Comparison of West Nile Virus infection rates in Culex tarsalis and exposure rate of horses in Montana using RT-PCR and ELISA

Abstract

Horses are more susceptible to West Nile virus (WNV) than human beings. In addition, for many ranchers in Montana, horses are their livelihood. In Montana, the only risk assessment tool for WNV is mosquito surveying. Testing horses across the state for WNV may contribute to a better estimate of high-risk areas. Also, since the numbers of human and horse cases are similar on a yearly basis, testing horses may predict the human risk. In this study, Culex tarsalis infection rates were compared with horse exposure rates in Montana. Mosquitos were collected from 46 sites on a weekly or biweekly basis. Collected mosquitoes were sorted, homogenized, and run through an RNA-extraction. WNV in mosquitoes was detected using RT-PCR. Horse serum was collected from the Helena area and analyzed for NS1 and envelope IgM and IgG antibodies to determine WNV exposure and/or vaccination. Three counties had positive Cx. tarsalis pools for WNV; the state infection rate was 0.108%. Two unvaccinated horses had positive IgM WNV antibodies. However, due to borderline results and vaccination contamination a horse exposure rate could not be calculated to make a comparison with the mosquito vector infection rate. Since horse positives occurred in an area where no positive pools were found, horse surveillance may be critical for detecting WNV hot zones.