The threat of African horse sickness virus in the UK: furthering our understanding of vector biology and how best to protect horses in the event of an outbreak

摘要

Recent changes in the global distribution of several vector-borne viral diseases have been linked to climate change and globalisation, leading to concerns that they will increasingly threaten northern Europe and the UK. African horse sickness (AHS) is a vector-borne viral disease of equids that is spread by Culicoides midges. The disease is associated with up to 95% mortality in naïve populations of horses and an outbreak in the UK would therefore have devastating consequences for both animal welfare and the equine industry. AHS has never been reported to occur further north than Spain, however it has been suggested that appropriate midge species and climatic conditions are now present in northern Europe to support an outbreak. Given the integral role of the vector in AHS epidemiology, data describing the distribution of Culicoides species throughout the UK is key to predicting the risk and potential spread of a disease outbreak. It is suspected that Culicoides species of the Obsoletus and Pulicaris groups would be most likely to act as vectors; however, only very limited data exist regarding the Culicoides species present on UK equine properties. Chapter 2 of this thesis aimed to describe the species of Culicoides collected by light-suction trapping on equine properties in the UK. Fourteen equine properties were identified and collection took place at each overnight for 3 sessions. The study identified the presence of potential AHS virus (AHSV) vector Culicoides species on both urban and rural equine properties within the southeast UK. PCR analysis revealed that engorged members of these species contained equine DNA, proving a direct vector-host interaction. Current recommendations for preventing the spread of AHS are limited. DEFRA currently suggest using topical deltamethrin for AHS control in the face of an outbreak; however, no data is available regarding its efficacy in the horse. The aims of Chapter 3 of this thesis were to investigate the effect of topical deltamethrin on the blood-feeding of Culicoides on horses and to investigate which species appear to preferentially blood-feed on horses. Three pairs of horses were placed in partially enclosed cages, which allowed samples representing the Culicoides interacting with individual horses to be collected. Four collection sessions were run before 1 horse from each pair was topically treated with 1% deltamethrin and another 4 sessions were completed. Collected Culicoides were identified and each midge examined to see if it had blood-fed. There was no significant effect of treatment on blood-feeding by Culicoides. The most abundant species collected were from the Obsoletus (44.3%) and Pulicaris (34.7%) groups. These species were also more likely to have blood fed than other species, supporting their potential role as AHSV vectors if the virus were to reach the UK.