Mosquito-borne Encephalitis

On October 5th, 1999 the Centers for Disease Control and Prevention (CDC) announced that the West Nile virus, and not the St. Louis encephalitis virus, is responsible for the outbreak of encephalitis in the New York City area.  The virus has been blamed for six deaths, and has sickened at least thirty people. EEE or West Nile Visrus was not detected in Ocean Beach or on Fire Island in 1999. The West Nile virus originates in Africa, and rarely occurs outside Africa, Australia, and the Middle East.  Like the St. Louis virus, the West Nile virus identified in New York is an arbovirus, spread by the bite of infected mosquitoes.  According to CDC officials, the virus is not directly transmitted between humans.

The primary transmitter of the West Nile virus in the U.S. outbreak is the female Culex pipiens mosquito, which acquires the virus when it bites an infected bird for the blood meal needed to produce mosquito eggs.  If the same mosquito subsequently bites a human, it can pass the virus to a human host.  The Culex pipiens mosquito is also a primary transmitter of St. Louis encephalitis.    

CDC researchers reversed the original diagnosis of the St. Louis virus as the cause of the New York cases after learning from pathologists at the Bronx Zoo that more than 20 flamingoes, herons, and bald eagles had died of an encephalitic virus.  Researchers knew that the St. Louis strain could not be the cause of the birds' deaths, because birds infected by the St. Louis virus are immune to its effects.  Pathological investigation revealed the presence of a virus never before seen in North America:  the West Nile virus.  After the Zoo's pathologists contacted CDC and New York health officials with their findings, scientists performed further genetic testing that revealed the West Nile virus to be the cause of human infection as well.

Scientists in the Northeast and Mid-Atlantic states are currently trapping mosquitoes to isolate Culex pipiens specimens and test them for the presence of the West Nile virus.  On September 22, 1999 Connecticut state officials confirmed that the virus had been found in mosquitoes trapped on a Greenwich golf course, as well as in the brain of a dead crow found in Westport.   New Jersey state health officials have also announced the presence of the virus in 17 dead crows found in four counties.  And in late October, officials confirmed that a dead crow found in Baltimore, Maryland, tested positive for the virus.  No human cases of West Nile virus have been reported in Connecticut, New Jersey, or Maryland.

According to wildlife disease specialists at the U.S. Geological Survey's National Wildlife Health Center, crows appear to be particularly vulnerable to the West Nile virus.  USGS researchers are coordinating with the CDC, New York City and  State health officials, and the U.S. Fish and Wildlife Service to conduct field investigations in the New York area.  They are also requesting the assistance of natural resource and conservation managers and local public health officials in the Northeast and Mid-Atlantic states in a joint national surveillance effort to document crow mortality. The approaching seasonal migration of birds to warmer southern climates raises the possibility of the virus spreading to other states.

By the end of October, most Northeast and Mid-Atlantic states will have experienced a first "hard frost," which signals the end of mosquito season.  Unfortunately, female mosquitoes of the Culex species born in late summer can hibernate until spring.  Thus, infected mosquitoes may, in fact, emerge next spring to begin the cycle anew.

Symptoms of viral encephalitis include:

To help protect yourself from mosquito bites: