Maryland hospitals have not seen any illnesses stemming from West Nile virus, which recently sparked an epidemic in New York and was detected in a dead crow found in downtown Baltimore this month, health officials said yesterday.
Despite this, state Health Secretary Georges Benjamin said he asked his department to investigate every case of encephalitis that has occurred since July -- a month before the New York outbreak -- to make sure that none was related to West Nile.
"We're going to look at every case of encephalitis in the state," Benjamin said. The health department will retest any frozen blood samples that were kept; otherwise, they will draw conclusions from medical records kept on the patients.
Since July 1, hospitals have diagnosed 22 cases of encephalitis but none has been tied to West Nile, said City Health Commissioner Peter Beilenson. An inflammation of the brain, encephalitis is usually caused by a virus but can also result from other infectious agents.
West Nile virus is transmitted by mosquitoes that feed on infected birds before biting humans. Health officials say there is now little risk of further transmission in Maryland because cool weather has driven mosquito populations down and caused surviving mosquitoes to become less active.
The mosquito that carries West Nile, culex pipiens, is active in the early morning and early evening -- and only when temperatures rise above 55 degrees, according to a spokesman for the state agriculture department. People who are getting bitten this late in the season are probably being attacked by tiger mosquitoes and other types that don't transmit the virus.
The epidemic that struck metropolitan New York in August marked the first time West Nile had been seen in North America. The virus killed seven people and caused at least 70 others to become ill. It also killed large numbers of birds, many of them crows.
The outbreak raised concerns that infected birds could spread the virus during their southerly migration. As a result, Benjamin and other health commissioners up and down the east coast told hospitals and laboratories to look out for suspicious cases and perform tests when possible.
"We have not been notified of any cases, and we're assuming there aren't any," Benjamin said. "But we're going to go back and check."
"It's an extremely low-risk situation at this time," he added. "I think we're extraordinarily lucky because of the cold weather. And we're lucky because we started looking for this so early."
Barbara Reynolds, a spokeswoman at the U.S. Centers for Disease Control and Prevention in Atlanta, said health officials will get a better idea of the long-term risk to the public this winter, as they survey more birds to find out what proportion may be infected.
"The more infected," Reynolds said, "the more danger there is."
Scientists are also trying to determine what proportion of infected people actually become ill. The symptoms are virtually identical to those seen in other types of viral encephalitis -- fever, stiff neck, headache and clumsiness -- making it hard to diagnose without sophisticated lab tests.
Dr. Diane Griffin, an encephalitis researcher at the Johns Hopkins School of Public Health, said the death rate from West Nile virus is low. About 10 percent of the people diagnosed with the disease died -- but the mortality rate was probably much lower because many people probably had the disease without knowing it.
The mortality rate might have been as low as 1 percent, she said.