Chapter 14 - Bluetongue virus in the insect host

Abstract

Bluetongue virus (BTV) is an arthropod-borne virus (arbovirus) that is transmitted biologically between its vertebrate hosts (ruminants) by certain species of Culicoides biting midge. Vector species of Culicoides are the only important means by which BTV transmission occurs, consequently the distribution of the virus is limited to those areas of the globe where these species occur and even within such regions the virus displays a seasonal incidence that reflects the abundance of the adult vectors at certain times of the year. BTV is the prototype of the genus Orbivirus in the family Reoviridae and exists as a number of serotypes, of which 24 have been identified to date. The virus infects all species of ruminant but severe disease usually occurs only in certain breeds of sheep. Once ingested by a competent vector, BTV attaches to the luminal surface of the midgut cells, infects these cells and replicates in them. Progeny virus particles then escape through the abluminal surface and 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 and release into the salivary ducts, transmission can take place. The whole cycle from infection to transmission takes between 10 and 15 days at 25°C but in a lesser time at higher temperatures and a longer time at lower temperatures.