Bluetongue virus transmission between ruminant host and Culicoides vectors: the importance of insect saliva proteins

Lead Research Organisation: The Pirbright Institute
Department Name: UNLISTED

Abstract

Bluetongue virus (BTV) can infect most ruminants but causes severe disease, in sheep, some species of deer and to a lesser extent in cattle. The virus is maintained in the field by transmission cycles between ruminant hosts and insect vectors - blood feeding midges (Culicoides spp.). Adults of most Culicoides have the potential to become infected with BTV. However their ability to transmit virus varies dramatically between species and only a small proportion of insects become infected (even from a known vector species). Any factors that can enhance infection of the insect, would also increase BTV transmission efficiency, and may influence vector status. In contrast, BTV transmission from vector to host is very efficient, requiring only a single insect bite to cause a severe clinical infection, while a much larger amount of virus is normally used to cause similar effects by needle inoculation. Culicoides saliva proteins cause a severe inflammatory response, and profound changes in the sheep skin, including formation of vesicles, synthesis (or release) of inflammatory modulators and massive recruitment of leukocytes. These changes may play an important part in the BTV infection mechanism in the sheep. Indeed cellular components of the ovine immune system can themselves become infected by BTV and may be involved in initial stages of infection and dissemination. Novel methods will be used to collect saliva proteins of Culicoides vector (and non-vector) species. Major protein components will be identified by AA sequencing. The effect of these proteins (particularly proteases) on virus structure and infectivity for different cells will be determined. The effect of saliva proteins and the inflammatory response, on virus titre and persistence in the sheep skin will be explored. The possibility of mechanical BTV transmission between host animals (by other insects, or by needle), or non-systemic transmission between co-feeding vector insects, will also be examined.

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