDC wants to determine whether tiger mosquitoes in Maryland and four other states might also have picked up the rare but dangerous encephalitis virus. But Maryland officials say the tiger mosquitoes remain uncommon in Maryland and they may not be able to find the 1,000 tiger mosquitoes the CDC will need to test for the virus.
"That would be [equal to] our whole collections for the last five years," said Dr. Jeannine Dorothy, an entomologist with the state Department of Agriculture. "They're finding a lot more in other states."
The hardy and aggressive tiger mosquito, named for the distinctive silver stripe down its back, has been found in the United States since 1985, mostly in eastern and southern states.
It was carried in on used tires imported from Asia.
The insects are efficient disease carriers elsewhere and have been linked to dengue fever outbreaks in Brazil.
Until Friday, tiger mosquitoes in the United States had not been linked to any viral disease of concern to public health officials.
The CDC reported Friday in the journal Science that some tiger mosquitoes (Aedes albopictus) collected last summer at a huge tire dump in Polk County, Fla., were found to be carrying the eastern equine encephalitis virus.
The CDC now plans to test for the virus in tiger mosquitoes in Maryland, Louisiana, Georgia, South Carolina and elsewhere in Florida.
This is "simply the first step in determining whether or not it is an important public health threat," said Dr. Carl J. Mitchell, chief of medical entomology and ecology for the CDC in Fort Collins, Colo.
"Until we obtain additional information, it would be very premature to unduly alarm the public," he said.
Public health officials worry about the tiger mosquito's potential as a disease carrier because it breeds near people, in tires and other water-filled containers, and it bites aggressively day and night.
The eastern equine encephalitis virus causes swelling of nervous tissue and the brain, damaging the central nervous system.
Its greatest threat is to unvaccinated horses.
Only 12 human cases were reported last year in the United States.
The encephalitis virus is maintained in a cycle of infection between freshwater mosquitoes and birds, Dr. Mitchell said.
When the birds leave the swamps, other species of mosquito bite them and transmit the virus through bites to people and horses.
In Maryland, the main threat to people and horses is from the salt marsh mosquito, Aedes solicitans, he said.
Dr. Henry A. Virts, Maryland's state veterinarian, said eastern equine encephalitis in Maryland is primarily a danger to horses and commercial pheasant flocks.
The number of cases in horses varies with vaccination rates and the number of mosquitoes, which depends in turn on the weather.
Fifty Maryland horses were diagnosed with eastern equine encephalitis in 1989, but only a handful of those cases was confirmed by lab tests, Dr. Virts said.
Two cases were confirmed in 1990, but none since then.
The disease is 80 percent fatal among horses, 30 percent fatal in humans.
The last reported human case in Maryland involved a Dorchester County man who died of the disease in 1989.
Single human cases also were reported in 1982, 1975 and 1970.
Most of the tiger mosquitoes in Maryland have been collected since 1987 in or around a tire shredding plant in West Baltimore.
Dr. Dorothy said the mosquitoes have been eradicated at the plant but still turn up in traps nearby. Their numbers reportedly are down so far this year.
Dr. Mitchell said he has asked Dr. Cyrus Lesser, of the Maryland Department of Agriculture's mosquito control section, to assist next month with the CDC's collection of tiger mosquitoes in Maryland.
The mosquitoes are typically collected in wide-mouthed vacuum devices. They are then frozen and shipped to the CDC in Fort Collins.