Future studies must focus on building an understanding of the prevalence of equine coronavirus infections, according to scientists who identified a fatal outbreak in horses in Switzerland.
In humans, coronaviruses are mostly linked to respiratory and gut-related diseases.
Equine coronavirus was found in 2000 to be the cause of diarrhoea in foals. More recently, it has also been linked with fever and gut-related diseases in adult horses.
Researchers from the University of Bern and the University of Zurich, writing in the journal Transboundary and Emerging Diseases, described an outbreak of equine coronavirus disease in adult horses, detected under a voluntary surveillance scheme.
The scheme allowed rapid and concerted action to diagnose and contain the disease, Melanie Hierweger and her fellow researchers reported.
They became aware of a disease outbreak on a horse farm in the Canton of Bern in January 2021. The farm housed 26 adult horses.
The previous month, one of the horses had developed a fever and showed signs of severe respiratory disease. Therapeutic interventions were unsuccessful and the animal was euthanized. Postmortem investigations were not performed.
In the following weeks, six further horses showed fever and poor appetite, but in contrast to the first horse, they developed gut-related disease and/or a low white blood cell count.
One of the horses was taken to the equine hospital at the University of Bern with fever and diarrhoea. The animal showed signs of toxic shock, with a fast heart rate and rapid breathing, as well as congested mucous membranes and an increased capillary refill time. The animal died shortly after arrival.
Post-mortem investigations found severe, acute, necrotizing enterocolitis. Bacteriological tests ruled out infection with Salmonella species., Clostridium perfringens and Clostridium difficile, and examination of the horse’s dung ruled out parasites as the cause.
A second horse from the farm was taken to the hospital with lethargy, diarrhoea and poor appetite. This horse fully recovered with supportive treatment and was discharged after 10 days.
At this stage, the emerging picture seemed to correlate with what has been reported for equine coronavirus infections, with the exception of the first horse. Molecular-based testing of faecal samples from the two horses brought to the hospital were positive for the virus.
Consequently, faecal samples from all other horses on the farm – four affected animals and 19 in-contact horses – were similarly tested. One of the horses who had shown clinical signs and two symptom-free in-contact horses were positive for the virus.
Measures were taken to contain the disease, with isolation of ill and positive animals, daily body temperature measurements of all horses on the farm, and specific hygiene measures, such as the use of medical gloves, boot disinfection, dedicated equipment, and restriction of movement for four weeks. No additional clinical cases were seen.
The study team delved further into the virus through further analysis of pooled samples from affected animals. Gene sequences were found to have a common origin and belonged to the same strain, designated as Equine Coronavirus CH21. The CH21 strain is closely related to other equine coronavirus strains.
Collectively, the findings support Equine Coronavirus CH21 infection as the cause of the outbreak.
The authors noted that equine coronavirus-associated disease outbreaks have not been reported previously in Switzerland, and cases have been identified only sporadically in Europe.
The outbreak, with badly affected animals and fatalities during one of the peaks in the Covid-19 pandemic in humans, had resulted in great uncertainty and concerns in the equestrian community and wider public.
A clear and concerted communication strategy was implemented under the surveillance system to ensure the release of evidence-based information.
The scientists said they had verified full-genome equine coronavirus sequences and found no evidence of spill-over infection from humans to horses or viral recombination events with SARS-CoV-2, the virus that causes Covid 19.
“Rather we found that Equine Coronavirus CH21 is highly similar to previously described equine coronavirus strains.”
The authors said the disease that affected the first horse was likely unrelated to equine coronavirus.
Testing in affected horses up to two weeks after convalescence and in healthy in-contact animals confirmed that prolonged viral shedding and subclinical infections do occur, corroborating previous reports. “This,” they said, “is an important finding and has vital implications on equine coronavirus disease control measures.
Collectively, they said, the findings show the value of surveillance schemes for infectious equine diseases, since they allow fast action to contain the outbreak.
“This study was able to characterize the genetic makeup of the equine coronavirus strain, but future studies must focus on developing a better understanding of the prevalence of equine coronavirus infections and associated disease pathogenesis.”
The study received funding from the Swiss Federal Food Safety and Veterinary Office and the Berne University Research Foundation.
Hierweger, M. M., Remy-Wohlfender, F., Franzen, J., Koch, M. C., Blau, D., Schoster, A., Nicholson, P., Gerber, V., Gurtner, C., Fouché, N., Unger, L., & Seuberlich, T. (2022). Outbreak of equine coronavirus disease in adult horses, Switzerland 2021. Transboundary and Emerging Diseases, 69, 1691– 1694. doi.org/10.1111/tbed.14501
The study, published under a Creative Commons License, can be read here.