Studies on Hibernating Populations of Culex pipiens from a West Nile Virus Endemic Focus in New York City: Parity Rates and Isolation of West Nile Virus

Abstract

A 3-year study was undertaken to examine the parity status, survival, and prevalence of West Nile virus (WNV) in overwintering populations of Culex pipiens pipiens collected from a hibernaculum located in a WNV endemic region in New York City. Nearly 6,000 females were collected from December through April. Parity rates were highest among females collected in December and January, ranging from 12.3% to 21.9%, depending on the year. In each year of the study, the proportion of parous females declined significantly during the course of the winter; the percentage of parous females found in April ranged from 0.9% to 10%. Results provide unequivocal evidence that parous Cx. p. pipiens females from this region of the northeastern US enter hibernacula in the fall in comparatively high proportions not previously recognized for this species, and while these females experience significant mortality during the winter, some survived to April to emerge in the spring. The absence of any detectible blood remnants in overwintering females reaffirms that blood feeding does not occur among diapausing females during the winter. The possibility that a portion of the diapausing population may be autogenous as a result of hybridization with sympatric belowground populations of Cx. p. pipiens "form molestus" is discussed. A single isolation of WNV was obtained in Vero cell culture from a pool of 50 females collected on January 11, 2007, representing an infection prevalence of 0.07% in the overwintering population in 2007 (n = 1,370 mosquitoes, 33 pools). No isolations of WNV were made from mosquitoes collected in 2008 (n = 1,870 mosquitoes, 190 pools) or 2009 (n = 1,767 mosquitoes, 184 pools). Findings provide further evidence for local overwintering of WNV in diapausing Cx. p. pipiens, albeit at very low rates, consistent with the paucity of WNV-positive mosquitoes detected in June and early July despite the emergence of females from hibernacula in early May in this region.

The experience of West Nile virus integrated surveillance system in the Emilia-Romagna region: five years of implementation, Italy, 2009 to 2013

“…modestus playing a bridge role in specific areas [7]. Generally, in temperate regions, it has been shown that the virus may overwinter in infected female mosquitoes as well as in residential birds, so there is no need for continuous re-introductions by migrating birds [8][9][10][11].…”

Experimental and Natural Vertical Transmission of West Nile Virus by California <I>Culex</I> (Diptera: Culicidae) Mosquitoes

“…In addition, WNV has been isolated from overwintering Culex adults in the Þeld (Nasci et al 2001, Bugbee and Forte 2004, Farajollahi et al 2005, Andreadis et al 2010, Unlu et al 2010. Because females destined for diapause purportedly do not blood feed (Mitchell and Briegel 1989), these Þndings suggest that vertical transmission of WNV must occur repeatedly in natural populations.…”

West Nile Virus

“…Therefore, there was also an increased likelihood that WNV can be transmitted by hibernating mosquitoes. This hypothesis has been supported by the analysis of overwintering mosquitoes, which were infected either experimentally or naturally [146][147][148].…”

“…This was demonstrated both by examining the progeny of experimentally infected mosquitoes as well as by virus isolation from male mosquitoes which must have been infected vertically because males do not take blood meals but rather feed on plant juice [130,150]. The transovarial transmission of the virus may be important for the maintenance of the infection cycle between mosquitoes and birds, as it has been shown that mosquitoes were infected with WNV during the winter diapause without having taken a blood meal [148]. The transmission rate to the gives serological evidence of an acute WNV infection.…”