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Testing Horses for Equine Herpesvirus Type 1 or Equine Herpes Myeloencephalopathy

The Cornell Animal Health Diagnostic Center (AHDC) offers several tests to diagnose clinically sick horses with the reportable disease Equine Herpesvirus Type 1 (EHV-1) and its neurologic form called Equine Herpes Myeloencephalopathy (EHM). Currently an outbreak of EHV-1 and EHM is occurring in the United States in horses that attended or were exposed to attendees of November 2025 events in Texas and Oklahoma. 

Veterinarians are encouraged to contact their State Animal Health Official if presented with a horse with acute neurologic disease. Diagnosis of EHV can be difficult as periods of viral shedding and viremia may vary - paired samples, including nasal swab and EDTA whole blood, are critical.  

EHV-1 Testing for Suspected Clinical Cases

The AHDC recommends submitting both EDTA whole blood in a lavender top tube as well as a nasal swab in a sterile non-additive tube (red or white top tube) with 0.5ml saline for EHV1 PCR.  Ship samples overnight in an insulated container with ice packs. Routine test turnaround is 1-2 business days. STAT testing is available M-F, with same day results if the AHDC STAT Testing Policy is followed. Contact the Diagnostic Center at (607) 253-3900 to determine availability of Saturday or holiday STAT testing. Post-mortem testing for EHV-1 can include EHV-1 PCR on nasal swab or CNS tissue, along with the collection of formalin fixed CNS tissue for histopathology. Detailed EHV1 PCR submission details can be found in the AHDC test and fee catalog.

The clinical signs of EHV-1 infection can include fever, acute respiratory signs, neurological signs and late-term abortion. The AHDC offers a variety of diagnostic plans and panels to comprehensively evaluate these clinical presentations, including:

EHV-1 Risk Evaluation Assay for assessment of EHV-1 immunity in healthy horses

The EHV-1 Risk Evaluation Assay quantifies special subsets of antibodies against EHV-1 in serum or plasma from horses and other equids to assess susceptibility to infection and clinical illness if exposed to EHV-1. It serves as a measure of protection against respiratory and neurologic illness by vaccine-induced or naturally acquired antibodies. The EHV-1 Risk Evaluation assay requires separated serum in a red top tube or a separated EDTA or heparinized plasma sample. Additional submission details are available in the AHDC test and fee catalog. Assay results can further be used to make informed decisions on EHV vaccination based on the current antibody results. Assay results can provide supportive information about the EHV-1 host immune status in EHV-1 outbreak situations in horses before they get exposed and during the first week after a potential exposure (EHV-1 antibodies take around 7-8 days to develop). Protection against abortion cannot currently be determined with this assay because the disease incubation period can be weeks or months prior to abortion. More information about this assay can be found on the AHDC website.

Additional Resources

Please visit the Equine Disease Communication Center (EDCC) for up-to-date outbreak information. For more information on the clinical signs, treatment, prevention and biosecurity recommendations for equine herpesvirus, visit the EDCC or American Association of Equine Practitioners website resources.  

Veterinarians are encouraged to contact the AHDC Veterinary Support Services (607-253-3900 or email vetsupport@cornell.edu) to discuss testing protocols.

Published Nov. 25, 2025