Date: 9 March 2000
From: Charles S. Calisher                                                                                          Moderator, Viral Diseases

In tomorrow's issue (vol. 49, no. 9) of Morbidity and Mortality Weekly Report, the U.S. Centers for Disease Control will report that the Division of Vector-Borne Infectious Diseases (DVBID) laboratories in Fort Collins has detected RNA of West Nile virus (WNV) in mosquitoes that have spent the winter in underground structures in New York City.

Although WNV RNA (envelope, NS-1 and NS-5 regions) were detected in three of 67 pools containing _Culex_ spp. mosquitoes, WNV has not been isolated from these pools. Thus, the persistence of WNV in mosquitoes in New York City has not yet been proven but the persistence of WNV RNA has. In an editorial note MMWR comments that the reason virus has not been isolated may be because (1) virus titer may be exceedingly low, (2) virus may be noninfectious in overwintering mosquitoes, (3) virus may have been inactivated during processing, (4) noninfectious WNV RNA, but not
infectious virus, may persist in the mosquitoes, or (5) the results may have been nonspecific.

The DVBID laboratories are continuing to attempt virus isolation and continuing other rigorous and thorough efforts to determine whether WNV is persisting in the New York area and to determine the likelihood of another outbreak of this virus and disease.

Although vertical transmission (from one generation to another) of WNV has been shown in experimentally-infected _Culex_ and _Aedes_ spp. mosquitoes and vertical transmission of WNV has been demonstrated in African _Culex univittatus_ mosquitoes, results of these studies in New York have not provided evidence for vertical transmission of WNV there.

We assume that further information will be forthcoming but, at least for now, all that can be said is that WNV RNA has persisted through the New York winter. Whether the virus has also persisted remains to be determined.