Effects of West Nile Virus Dose and Extrinsic Incubation Temperature on Temporal Progression of Vector Competence in Culex pipiens quinquefasciatus

Abstract

Culex pipiens quinquefasciatus were fed blood containing either 7.0 ± 0.1 logs plaque-forming units (pfu)/ml (high dose) or 5.9 ± 0.1 logs pfu/ml (low dose) of West Nile virus and held at extrinsic incubation temperatures (EIT) of 28°C or 25°C. Approximately 20 mosquitoes per dose were collected after incubation periods (IP) of 4, 6, 8, and 12 days postinfection (dpi). Infection rates were influenced by EIT and virus dose but not by IP. Body titer was significantly higher for mosquitoes fed the high dose and held at 28°C at the later IPs (6, 8, and 12 dpi). However, leg titer was significantly higher for mosquitoes at the later IPs but did not differ between EITs or doses. Because infection rates varied with EIT and dose, there is likely a midgut infection barrier influenced by these factors that is not influenced by IP. Dissemination rates were influenced by all 3 factors consistent with the presence of a midgut escape barrier. Dissemination rate, body titer, and leg titer were dependent on IP, indicating the need to investigate multiple time points in vector competence studies to elucidate critical events in infection and dissemination. KeywordsCulex pipiens quinquefasciatus; West Nile virus; dose; vector competence; temporal progression West Nile virus (WNV, family Flaviviridae, genus Flavivirus) is cycled between wild birds and ornithophilic mosquitoes in the genus Culex (Hayes 1989, Day 2005. Culex pipiens quinque-fasciatus Say has been found infected with WNV in the field (Rutledge et al. 2003, Godsey et al. 2005, is a competent laboratory vector of WNV (Sardelis et al. 2001, Goddard et al. 2002, and is considered an important WNV vector in the USA.Vector competence is influenced by both extrinsic and intrinsic factors (Hardy et al. 1983). Extrinsic factors include extrinsic incubation temperature (EIT) (Hardy et al. 1983, Dohm et al. 2002 and virus dose (Kramer et al. 1981). Biological factors include mosquito species (Goddard et al. 2002), mosquito population (Richards et al. 2009), and virus strain (Moudy et al. 2007). Extrinsic and intrinsic factors may also influence the extrinsic incubation period (EIP) and affect vector competence (Hardy et al. 1983, Dohm et al. 2002, Reisen et al. 2006, Kilpatrick et al. 2008. The EIP begins when a virus is ingested with a blood meal. The virus infects mosquito midgut epithelial cells and disseminates out of the midgut, and the EIP ends when the virus is transmitted to a susceptible host. Mosquitoes were bloodfed as described elsewhere (Richards et al. 2007) with the exception that the virus used was freshly propagated in Vero cell culture. Mosquitoes were allowed to feed for 30 min on cotton pledgets soaked with a high or low dose of WNV mixed with citrated bovine blood (Hemostat, Dixon, CA) that had been warmed (35°C) for 10 min. Virus doses used were within the range of viremias commonly found in WNV-infected birds in Florida (Komar et al. 2003). Two aliquots (0.1 ml each) of the heated blood were placed into separate tubes of 1 ml of BA-1 diluent prio...

West Nile Virus Vector Competency of Culex quinquefasciatus Mosquitoes in the Galápagos Islands

“…Studies in Florida showed much higher infection and dissemination rates at one-week post-exposure at comparable or lower doses, even at lower temperatures (transmission rates were not mentioned). 30 Similarly, although studies in California (which did cite transmission rates) showed variability in results on the basis of comparable viral titers, mosquitoes there tended to have higher vector competency than those of Galápagos. 32,34 In our own study, transmission ability in the first two weeks of infection was less than 10%, which suggests a lower WNV risk for Galápagos fauna if other more-competent mosquito vectors are not present.…”

“…32,[36][37][38] Generally, longer incubation periods, higher viral doses, and higher temperatures are expected to yield higher rates of infection. 30,32,39 However, Cx. quinquefasciatus in Australia was shown to be a highly competent vectors when exposed to a dose of only 10 4.0±0.…”

“…quinquefasciatus populations are reported as competent vectors of WNV, and in southern United States this is the principal mosquito species associated with WNV. 28,30 Infection rates of up to 100% have been observed (Florida and Texas), and high WNV transmission rates of up to 52% (southern California), calculated as a proportion of the number of mosquitoes tested. 30,[32][33][34][35] There are noticeable influences on mosquito susceptibility to WNV infection including extrinsic (e.g., initial viral dose, temperature, and length of incubation) and intrinsic (e.g., genetic strain, midgut physiology) factors.…”

“…28,30 Infection rates of up to 100% have been observed (Florida and Texas), and high WNV transmission rates of up to 52% (southern California), calculated as a proportion of the number of mosquitoes tested. 30,[32][33][34][35] There are noticeable influences on mosquito susceptibility to WNV infection including extrinsic (e.g., initial viral dose, temperature, and length of incubation) and intrinsic (e.g., genetic strain, midgut physiology) factors. 32,[36][37][38] Generally, longer incubation periods, higher viral doses, and higher temperatures are expected to yield higher rates of infection.…”

“…quinquefasciatus known from elsewhere indicates that this mosquito would act as a bridge vector for WNV between bird and mammals in Galápagos. 23,30,31 North American Cx. quinquefasciatus populations are reported as competent vectors of WNV, and in southern United States this is the principal mosquito species associated with WNV.…”