Equine Herpesvirus 1 – What you need to know when facing current outbreaks

Outbreaks of diseases are scary.  The disease can be a brand new virus, a bacterial “superbug”, a virus that has mutated, a parasite that has spread to a new geographic location – you get the idea.  The recipe for a contagious disease outbreak calls for two basic things:

Ok, we aren't talking about an outbreak of zombies, but horse disease outbreaks are scary too! Source:  inkace.com
Ok, we aren’t talking about an outbreak of zombies, but horse disease outbreaks are scary too!
Source: inkace.com

1) a susceptible population of living things (humans, animals, plants, etc) that do not have immunity to the pathogen

2) a pathogen (virus, bacteria, etc) that is easily transmitted between individuals it infects

The disease does not have to be deadly, its main goal is just to replicate in its host.  For instance, the flu virus is not often deadly but it transmitted very easily between individuals.  This ability allows the virus to find a new host before it is destroyed by the original host’s immune system.  The ease of transmission causes a lot of individuals to become sick in a short amount of time.  Additionally, if the pathogen can replicate in its host, and that individual becomes contagious before they realize they are sick, the disease has an extra advantage.

So how does all of this figure into the current outbreaks of Equine Herpesvirus-1 (EHV-1)?  Well, first I am going to discuss a little bit of background information on this virus (which is not a new virus, by the way).  Then I will delve into how it has evolved to cause current outbreaks of disease.  Finally, I will go over ways horse owners can prevent and prepare for an outbreak of EHV-1.

What is EHV-1?

Source: Gluck Center at University of Kentucky
An electron photomicrograph of EHV-1          Source: Gluck Center at University of Kentucky

EHV-1 stands for Equine Herpesvirus Type 1.  There are several types of Equine Herpesviruses but the main ones that cause serious health problems  in horses are Types 1, 3, and 4.  In this post I am only going to discuss EHV-Type 1.  EHV-1 can cause 4 different manifestations or clinical presentations:

1.  Neurologic – aka Equine Herpesvirus Myeloencephalopathy (EHM).  This form affects the nervous system (brain and spinal cord).

Just to be clear – EHM is a form of EHV-1 – not a separate disease.

2.  Respiratory – Affects the respiratory system (lungs, sinuses, trachea, nasal passages, etc.)

3.  Abortive – Affects the reproductive tract of a pregnant mare causing fetal death and subsequent abortion

4.  Neonatal – Newborn foal is born apparently normal but then becomes severely ill 1-2 days after birth.  Despite the best supportive care these foals often die from severe respiratory disease and it’s complications.

How does EHV-1 affect the horse?

The virus can affect 3 different types of body systems: neurologic, reproductive, and respiratory.  Once a horse is infected with the virus, the virus travels to the lymph nodes and infects certain types of white blood cells.  The infected cells are released out into circulatory system, where they travel in the blood to different sites in the body.  They can travel to the lungs, uterus, or spinal cord.

One key point here is that it is unknown why the virus attacks certain areas of the body.  There is research currently being performed that is attempting to figure out why an outbreak of neurologic disease will affect one group of horses, but another group may only show up with respiratory disease.  One recent finding is that there seems to be some genetic varieties of the virus that tend to cause the neurologic form more often, but not all cases of EHM are caused by these mutant varieties.

How is EHV-1 transmitted or spread?

EHV-1 has several of what I like to call “superpowers”.  The way it is transmitted is one of its superpowers.  Most of the time the virus is transmitted by horse-to-horse contact.  We all know how curious horses like to smell everything and everyone, especially new horses.  In fact, most horses introduce themselves by intensely smelling one other’s nose – it’s like they are smelling each others breath!  EHV-1 (cleverly) is easily transmitted through nasal secretions from one horse to another.

Horses naturally communicate with one another by touching noses and smelling.  EHV-1 has adapted so that it is transferred to a new horse when this happens.   Source: thehorse.com
Horses naturally communicate with one another by touching noses and smelling. EHV-1 has adapted so that it is transferred to a new horse when this happens.
Source: thehorse.com

However, EHV-1 has also evolved to be able to survive on the surfaces of objects that are contaminated by an infected horse (tack, human clothing, stalls, fences, etc).  And that’s not all – the virus can live (in the ideal conditions) up to 21 days on an object!  Furthermore, foals can be infected in utero and other horses can be infected by contact with an infected placenta or fluids from the mare.

So I can just keep my horse away from sick horses and he will be fine, right?

Wrong :(.  Another superpower of EHV-1 is that it can infect a horse and the horse can become contagious to others even before it shows physical symptoms of being ill.  By the time you see that the horse is sick he could have already infected all of his pasture mates.

A third superpower of EHV-1 is it’s ability to become latent (dormant).  A horse can be a “carrier” of this virus (a reservoir) and not show any outward abnormalities.  At some point, though, the virus can be reactivated in the carrier horse – who can now spread the disease to others.  Things like stress or a different illness can cause EHV-1 to be reactivated.  It is no wonder that  weaning, training at the racetrack, and traveling for shows are common times where we see outbreaks of EHV-1.

What about vaccines?

Vaccination is an excellent way to prevent spread of EHV-1 and has worked wonders for the breeding industry in preventing abortion outbreaks.  It also seems to protect against respiratory disease.  However, there is no current vaccination that is able to completely prevent the neurologic form of the disease.  This is a hot area of research and scientists are working on ways to improve the current vaccines or possibly manufacturing new vaccines to protect against EHM.

Source:  The Gluck Center at the University of Kentucky
Source: The Gluck Center at the University of Kentucky

EHV-1 has been around for a long time.  Why the big concern about it now?

The concern over the recent outbreaks has to do with the fact that EHM (caused by EHV-1) is an emerging disease.  The neurologic form of EHV-1 (EHM) used to be quite rare.  However, in the early 2000’s we started to see outbreaks of horses affected with EHM in various spots across the US and the world.  The virus has seemed to change in its severity and/or behavior in affected animals.  Furthermore, since vaccinations do not protect against this form of the virus, there is no way to systematically control it.  And remember how easily it is spread (its superpowers)?  The fact that the virus can spread to hundreds of horses without seeing a single horse look sick makes it a big cause for concern.

What are the clinical signs of EHV-1 and EHM?

Screen Shot 2015-04-24 at 10.59.00 AM

The signs listed in the tables above are seen with lots of diseases – not just EHV-1 or EHM!  One important characteristic is that your horse needs to have made contact with new horses or their bodily fluids, and that the onset of the neurologic signs is very fast.  Usually the signs show up and progress for 24-48 hours and then stop progression.  After that 48 hour period, the worst appears to be over and how bad each individual horse is affected will determine their prognosis.  If the horse goes down and can’t get back up then the prognosis is grave.  If the horse is only minimally affected then the odds are good that they will gradually recover.

What about treatment?

There is currently no specific treatment for EHV-1 and EHM.  Because they are viruses, antibiotics will not fight them.  However, sometimes horses are given antibiotics to prevent opportunistic bacterial infections because the horse’s immune system is working overtime on fighting the virus.

There is no specific treatment for EHV-1 and EHM, so we rely on supportive treatments (like IV fluids) assist the horse in it's recovery.
There is no specific treatment for EHV-1 and EHM, so we rely on supportive treatments (like IV fluids) assist the horse in its recovery.

The treatments your horse will receive are all supportive – meaning they are just supporting his basic bodily functions until his immune system takes care of the virus and healing takes place.  Examples of supportive treatments are IV fluids and anti-inflammatory drugs.

How can I prevent my horse from getting EHV-1 or EHM?

Even though the vaccine does not specifically protect against the neurologic form of EHV-1 (EHM), it is still a good idea to vaccinate your horse for EHV-1.  The vaccination does protect against the respiratory form and the abortive form of EHV-1, so your horse will be protected against those manifestations of this disease.  The vaccine does also seem to decrease viral shedding, so if your horse does become infected with the neurologic form you can help prevent shedding and infecting other horses with the virus.

Another was to prevent EHV-1 and EHM is to practice good biosecurity around the farm.  This means that any incoming horses are isolated from your herd for a period of at least 21 days.  That means that nothing that the isolated horse touches is allowed to touch anything that your herd horses may touch (tack, buckets, stalls, fences, etc).  Wash your hands between caring for your new horse and the rest of the herd.  Take the temperature of the new horse twice daily to make sure he does not spike a fever.  Isolating new horses protects against many diseases, including all the forms of Equine Herpesvirus and Strangles.

Horses that travel to shows or clinics should also be isolated from your herd when returning.  It is a good idea to take the temperature of your horse twice daily for 14 days before returning him to the herd.  If he develops a fever, call your vet.

There is a ton of information on the internet regarding what happens if your horse is at a show when there is an outbreak and various scenarios of the like.  Sources that are reputable include AAEP and APHIS.USDA.gov.  Another thing you can do is call your vet and see if they know about any current outbreaks in the area.  And it is never a bad idea to contact the shows and places you are traveling to see if they know of any outbreaks or precautions you should take to protect your horse.


The Dreaded “Down” Horse

I dread several types of veterinary cases every winter – cow with a prolapsed uterus, cow with dystocia (difficulty calving), horse lacerations (trying to suture in freezing temps is NOT fun).  However, one of the worst calls that I get begins with “Doc, I have a down horse.”  Ugh.  Let me explain why the “down horse” case is frustrating to every veterinarian.

FYI – A “down” horse refers to a horse that is recumbent (lying down) and can’t rise and stand up.

Reason #1: The cause is rarely clear-cut and easy to explain

Logically, most people want to know why there horse is down and can’t get up.  It may seem like a simple problem to figure out but in reality it is quite complex.  Sometimes the horse is laying down because they were colicking all night and they are simply exhausted and painful.  Colic can also lead to electrolyte imbalances because the horse typically sweats (losing water and electrolytes), does not drink, and loses electrolytes in the GI tract as it becomes inflamed and “leaky”. Examples of electrolytes that can cause severe symptoms and lead to the horse not being able to stand are potassium and calcium.

Vaccination against neurological diseases such as Rabies and West Nile Virus can help prevent dealing with a "down" horse.   Sourse: www.thehorse.com
Vaccination against neurological diseases such as Rabies and West Nile Virus can help prevent dealing with a “down” horse.
Source: http://www.thehorse.com

Trauma can lead to a “down” horse.  Examples of trauma include fractures of the leg and pelvic area, fractures of the neck and spine, and compression of the spine.  Certain diseases that affect the neurologic system (brain and spinal cord) also can cause a horse to be “down”.  Equine Protozoal Myeloencephalitis (EPM), Rabies, West Nile Virus, and Botulism are just a few examples of diseases that can affect the neurologic system and result in a down horse.  Nutrition is also a factor – hypoglycemia/low blood sugar (mostly in foals), vitamin E & selenium deficiencies, and general starvation or malnutrition can contribute or cause a horse to be unable to stand.  Finally, toxins in feed or plants are always a possibility.

As you can see, the wide variety of possibilities make determining the cause of a “down” horse make a very difficult job for your veterinarian.  Furthermore, testing and diagnosis of many of these diseases/disorders is costly, takes several days to weeks for results, and may not make a difference in the treatment and/or prognosis of your horse.

Reason #2:  The outcome is usually not great

Unfortunately most horses that are “down” have a serious medical problem(s).  Often the treatment of such problems involves intensive care and monitoring plus medications that need to be administered by a veterinarian.  Even with the best treatment many of these cases do not end well because of complications – pressure sores, anorexia, infection, etc.

"Down" horses often have chronic issues that may be difficult to treat and/or diagnose. Source: www.wholehorseveterinaryservices.com
“Down” horses often have chronic issues that may be difficult to treat and/or diagnose.
Source: http://www.wholehorseveterinaryservices.com

Many of the causes of a “down” horse are diseases or disorders that are chronic or insidious – meaning they do not show up overnight.  Things internally have been going downhill for some time and now have gotten to the so-called breaking point.  For instance, the majority of the cases I see involve old, thin horses in the winter.  As the horse ages multiple factors influence weight loss – teeth problems, metabolism and immune system changes, place in herd hierarchy, etc.  The horse may be fed large amounts of senior feed and high-quality hay, yet still is severely underweight (the topic of weight loss in horses is very complex and not going to be discussed in this article, but I want to make the point that many old horses are thin despite the best care from the owner).

A typical scenario is as follows: Old horse lays down to sleep for a bit but finds that it has difficulty getting up.  There may be ice on the ground or arthritis might make it hard to his legs underneath him.  The horse struggles to get up for hours, which causes him to burn up energy and sweat.  The sweat makes the horse colder and the body continues to burn through energy to warm the horse back up.  As the body runs out of fuel, the horse’s internal body temperature goes down and organs begin to cease functioning.  The pressure of the body on the cold, hard ground may cause damage to nerves on the down side of the horse, making it even harder to move.  The owner then finds the horse in this state and calls the vet.

At some point in this cascade there is not anything medicine can do to help the horse.  Since people typically can’t watch their horse 24 hours a day, horses often are found when the problem has been going on for some time (usually all night) and the horse is beyond help.  No amount of heat or food or anti-inflammatories will make the horse get up.  Which is why vets dislike to hear that a horse is “down”.  We hate bad outcomes and putting animals to “sleep”.

Reason #3: Treatment is complicated and costly

Most vets get nauseous when they have to start talking about the cost and difficulty of treating a patient, especially if they don’t have a clear-cut diagnosis.  When you have a horse that is “down” your vet will probably go through a list of possible causes and explain which are the most likely culprits.  They may suggest testing for specific diseases or disorders.  If they feel that there is a possibility to save the horse then they will most likely explain they type of care the horse will need to receive in order to survive.

Because horses are large animals with a heavy body mass they should not lie down for prolonged periods of time.  The mass of the body will start to put pressure on the skin, muscle, and nerves of the down side of the horse.  Also, if the horse is laying on one side the lungs can not function properly because they will not inflate evenly.  The lack of blood flow and oxygen to these tissues causes them to die off, which is termed “pressure necrosis”.  Pressure necrosis can cause skin ulcers (pressure ulcers or bed sores) and muscle or nerve damage.  Necrosis of large amounts of muscle or major nerves can result in permanent lameness or gait abnormalities.  To prevent the complications that develop when a horse is unable to stand, most horses are placed in a sling to take pressure off the body.

Care for the horse in a sling is complex and time-consuming. Source:  www.newenglandequine.com
Care for the horse in a sling is complex and time-consuming.
Source: http://www.newenglandequine.com

Taking care of a horse in a sling is no easy task.  They require frequent monitoring to make sure they do not get injured or caught up in the straps.  They must be checked vigilantly for pressure sores where the sling touches the body.  Intake of food and water have to be checked as well as fecal and urine output.  The sling should be taken off regularly which means that it also has to be put back on – not an easy task even if two people are working at it.  Most horses will receive IV fluids and antibiotics, and some will even have to be fed through an IV port.

Owners that decide they want to proceed with treating the down horse need to be prepared to have the horse transported to a facility that can properly care for such a patient.  Companies that specialize in equine rescue will have the equipment needed to transport the horse, however such transport may be costly.  Once the horse arrives at the hospital the veterinary team will determine the treatment plan and most likely come up with an estimate for the owner.  These cases can run upwards of $10,000, which is money many people do not have sitting around to spend.

Reason #4: The situation is traumatic for owners

As with any emergency or life/death situation, horse owners may panic when they find their horse down.  It is important to call your veterinarian when you find that your horse will not rise when prompted, but there are some things you may try before your vet arrives.  One task is to make sure the horse is not just  “cast” in the stall, meaning he is stuck against a wall and can not get his feet underneath himself.  A couple of articles have been written on how to “uncast” the horse – check out this article from horseandhound.co.uk or this one from California Thoroughbred.  If the horse is out in an open area and down then you can try rolling it to its other side.  Sometimes the horse has to be on its “good” side in order to rise, especially if the horse has arthritis.  You can download this Word Document at www.saveyourhorse.com/rollhorse.doc to see pictures and explanations of how to “roll” your horse over.  Once the horse is on its other side try prompting him again to get up – it may just work!

This horse is "cast" in the stall and needs to be rolled over in order to stand.   From:  horses-and-horse-information.com
This horse is “cast” in the stall and needs to be rolled over in order to stand.
From: horses-and-horse-information.com

However, if the horse does not get up, owners often continue to panic, which is understandable.  One minute the horse is fine, then the next it can’t get up – what could be going on?!?  The vet comes out and says “We are not exactly sure, but XYZ is probably the likely culprit.  We can run these tests but they cost $$$ and take a week to come back.  We can try XYZ treatment, but if that doesn’t work then you may have to make a decision.”  The owner thinks “Wait a minute….you are saying that you don’t know why my healthy normal horse won’t get up and want me to spend a bunch of money running tests (which also may tell us nothing), and you can try a couple of treatments on the farm but if they don’t work then I will have to put him to sleep??  He was fine yesterday!“.  The vet explains the costs of intensive care for horses in slings and the owner finds that there is no way they can afford that.  This leaves the owner in a very difficult position where they don’t feel comfortable putting their horse to sleep without giving him a chance, but they don’t want him to suffer.  They feel guilty for giving up on the horse and just don’t understand what could have happened to cause this situation.

On the other hand, there are owners who don’t know that prolonged laying down is not good for horses and they may leave the horse like that for hours or days.  Then the vet comes out and says they should have called earlier and now things have progressed too far so the horse has to be euthanized.  This owner also feels guilty and horrible because they honestly did not know that horses couldn’t survive laying down for days.

Do you see why I dread these cases so much?

Next week I will try and post something a bit more upbeat, but because I had to euthanize a down horse earlier this week I felt this topic needed to be discussed.  Have a great weekend!