The Harry M. Zweig Memorial Fund for Equine Research

Equine Lyme Disease: Antibiotic Treatment of B. burgdorferi
Persistent Infection

Dr. Yung-Fu Chang

Lyme disease is the most prevalent tick-borne disease of humans and animals in the United States. Most of the reported cases have been in the northeast, the upper mid-west, and California. Lyme disease is caused by a tick-borne bacterial spirochete Borrelia burgdorferi which infects several domestic animals including horses, cattle, dogs, and cats. Clinical signs are non-specific and include fever, stiffness, muscle pain, and swollen joints. The diagnosis of Lyme disease in animals is currently based on a combination of history, clinical signs, response to antibiotic therapy, risk of probable exposure, and blood tests. It should be emphasized, however, that the results of blood tests do not always correlate with disease status.

Borrelia burgdorferi is maintained in a complex life cycle of small wild mammals and immature stages of the black legged tick, Ixodes scapularis (formerly called Ixodes dammini) and Ixodes pacificus. Larval and nymphal stages of the tick acquire the organism when they feed on infected mice.

Lyme disease develops when the infected nymphs feed and transmit infection via the skin to humans and animals such as horses, particularly during the summer months. Adult ticks feed and mate on deer during the fall and spring. The female ticks drop off, laying eggs on the ground. After several weeks the eggs hatch into larvae, initiating a two-year life cycle. It is now understood that deer are not a reservoir for Lyme disease, but merely the host for the adult stages of the tick.

A very large number of horses is treated presumptively each year for Lyme disease. Treatment is often prolonged, expensive and carries some chance of toxicity. Virtually all cases of equine Lyme disease are treated with doxycycline, tetracycline or ceftiofur. Based upon treatment of Lyme disease in other species, any use of these antibiotics in horses could be justified. Unfortunately, other than the rare equine case that has a pre- and post-treatment synovial biopsy there is no evidence to suggest which drug would be most effective in treating Borrelia infection and its associated synovitis. Borrelia has a strong predilection to infect skin and synovial membranes in the horse, as has been shown in our previous experimental work. This finding helps support the belief of equine practitioners that stiffness/lameness is a common sign of Lyme disease in the horse. The high infection rate of synovial membranes in our experimental equine studies provides an excellent model to determine efficacy of different antimicrobials in treating Lyme synovitis/arthritis. By using this model and treating an appropriate number of ponies with each of these antibiotics (listed above) we will be able to compare efficacy of the drugs in treating patients infected by the Lyme agent. This should answer many questions regarding treatment of Lyme disease in the horse for both veterinarians and horse owners.

Ponies will be exposed to B. burgdorferi-infected ticks to mimic natural infection. The animals will be observed for clinical signs (fever, lameness, or any other course of clinical manifestations of disease). Blood tests and skin biopsies will be used to monitor the infection. Twelve weeks after tick exposure, ponies will be treated with doxycycline and ceftiofur. Beginning ten months after tick exposure, two animals per week will be euthanized and subjected to gross and histopathologic examination. Culture and a DNA test (polymerase chain reaction) will be done on various tissues collected by biopsy and during pathologic examination of the animals after death. This information is very critical to both veterinarians and horse owners for effective treatment of equine Lyme disease.