Equine Ulcerative Genital Lesions Associated with Parapoxvirus

In 2026, a widespread outbreak of ulcerative dermatitis of unknown aetiology occurred in breeding horses in Ireland. The lesions resembled those associated with equine herpesvirus 3 (EHV3) infection, i.e., papules, vesicles, and ulcers on the penis of stallions and on the vulval and anal areas of mares. This study aimed to identify the probable causative agent. The methods applied included metagenomic sequencing of DNA, phylogenetic analysis, and real-time PCR. Metagenomic analysis directly from a penile sample confirmed the presence of equine parapoxvirus (EqPPV) DNA. Phylogenetic analysis indicated that it clustered closely with EqPPV first identified in Finland in 2013 from a horse with proliferative dermatitis, and associated with outbreaks of pastern dermatitis in trotting racehorses in 2021/2022. Comparison of the amino acid sequences of the DNA polymerase gene showed that the EqPPVs from Finland and Ireland share 99% identity in contrast to 76–80% with other members of the parapoxvirus genus. A specific PCR test for EqPPV, adapted from that developed in Finland, successfully identified viral DNA in samples from 22 of 25 suspect cases that tested negative for EHV3. In conclusion, this study is the first documented outbreak of genital lesions in breeding horses associated with EqPPV.