Transmission of EHV-1 by Horses with EHV-1 Myeloencephalopathy: Implications for Biosecurity and Review

Although myeloencehalopathy due to Equine Herpesvirus-1 occurs infrequently, multiple horses may be affected during an outbreak and mortality may be high. In January of 2003, six horses from a large outbreak of EHV-1 respiratory and neurological disease were referred to The Ohio State University Veterinary Teaching Hospital for treatment of incapacitating myeloencephalopathy. Following introduction of these horses, we documented transmission of EHV-1 to five bystander horses in our hospital and indirectly to a sixth horse at a distant site. We concluded that one or more of the horses from the outbreak was shedding EHV-1 while displaying signs of myeloencephalopathy. Our experiences demonstrated the erroneous nature of the commonly held assumption that horses with EHV-1 myeloencephalopathy are no longer shedding virus. Horses with EHV-1 myeloencephalopathy should be strictly quarantined. Vigorous biosecurity measures should be instituted, including quarantine of the affected premises for a minimum of 21 days following the last clinical evidence of active EHV-1 disease.