Lyme Disease

Lyme disease in horses is one of the most confusing diagnostic and treatment situations for owners to navigate.

First, is your horse actually infected? If you live in an area where ticks are common, then your horse is probably frequently exposed to ticks carrying the Lyme spirochete (borrelia burgdorferi). Studies of asymptomatic horses make clear that horses routinely have positive titers. If we checked our horses’ titers regularly rather than only when symptomatic then we would see that titers naturally fluctuate as horses are exposed and the immune system responds. Titers measure the immune response — not the spirochete itself. If the immune system is focused on dealing with the spirochete, then titers will be measurable and may be quite high – but the horse may not be infected.

The most commonly used blood test is the Cornell multiplex assay; it yields three titers that reflect timing of exposure.

The test also can yield equivocal titers that are borderline for infection; re-testing may be recommended to determine if the titer is going up or down. But even a high titer may not mean infection. Titers can only be interpreted in the context of symptoms — if a horse has symptoms consistent with Lyme disease and all other potential causes of symptoms have been ruled out then a Lyme infection is possible. See below for examples of different titer outcomes.

What are Lyme symptoms? The Merck Manual only recognizes three — uveitis, neurological symptoms, and cutaneous lymphoma — and these are all relatively rare. However, owners report a wide range of symptoms:

• Full body stiffness and/or lameness in more than one leg
• Shifting or intermittent lameness
• Leg-shifting
• Muscle tenderness, especially sore back
• Lethargy/fatigue
• Sensitivity to cold
• Stiffness
• Skin sensitivity
• Weight loss
• Change in behaviors (e.g., horse becomes spooky or withdrawn, will not go forward, resists changing gaits, will not pick up one lead, etc.)
• Poor or inconsistent performance

In the presence of a positive titer and any symptoms most vets will advise treatment.

The most commonly used oral treatments are doxycycline or minocycline. Neither has been shown to definitively clear the spirochete. Doxycycline was tested in four Icelandic ponies that were deliberately infected — only one pony cleared the spirochete — a 75% failure rate. There are multiple problems with this study. In addition to the small sample size, the ponies were never symptomatic; in the real world, they would never have been tested. And ponies are not physiologically identical to horses, making it unclear whether these findings are relevant to horses.

The lack of efficacy for doxycycline is probably the result of its poor bioavailability in horses with >90% of the drug passing through the horse unabsorbed. Minocycline is better absorbed, ranging from 16 to 40% depending on when the horse last ate — but no studies have shown it actually clears the spirochete either (Merck Manual, 2022; Tetracyclines Use in Animals).

Owners frequently report that after beginning treatment symptoms improve. These medications exert strong anti-inflammatory effects that can decrease symptoms and make the horse feel better. It is possible that the inflammation decrease helps the immune system mount a stronger response. But disentangling effects on the spirochete vs. the anti-inflammatory effects is not possible because titers do not reliably decrease with treatment. In fact, they may remain elevated for months or years after treatment in horses who appear to be well.

A common misinterpretation of symptoms that reappear once treatment has ceased is that the infection is still present – but it may be that the removal of anti-inflammatory action of the antibiotics is the cause of symptom return. Because titers may or may not change in response to treatment, using titers to track treatment efficacy is not advised.

The Lyme disease guideline from the American Association of Equine Practitioners (AAEP; 2020), notes:

“Several options exist for treatment of horses with suspected Lyme disease, and a gold standard protocol has not yet been developed. Tetracycline antimicrobials are commonly administered for 7 to 30 days (oxytetracycline 6.6 mg/kg IV q24h; or doxycycline 10 mg/kg PO q12h; or minocycline 4 mg/kg PO q12h). It should be noted that tetracycline antimicrobials exert anti-inflammatory effects through their inhibition of matrix metalloproteases, which may contribute to improvement in clinical signs during treatment independently of any effects on B. burgdorferi. Accordingly, relapse of clinical signs following treatment should not necessarily be interpreted as failure to clear the organism, as the improvement may have been due to the medication’s anti-inflammatory properties alone…. Titers should not be used alone to assess response to treatment or as deciding factors for prolonged therapy, as many treated horses maintain positive titers for months to even years.”

And unfortunately these medications have significant side effects. Doxycycline hyclate (typically used for oral dosing) is extremely acidic and can irritate the esophagus and stomach, eroding the protective mucous barrier. Depending on the form, minocycline also can be quite acidic. Omeprazole (Gastrogard or Ulcergard) may be recommended to offset this damage but it can only turn off stomach acid — possibly creating gut comfort but doing nothing to restore the gut lining.

More seriously, even short durations of doxycycline can damage the gut microbiome, reducing overall flora diversity by killing off some organisms and reducing populations that take care of the protective mucus barrier and that digest fiber. Populations of unwanted organisms increase. These changes can persist for weeks or months after treatment and are believed to create risk of colitis in vulnerable horses. Horses who are treated repeatedly in a misguided effort to bring titers down may suffer these consequences long-term. The gut impact of minocycline has not been studied in horses but given its similarity to doxycycline, the consequences are probably similarly negative.

What to do?

Support overall health with a diet that covers key nutrient needs and eliminates pro-inflammatory and toxic ingredients

• Avoid GMO high omega-6 fatty acid ingredients that promote inflammation and are contaminated with herbicides (i.e., soy, corn, wheat middlings, rice bran)
• Avoid “fillers” that provide no nutritional value (i.e., soybean hulls, peanut hulls)
• Ensure adequate coverage of essential amino acids and key trace minerals
• Ensure adequate coverage of anti-oxidants
• Ensure adequate protein, carbohydrate, and fat content for horse’s breed, stage of life, work, and medical history

If you choose to treat with oral antibiotics, then maximizing gut protection is imperative. Key steps include providing:

• Nutrients that protect and nourish the gut lining to control gastrointestinal damage and discomfort
• Nutrients that buffer excess acid without turning it off to avoid turning off digestion
• Antibiotic resistant probiotics to offset diminished flora
• Immune boosting through the gut wall as well as direct immune stimulation to activate and empower the immune system
• Robust gut support long-term to ensure complete healing of gut mucosa, repopulation of desirable flora, and preservation of immune strength

Chronic Lyme

• Avoid chasing titers with repeated antibiotic treatment; this strategy will have long-lasting negative effects on the gut mucosa and the microbiome
• Maintain a multi-faceted program of immune boosting to keep the immune system strong and effective; this strategy also minimizes symptom flares in most horses
• Consider holistic treatment alternatives that avoid the gut toxic effects of antibiotics
• See below for an example of a high-achieving chronic Lyme horse effectively managed for a decade using this strategy; this is Cruiser, a 20 year old Thoroughbred carefully managed by devoted owners Reba and Lisa Merritt.

Need help with a Lyme horse? Please Reach Out

References:

American Association of Equine Practitioners. (2020). AAEP Infectious Disease Guidelines: Borrelia burgdorferi infection and Lyme disease.

Bernardino, A. et al. (2009). The antibiotics doxycycline and minocycline inhibit inflammatory responses to the Lyme disease spirochete Borrelia burgdorferi. J Infect Dis.;199: 1379–1388.

Bryant, J. et al. (2000). Study of intragastric administration of doxycycline: Pharmacokinetics including body fluid, endometrial and minimum inhibitory concentrations. Equine Vet J.; 32; 2233-238.

Chapuis, R. et al. (2023). Characterisation of faecal microbiota in horses medicated with oral doxycycline hyclate. Equine Vet J.; 55(1); 129-141.

Chang Y. et al., (2000). Experimental infection of ponies with Borrelia burgdorferi by exposure to Ixodid ticks. Vet Pathol., 37:68–76.

Chang, Y. et al. (2005). Antibiotic treatment of experimentally Borrelia burgdorferi-infected ponies. Vet Microbiol. 107(3-4):285-94

Cornell Diagnostic Center; https://www.vet.cornell.edu/…/pro…/lyme-multiplex-horses

Davis, J. et al. (2006). Pharmacokinetics and tissue distribution of doxycycline after oral administration of single and multiple doses in horses. Am J Vet Res.; 67; 310-316.

Divers, T.J. et al. (2018). Borrelia burgdorferi infection and Lyme disease in North American horses: a consensus statement. J Vet Intern Med., 32(2); 617-632.

Durrani, A. et al. (2011). Retrospective study on seroprevalence of Borrelia burgdorferi antibodies in horses in Minnesota. J Equine Vet Sci.;31:427–429.

Echeverria, K. et al. (2017). Effect of feeding on the pharmacokinetics of oral minocycline in healthy adult horses. J Vet Pharmaol Ther.; 41(1):e53-e56.

Funk R et al. (2016). Seroprevalence of Borrelia burgdorferi in horses presented for coggins testing in southwest Virginia and change in positive test results approximately 1 year later. J Vet Intern Med;30:1300–1304.

Merck Manual; https://www.merckvetmanual.com/…/tetracyclines-use-in…

Neely, M. et al. (2020). Seroprevalence and evaluation of risk factors associated with seropositivity for Borrelia burgdorferi in Ontario horses. Equine Vet J., 53(2); 331-338.

Wagner B. et al. (2013). Antibodies to OspC, OspF and C6 antigens as indicators for infection with Borrelia burgdorferi in horses. Equine Vet J.; 45:533–537.