Lyme Disease in Horses

Lyme disease is increasing at an alarming rate – not only in our horses, but all of our pets and even in the human population.  My horse has it; I recently pulled blood for a Lyme titer to see if she was having a flare-up.  Thankfully, she was not, and we haven’t had any Lyme-related issues in over four years.

Being one of the fastest growing arachnid diseases, I wanted to look more into exactly what Lyme disease is and how I can better protect myself and my animals.  In my mare’s case, I want to make sure I am managing the disease as best as I can.

Lyme disease is caused by the bacterial spirochete “Borrelia burgdorferi.”  Early in their life cycle, ticks can pick up this bacteria by feeding on infected mice.  The ticks then later transmit it to their next host, be it a human, horse, dog, cat, etc.

The nasty little villains!

Every animal reacts differently to Lyme Disease.  The most common symptoms include:

  • Lameness in the joints; swollen joints
  • “Shifting lameness” – the horse may appear to be lame in one limb one day and a different one the next.
  • Fever
  • Change in behavior, to include aggression
  • Muscle wasting – horses that are in shape or normally well muscled suddenly lose muscle tone, or have difficulty putting muscle on
  • Muscle tenderness
  • Lack of appetite
  • Chronic weight loss; difficulty gaining weight
  • Increased sensitivity to sensory stimuli
  • Neurological issues

Your horse may show one, several, or even none of these symptoms.  I have seen a variety myself; my horse, for example, will feel sore all over, unable to really use herself properly.  That is her only symptom.  A friend’s horse was at the opposite end of the spectrum; she lost all of her muscling rapidly, went off her feed, and had a low grade fever.  Her onset was fairly rapid and her symptoms dramatic.  Still another horse I had, a mild mannered gelding, became very aggressive and very tight in his back under saddle.

These are just a few examples of the many experiences with horses and Lyme.  You can see why I often recommend having a Lyme titer done.  Titers usually run around $100 and the results are fairly quick.  A titer measures the antibody levels in your horse.  About 3-5 weeks after infection, your horse will develop antibodies to help fight the spirochete.  There are many different types of test available, but generally a multiplex test is becoming more popular.  The multiplex test can detect different levels of the antibody and determine if there is an early (new) infection or a chronic infection.

My mare’s recent multiplex results.  You can see she has a chronic result (meaning she’s had Lyme for over 5 months).  The low value, or equivocal, is a good result.

A spirochete is a corkscrew shaped bacteria that has the ability to alter their structure in response to the reaction from the host.  Due to this ability, the bacteria can “hide” from the body and remain dormant.  This is one reason why once a horse has Lyme disease, they will always have it.  The bacteria will remain dormant until the ideal opportunity presents itself to reappear.  This opportunity may be a suppressed immune system, stressed horse, or reinfection from a new tick bite.

Altering their structure in response to the host body is also the reason that every horse shows different symptoms.  This bacteria in particular is attracted to collagen, hence the numerous joint related symptoms.  There is also a high amount of collagen in the eye, brain, and skin, so vision, neurological, and skin issues are also possible symptoms.

Finally, the bacteria’s ability to react and adapt is why horses all respond differently to different treatment options.  In my next post, I will talk about the different treatment options and how you can protect your horses.


11 thoughts on “Lyme Disease in Horses”

  1. Wow! Thank you so much for writing this and I look forward to the next post. I have a mare recently diagnosed with Lymes after not knowing what she had for almost 6 months as her symptoms were sore muscles, spookiness, and aggressiveness. I am wondering if even after treatment and a negative repeat lab test, can the damage done still linger.


  2. Hi! As someone who has studied tick-borne diseases this is one people recognise the most when I give talks. Something to look out for is co-infection with Anaplasma phagocytophilum. This bacteria is transmitted by the same tick (Ixodes scapularis) as the one that transmits Lyme. When animals are co-infection with both bacteria, the symptoms for both are worse because they’re fighting 2 wars at once. The good news is the same antibiotic used to treat Lyme also treats Anaplasma. Best of luck with you critter!

    Liked by 1 person

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