Seasonal Transmission of Bluetongue Virus by <I>Culicoides sonorensis</I> (Diptera: Ceratopogonidae) at a Southern California Dairy and Evaluation of Vectorial Capacity as a Predictor of Bluetongue Virus Transmission

Abstract

Vectorial capacity of Culicoides sonorensis Wirth & Jones for the transmission of bluetongue (BLU) virus was examined at a southern California dairy from January 1995 to December 1997. Insects were collected one to two times per week in five CDC-type suction traps (without light) baited with CO2 at a constant release rate of 1,000 ml/min. BLU virus was detected in midges collected from May through December with an estimated overall infection rate of 0.08%. The BLU virus infection rate of field-captured midges was not correlated with sentinel calf seroconversions to BLU virus. Sentinel calf seroconversions were highly seasonal, occurring from August through November with most calves seroconverting during September and October. Vector competence of field-collected nulliparous flies fed a locally acquired serotype of BLU virus in the laboratory was stable among years (17-23%). Vectorial capacity was strongly correlated with BLU virus transmission (measured by sentinel calf seroconversions) during 1995 and 1996, but not during 1997. Host biting rate estimated for traps nearest to the sentinel calves was the index best correlated with BLU virus transmission for all study years and was most highly correlated with sentinel seroconversions 4 wk later. The utility of vectorial capacity and its component variables is discussed for this system.

The combination of abundance and infection rates of Culicoides sonorensis estimates risk of subsequent bluetongue virus infection of sentinel cattle on California dairy farms

“…These traps were deployed 2 hours before sunset and removed 2 hours after sunrise, and the positions of the different types of suction traps were alternated each week to correct for position effects, essentially as previously described (Gerry et al, 2001;Gerry et al, 2009;Mayo et al, 2011). The Culicoides midges captured were identified on the basis of their morphological characteristics and wing patterns using a stereomicroscope (Wirth et al, 1985).…”

“…The abundance of vector Culicoides midges at specific sites has been traditionally assessed utilizing traps artificially baited with CO 2 , light of appropriate wavelength, semiochemicals, or a combination thereof (Du Toit, 1944;Barnard, 1980;Mullens, 1985;Meiswinkel et al, 2007). While collection of insects by these trap methods is relatively cost-effective and convenient, the insect activity and infection prevalence determined using these trap methods may be poorly correlated to the biting rates and infection prevalence of truly host-seeking insects (Gerry et al, 2001;Gerry et al, 2009). These parameters are critical for accurately assessing and modeling the dynamics of virus transmission to livestock (Gerry et al, 2001;Carpenter et al, 2008;Baylis, 2009;Guis et al, 2011).…”

“…While collection of insects by these trap methods is relatively cost-effective and convenient, the insect activity and infection prevalence determined using these trap methods may be poorly correlated to the biting rates and infection prevalence of truly host-seeking insects (Gerry et al, 2001;Gerry et al, 2009). These parameters are critical for accurately assessing and modeling the dynamics of virus transmission to livestock (Gerry et al, 2001;Carpenter et al, 2008;Baylis, 2009;Guis et al, 2011). In contrast, enclosure trapping or mechanical aspiration of Culicoides midges directly from "bait' livestock has shown biting rates that can be several fold greater than the capture rate obtained using the conventional CO 2 baited suction trap, perhaps because mammalian -feeding Culicoides midges also respond to stimuli other than CO 2 (Mullens and Gerry, 1998;Gerry et al, 2009;Viennet et al, 2011).…”

Occurrence of Genetic Drift and Founder Effect during Quasispecies Evolution of the VP2 and NS3/NS3A Genes of Bluetongue Virus upon Passage between Sheep, Cattle, and Culicoides sonorensis

“…Sheep and calves that were seronegative to BTV as determined by competitive enzyme-linked immunosorbent assay (cELISA) (Blueplate Special; DiagXotics) were obtained from northwestern California, a region that is free of BTV infection. A laboratory colony of C. sonorensis Wirth and Jones insects was established, using standard rearing methods (41), from a southern California field population that was susceptible to BTV infection (21). Larvae were reared to adults, and 1-to 4-day-old adult flies were orally infected with BTV by being fed on defibrinated sheep blood spiked with BTV FI10O90Z at a titer of 10 6.7 50% tissue culture infective doses (TCID 50 ) per ml (41).…”

Bluetongue virus genetic and phenotypic diversity: Towards identifying the molecular determinants that influence virulence and transmission potential

“…In the latter case it should be emphasized that the relationship between the virus load and the efficiency with which the virus is transmitted is still unclear (Carpenter et al, 2008). Other potential pitfalls of oral susceptibility studies in Culicoides include difficulties in standardizing the titre of virus in the blood meal prior to blood feeding , the generally low oral susceptibility of Culicoides for BTV (Bellis et al, 1994;Gerry et al, 2001), as well as the potential varying oral susceptibility of field caught Culicoides based on their geographic origin (Carpenter et al, 2006;Jones and Foster, 1978;Venter and Paweska, 2007).…”