Lyme Disease Table Topic (AAEP 2011)

The Lyme Disease Table Topic at this year's American Association of Equine Practitioners (AAEP) Annual Meeting, held Nov. 18-22 in San Antonio, Texas, was facilitated by Amy Grice, VMD, partner at Rhinebeck Equine LLP, in Rhinebeck, N.Y., and Bettina Wagner, DVM, PhD, associate professor and Director of Serology/Immunology at Cornell University's College of Veterinary Medicine. Lyme disease is caused by the spirochete Borrelia burgdorferi, and bacteria are transmitted to horses by infected ticks. Table Topic participants discussed clinical signs of disease, treatment options, diagnostic tests, and vaccination against Lyme disease. Behavioral changes and increased skin sensitivity, both with rapid onset, were the most common clinical signs seen in horses with potential Lyme disease. In addition, most practitioners observed weight loss and shifting leg lameness regularly. Attendees discussed special clinical conditions such as neuroborreliosis (a disorder of the central nervous system), a recently described pseudolymphoma (a benign disorder of lymphoid cells), and uveitis (inflammation of the uvea of the eye) cases as less frequently described clinical outcomes of infection with B. burgdorferi. Discussed briefly were differential diagnoses such as equine protozoal myeloencephalitis (EPM) and infection with Anaplasma phagocytophilum.

Most practitioners treat horses with clinical signs and a confirmed serological antibody titer to the pathogen with either oral administration of the antibiotic doxycycline for 30-45 days, intravenous (IV) administration of the antibiotic oxytetracycline for 30 days, or a combination of oxytetracycline IV for a variable period, followed by an oral doxycycline treatment for a total treatment time of 30-60 days. Very few practitioners reported adverse reactions to treatment; the most common side effect was soft manure. Many of the doctors treat concurrently with pre- or probiotics.

Diagnostic tests to support Lyme disease diagnosis in horses all identify antibodies to B. burgdorferi as indicators of infection. Available assays include ELISA followed by Western blot offered by several diagnostic laboratories, the stallside SNAP 4Dx test (IDEXX Laboratories), and the new Lyme multiplex assay performed at Cornell University. Practitioners discussed the advantages of the multiplex assay to differentiate between acute and chronic infection. During the early stage of infection clinical signs are rather unlikely to occur. Researchers at Cornell have observed very good success of antibiotic treatment in the early infection stage, sometimes resulting in the complete disappearance of antibodies to B. burgdorferi. Follow-up testing for horses in the early infection stage can be performed six to eight weeks after treatment starts. For chronic cases, veterinarians should confirm treatment success by retesting no sooner than three months, but it is not uncommon for antibodies to persist and wane slowly over many months.

Several practitioners also vaccinate horses off-label with one of the three available Lyme vaccines for dogs. These vaccines are not approved for horses but are the only available vaccines for trying to protect horses housed in Lyme endemic areas from infection with the pathogen. Efficacy studies of canine vaccines in horses are not yet available, but a research vaccine similar to these commercial products was found to induce antibodies in a small number of research ponies.

To obtain more information about horses' antibody responses to vaccination and the decrease of antibody levels after antibiotic treatment of horses with Lyme disease, the facilitators encouraged the practitioners in attendance to provide detailed information about treatment or vaccination on their submission form if samples are submitted to Cornell University for Lyme multiplex testing. Details on clinical signs, when the horse was vaccinated, when and for how long it was treated, and which vaccine or antibiotic treatment was used are vital to helping obtain the clinical data needed to better assist veterinarians treat and prevent Lyme disease.

This Table Topic was moderated by Bettina Wagner, DVM, PhD, associate professor and Director of Serology/Immunology at Cornell University's College of Veterinary Medicine; and Amy Grice, VMD, partner at Rhinebeck Equine LLP, in Rhinebeck, N.Y. Wagner and Grice also authored this piece. 

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