Bridging the gap: Persistence of Culicoides-borne viruses over winter in temperate regions

In order to persist from year to year in temperate regions, vector-borne viruses must overcome a period when the adult vector population is low or absent. Proposed overwintering mechanisms involve vector ecology and virus:vector and virus:host interactions. Here we present studies of factors that may facilitate the transmission of bluetongue virus between ruminants and Culicoides to bridge a seasonal vector free period.

Firstly, we examined the seasonality of Culicoides adult emergence from semi-permanent larval habitats. Collection of more than 96,000 Culicoides from 267 emergence trapping events across two years revealed clear evidence of bivoltinism peaks of both female Culicoides obsoletus complex and male C. obsoletus emerging from dung heaps. Emergence of adult Culicoides during the winter period (November-April) was negligible. The thermal time required for development was estimated to be 151.5 day-degrees.

Secondly, we investigated the duration of infectivity of BTV-infected cattle to Culicoides and therefore the period over which transmission could be maintained by the mammalian host. Culicoides sonorensis were fed on four calves infected with BTV at 4, 7, 14, 20 and 26 days post infection. The peak in vector infection rates coincided with the peak in vireamia within the animals, with effective transmission to vectors to at least 14 days post infection. At 20 and 26 days post infection, however, very few Culicoides individuals were infected, despite the presence of high levels of virus genome in the blood of the animals. Therefore transmission from hosts infected the previous season may not be sufficient to allow BTV overwintering.