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

Abstract

Culex (Diptera: Culicidae) mosquitoes, the primary summer vectors of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), also may serve as overwintering reservoir hosts. Detection of WN viral RNA from larvae hatched from eggs deposited by infected females during late summer and fall may provide evidence for the vertical passage of WNV to overwintering cohorts. To determine whether vertical transmission to the overwintering generation occurs in populations of Culex mosquitoes throughout California, larvae from naturally infected females were tested by family for WN viral RNA by real-time quantitative reverse transcription-polymerase chain reaction during August through October 2011. Viral RNA was detected in 34 of 934 Culex tarsalis Coquillett and Cx. pipiens complex females that laid viable egg rafts. From these egg rafts, Þrst-instar larvae from nine families tested positive, yielding an overall Þeld vertical transmission rate of 26% (n ϭ 34). To determine whether the WNV may be lost transtadially during development to the adult stage, Þrst-instar larvae and adult progeny from experimentally infected Cx. pipiens complex females were assessed for the presence and quantity of WN viral RNA. Most (Ϸ75%) WNV infections were lost from positive families during larval development to the adult stage. In Þeld and laboratory studies, only infected mothers with mean cycle threshold scores Յ20 vertically transmitted WNV to larval progeny, adult progeny, or both. In summary, vertical transmission of WNV was detected repeatedly in naturally infected Culex mosquitoes collected throughout California during late summer and fall, with females having high titered infections capable of passing WNV onto their progeny destined for overwintering.

Phenotypic Variation among Culex pipiens Complex (Diptera: Culicidae) Populations from the Sacramento Valley, California: Horizontal and Vertical Transmission of West Nile Virus, Diapause Potential, Autogeny, and Host Selection

“…Although WNV was not detected in adult progeny of colonized f. pipiens females, another colonized strain of f. pipiens from the Sacramento Valley, CA, successfully transmitted WNV to larval and adult progeny in a previous study. 68 In this study, f. molestus females were uniparous (after autogenous oviposition) before imbibing an infective bloodmeal, perhaps allowing increased dissemination of WNV to the reproductive tract, as vertical transmission rates have been shown to increase with the number of gonotrophic cycles completed by infected females. 67 The frequency of vertical transmission also may be influenced by rearing temperature and mosquito species and strains.…”

“…pipiens complex populations in California appear to be competent vectors for WNV, the principal question is whether WNV can persist in these populations during winter. Both f. molestus and f. pipiens 68 from California are capable of vertically passing virus to their offspring during late summer. Because the majority of Cx.…”

Rift Valley fever virus: strategies for maintenance, survival and vertical transmission in mosquitoes

“…Assuming virus is passed to the egg, it must then pass transstadially from egg to larvae, to pupae and to adult, disseminating to the salivary glands or ovaries for onward HT or VT. Hardy et al [110] proposed that virus is inactivated during metamorphosis, which was supported by a reduction in viral titres transstadially during development [111]. In contrast, LACV transmission to progeny of orally infected Aedes triseriatus indicated that viral load was higher in larvae compared to eggs [106] and similarly increased in each stage from egg to adult with most larval and adult tissues supporting replication [112].…”

“…Thus, for these mosquitoes to become infected with WNV they require infection by VT, not oral infection, prior to overwintering. VT of WNV by Culex mosquitoes and subsequent detection in overwintering populations has been recorded [111,128]. Therefore, when considering RVFV in temperate regions outside Africa, it may be necessary to consider a similar overwintering model.…”

Detection of Persistent Chikungunya Virus RNA but not Infectious Virus in Experimental Vertical Transmission in Aedes aegypti from Malaysia

“…1 Vertical transmission of arboviruses occurs from an adult female mosquito to its eggs and progeny. This may be an important source of maintenance for arboviruses such as dengue virus [3][4][5] and West Nile virus, 6,7 particularly during unfavorable environmental conditions. Inconsistent findings are reported for the existence of vertical transmission of CHIKV; if it does occur, it is infrequent.…”