11/20/2025
EQUINE HERPESVIRUS MYELOENCEPHALOPATHY (EHM) & EHV-1- FREQUENTLY ASKED QUESTIONS
1. How do we handle horses returning from events where they may have been exposed to EHV-1?
• These horses should be isolated from any other horses when they return to their home facility. Isolation requires housing them away from other horses, using different equipment to feed, clean and work with them than is used with non-isolated horses, and rigorous hygiene procedures for horse handlers (hand hygiene, wearing separate clothes when coming in contact with isolated horses, etc.). Please discuss this with your veterinarian.
• We strongly advise owners to call their veterinarian to discuss how long to keep the horses isolated at home. A minimum recommended isolation period is 21 days.
• These horses should have their temperature taken twice a day, as elevated temperature is typically the first and most common sign of infection. Horses with elevated re**al temperatures (greater than 101.5 F) should have a nasal swab and blood submitted by your veterinarian for EHV-1 PCR.
• If a horse develops a fever and is found to be shedding EHV-1, then the level of risk to other horses on the premises increases significantly. Affected farms should work closely with their veterinarian to manage the situation.
• The American Association of Equine Practitioners (AAEP) has an extensive set of Equine Herpesvirus (EHV) Control Guidelines that veterinarians can use as a resource.
2. What do we do if we already have a potentially exposed horse on a farm?
• The exposed horse should still be isolated, even if it may have already been in contact with other horses. Start isolation procedures to stop further exposure.
• It is very important to separate horses from different groups to accomplish this. Try to isolate the suspect horse without moving other horses from one group to another. Segregation of horse groups is the key, because this will help reduce spread if an outbreak starts.
• Check temperatures of all horses on the farm twice daily (fever spikes can be missed if you check once daily). If fevers are detected, then isolate the horse and test for EHV-1.
3. What anti-viral treatments can I use against EHM on a farm?
• If EHM is present on a farm, then the risk of other horses developing EHM at that farm is greatly increased. Stringent quarantine and biosecurity procedures must be implemented immediately.
• Treatment of horses with clinical neurological disease (EHM) is largely supportive—the use of anti-viral drugs is not known to be of value at this stage, but are often administered. Use of anti-inflammatory drugs and good nursing care with sling support if necessary is recommended.
• For horses that develop fever, test EHV-1 positive, or have been exposed to an EHM horse, anti-viral drugs may decrease the chance of developing EHM.
• Speak with your veterinarian regarding all medications and their suitability for your horse. We currently recommend Valacyclovir (Valtrex™) for prophylactic therapy at a dose of 30 mg/kg q 8 hr for two days, then 20 mg/kg q 12 hr for 1–2 weeks.
• The use of valacyclovir in horses that have already developed signs of EHM is questionable at this time. In that circumstance, the use of intravenous ganciclovir is preferable as it may have greater potency against the disease. The dose of ganciclovir is 2.5 mg/kg q 8 hr IV for one day then 2.5 mg/kg q 12 hr IV for one week.
• Administration of a zinc-containing supplements may be beneficial based on one epidemiologic study that found a decreased risk of EHM associated with owner-reported dietary zinc supplementation.
4. Is there any value to using booster vaccination against EHV-1 at this time?
• Unfortunately, there is not a licensed EHV-1 vaccination product with a label claim for prevention or control of EHM.
• EHV-1 vaccines have been shown to reduce nasal shedding and, in some cases, reduce viremia. These products may therefore have some theoretical value against EHM by reducing viremia, and certainly against spread of the virus by reducing viral shedding in the environment.
• If horses on the farm are previously vaccinated against EHV-1 then booster vaccination should quickly increase immunity, and perhaps reduce spread of EHV-1, if it is present.
• Vaccination in these circumstances is controversial, as some authorities speculate that immunity to EHV-1 may play a role in the development of EHM. While this is unproven, it remains a possibility. The use of vaccination is therefore a risk-based decision.
• Vaccination has no value as a treatment in affected horses.
Check the EDCC alerts page at https://equinediseasecc.org/alerts for updates regarding reported cases of EHM.
