EHV-1 Outbreak Myths Debunked

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 Four of a series of posts in relation to the EHV-1 Virus so be sure to check back often!!

(Part Four) Myth #1 – OMG! This is a mutant strain of the virus and there is no vaccine!
Truth – No one knows yet if this is an EHV-1 virus that has undergone some kind of mutation. It is also not known yet if it is the same strain that was isolated in the Findlay, OH outbreak in 2003. Right now, there are several labs at universities in the US working on trying to grow the current virus and identify it.
There are several vaccines on the market that induce immunity to EHV-1. These vaccines are labeled to prevent the respiratory and abortion forms of the rhinovirus. It is true that they are NOT labeled to prevent the neurologic form of EHV-1. However, most horses start with the respiratory form and then, in some horses, it can become the neurologic form. Doing what you can to induce immunity to the respiratory form by vaccinating makes good sense. Look back at previous blog entries for the vaccines that are currently being recommended.

The word “mutant” is probably a strong term for most people to really understand. Rhinoviruses, like flu viruses, change and adapt, which makes it harder for the immune system to pick up on them when they get in the body. This possibly is what makes them able to get across the blood-brain barrier and cause the neurologic infection. This is why EHV-1 persists and infect so many horses year after year. In the viral world, yes, we call these changes mutations, but it isn’t like it is some crazy HUGE change in the virus that has never been seen before, or a virus from some other planet. Even very small changes to just one protein on the outside envelope technically labels a virus as mutated.

Myth #2 – OMG! If my horse gets this virus it is gonna DIE!  
– Take a deeeeeeeep breath.  Veterinary medicine has come a LONG way in a very short few years in the rapid diagnosis and treatment of EHV-1.

We have diligent owners and trainers who are taking temperatures twice a day looking for the fever spike in a possibly infected horse, hoping to catch it early if a horse presents with a fever. If we can identify it early, medications can be used to possibly prevent the horse from getting the neurologic form. It should be noted that there is no *specific* anti-EHV-1 treatment available, most medications used are in response to the symptoms that the horse shows.

We now have a great nasal swab test that can quickly determine if a feverish horse is shedding the virus. We have many drugs that can be used to help to manage the clinical signs and antiviral to use early in the disease process to try to prevent the neurologic form.

We have made great strides in equine intensive care facilities – they are able to provide slings for horses that cannot stand and IV fluids and nutrition for horses that cannot drink or eat, urinary catheters for those who cannot urinate, helmets to prevent head injuries when they fall, and more! Often, it is these things that are the hardest to manage in a down or unstable horse. The medications are the easy part! If the horse can be supported systemically through the neurologic crisis, it may survive EHV-1.

It is true that many horses do succumb to the neurologic form of EHV-1 regardless of treatment they receive. It should be noted that euthanasia is often chosen because of the management/ safety issue – it is just REALLY hard and unsafe to manage a down or unstable neurologic horse in a normal barn environment without a lot of equipment and knowledgeable personnel to work with the horse.

Economics also plays a huge factor in euthanasia rates of EHV-1 horses. It is simply VERY expensive for owners to treat a neurologic horse with ANY disease, let alone EHV-1. The amount of round-the-clock care and drugs needed is phenominal.

Myth #3 – The virus can spread through the air for miles, and lives in the dirt for years! I’m never going anywhere again with my horses!
– EHV-1 viruses are actually pretty weak viruses in the grand scheme of things. While they are spread by nasal secretion contact and can go through the air, it will usually not travel very far before it is inactivated. Estimates are about 50 feet before it is inactivated.

A recently infected horse will normally shed EHV-1 via nasal secretions for about 10 days or so. We know EHV-1 can live in the environment on surfaces for short periods of time, but it has not really been determined what that time frame is – I have read estimates from 7 days all the way to 35 days. Common disinfectants will kill the virus on skin, surfaces and equipment.

Myth #4 – It came from a horse in Canada. Damn Canadians.
– It may have. Or it may have come from ANY other horse on the grounds at the Ogden, UT. It very well may have started with a horse that wasn’t sick at all!

EHV-1 is a virus that has been around for a long, long time in the US and Canada in most populations of horses. Horses normally get exposed to EHV-1 viruses early in life, get a bit of a respiratory infection, get over the disease and have no further problems.

Some horses become what is known as persistently infected with the rhinovirus. They become intermittent shedders of EHV-1. They harbor the virus in their respiratory tracts for years. Then during times of immune system stress, they will shed EHV-1 virus through nasal secretions, and then infect others. This is what keeps the EHV-1 virus going year after year, and causing repeated outbreak problems all over the continent.

It is really hard to test horses to see if they are persistently infected with EHV-1, because they only shed the virus during very short periods of time. It’s hard to time the testing – which requires a nasal swab of infected nasal secretions to diagnose.

While it is likely it has started with a horse at the Ogden, UT cutting show last week, it would be nearly impossible to determine which horse was the sentinel (first) horse to shed the virus.

For the record, I’m Canadian. We are not all bad. ; – )

Photo Credit: Ruhlmann by Flashpoint Photography