Equine Lyme disease: Significant or coincidence (Proceedings)

Borrelia burgdorferi is a common infection in horses in parts of the U.S and Europe. The infection may be persistent in many horses. There have been at least 3 retrospective clinical studies (with control groups) that correlate seropositivity for Borrelia with clinical signs. One study looked specifically at equine recurrent uveitis (79 horses with ERU and 153 controls) and seropositivity to Borrelia and found no correlation between the two. Two studies looked at correlation between seropositivity to Borrelia and signs of musculoskeletal disease, often attributed to Borrelia infection. Those signs included: shifting lameness, stiffness, behavioral changes and inability to perform work. In 1 of the studies, there was a significant correlation between the 2, while in the largest study involving 1618 horses sick for any reason and 400 healthy horses, an association between Borrelia seropositivity and signs commonly attributed to Lyme disease was not found. Our experimental studies support a possible preferred migration of the organism through connective, perineural, and perivascular tissue in the skin, fascia, muscle and synovial membranes, with the synovial membrane most commonly affected. There is mild pathology in these areas which could cause hyperesthesia and lameness, two of the most commonly reported (by owners and veterinarians) clinical signs. None of the ponies had clinical signs though; it may be that clinical signs do not exist in equines or that the clinical signs are mild and/or can only be recognized in certain horses. Unfortunately, response to antibiotic treatment has not helped with the question proposed in this title, since we have found that the most commonly used antibiotics for treating Lyme disease in the equine have rather potent anti-inflammatory properties and it is common to get a clinical response to treatment of stiff or shifting leg lame horses regardless of the Borrelia status. It has been our limited experience that clinical signs believed to be associated with Lyme disease are most commonly reported in performance horses that are used for eventing (Dressage, stadium jumping, 3-day eventing). Although not the only possible explanation, one explanation for this observation would be that hyperesthesia and mild lameness/stiffness are more likely to be noticed in this type of horse. We have almost never been referred a race horse with a presumptive diagnosis of Lyme disease. We have observed muscle wasting and markedly swollen joints in one horse with Borrelia infection and dramatic response to treatment, but these signs have not been common in horses diagnosed with Lyme disease. Fever and edema, both responsive to tetracycline therapy, may be found in horses just prior to Lyme seroconversion, but these signs are most likely a result of Anaplasma phagocytophilia infection. A. phagocytophilia is found concurrently with B. burgdorferi in many Ixodes ticks resulting in dual infection in the horse. Most of the current diagnostics for Borrelia infection appear to be both sensitive and specific. High ELISA titers to whole cell Borrelia or selected recombinant proteins nearly guarantee previous or current infection, unless the horse has been vaccinated. Positive Western blot tests using Borrelia-specific antigens will help separate vaccination antibody from infection antibody. The SNAP 3DX® appears to correlate well with infection if any (even mild) color change is recorded as a positive. Our treatment studies suggest that tetracycline given intravenously is superior to orally administered doxycycline or intramuscular ceftiofur. The superiority of tetracycline over doxycycline might be related to expected higher tissue concentrations following intravenously administered tetracycline since doxycycline has been shown to have low bioavailability when given per os to horses. Some of the ponies treated with doxycycline or ceftiofur had a significant decline in antibody level during treatment, but antibody level increased after treatment was discontinued in most of the ponies treated with these two drugs. In order to be reasonably confident the organism is no longer present, KELA ELISA titers should drop to very low levels (< 110) and remain low for 2 months after discontinuing treatment. Although tetracycline was highly efficacious in this study, its efficacy (in decreasing titers) has not been good in naturally-occurring field cases! It is unclear if this is because horses have been infected for longer (than the experimental ponies) periods of time prior to beginning therapy, with potential for antibiotic resistant cyst formation and/or the poor response could also be explained by re-infection, or persistent antibody production due to immune mimicry. I do not recommend treating horses based solely upon a positive test!

Many questions regarding Lyme disease in the horse remain unanswered. Our experimental studies and previous cross-sectional studies on seropositivity vs. clinical signs do not definitively determine the significance, if any, of Lyme disease in the horse. Further in-vitro studies are planned, but unless high level event horses are used in experimental infection studies, this question is unlikely to be answered any time soon, I am sorry to say.

References

Chang YF, et al. Vet Pathol 2000;37(1):68.

Chang YF, et al. Vet Microbiol 2005;107(3-4):285.