Prevalence of latent alpha-herpesviruses in Thoroughbred racing horses

The objective of this study was to detect and characterize latent equine herpes virus (EHV)-1 and -4 from the submandibular (SMLN) and bronchial lymph (BLN) nodes, as well as from the trigeminal ganglia (TG) of 70 racing Thoroughbred horses submitted for necropsy following sustaining serious musculoskeletal injuries while racing. A combination of nucleic acid precipitation and pre-amplification steps was used to increase analytical sensitivity. Tissues were deemed positive for latent EHV-1 and/or -4 infection when found PCR positive for the corresponding glycoprotein B (gB) gene in the absence of detectable late structural protein gene (gB gene) mRNA. The EHV-1 genotype was also determined using a discriminatory real-time PCR assay targeting the DNA polymerase gene (ORF 30). Eighteen (25.7%) and 58 (82.8%) horses were PCR positive for the gB gene of EHV-1 and -4, respectively, in at least one of the three tissues sampled. Twelve horses were dually infected with EHV-1 and -4, two carried a latent neurotropic strain of EHV-1, six carried a non-neurotropic genotype of EHV-1 and 10 were dually infected with neurotropic and non-neurotropic EHV-1. The distribution of latent EHV-1 and -4 infection varied in the samples, with the TG found to be most commonly infected. Overall, non-neurotropic strains were more frequently detected than neurotropic strains, supporting the general consensus that non-neurotropic strains are more prevalent in horse populations, and hence the uncommon occurrence of equine herpes myeloencephalopathy.