Prevalence of latent, neuropathogenic equine herpesvirus-1 in the Thoroughbred broodmare population of central Kentucky

Summary Reasons for performing study: An emerging problem of equine herpesvirus‐1 (EHV‐1) infection in horses in the USA is a high‐mortality myeloencephalopathy that commonly occurs where large numbers of horses are stabled. EHV‐1 isolates recovered from recent neurological outbreaks represent a mutant virus strain that possesses enhanced neuropathogenicity. A central question of EHV‐1 myeloencephalopathy is the latency carriage rate for these mutants of EHV‐1 in USA horse populations. Objective: To estimate the prevalence of neuropathogenic strains of EHV‐1 as latent infections in the Thoroughbred broodmare population of central Kentucky. Methods: Submandibular lymph nodes (SMLN) were collected during post mortem examination of 132 Thoroughbred broodmares. Total DNA purified from SMLN tissue was tested for the presence of latent EHV‐1 DNA by an ultrasensitive magnetic bead‐based, sequence‐capture, nested PCR method. Differentiation of active from latent infections by EHV‐1 was achieved by detection of transcripts of EHV‐1 glycoprotein B by reverse transcription PCR. Results: Latent EHV‐1 DNA was detected in the SMLN tissues of 71 (54%) of the 132 mares submitted for necropsy. Thirteen (18%) of the 71 latently infected horses harboured the neuropathogenic biovar of EHV‐1. Of the 13 horses latently infected with an ORF30 mutant strain of EHV‐1, 11 also carried a latent, wild‐type strain of the virus in their SMLN tissues. Conclusions: Neuropathogenic strains of EHV‐1 have established a significant presence in the Thoroughbred broodmare population of central Kentucky as latently infected carrier horses. The data also indicate that a highly sensitive DNA detection method is required to identify many instances of EHV‐1 latency. Potential relevance: The presence of a relatively large biological reservoir of latent, neuropathogenic EHV‐1 has the potential for posing emerging equine health and economic threats to the future prosperity of the USA horse industry.