EHV-1 – What Do I Need To Know RIGHT NOW?

Friday, May 20, 2011
Posted by Tanis MacDonald, DVM

Tanis MacDonald, DVM mantains a blog that is both well-written and informative and we are excited to be featuring many of her articles on our Horse Health section in the weeks and months to come.  She has a natural ability to take a seemingly complicated topic and break it down into easy to understand pieces suitable for equine enthusiasts of all ages and experience levels.  Today's article is one of several parts in relation to the EHV-1 Virus so be sure to check back often!!

(Part One) EHV-1 – What Do I Need To Know RIGHT NOW?

For the purposes of this post right this moment, I am not going to get into a lot of scientific details about Equine Herpes Virus 1 (EHV-1) – you can find the science with a Google search. What I am going to give you the down-and-dirty version of how it works and what you need to know RIGHT NOW to prevent this in your barn.
EHV-1 can cause 4 kinds of disease in a horse – but we are going to focus on the respiratory version and the neurologic version. In many horses that contract EHV-1 viruses, they spike high fevers, become lethargic, have clear runny noses and generally feel like crap. Many horses only get the respiratory version and clear the virus after a few days and have no further problems with it.

In some horses, and no one understands why they do this, an EHV-1 infection can become the neurologic form. Literally, a mutated form of the virus gets into the spinal fluid and causes a myeloencephalopathy – a severe viral brain and spinal fluid infection. This infection is what causes the horses to become suddenly neurologic.

The EHV-1 virus is RIDICULOUSLY contagious. It spreads via nasal secretions, and horses can contract it by touching the nose of another horse, through the air or from simply being in a barn that was previously occupied by a horse that was shedding the EHV-1 virus.

The current outbreak has been linked to a cutting horse show in UT and linked cases are now popping up in several other states. As such, several vet school hospitals are closed or not seeing emergency patients, many horse shows are being cancelled, etc.


The first step in prevention of EHV-1 is to take a deep breath, and STAY HOME. There is no such thing as an Emergency Horse Show. This is not the time to go visit your friends down the street with horses. This is not the time to load up and meet 20 different trailers at a trail ride and head out for the day. The BEST way to prevent EHV-1 exposure is to quarantine your horses and barn from other equine traffic.

If you have been in contact with horses associated with the Utah show, you should start to take temperatures on your horses twice a day. The fever spike is one of the very first signs that happens in horses. If your horse spikes a fever, you need to contact your equine vet IMMEDIATELY for further instructions and testing.


There is NO EHV-1 vaccine that is labelled for the prevention of the neurologic form of EHV-1. However, several are labelled for the prevention of the respiratory form. This is the “rhino vaccine” that your horse normally gets in its regular vaccine series. It is KEY to note that the rhino vaccines generally have a very short efficacy even under normal circumstances. If your horse is high risk, or has had exposure to horses or barns that are affected, even if your horse just had a recent “flu/rhino” vaccine in its spring series, it would be a good idea to consider boosting your horse with a rhino-only vaccine.

The three most effective rhino vaccines to use in the face of an outbreak are Rhinomune (modified live virus), Pneumabort-K (killed vaccine), Prodigy or Calvenza EHV.

I recommend that Rhinomune be used only in non-pregnant animals because it is a modified live vaccine. It should be noted that there is a small chance of a horse developing colicky symptoms secondary to the Rhinomune vaccine administration, so monitoring the horse after the vaccine is important.

Pneumabort-K, Prodigy and Calvenza EHV are killed vaccines that are labelled for pregnant mares, so is definitely safe for them. It can also be used in stallions, geldings, and non-pregnant mares as well with no problems as an off-label use. Pneumabort K can be a high-reaction vaccine, and often makes a lump in the muscle where it is given.

Again, the key here is to try to boost immunity to prevent infection in any form of EHV-1 so that the horse doesn’t develop the neurologic form.

The reason these vaccines are the ones recommended is because they are the most potent vaccines against EHV-1 – they contain the most dead or modified live EHV-1 virus per dose. In the face of an outbreak, it is important to boost those antibodies hard and fast and these vaccines can do that. If you cannot get ahold of either of these, the third choice would be to reboost one of the normal Flu/Rhino vaccines that are on the market.


The immune booster, Zylexis, is labelled for the treatment of the respiratory form of EHV-1. The idea is to help boost the horse’s natural immune system and make it stronger, therefore making it easier for the horse to fight the virus naturally. Zylexis is given on day 0, 2 and 9 as a IM injection.

Another immune booster is EqStim, which is similar to Zylexis only given IV instead.


Again, to reiterate, stay calm and stay home.

Quarantine your barn from unnecessary horse traffic. Do not travel with your horse unless under emergency circumstances.