There’s no question that Lyme disease, the clinical manifestation of infection with the bacteria Borrelia burgdorferi, is a topic of great concern to many in the mid-Atlantic region. Folks have good reason to worry about Lyme, as the infection is spreading with the deer ticks that transmit it, and this year is supposed to be a bumper crop year for the ticks because of last year’s mild winter. The American College of Veterinary Internal Medicine (ACVIM) recognizes the importance of this disease in horses and has dedicated this year’s Consensus Statement (a document summarizing the results of the recent relevant research by a panel of well-established experts in the field) to Borrelia in horses. Here are some highlights from the statement, which will be available by open access on https://www.ACVIM.org this fall.
Diagnostic Challenge: The diagnostic tests used to help establish if a horse may have Lyme test the horse’s immune response to having been exposed to the bacteria. The titer only confirms that the horse has been exposed to the bacteria, not that the bacteria is causing whatever signs the horse is showing. One study showed that over 1/3 of normal horses in Southwest VA had positive antibody titers for Lyme. Similarly, in research settings, ponies are experimentally exposed to infected ticks and mount a strong antibody response, but do not show any signs of illness. Therefore, it is likely that the vast majority of horses that are exposed will fight the infection on their own, but will “test positive.” This has led to the recommendation that healthy horses not be “screened” for Lyme on purchase exams or that horses with vague signs have other causes of their signs ruled out before considering Lyme. To date, the disease syndromes that have been proven to be caused by Lyme include neurologic disease with fever, uveitis, and pseudolymphoma. Other syndromes that have been extrapolated from human symptoms of Lyme (fatigue, muscle aches, shifting leg lameness, back soreness) may be Lyme related in the horse, but we do not at this time have evidence that they are.
Treatment: The panel did not have specific recommendations regarding specific antibiotics or duration of treatment, except to say that titers should NOT be used to determine when to stop treatment or to monitor response to treatment. According to the panel, a horse’s prognosis is not worse if its titer stays high, so following any changes in titer levels is not recommended.
Prevention: The best prevention of Lyme is preventing or minimizing the horse’s exposure to ticks. This may include the use of insect repellants and pasture management. The OspA vaccine used in dogs has been shown to protect ponies from experimental infection, but the duration of OspA antibodies is relatively short (5 months), which may make several time a year vaccination necessary to maintain protective levels. A similar vaccine was pulled from the human market because it caused immune mediated arthritis in a significant number of people using the vaccine. The risk of this complication is unknown in horses. Because the actual disease rates due to Lyme are still unknown in horses, the panel did NOT recommend vaccination of horses as a means of prevention at this time.
Many controversies remain around Lyme in horses as they do for people. More work needs to be done to better characterize the disease and specify treatment and prevention. It will also be important to gain information about the effect of other tick bourne illnesses, such as Anaplasma, on Borrelia infections in horses.