The barriers to bluetongue virus infection, dissemination and transmissionin the vector, Culicoides variipennis (Diptera : Ceratopogonidae)

Transmission of bluetongue virus (BTV) by a vector species of Culicoides wa s studied using immunohistochemistry, virus titration and in vitro transmis sion tests. Adult female C. variipennis were used from two colonies that ar e either "transmission competent" or "transmission refractory" after oral i nfection with BTV. Intrathoracic (IT) injection of BTV into the haemocoel a lways resulted in a fully disseminated infection and transmission of virus in saliva. However, after ingestion of an infectious blood meal, only 30% ( approximately) of midges from either colony became persistently infected. A lthough none of the orally infected insects from the "refractory" colony we re able to transmit virus, 12% of those from the "competent" colony (contai ning greater than or equal to 10(3.0)TCID(50) of virus/midge) did transmit BTV in their saliva. The most important barriers to BTV transmission in Cul icoides vector species appeared to be a mesenteron infection barrier (MIB), which controls initial establishment of persistent infection, a mesenteron escape barrier (MEB) which can restrict virus to gut cells and a dissemina tion barrier (DB) which can prevent virus which enters the haemocoel from i nfecting secondary target organs. Culicoides variipennis do not appear to p resent either a salivary gland infection barrier (SGIB), or a salivary glan d escape barrier (SGEB) to BTV.