West Nile Virus in Overwintering Culex Mosquitoes, New York City, 2000

Roger S. Nasci, Harry M. Savage, Centers for Disease Control and Prevention, Fort Collins, Colorado, USA; Dennis J. White, New York State Department of Health, Albany, New York, USA; James R. Miller, New York City Department of Health, New York, New York, USA; Bruce C. Cropp, Marvin S. Godsey, Amy J. Kerst, Centers for Disease Control and Prevention, Fort Collins, Colorado, USA; Paul Bennett, New York City Department of Environmental Protection, New York, New York, USA; Kristy Gottfried, Robert S. Lanciotti, Centers for Disease Control and Prevention, Fort Collins, Colorado, USA


Emerging Infectious Diseases. 2001;7(4) 

In This Article


Detection of WN viral RNA in three pools and isolation of live WN virus from one pool of overwintering Cx. pipiens mosquitoes in New York City indicated that WN virus persisted in vector mosquitoes at least through midwinter, suggesting that the virus would persist until spring and emerge with mosquitoes to reestablish an enzootic transmission cycle in the area. Transovarial transmission of WN virus and preservation of the virus in hibernating mosquitoes are not thought to play an important role in the maintenance of the virus in nature[16,17]. However, our observations indicate that approximately 0.04% of the overwintering Culex mosquitoes collected at Fort Totten carried viable WN virus, and 0.1% contained WN viral RNA. This finding suggests that WN virus infected, hibernating Cx. pipiens were relatively common where virus activity was intense the previous season and likely play an important role in persistence of the virus in an area. This infection rate is similar to rates observed for another flavivirus, St. Louis encephalitis virus, in overwintering Cx. pipiens collected in Maryland, where 0.3% were infected (1 isolate from 312 tested), and Pennsylvania, where 0.2% were infected (1 isolate from 406 tested)[18].

What is unclear is the mechanism that produced these infected overwintering mosquitoes. Transovarial transmission of the virus from an infected female to her offspring, which then enter diapause (hibernation physiology and behavior) as adults and survive the winter without taking a blood meal, is supported by evidence from the field and laboratory[6,8]. Alternatively, Cx. pipiens infected by feeding on a viremic vertebrate host may have survived the winter. Though blood-fed adult Cx. pipiens survive winter conditions[19], they are not considered an efficient mechanism for virus persistence[10]. Regardless of the underlying mechanism, WN virus persistence in Cx. pipiens clearly contributes to the maintenance of WN virus through the winter season. Future research should address the mechanisms of WN virus maintenance and potential involvement of other mosquito species that may be important vectors in other regions of North America.