The replication of bluetongue virus in Culicoides vectors.

BTV is maintained in nature by an endless series of alternating cycles of replication in Culicoides midges and various mammalian ruminant species. Experimentation has shown that the ability of the virus to infect Culicoides persistently and be transmitted by them is restricted to a relatively small number of species. In essence, therefore, the world distribution map of BTV is little more than a distribution map of competent insect vectors. Once ingested by a competent vector, BTV attaches to the luminal surface of the mid-gut cells, infects these cells and replicates in them. Progeny virus is then released through the basement lamina into the haemocoel from where the secondary target organs including the salivary glands are infected. Subsequent to virus replication in the salivary glands transmission can taken place. The whole cycle from infection to transmission takes between 10-15 days at 25 degrees C and individual vectors once infected usually remain so for life. Not all female midges within a vector species are susceptible to infection with BTV, or if infected, are competent to transmit the virus. A series of barriers or constraints exists within certain individuals of a vector species which either prevents virus infection or else restricts it in such a way as to stop transmission. Each population of a vector species of Culicoides has a variable proportion of these so-called refractory midges. The refractory and susceptible traits for BTV within a vector species are under genetic control, and by selective breeding, highly susceptible or completely insusceptible populations can be obtained. However, the mechanisms by which these traits are expressed are poorly understood. Further studies are therefore urgently required to determine the precise biochemical nature of these mechanisms and their mode of operation.