Q: My warmblood mare was recently diagnosed with Lyme disease, and she also has uveitis (perhaps coincidental, perhaps related—who knows). It seems like treatments for Lyme are different depending on who you talk to, and I’m having trouble getting answers to a long list of questions.

Do you recommend blood testing every six months? What type of test? Is there a vaccine for it, is there one being developed, or is that even possible? It’s frustrating to know my mare has a high titer but not know when she was infected. Did she have it when I bought her, did it develop years later, or was it this spring? Is it unusual for her to have uveitis, but no other symptoms? What is the most effective approach to treatment?

A: Lyme disease, caused by the bacterium Borrelia burgdorferi and transmitted to horses by Ixodes spp. ticks, is a controversial topic in equine medicine today because the relationship between infection and true clinical disease is difficult to define. Roughly 45 percent of horses in the northeastern United States will test positive for B. burgdorferi antibodies, but most of these horses show few to no clinical signs. Even ponies who were experimentally infected with B. burgdorferi in a laboratory showed no clinical signs despite high titers.

So to diagnose a horse with Lyme disease we must satisfy four criteria:

• potential exposure to an infected tick, or a history of a known tick bite.

• clinical signs consistent with Lyme disease. These can be vague and nonspecific but most commonly include weight loss, shifting leg lameness, hypersensitivity of the skin, muscle soreness and lethargy. (There are several reports of uveitis associated with Lyme disease, but these cases all had additional, more typical clinical signs. Unfortunately, a definitive connection between uveitis and B. burgdorferi infection has been made only with postmortem testing.)

• absence of other disease. Other causes of the clinical signs must be ruled out.

• high antibody titers in the blood.

Testing for B. burgdorferi infection has advanced in recent years. The most comprehensive test currently available is the equine Lyme multiplex assay, offered by Cornell University. This assay looks for three distinct antigen proteins (Osp A, C and F) that fluctuate in intensity as the disease progresses:

Osp A is found in horses who have been vaccinated, which differentiates them from those who have positive antibody titers from a natural infection. (While no Lyme vaccine is currently approved for horses, many people use the canine vaccines off-label, although equine safety and efficacy studies have not been performed.)

Osp C antibodies will be present in horses who have acquired an infection recently. The antibodies can be detected as early as three to five weeks after the initial exposure; these levels will decline within seven to 11 weeks and are undetectable after five months.

Osp F antibodies are an indication of chronic infection; these begin to rise two to three months after infection and remain at high levels thereafter.

By referring to the timeline of these antibody fluctuations, we can estimate whether an infection is acute or chronic. The older ELISA or Western blot tests are still good screening tools for infection, but the multiplex test gives us far more information.

Once a diagnosis is made, a treatment protocol can be selected. Two antibiotics—intravenous (IV) oxytetracycline and oral doxycycline—have been widely used to treat Lyme disease. In experimentally infected ponies, the IV oxytetracycline led to a larger decrease in antibody titers than did the oral doxycycline. However, in a study done on horses who were infected naturally, both options performed equally as well, although the decrease in antibody titer was not as large as was seen in the experimental model. Horses in the field may not respond to treatment as readily as the ponies infected in the lab because they may have repeated exposures to the bacteria or perhaps because their cases are not treated as soon after the initial infection.

Many veterinarians use the protocol of IV oxytetracycline for up to five days followed by oral doxycycline for anywhere between 14 and 30 days. Response to treatment can be difficult to interpret, in part because the antibody titers may not decrease as much as expected, but also because the medications have anti-inflammatory properties that may cause the resolution of clinical signs rather than a resolution of the infection.

I would recommend routine screening every three to six months to monitor your mare’s titers and differentiate between acute and chronic infections. If high titers seem to be causing clinical signs in your mare, then I would recommend treatment. However, I would caution against administering antibiotics simply to prevent clinical signs from arising. Anytime we give antibiotics to a horse (IV or oral) we risk disrupting the gastrointestinal flora and causing colonic inflammation and diarrhea. In addition, the overuse of antibiotics is leading to increased resistance in bacterial populations. It is far better to use routine testing to monitor for exposure and reserve treatment for when clinical signs coincide with high titers.

Joan Norton, VMD, DACVIM
Norton Veterinary Consulting and Education Resources
Noblesville, Indiana

This article first appeared in EQUUS issue #453