Vets Consider ‘Chronic’ Lyme in Horses

Editor's Note: Updated July 7, 2008, with original information from Dr. Rick Mitchell. Quotes in the original version did not accurately reflect Dr. Mitchell's comments.)

The Infectious Diseases Society of America (IDSA) recently announced that it will be reviewing the case for "chronic" Lyme disease, as well as the treatments for ongoing symptoms of the tick-borne infection in humans. On the equine side, veterinarians are also investigating whether Lyme disease in horses can be considered a chronic condition, and, if so, how best to handle it.

According to Diana Olson, director of communications for the IDSA, the society's current guidelines do not recognize a chronic Lyme disease condition or the effectiveness of prolonged treatments for chronic clinical signs.

Rick Mitchell, DVM, of Fairfield Equine Associates in Connecticut, said, "The problem is establishing a definitive diagnosis of 'chronic Lyme disease' in the horse. While I acknowledge that horses are infected with the organism, they most often resist clinical disease and simply demonstrate an immune response that can be measured. Having been involved in advance-level lameness diagnostics for many years, I have all too often seen horses presented that had been treated for chronic Lyme disease repeatedly that actually had real orthopaedic problems or other real medical issues (gastric or colonic ulcers for example), which once properly addressed, symptoms went away. There would appear to be a group of horses that react negatively to this organism and have difficulty getting rid of it, but they are hard to identify. Additionally, there may be some other yet unidentified organism that is cross-reacting with the immune assays. It's very difficult to pursue obscure soreness complaints, but easy to prescribe Doxycycline, which has anti-inflammatory properties in the first place. So vets just say 'could be Lyme' and put the horses on Doxy. Often doses are insufficient for true antibiotic function (should be 10 mg/kg twice daily).

"We have had a few apparently recurrent cases that have demonstrated Lyme organisms on PCR analysis of the synovial membranes (joint membrane biopsy), so I have to admit that some horses would appear to be chronically infected," Mitchell continued. "Occasionally these horses will respond to high levels of IV oxytetracycline (5 gm once or even twice daily) for 2-3 weeks followed by more oral doxycycline for another month or two. Concurrent use of prednisolone has minimized 'kill off' side effects. Many will gradually improve following this therapy. Others just need to stay on a long term course of Doxy and/or other anti-inflammatory meds.

"We have never created an intractable diarrhea in the hospital environment using IV oxytet as mentioned above," noted Mitchell. "Hydration is a big concern, and we must watch their kidney function closely."

Yung-Fu Chang, DVM, Dipl. ACVM and Thomas Divers, DVM, Dipl. ACVIM, ACVECC, who specialize in infectious diseases at Cornell University, acknowledge the existence of chronic Lyme infection in equids and said that prolonged treatments might benefit some horses showing ongoing clinical signs.

In an antibiotic treatment trial with horses experimentally infected with Lyme's causative bacterium, Borrelia burgdorferi, the researchers found that clinical signs recurred in some of the treated horses several months after researchers discontinued antibiotics. As a result, they now suspect that B. burgdorferi hides in niche areas that antibiotics cannot easily penetrate, such as in connective tissues.

Chang and Divers added that prolonged antibiotic treatment might not be indicated unless clinical signs return or there is evidence that antibodies against bacteria are decreasing after the first month of treatment.

Clinical signs might include muscular tension, stiffness, and behavioral changes.

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Elizabeth Troutman

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