Misunderstood, Misused, & Misdiagnosed Disease #2: Lyme Disease

I hope you enjoyed reading about Misunderstood, Misused, & Misdiagnosed Disease #1: EPM.  In that post I explained how some horse enthusiasts (trainers, owners, etc) have used this disease to explain any number of abnormalities that their horse is exhibiting.  EPM is quite a convenient disease to blame for oddities because it affects horses in so many different ways.

Another disease that I have found that is easy to over-use as a diagnosis is Lyme Disease.  In this post I would like to briefly explain what Lyme disease is, why it is easy to misuse as a diagnosis, and how a veterinarian evaluates a horse to determine if the Lyme Disease is responsible for the clinical abnormalities that the horse exhibits.  I will not get into the complex pathology and treatment options of Lyme Disease in this post.


Lyme Disease is one of several nasty diseases that can affect multiple species, including humans, dogs, and horses.  It is caused by spirochete bacteria named Borrelia burgdorferi (Bb).  These spiral-shaped bacteria are transmitted to humans and animals by species of Ixodes ticks.  The deer tick or blacklegged tick (Ixodes scapularis) transmits Lyme Disease to humans and animals on the east coast and north central United States, while the western blacklegged tick (Ixodes pacificus) is responsible for cases on the west coast of the US.

The Blacklegged Tick can carry Bb bacteria and typically infects animals and humans on the east coast and upper midwest of the US.  Source:  cdc.gov/lyme/
The Western Blacklegged Tick (Ixodes pacificus) can transmit Lyme Disease to animals and humans in the western US.  Source: cdc.gov/ticks/







A tick becomes a host for Bb when it sucks blood from an infected animal or human.  That tick can infect others when it attaches to another being and begins to feed on them.  Ticks salivate excessively during feeding, and the bacteria are present in the saliva.  Usually the tick needs to feed for over 24 hours before the Bb bacteria are transmitted to the host.  The longer the tick feeds, the greater the chance that Bb bacteria will be transmitted to the host.

Once transmitted, the Bb bacteria will be attacked by the body’s immune system.  This attack is what causes the symptoms that are present when an animal has Lyme Disease.  Symptoms/clinical signs are different for humans versus dogs versus horses.  For the purpose of this post, we will only focus on horses.

Now I’d like to focus on what makes Lyme Disease a Misused, Misunderstood, and Misdiagnosed disease.

#1 – Large Variety of Clinical Signs

Below is a graphic listing some of the clinical signs that have been seen in horses with Lyme Disease.


Important factors to consider:

  • Not every horse infected with Lyme Disease will develop all of these signs.
  • Several of these symptoms are difficult to recognize (low-grade fever, skin sensitivity).

“For some reason humans like to have one explanation for everything, so our minds will want to look for one disease that causes all of these problems.  Lyme Disease is such an easy way to explain all of these issues and it is treatable!  However, things are not usually that simple.”

#2 – Many other (and sometimes more common) diseases show similar clinical signs

Lets look at a few examples.

Case 1:  A performance horse has had chronic weight loss, poor performance, and some behavior changes (lethargy)  over the past 6 months.  Based on the clinical signs listed above it seems like the horse is a shoe-in for Lyme Disease diagnosis.  However, as we gain some more history on the horse we find that it is a nervous horse and many times will not eat his grain.  Based on this information, the vet decides to perform endoscopy and look at the stomach for evidence of ulcers.  In this case it is highly more likely that gastric ulcers are causing the horse to not eat, lose weight, and become more lethargic.

Case 2:  An older horse is a pasture ornament for 90% of the time, but sometimes the owner really feels like taking him camping and riding for 3-4 days over rugged terrain.  The horse always is sore and lame after the first day, but it is a different leg each time.  He seems fine after getting home and spending some time resting though.  Again – Lyme Disease fits right in with a reasonable diagnosis.  But after using some common sense, we can assume that the owner needs to keep her horse in shape and not expect him to be a weekend warrior.

Muscle soreness is a common clinical sign for horses with Lyme Disease, as reported by veterinarians. However, there are many other more common causes for muscle soreness that should be ruled out first before jumping to Lyme Disease. Source: equisearch.com

There are myriads of issues that can have the same symptoms as Lyme Disease, and sometimes more than one problem can be happening at once.  For example, a mare might be in heat (which causes behavior changes and hypersensitivity) and have a injured tendon (causing lameness and swelling).  For some reason humans like to have one explanation for everything, so our minds will want to look for one disease that causes all of these problems.  Lyme Disease is such an easy way to explain all of these issues and it is treatable!  However, things are not usually that simple.

# 3:  Lyme Disease is not easy to definitively diagnose

Like EPM, a diagnosis of Lyme Disease is not simply a test with a positive or negative result.  To reach a diagnosis of Lyme Disease, the vet has to use deductive reasoning and their experience.  For an example I would like to point to a graphic used by veterinarians when trying to make a diagnosis of Lyme Disease in a dog.

When a dog presents to a vet with lameness and fever, they must use powers of deductive reasoning to determine the diagnosis of Lyme Disease because there are many other disorders that cause lameness and fever.  The same goes when examining horses.  Source:  ACVIM Consensus Statement, Journal of Veterinary Internal Medicine 2006.

When a vet is presented with a horse that is a Lyme Disease suspect (is showing appropriate clinical signs) they must ask themselves several questions:

  1.  Does the horse live or has it been to an area where ticks that carry Bb are present or endemic?
  2. Have ticks been found on the horse?
  3. Have other disorders that cause similar signs be ruled out?

If the answer(s) is(are) yes to those questions, then the next step is to draw a blood sample and send it in for testing.  Usually the test is looking for the presence of antibodies to Bb present in the horse’s blood.  The presence of the antibodies indicate that the horse has been EXPOSED to the Bb bacteria.  It does not tell us if the horse currently is infected and fighting Lyme Disease.  For that answer we have to look at the number and type of antibodies present in the blood.

Large numbers of antibodies in the blood typically indicate an active infection, but these numbers are all relative.  There are no exact cutoff numbers that say “if your horse has X amount of antibodies then they should be treated for Lyme disease”.  The decision on whether to treat or not is best left up to your veterinarian, who has clinical training and experience in determining what horses might benefit from treatment.  For more information on testing and interpreting results, I refer you to this nice summary from Cornell University.


To summarize:  Diseases that have many vague and intermittent symptoms are easy to jump to as a diagnosis for any ailment your horse may face.  This is especially true when there is no diagnostic test that is clear cut, and when treatment may or may not improve the clinical signs of the disease.  Too often these type of maladies are misused by horse enthusiasts to explain all of the problems that they see in their horse, mostly because the disease is misunderstood.  It would be best for the horse if we can keep an open mind and scientifically search for a correct diagnosis, instead of conveniently jumping to diseases like EPM and Lyme Disease.





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!!