ŠAMNH   A nasty new face in town In the fall of 1999, public health officials realized that a new virus had reached New York City. Called West Nile, this mosquito-transmitted virus kills some kinds of birds and causes encephalitis, a serious brain inflammation, in less than 1% of people who become infected, mostly the elderly. It had never before been found in the Western hemisphere. Epidemiologists breathed a sigh of relief when the first frost hit the East Coast in October 1999. This signaled the end of mosquito season. By then, 7 people in the New York City metropolitan area had died of West Nile virus (WNV), and 55 others had gotten seriously ill. As many as 1,900 additional Queens residents were infected but did not develop encephalitis. Exotic zoo birds, American crows, and horses were also affected, and quite a few died. But female mosquitoes of various Culex species--the predominant carriers of the virus--that are born in late summer can hibernate until spring. If infected mosquitoes are among those lying dormant, the virus can survive the cold season and re-emerge when the insects do. Over the winter, scientists at various state and federal agencies therefore developed guidelines and comprehensive state plans to detect the virus, and, if necessary, prevent or control any West Nile outbreak in 2000. January-February 2000: Did the virus survive the winter? Investigators collect more than 2000 overwintering Culex mosquitoes from sewers, underpasses, and municipal swimming pools in Queens and the Bronx to test them for the presence of the virus. February 5: What killed the red-tailed hawk? A dead red-tailed hawk that had toppled out of a tree in Westchester County is taken to a wildlife hospital, where it tests positive for West Nile. No other infected birds had been found since November 5th of 1999. How could the hawk have been infected during the winter by a virus only known to be transmitted by mosquitoes? Could it have lived with the virus in its system since the summer? Or had it been bitten somewhere warmer and then flown north? (If so, it would mean that the virus had established itself somewhere in the southern part of the country, and could be regularly reintroduced by migratory birds.) March 9: Mosquitoes test positive in Queens 3 pools of mosquitoes collected from the walls of historic Fort Totten earlier in the year are found to have low but detectable levels of WNV genetic material. It is uncertain whether the identified amounts would be sufficient to transmit the virus. March 17: Live virus detected in mosquitoes Further testing of one of the pools confirms the presence of live West Nile virus. "This means the virus has survived the winter in a viable form," said Roger Nasci of Atlanta's Center for Disease Control. "Now the question is, have enough of them gone into hibernation with the infection so that the cycle will continue," explained Dr. Andrew Spielman of the Harvard School of Public Health. Surveillance and prevention activities now go into high gear, with an emphasis on trying to kill mosquitoes before they hatch.      1 of 6