Temperature and Transmission of Arboviruses

Emma Jane Wittmann
PhD Thesis Culicoides biting midges Bluetongue African Horse Sickness Epizootic Haemorrhagic Disease Virus


Culicoides biting midges are economically important as vectors of several arboviruses of domestic and wild animals. The most important of these viruses are bluetongue virus (BTV), which infects ruminants, and African horse sickness virus (AHSV), which infects equids. Climatic factors can affect the capacity of Culicoides to transmit these viruses by influencing the size of adult Culicoides populations and the proportion of adults within a population capable of transmitting the viruses. Here, I report the results of a series of studies investigating the influence of one such factor, temperature, on virus transmission. The optimum temperatures for recruitment of adult midges from the immature stages were estimated to be 25-30°C for C. nubeculosus and 25-35°C for C. variipennis sonorensis, while the minimum temperatures for development were 8.1°C and 10.7°C respectively. The proportion of adult C. variipennis sonorensis capable of transmitting BTV and AHSV, as well as epizootic haemorrhagic disease virus (EHDV) was greatest at 27-30°C. Thus although longevity of adult C. variipennis sonorensis was reduced at these high temperatures, this was more than compensated for by the accompanying decrease in the duration of the viral extrinsic incubation period (EIP). In contrast, at cooler temperatures (15-17°C) adult longevity was extended, but the EIP was disproportionately prolonged meaning that few adult midges were capable of virus transmission. The impact of temperature on the vector competence of Culicoides vector populations (i. e. proportion of midges with virus susceptible genotypes that develop susceptible phenotypes) varied with the virus species and serotype. Vector competence increased with temperature for C. variipennis sonorensis infected with AHSV4 or EHDV1, whereas temperature had no effect on vector competence of C. variipennis sonorensis infected with BTV 10 or BTV16 and C. imicola infected with AHSV8. In addition, I found that exposure of immature C. nubeculosus (a non-vector species) to temperatures close to their upper lethal limit (33°C) could induce vector competence for BTV and AHSV. The distribution of Culicoides species is also influenced by climate and here I show that the range of C. imicola (principally an Afro-Asian species) in Europe is limited by temperature. The information from these studies can be used to assess how global warming will affect the distribution and seasonal occurrence of BTV and AHSV