Molecular prevalence of equine alphaherpesvirus-1 shedding in healthy broodmares in Ontario

Equine herpesvirus-1 (EHV-1) is ubiquitous in the horse population, but prevalence estimates have ranged from 3 to 88% depending on the population and method of sampling. No prevalence studies have been carried out in Ontario, Canada. The objective of this study was to measure the prevalence of EHV-1 shedding in healthy broodmares in Ontario. A total of 381 mares from 42 farms in Ontario were sampled, including pregnant and barren broodmares. Samples were collected from the nose, vagina, and blood of each mare up to 6 times from December 2016 through October 2017 using a cross-sectional study design. The EHV-1 glycoprotein B (gB) copy number was measured using droplet digital polymerase chain reaction (ddPCR). A survey was completed at time of sampling regarding signalment, pregnancy status, and vaccination. Overall, 85% of the mares sampled were positive for EHV-1 from at least one site, on at least one occasion. Samples were positive 8.1%, 15.8%, and 17.2% of the time from the nose, vagina, and white blood cells, respectively. Pregnant mares had increased odds of shedding virus from the nose [odds ratio (OR) = 1.50, 95% confidence interval (CI): 1.03 to 2.18, P = 0.037]. Vaccination only reduced the odds of virus presence in blood (OR = 0.70, 95% CI: 0.49 to 0.99, P = 0.043). Advanced gestation appeared to also have a sparing effect on virus presence in blood (OR = 0.89, 95% CI: 0.85 to 0.93, P < 0.001). Most mares in Ontario were positive for EHV-1 despite being healthy and vaccinated, although the amount of viral DNA detected was extremely low. In addition, the vagina was identified as a source of viral shedding.