Retrospective assessment of equine herpesvirus type 1 (EHV-1) genotype-associated clinical disease in Ontario.

Background: Equine herpesvirus type 1 (EHV-1) is an important viral pathogen in horses, associated with diverse clinical manifestations including fever, upper and lower respiratory disease, neurological signs, ocular disease, and abortion. Currently, EHV-1 variants (genotypes) are classified based on a single nucleotide polymorphism mutation of the open reading frame 30 (ORF30). Objective animals and procedure: The aim of this retrospective study was to evaluate the EHV-1 genotypes present in horses with various clinical presentations, including respiratory disease, abortion, and neurological disease, at a referral center in the province of Ontario. Results: Equine herpesvirus type 1 was detected in 11.7% of the tested horses, with clinical presentations including abortion (21%), myeloencephalopathy (64%), fever (11%), pneumonia (3.5%), and asymptomatic cases (3.5%). The neuropathogenic strain was more commonly present in horses with abortions and fevers. Equine herpesvirus myeloencephalopathy was associated with the non-neuropathogenic strain in 50% of the cases and the neuropathogenic strain in 44% of the cases. However, in 6% of the cases, the viral genotype was unknown. Conclusion and clinical relevance: These findings highlighted that this is a multifactorial problem and clinical disease may not be solely attributed to viral genome. This variability reinforced the need for continued research into the behavior and epidemiology of EHV-1.