EHV-1 – What It Is, How It Spreads

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 Part Two of several articles in relation to the EHV-1 Virus so be sure to check back often!!

(Part Two) EHV-1 is a virus. It is Equine Herpes Virus Type 1. It has been around in the US and Canada for a LONG time. It is NOTHING new. It is what is known as the “rhino virus” or “rhinopneuomitis” – they are the same thing. It is NOT a flu virus.
EHV-1 can cause 4 kinds of disease, but really we are concerned with 2 kinds at this point – the respiratory and the neurological. What happens is the EHV-1 virus is shedded from an infected horse via nasal secretions. It is probably coming from a horse that has been an EHV-1 carrier its whole life. It is hard to know what horses are carriers and what horses are not because they do not shed the virus all the time, so it makes it hard to test for it.

EHV-1 viruses change frequently, and what happens is a mutated version is shedded and causes a high fever respiratory disease in some of the horses that it contacts. It could be that the horse has been harboring the mutated version for a long time and never was sick, or it could be that they have a “wild” form of the EHV-1 virus that mutated in that horse and then they shed it. This part is not understood.

EHV-1 is spread from horse to horse via nasal secretions – so through the air, nose to nose contact, being in a stall previously occupied by a shedder, hands and clothing of people who handle or work with horses.

The respiratory form of EHV-1 starts with a high fever, clear runny nose, off feed, lethargy and generalized feelings of sickness. MOST horses can be treated symptomatically, will clear the EHV-1 virus within a few days, recover, and have no other problems.

In a small number of horses, what happens is the mutated virus gets into the brain and spinal canal, causing the neurologic form of EHV-1. No one understands why it happens in some horses and not others. Some horses can be infected with a mutated EHV-1 virus and not be sick at all! Horses that get the neurologic form can now be treated with antivirals and other drugs, but it is harder for them to recover and most will succumb to the neurologic form disease.

There are cases of the neurologic form of EHV-1 reported across the US every year, and in Canada as well, but most of the time it seems to be within racehorse populations, so that is why the stock breed community doesn’t hear much about it. It’s kind of a shock to you all right now, but for me, coming from a Standardbred and TB racing world here in DE, I hear about outbreaks frequently.

Quarantine and good biosecurity practices are the number one key in preventing the spread of EHV-1. If you don’t have any exposure now, and don’t have to travel with your horse, this is the time to stay home.

The EHV-1 vaccines that are currently available are only labeled for the respiratory and abortion forms (this is one of the other forms of EHV-1). HOWEVER, the idea is that with current vaccines, if we can get a horse to build good antibodies to rhinoviruses in general, then hopefully it will help protect the horse from getting infected with EHV-1 at all, and possibly protect against the neuro version.

The current vaccines suggested for use during an outbreak are Pneumabort K, Prodigy, Calvenza EHV or Rhinomune. All three are what is known as high antigen load vaccines – in other words, they are CHOCK FULL of dead or modified live EHV-1. It is important to know that Rhinomune is a modified live virus vaccine and I do not recommend it be used in a pregnant mare. However, even though the other 3 are specifically labeled for broodmares, they can be used in all other populations of horses – stallions, geldings, show horses, babies, minis, donkeys, etc.

Another product that is labelled for protection against the respiratory form of EHV-1 is Zylexis. It is an immune booster – the idea being that you stimulate the horse’s immune system to “wake up” and be on high alert for incoming disease. It is given IM in 3 doses on Days 0, 2 and 9.

The normal quarantine period is about 14 days for EHV-1 to stay alive in the environment. This is why every time a new case pops up in a barn, the quarantine period resets to 14 days. Some vet facilities and state vets use 21 days as a precaution to make sure it is unlikely that any live virus is still in the environment.