Effects of Temperature on the Transmission of West Nile Virus by <I>Culex tarsalis</I> (Diptera: Culicidae)

TLDR

Culex tarsalis females were infected with the NY99 West Nile virus strain and incubated at 10–30 °C, with transmission assessed by capillary tube assay and the median extrinsic incubation period (EIP50) estimated via probit analysis; monthly EIP durations were then modeled from field temperatures and linked to gonotrophic cycle timing and WNV activity. The NY99 West Nile virus strain required warm temperatures for efficient transmission, with an extrinsic incubation period of 109 degree‑days and a zero‑development threshold of 14.3 °C, and field data showed that rising temperatures shortened the EIP, enabling transmission within two or fewer gonotrophic cycles and correlating with epidemic spread during 2002–2004.

Abstract

Culex tarsalis Coquillett females were infected with the NY99 strain of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) and then incubated under constant temperatures of 10-30 degrees C. At selected time intervals, transmission was attempted using an in vitro capillary tube assay. The median time from imbibing an infectious bloodmeal until infected females transmitted WNV (median extrinsic incubation period, EIP50) was estimated by probit analysis. By regressing the EIP rate (inverse of EIP50) as a function of temperature from 14 to 30 degrees C, the EIP was estimated to require 109 degree-days (DD) and the point of zero virus development (x-intercept) was estimated to be 14.3 degrees C. The resulting degree-day model showed that the NY99 WNV strain responded to temperature differently than a lineage II strain of WNV from South Africa and approximated our previous estimates for St. Louis encephalitis virus (family Flaviviridae, genus Flavivirus, SLEV). The invading NY99 WNV strain therefore required warm temperatures for efficient transmission. The time for completion of the EIP was estimated monthly from temperatures recorded at Coachella Valley, Los Angeles, and Kern County, California, during the 2004 epidemic year and related to the duration of the Cx. tarsalis gonotrophic cycle and measures of WNV activity. Enzootic WNV activity commenced after temperatures increased, the duration of the EIP decreased, and virus potentially was transmitted in two or less gonotrophic cycles. Temperatures in the United States during the epidemic summers of 2002-2004 indicated that WNV dispersal and resulting epicenters were linked closely to above-average summer temperatures.