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Questions & Answers About Equine Herpes Virus

Quarter Horse News interviewed Dr. Paul Morley, DVM, PhD, Diplomate American College of Veterinary Internal Medicine and director of Hospital Bio-security at Colorado State University, about the equine herpes virus.

What is the incubation period for the virus:

That is a difficult question to answer for the herpes virus. It is a class of virus where it causes a latent infection, which means that once exposed, the virus is maintained in the cells of the host. The particular herpes virus that we’re talking about in this particular outbreak is herpes virus 1 is closely related to the cold sore virus in people, herpes simplex; if you’ve had a cold sore before then, you can get it back again.

Horses are infected with EHV-1 and EHV-4 and other herpes viruses very early in life. Research would suggest that all horses are infected before they are 18 months of age. At the same time, horses are re-exposed throughout their life, so it is hard to say whether they got it from recrudescence – like when a cold sore comes back or if they were acutely exposed. There is some thought that horses show signs more likely from an acute exposure in those circumstances, typically under experimental conditions, where you take horses and give them the virus, the earliest that you would see signs is from 12-48 hours.

What is the latest time that you would see signs of infection?

It’s very hard to say because all horses are latently affected.

How stable is the virus?

It doesn’t die immediately after being shed by the horse, but the virus is not stable. It’s an envelope virus, which means that it is very susceptible to drying. In general, heat, drying and UV light all will kill the virus relatively quickly.

How is it spread?

Herpes virus, although they can be spread by respiratory secretions like coughing, the more common way herpes virus is spread is by contact, either direct nose-to-nose contact, someone’s hand or a shared feed bucket. Use common sense about separating horses and washing hands.

Can the virus be transferred by people to different horses?

If you went in to a horse’s stall and the horse nuzzled you and you got some nasal secretions on your shirt and then you went in to the next stall and your other horse did the same thing you could easily transmit it like that.

What precautions should you take?

Wash your hands before and after you handle the horse. Don’t use the same grooming tools, the same tack or share buckets with different horses – particularly not share with different stables. In a situation like this you wouldn’t want to allow nose-to-nose contact amongst your horses particularly if it’s with a different group of horses. If you are bringing new horses on to a premises, regardless of whether they’re your own horses, or someone else’s, for example if you take your horse to a show and then bring it home we would recommend that you keep it separate to the other horses on the premises.

Take your horse’s temperatures twice a day. Fever is one of the first signs we see in equine herpes virus infections so taking temperatures is and easy and good tool for monitoring what’s going on. Be concerned if your horse is over 101.5F-102F. Call your veterinarian immediately if they are.

Could an infected horse in the loping pen transmit it to another horse that’s warming up?

It’s possible with herpes virus but transmission is more likely through contact. The greater the distance between the horses the less likely it is – it’s the same as if you had an individual cough right in to your face then you’d be more likely to catch it then if you were 20 feet away.