Horse with fever and diarrhea – could it be PHF?

It is late summer/early fall here in the United States.  The weather can quite variable – one minute its hot and sunny, and the next day it cold and blustery.  During this time of the year veterinarians in certain portions of the country will also start seeing an uptick in cases of horses with fever and/or diarrhea.  I think this would be an excellent time to discuss how your veterinarian might approach the diagnosis and treatment of your horse if it develops these symptoms.  I also think it is pertinent to explain why your vet might lean towards one diagnosis over another, depending on the season.  So, lets go through a typical case that I see during this time of year….

Later summer to early autumn is the classic time to see cases of Potomac Horse Fever in the United States.
Later summer to early autumn is the classic time to see cases of Potomac Horse Fever in the United States. Source:

(phone rings)

Horse Owner:  Hey Doc, I noticed that my mare did not finish her grain this morning and now she won’t even look at her food tonight.  She also is acting dumpy.

Me:  I’m sorry to hear that.  Have you seen her pass any feces?

Horse Owner:  Not yet.  She’s not really acting colicky though.  She just stands in her stall with her head in the corner.  Now that I’m watching her, I can see it looks like she is breathing fast and her nostrils are flaring.

Me:  Oh, well that sounds like a fever.  Can you take her temperature?

Horse Owner:  I don’t have a thermometer and I really don’t feel comfortable doing that.  Can you just come see her?

Me:  OK.  Just keep her in the stall, pull her food away, and keep water available.

(Later, at the barn)

Horse Owner:  I’m really worried about her Doc.  What do you think is going on?

Me:  Well she definitely is fighting something.  Her temperature is 105.0°, which is pretty high.  Anything over 101.5° is what I would consider a fever.  Her lung and heart sounds are normal, but their rate is increased.  Her gut sounds are very active, which means that colic is less likely.  I’m a bit concerned about her gum color though – its kind of pale with a dark purplish line just above the teeth.  It’s what we call a “toxic gum line”.

A "toxic" gum line in a horse refers to the darker pink to purple color above the teeth in contrast to the light pink above that. Source:
A “toxic” gum line in a horse refers to the darker pink to purple color above the teeth in contrast to the light pink above that.

Horse Owner:  Toxic gum line?!!  Do you think she ate some poison???!!

Me:  No, no, nothing like that.  The term “toxic” refers to endotoxins, which are toxins that normally are present in certain bacteria.  When the membrane that surrounds the bacteria is disrupted, then these toxins are released into the circulatory system.  The toxins can cause a lot of the clinical signs that your horse is exhibiting – fever, gum color abnormalities, and an elevated heart rate.

Horse Owner:  Do you think she has a bacterial infection?

Me:  I am not sure at this point.  Endotoxemia can be caused by bacteria that normally live inside the horse’s GI tract and release small amounts of endotoxins as they die naturally.  In a normal situation the GI tract has a nice barrier that prevents the endotoxins from getting into the bloodstream.  However,  if the GI tract gets inflamed or loses blood supply, then the endotoxins can get into the bloodstream.  They basically cause a massive inflammatory reaction because the body is trying to get rid of them.  So we can see these symptoms if anything causes the intestinal lining to become inflamed or lose blood supply.

Horse Owner:  So how do we treat this problem?

Me:  First we start by administering anti-inflammatory medication.  This will help decrease her fever and make her feel better.  It also is to prevent laminitis/founder – which is also a side effect of endotoxemia.  Second, I would like to take a blood sample and run a complete blood count and serum biochemistry panel.  This will give me an idea of the severity of the inflammation as well as tell me how her vital organs are functioning.

(The next day I call the owner.)

Me:  How is your horse doing today?

Horse Owner:  She still is pretty dumpy today and now I think she has diarrhea.  Its in her tail and on the stall walls.  Her temperature is normal though!

Me:  I had a feeling this would happen.  The results from the blood work came back showing very low white blood cell counts, which most likely means that her body is fighting an infection and the blood cells have moved out of circulation and to the area of infection.  Her protein and red blood cell counts are up, which means that she probably is also a bit dehydrated.  The good news is her kidneys and liver are functioning.

Horse Owner:  OK.  What kind of infection?

Me:  Based on the time of year (late summer/fall) and her clinical signs (fever followed by diarrhea), we most likely are dealing with Potomac Horse Fever.  According to our records it looks like we didn’t vaccinate for that this year.

Horse Owner:  Yeah, I know.  I was tight on money at that time so I skipped that vaccine.  Plus, I thought you said that the vaccine wasn’t going to protect her from it.

Me:  It’s true that the vaccine is not 100% effective, but horses that are vaccinated and then infected tend to have a less severe form of this disease, at least according to many equine vets out there (myself included).  So we do recommend that vaccine so that if a horse is infected they have a better chance of coming through the disease more quickly and with less side effects.

Horse Owner:  How did she get Potomac Horse Fever (PHF)?  None of my other horses are sick and she hasn’t traveled anywhere.

Me:  Potomac Horse Fever is a disease that is transmitted by the horse ingesting insects or bat feces that are infected with a bacteria called Neorikettsia risticii.  The life cycle is kind of complicated, but basically the bacteria live inside of a parasite (cercariae) that infects fresh-water snails.  When the weather is warm the parasite leaves the snail and gets eaten by the larvae of aquatic insects (May Fly, Caddis Fly, Dragonfly).  Inside the insect larvae, the parasite enters a different part of it’s life cycle and becomes a metacercariae, which is still housing the bacteria (N. risticii) inside of itself.

Potomac Horse Fever Life Cycle Source:
Potomac Horse Fever Life Cycle

If a horse accidentally ingests a metacercariae, the parasite will pass through the stomach untouched because it is acid resistant.  The parasite carries the bacteria all the way to the intestines of the horse.  The bacteria then infect the cells that line the intestines and cause massive damage to the intestines.  This is what causes the horse to have diarrhea.  Also, a large portion of the white blood cells leave the blood circulation and to attack the bacteria – which is why their counts are low.

Horse Owner:  So the horses get the bacteria from insects in the environment, not other horses?

Me:  Technically, a horse could get the bacteria from another horse if he ate large amounts of the infected horse’s feces, but that is highly unlikely.

Horse Owner:  What about humans or other animals?

Aquatic insects as seen above can carry the parasite that carries the bacteria that causes PHF.   Source:
Aquatic insects as seen above can carry the parasite that carries the bacteria that causes PHF.

Me:  Humans do not seem to get this disease.  There is some thought that bats may carry the disease, but it hasn’t been seen in other animals.

Horse Owner:  I thought horses had to be near water to get Potomac Horse Fever.  Is that not true?

Me:  Unfortunately that is not true.  Any place where there is enough fresh water for insect larvae to live will do.  This includes areas under stall mats, water tanks, etc.

Horse Owner:  So now what should we do?

Me:  Luckily there is an antibiotic that is highly effective for treating PHF – oxytetracycline.  Additionally I recommend some IV fluids to help with hydration, especially since she was already dehydrated last night.  We will keep her on the anti-inflammatory medication to control her fever and prevent laminitis.  Also, it would be a good idea to ice her feet to prevent laminitis.  Most horses make it through the fever and diarrhea just fine (with appropriate treatment).  The part that can be deadly is usually the laminitis.

Horse Owner:  Well I guess I can load her up and take her to stay with you guys at the hospital until she can come home.  Is there anything I can do to prevent my other horses from getting PHF?

Me:  Number one – vaccinate your horses before the warm weather gets here.  The best time is springtime, however in some areas of our country they recommend a booster in the fall.  Number two – practice insect control measures.  Limit places where standing water accumulates and keep the environment as clean and dry as possible.  You should also turn off lights at night to discourage bugs from hanging around the barn and/or sheds.

(End conversation)

I hope you all found this change of pace in my blog refreshing!  Again, please let me know if there are any topics of interest that you would like me to discuss.  Thank you!!


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

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:
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.

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  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.