Effect of temperature on African horse sickness virus infection in Culicoides

This paper shows that both the infection rate and the rate of virogenesis of African horse sickness virus (AHSV) within vector Culicoides are temperature dependent. As temperature is reduced from permissive levels the lifespan of the vector itself is extended but the rate of virogenesis decreases and infection rate falls dramatically so that at 10 degrees C virtually all midges are free from virus by 13 days post infection (dpi). When vectors that had been kept at this temperature for 35 days were moved to a permissive temperature for 3 days; however, the apparent zero infection rate increased to 15.5%. It therefore appears that at low temperature (< or = 15 degrees C) AHSV does not replicate but virus may persist in some vectors at a level below that detectable by traditional assay systems and when the temperature later rises to permissive levels virus replication is able to commence. On the basis of this information an overwintering mechanism for AHSV is suggested. The temperature at which the immature stages of Culicoides are reared may also influence infection with AHSV. A 5-10 degrees C rise in larval developmental temperature resulted in an increase in oral infection rate of a normally non-vector species of Culicoides, from < 1% to > 10%. A mechanism is suggested.