Abstract
BackgroundWest Nile virus (WNV) is a single-stranded RNA virus that can cause neurological disease in both humans and horses. Due to the movement of competent vectors and viraemic hosts, WNV has repeatedly emerged globally and more recently in western Europe. Within the UK, WNV is a notifiable disease in horses, and vaccines against the virus are commercially available. However, there has been no investigation into the seroprevalence of WNV in the UK equine population to determine the extent of vaccination or to provide evidence of recent infection.MethodsEquine serum samples were obtained from the Animal and Plant Health Agency’s equine testing service between August and November 2019. A total of 988 serum samples were selected for horses resident in South East England. WNV seroprevalence was determined using two enzyme-linked immunosorbent assays (ELISAs) to detect total flavivirus antibodies and WNV-specific immunoglobulin M (IgM) antibodies. Positive IgM results were investigated by contacting the submitting veterinarian to establish the clinical history or evidence of prior vaccination of the horses in question.ResultsWithin the cohort, 274 samples tested positive for flavivirus antibodies, of which two subsequently tested positive for WNV-specific IgM antibodies. The follow-up investigation established that both horses had been vaccinated prior to serum samples being drawn, which resulted in an IgM-positive response. All the samples that tested positive by competition ELISA were from horses set to be exported to countries where WNV is endemic. Consequently, the positive results were likely due to previous vaccination. In contrast, 714 samples were seronegative, indicating that the majority of the UK equine population may be susceptible to WNV infection.ConclusionsThere was no evidence for cryptic WNV infection in a cohort of horses sampled in England in 2019. All IgM-seropositive cases were due to vaccination; this should be noted for future epidemiological surveys in the event of a disease outbreak, as it is not possible to distinguish vaccinated from infected horses without knowledge of their clinical histories.Graphical
Highlights
West Nile virus (WNV) is the causative agent of West Nile fever in humans and encephalitis in both humans and horses [1, 2]
Of the 988 sera samples, 274 tested positive for flavivirus antibodies by competition ELISA (cELISA) (Fig. 1) giving a seroprevalence of 27.7% and highlighting that 72.3% of the cohort were seronegative and potentially susceptible to WNV infection
Two of the positive cELISA samples tested positive by the immunoglobulin M (IgM) enzymelinked immunosorbent assays (ELISA)
Summary
West Nile virus (WNV) (genus Flavivirus, family Flaviviridae) is the causative agent of West Nile fever in humans and encephalitis in both humans and horses [1, 2]. It is a single-stranded RNA virus which is maintained in a natural enzootic cycle between mosquito vectors and avian reservoirs. Several competent mosquito WNV vectors are not host specific and this can result in transmission to and disease in both humans and equines. One method for the early detection of WNV is the serosurveillance of sentinel horses, as reported from Spain [4], Germany [5] and Africa [6], for example. There has been no investigation into the seroprevalence of WNV in the UK equine population to determine the extent of vaccination or to provide evidence of recent infection
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