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: absorbine.com

(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: ckequinehosiptal.com
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: ckequinehosiptal.com

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: threeoaksequine.com
Potomac Horse Fever Life Cycle
Source: threeoaksequine.com

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

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!!

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