Effect of temperature on survival and rate of virogenesis of African horse sickness virus in Culicoides variipennis sonorensis (Diptera: Ceratopogonidae) and its significance in relation to the epidemiology of the disease

Abstract

Culicoides variipennis sonorensis Wirth & Jones and C. nubeculosus (Meigen) were orally infected with African horse sickness virus (AHSV) type 9 and subsequently incubated at 10, 15, 20 and 25°C (R.H. 80%±10%). A time course of infection rates and virus titres was recorded by assaying flies individually or in pools, and survival rates of flies were also estimated. Survival rates at 10, 15 and 20°C were very similar and 80-90% of flies remained alive after 14 days; at 25°C after the same period survival was reduced to 40%. None of the C. nubeculosus became persistently infected with AHSV, but the virus took longer to clear as the incubation temperature dropped. At temperatures of 10, 15, 20 and 25°C virus was undetectable on days 12, 8, 5, and 4 days post infection (dpi), respectively. In C. v. sonorensis both the infection rate and rate of virogenesis were related to temperature. At 25°C a maximum mean titre of 10 43 TCID 50 /fly was reached by 9 dpi and the infection rate remained between 60 and 80%. At 20°C virogenesis was slower and a maximum mean titre of 10 43 TCID 50 /fly was reached only after 23 days; the infection rate was also reduced to 50-70%. At 15°C there was an overall decline in virus titre with time, although between 12 and 15 dpi some pools of flies contained 10 30 -10 40 TCID 50 /fly, demonstrating that virogenesis can occur. The infection rate at this temperature decreased dramatically to 0-15% after 9 dpi. At 10°C there was no detectable virogenesis and all pools tested at 13 dpi were negative. The apparent infection rate dropped to 0-5% between 13 and 55 days post infection. However, when surviving flies were then returned to 25 °C for 3 days the infection rate increased to 15.5%. It therefore appears that at low temperatures the virus does not replicate but infectious virus may persist at a level below that detectable by the usual assay systems. The implications of these findings for the epidemiology of AHS are discussed.

Olfaction in vector-host interactions

“…Due to their small size, adult Culicoides are particularly susceptible to desiccation and even brief periods at low humidity can reduce longevity (Murray 1991 (Mullens 1995, Wellby et al 1996, Wittmann 2000, as stated previously, suitable changes in temperature may allow an increase in viable vectors. Arndt (1995) investigated whether air pollutants react with pheromones, which would have potential ecological and economic consequences.…”

African horse sickness

“…AHSV infection rates of vector Culicoides and rates of virogenesis within them are temperature dependent [132]. As temperature increases infection rates also tend to increase, virogenesis is faster and transmission can occur sooner.…”

“…Furthermore, replication of AHSV does not seem to occur below 15 °C and at temperatures below this level the apparent infection rate rapidly falls to zero [132]. However, when midges are maintained for extended periods at these cooler temperatures and then transferred to temperatures within the virus permissive range, "latent" virus that has presumably persisted at very low levels in some individuals, commences replication and rapidly reaches sufficiently high titres for transmission to occur [132].…”

Serological evidence of African horse sickness virus infection of donkeys in Karamoja sub-region, North-eastern Uganda

“…AHSV apparent infection rate rapidly fall to zero at temperatures below 15°C since virus replication does not seem to occur below this temperature (Wellby et al, 1996). However, overwintered midges could harbor "latent" virus in some of these surviving midges that will commence replication and transmission should temperatures rise to permissive levels for example during spring.…”