Arbovirus Transmission by Culex nigripalpus in Florida, 2005

Abstract

Understanding the transmission patterns of West Nile and St. Louis encephalitis viruses (family Flaviviridae, genus Flavivirus, WNV and SLEV) could result in an increased ability to predict transmission risk to humans. To examine transmission patterns between vector and host, we trapped mosquitoes in three Florida counties from June to November 2005 by using chicken-baited lard can mosquito traps. These traps were used to monitor for presence of WNV and SLEV in mosquitoes and subsequent transmission of these viruses to chickens. In total, 166,615 female mosquitoes were sorted into 4,009 pools based on species and bloodfed status, and they were tested for presence of WNV and SLEV. Sera from 209 chickens were tested for WNV and SLEV antibodies. We detected eight WNV-positive Culex nigripalpus Theobald mosquito pools; SLEV was not detected in any pools. Six positive pools were collected in August and September from Duval County, one pool in September from Manatee County, and one pool in November from Indian River County. Of the eight chickens potentially exposed to WNV, antibodies were detected in only one chicken, indicating a low rate of transmission relative to the observed mosquito infection rates. Low virus transmission rates relative to infection rates would suggest that using sentinel chicken seroconversion data as a means of arbovirus surveillance may underestimate the prevalence of WNV in the mosquito population. However, using mosquito infection rates may overestimate the risk of arboviral transmission. A variety of factors might account for the observed low level of transmission including a lack of viral dissemination in mosquito vectors.

Ecology of Potential West Nile Virus Vectors in Southeastern Louisiana: Enzootic Transmission in the Relative Absence of Culex quinquefasciatus

“…nigripalpus collected in chicken-baited lard can traps placed at three widely dispersed sites in Florida. 41 This study also noted a low rate of WNV transmission to the chickens restrained within the traps, despite seven of eight virus-positive pools containing only blooded females, a finding that corroborates previous work. 17 Vitek and others 41 speculate that the lack of virus dissemination within the infected mosquitoes may be responsible for the low transmission rate, but their experimental protocol did not allow them to test this idea.…”

“…41 This study also noted a low rate of WNV transmission to the chickens restrained within the traps, despite seven of eight virus-positive pools containing only blooded females, a finding that corroborates previous work. 17 Vitek and others 41 speculate that the lack of virus dissemination within the infected mosquitoes may be responsible for the low transmission rate, but their experimental protocol did not allow them to test this idea. 41 In southern Florida, onset of human cases and the appearance of anti-WNV antibodies in sentinel chickens are temporally and spatially associated with prior cycles of drought and moisture in a manner similar to that seen with SLEV, thus providing strong evidence that Cx.…”

“…17 Vitek and others 41 speculate that the lack of virus dissemination within the infected mosquitoes may be responsible for the low transmission rate, but their experimental protocol did not allow them to test this idea. 41 In southern Florida, onset of human cases and the appearance of anti-WNV antibodies in sentinel chickens are temporally and spatially associated with prior cycles of drought and moisture in a manner similar to that seen with SLEV, thus providing strong evidence that Cx. nigripalpus transmits WNV in that area.…”

Sources of Error in the Estimation of Mosquito Infection Rates Used to Assess Risk of Arbovirus Transmission

“…nigripalpus showed that transmission is low in relation to estimated infection rates, 38,39 and many factors that we studied here were considered to explain this occurrence. These field studies support the idea that incorporation of information on the mosquito population (age structure, abundance and estimated infection rate), virus (strain, species-specific dissemination rates), and the environment (temperature) are necessary to make assessments of the risk of virus transmission.…”

“…In that way, one would have an estimate of mosquito infection paired with a measure of transmission. 38,39 However, these methods are seldom feasible, practical, or affordable for long-term surveillance programs.…”