Effects of Temperature on the Transmission of West Nile Virus by <I>Culex tarsalis</I> (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.

Arboviruses in North Dakota, 2003–2006

“…Previous laboratory and field studies identified Cx. tarsalis as one of the most important vectors in western United States, [22][23][24][25] including North Dakota. 26 Our current investigations confirm this conclusion, but we also documented Ae.…”

“…36 It was relatively abundant in North Dakota from the end of June through the end of August, had a relatively high IR during the summer, is efficient in transmitting this virus horizontally, 22,24,25,37 and is relatively efficient in transmitting this virus vertically. 24,38,39 Culex tarsalis was collected during the weeks of June 2 to October 6, similar to collections made in Grand Forks, North Dakota, 26 Figure 4).…”

Climate change impacts on West Nile virus transmission in a global context

“…Increased temperatures cause an upsurge in the growth rates of vector populations, decrease the interval between blood meals, shorten the incubation time from infection to infectiousness in mosquitoes, accelerate the virus evolution rate and increase viral transmission efficiency to birds [22,23,30,32,35,36].…”

West Nile Virus Activity in a Winter Roost of American Crows (Corvus brachyrhynchos): Is Bird-To-Bird Transmission Important in Persistence and Amplification?

“…Since the virus's emergence, WNV has been a frequent cause of viral encephalitis in equines and humans within the United States (Azad andThomas 2004, Petersen et al 2012), and infection has been recorded naturally in $326 avian species, spanning across most native North American avian orders (Marra et al 2004, Centers for Disease Control [CDC] 2009). Despite widespread impact on both avian and human communities, gaps remain in our understanding of WNV transmission dynamics, particularly how the virus persists in temperate climates when cold temperatures preclude viral replication and diminish vector blood-feeding activity (Reisen et al 2006b). Overwintering mechanisms are critically important because they determine the prevalence of virus at the termination of winter and thereby may dictate the force of transmission during vernal amplification.…”

“…Ornithophagic species within the genus Culex are important vectors and responsible for zoonotic WNV transmission among birds (Molaei et al 2006). Culex in California are quiescent or diapause during cold winter months (Nelms et al 2013a,b), and low temperatures hinder WNV replication within the overwintering mosquito hosts (Dohm et al 2002, Reisen et al 2006b). Nevertheless, researchers in New York State documented American Crow (Corvus brachyrhynchos; hereafter "crow") mortality during February and March of 2005 (Dawson et al 2007).…”