A severe equine herpesvirus type 1 (EHV-1) abortion outbreak caused by a neuropathogenic strain at a breeding farm in northern Germany

A particularly severe equine herpesvirus type 1 (EHV-1) abortion outbreak occurred at a breeding farm in northern Germany. Sixteen of 25 pregnant mares that had received regular vaccination using an inactivated vaccine aborted and two gave birth to weak non-viable foals in a span of three months, with 89% of cases occurring within 40 days after the initial abortion case. Virological examinations revealed the presence of EHV-1 in all cases of abortion and serological follow-up in mares confirmed recent infection. Molecular studies identified a neuropathogenic variant (Pol/ORF30 A2254 to G2254) that belonged to geographical group 4 of EHV-1 isolates. The abortion outbreak was preceded by a case of mild ataxia of unknown cause in a mare that aborted four months after the ataxic episode. Although vaccination of pregnant mares did not prevent abortion, good EHV-1 immune status of the population at the time of outbreak may have had an impact in the failure of manifestation of the neurological form of the disease.