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

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.

Climatic effects on mosquito abundance in Mediterranean wetlands

“…Moreover, mosquito abundance has been directly related to the basic reproductive rate R o (the total number of secondary cases arising from one infective case in a susceptible population) and the vectorial capacity C (the daily rate at which secondary cases arise from a currently infective case) [7,10]. Additionally, temperature alters the replication of the virus in the mosquito – the extrinsic incubation period (EID) – and therefore infection rates [48,65]. In this work, we focus on how climate change could influence transmission by altering mosquito vector abundance.…”

West Nile Virus: Review of the Literature

“…6 Increased ambient temperature shortens the incubation time from infection to infectiousness in mosquitoes and increases viral transmission efficiency to birds, both critical factors for arboviral amplification. 7,8 At smaller scales, urban and agricultural land covers, 9 rural irrigated landscapes, 10 increased temperature, 11 increased rainfall, 12 decreased rainfall, 12 and several socioeconomic factors such as housing age and community drainage patterns, 13 per capita income, 10 and density of poorly maintained swimming pools 14 relate to higher incidence in some locations. Nevertheless, considerable challenges remain in predicting how, when, and where these factors will combine to produce the focal, intense outbreaks that now characterize West Nile virus ecology in the United States.…”

Impacts of Increasing Temperature on the Future Incidence of West Nile Neuroinvasive Disease in the United States

“…WNV disease outbreaks have been associated with climate variables, including temperature and precipitation, in a number of studies [10] [19]. While the nature and strength of the observed associations have varied in these studies according to the region and lag-times among other factors, above-normal temperatures have been among the most consistent predictors of outbreaks, due in part to the acceleration of viral incubation in mosquitoes and increased mosquito reproduction rates at higher temperatures [19] [20] [21] [22] [23].…”