Frequently Asked Questions Concerning EHV-1

Colorado State University has released some answers to frequently asked questions about EHV-1 and how to treat it.

Frequently Asked Questions
Information from Colorado State University

1. How do we handle horses returning from events where they might have been exposed to this infection?

For horses that might have been exposed to the risk of infection, there are some steps to take to mitigate the risk at their home facility. Even if these horses are returning home from events at which no disease was reported, and even if these horses appear healthy, precautions are needed at this time as these horses could bring it home and spread it at their home farm – this is the classic way this disease spreads:

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 that is used with any other horses, and rigorous hygiene procedures for horse handlers (hand hygiene, wearing separate clothes when contacting the horses, etc.). Please discuss this with your veterinarian.

We strongly advise owners to call their vets to discuss how long to keep the horses isolated at home, but even if they don't develop fevers this should be at least 14-21 days.

These horses should have their temperature taken twice a day, as temperature is typically the first and most common sign of infection – horses with elevated temperatures (101.5 degrees Fahrenheit or greater) should be swabbed by your vet to find out whether they are shedding EHV-1.

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. Those affected farms should work closely with their veterinarian to manage that situation, if it develops. 
2. What do we do if we already have a potentially exposed horse on a farm?

It still makes sense to isolate this horse from other horses. Even though it might have already been in contact with them, start isolation procedures to stop further exposure. It is important to not mix 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 you 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 test for EHV-1.

The value of starting healthy horses on anti-viral treatment when there is no evidence of disease on the farm is questionable. The treatment is expensive, the drug (Valtrex - valacyclovir) may have limited availability, and prophylactic therapy against equine herpesvirus myeloencephalopathy will only work while drug is being administered. Therefore it is more likely to be effective if administered when fever is first detected.

3. What anti-­viral treatments can I use against equine herpesvirus myeloencephalopathy on a farm?

If EHM is present on a farm, then the risk to other horses 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. Use of anti-inflammatory drugs is recommended: flunixin meglumine (0.5 to 1 mg/kg, IV, q 24 hours).

For horses on the farm that develop fever, test EHV-1 positive or have a high risk of exposure, anti-viral drugs may decrease the chance of developing EHM.

Currently, the treatment of choice in a febrile EHV-1-infected horse to prevent the development of EHM is valacyclovir (Valtrex), given orally. The use of oral acyclovir is unlikely to be of any value, as it is not absorbed from the GI tract.

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 one to two weeks). Maintain on higher dose rate if the horse is still febrile. This is an expensive drug, and daily treatment costs can typically be $20-$300 per day. Generic forms of valacyclovir might be available and might be marginally cheaper.

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 might 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.)

4. Is there any value to using booster vaccinations against EHV-1 at this time?

Unfortunately, there is no evidence at this time that current EHV-1 vaccines can prevent EHM.

The more potent EHV-1 vaccines have been shown to reduce nasal shedding and in some cases reduce viremia. These products might have some theoretical value against EHM (by reducing viremia), and certainly against spread of the virus.

The more potent EHV-1 vaccines include: Rhinomune, or Calvenza EHV, Boehringer Animal Health; Pneumabort-K, Pfizer Animal Health; Prodigy Intervet Schering-Plough Animal Health.

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. Additional sources of information are listed below. Until we know more about this outbreak, caution is recommended at all times to reduce the spread of infection. Movement of horses on and off farms should be limited whenever possible.