Equine parapoxvirus: Re-emerging pathogen? Immune response and seroprevalence in horses in Finland

Parapoxviruses (PPVs) infect various animal species and humans worldwide. Equine parapoxvirus (EqPPV), first identified in one sick horse in Finland in 2013, was recently recognized as the sixth known PPV species. EqPPV is potentially zoonotic. During the winter of 2021−2022, EqPPV caused a widespread pastern dermatitis outbreak in racehorses in Finland, with earlier reports of similar, unconfirmed outbreaks in 2015 and 2019. This study aimed to develop a serological test for EqPPV and investigate the immune response and seroprevalence of the virus using both clinical and archived equine serum samples. We developed an immunofluorescent assay based on recombinant envelope proteins, and validated it with western blot analysis. The results showed that EqPPV provokes a rapid immune response within days of clinical signs. Two horses were tested one year after a PCR-verified EqPPV infection, and they retained stable IgG titres. Seroprevalence in archived samples peaked in 2015 and 2022, suggesting that EqPPV may have caused a previous outbreak. These findings imply that EqPPV is a re-emerging pathogen capable of causing significant epidemics.