Equine dermatitis outbreak associated with parapoxvirus in Finland

Parapoxviruses (PPV) cause skin and mucous membrane lesions in several animal species, and are often zoonotic. Equine PPV (EqPPV) is initially described in humans in the United States and later in a severely sick horse in Finland in 2013–2015. In 2021–2022, a large-scale pustulo-vesicular pastern dermatitis outbreak occurred in horses around Finland.
This study aimed to identify and analyze the causative agent, describe the disease, and prepare for future epidemics by studying disease transmission and developing diagnostics.
Samples to identify causative agent were taken from skin lesions of acutely ill horses during the outbreak. Epidemiological survey was conducted to gain understanding on the epidemiology, clinical signs, treatment, distribution, risk factors, and zoonotic potential.
Skin samples from 26 horses representing 11 stables were subjected to bacteriological, histological and virological analysis. Epidemiological survey was conducted online and horse farms in Finland were invited to participate.
EqPPV was identified as a probable cause of the epidemic and co-infections with several potentially pathogenic and zoonotic bacteria were observed. Almost complete virus genome with little sequence variation was sequenced and annotated from several samples. Histopathologically, suppurative and ulcerative dermatitis was diagnosed. Cases were reported in entire Finland. Having racehorses and contact with racecourse were significant risk factors for the infection. Skin symptoms consistent with PPV infection were reported in humans in case stables significantly more often than in control stables.
Samples from the humans taking care of sick horses and presenting typical skin signs for PPV infection were not obtained. Therefore, zoonotic potential of EqPPV could not be confirmed.
The disease spread quickly among racehorses in the entire country in winter 2021-2022. EqPPV had previously only been described in one horse in 2013 and had not been shown to cause epidemics, making it mostly unknown to racing community and clinicians. Reports of skin lesions in humans and zoonotic nature of other PPVs makes this disease a potential zoonosis, highlighting a need for OneHealth approach. The epidemic pinpointed a lack of preparedness to epidemics caused by new pathogens and this study, together with efficient communication, enables a better preparedness for future outbreaks.