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Swiss researchers have identified a novel virus in
the brains of four horses that were
euthanased after developing neurological
disease, prompting investigations into whether the virus may represent an
emerging cause of equine central nervous system (CNS) disease.
Since late July 2026, equine referral hospitals at the Universities of Bern and Zurich have seen an increase in horses presenting with neurological signs including ataxia, muscle fasciculations, hypersensitivity and lethargy. Extensive diagnostic investigations failed to identify recognised infectious causes of neurological disease in many of these cases.
Researchers at the Swiss Institute of Virology and Immunology (IVI), working with the Vetsuisse Faculty at the University of Bern, subsequently detected genetic material from a Shamonda-like virus, an Orthobunyavirus in the Simbu serogroup, in brain tissue collected from four affected horses following post-mortem examination. Three cases originated from Zurich and one from Bern. This is the first reported detection of a Simbu serogroup Orthobunyavirus in horses in Europe.
The virus has been confirmed in post-mortem brain tissue from four horses with neurological disease. It has not yet been found in blood or cerebrospinal fluid from affected live horses, so ante-mortem diagnosis is currently not possible. Antibody testing can show previous exposure, but cannot confirm that the virus caused the neurological disease. There is currently no specific treatment, and affected horses receive supportive care.
Like other members of the Simbu serogroup, the virus is believed to be transmitted by Culicoides biting midges. Related viruses, including Schmallenberg virus, are well recognised in European livestock.
At present, there is no evidence that the Shamonda-like virus causes neurological disease in horses. Although its detection in affected brain tissue makes it a plausible candidate, researchers have not established a causal relationship. Further studies are needed to determine whether the virus is responsible for the recent increase in neurological cases in Switzerland or whether its presence is incidental.
Veterinarians investigating horses with neurological signs should therefore continue to consider established differential diagnoses, including equine herpesvirus (EHV), West Nile virus (WNV) and tick-borne encephalitis, while research into the Shamonda-like virus continues.
The UK's Equine Infectious Disease Surveillance (EIDS), based at the University of Cambridge, reports that there is currently no evidence of Shamonda-like virus-associated disease in horses in the UK. However, because infected Culicoides midges may be transported long distances by wind, and horses continue to move between mainland Europe and the UK, the situation is being monitored closely.
For more details, see:
https://www.ivi.admin.ch/en/neues-orthobunyavirus-der-simbu-serogruppe-bei-rindern-entdeckte
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