Causes: Lyme disease is caused by infection with spirochete-shaped bacteria called Borrelia burgdorferi. The disease is passed to horses, people, and dogs through insect vectors, namely ticks. The particular culprits are the Ixodes ticks (black-legged and deer ticks), which flourish in the Northeast, mid-Atlantic, upper Midwest, and in pockets of the Northwest. Transfer of bacteria from a tick bite to a horse generally requires about 24 hours of tick attachment. Finding and removing ticks immediately improves the chances that your horse won’t develop an infection. If a tick is present for longer than 24 hours, Borrelia spreads through the horse’s connective tissue and into the blood to cause systemic infection.
Diagnosis: A horse infected with Lyme bacteria often has a variety of non-specific signs: Low-grade fever, shifting limb lameness, muscle tenderness, muscle wasting, weight loss, stiff gait, incoordination, lethargy, behavioral changes, increased skin sensitivity to touch referred to as hyperesthesia, and uveitis (inflammation of the eye tissues surrounding the pupil). Joint swelling can occur but is less common in horses than in dogs and people. Other possible signs include effects on cranial nerves, such as difficulty swallowing, facial weakness, laryngeal dysfunction, and respiratory distress. Lyme-induced neurologic signs are referred to as neuroborreliosis; it is not uncommon to see concurrent signs of Lyme-induced uveitis and neuroborreliosis. Not all horses bitten by an infective tick actually develop clinical disease, but they do develop antibodies, which are identified through testing. Testing methods are able to detect antibodies indicative of exposure, but such serological tests are not reliable until 3-4 weeks following a tick bite. Exposure does not always mean a horse develops an active infection. The experts recommend testing only horses with clinical signs suggestive of Lyme disease.
Treatment: Tetracycline and derivatives (doxycycline and minocycline), and β-lactam medications (penicillin and cephalosporins) are used to treat Lyme disease.
Prevention: Tick control is essential for prevention. Check your horse often and remove ticks as soon as possible. Environmental control using “tick-scaping” practices minimizes tick access to horses. Ticks prefer woodlands and vegetation of abundant tall grasses or leaf piles, especially in the transition zones of these ecosystems. Ticks tend to avoid dry, sunlit, regularly disturbed, and clean areas that are well maintained. Keep horses away from wooded areas and transition zones into wooded areas, mow pastures, and clear leaves and debris. Off-label use of canine Lyme disease vaccine has been attempted in horses, but protective antibody levels drop significantly by four months.