Borrelia burgdorferi, the causative agent of Lyme disease, 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. 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.
Research has shown that ticks must be attached for at least 24 hours to transmit their disease. Each stage of the tick, larva, nymph, and adult are capable of transmitting B. burgdorferi. The tick holds the organism in its simplified digestive system, known as a hindgut. A tick attaches to a host with its very sharp mouthparts. It then salivates and regurgitates into the host an anti-clotting agent so it can get a ready supply of host blood. While the tick is attached to the host it uses its mouth parts to suck and ingest blood of the host. It then sends this blood to its hindgut where it mixes with the organism. The tick then regurgitates back into the host as it continues to feed. If the tick had the Borrelia organism in its hindgut, it has now been deposited in the host. Not only does the host become infected, but is now serves as reservoir of Borrelia for the next tick to ingest when it attaches and continues to spread the disease.
Lyme disease infects humans and several domestic animals including horses, cattle, dogs, and cats. Like humans, horses are incidental, dead end hosts. Clinical signs are non-specific and include fever, stiffness, muscle pain, and swollen joints. Often the lameness seems to “wander” from limb to limb. Changes in behavior and skin sensitivity (tactile hyper-aesthesia), both with rapid onset, are common clinical signs seen by many practitioners in horses with potential Lyme disease. Borrelia has a strong predilection to infect skin and synovial membranes in the horse, as has been shown in experimental work by researchers at Cornell University. This finding helps support the belief of equine practitioners that stiffness/lameness is a common sign of Lyme disease in the horse. Neurological signs such as depression, dysphagia, head tilt and encephalitis were reported in chronic cases. Most recent reports describe horses with a Borrelia-associated pseudolymphoma or Borrelia-associated uveitis.
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.
Other causes of vague lameness and performance issues such as equine protozoal myelitis and other causes of neurological disease, osteoarthritis, tendon/ligament injuries and rhabdomyolysis need to be rules out as well.
Patient-side canine Lyme SNAP tests are being used to identify exposure, but cannot determine is the horse has been exposed in the past, is currently infected, or is positive due to vaccination. Western blot testing has been the gold standard, but results take several days to acquire and the test can be ambiguous.
The Equine Lyme Multiplex Assay is one of the better means of identifying horses with Lyme disease. This test assays for three outer surface proteins (Osp) of B. burgdorferi. Studies have shown that the Osp antigen expression on the bacterial surface changes in response to tick feeding and again after infecting a warm-blooded host. Horses develop antigens to these Osp proteins in response to infection. Osp A is positive in vaccinated horses. The protein is expressed normally only in the tick, so unvaccinated horses usually have no Osp A antigen. Osp C is the best indicator of early infection. These antigens develop 3 weeks after infection and decline at 7-11 weeks. Osp F is an indicator of chronic infection. Antibodies become detectable at 5-8 weeks post infection and remain high.
Virtually all cases of equine Lyme disease are treated with doxycycline, tetracycline or ceftiofur. Based upon treatment of Lyme disease in other species, any of these drugs should be effective. Cornell is studying which antibiotic may be most effective.
Body soreness, muscle soreness, and/or joint pain can be controlled with non steroidal anti-inflammatory agents (NSAIDs). Inflammed joints are at risk for developing chronic problems or cartilage issues from the inflammatory mediators within the joints. Therefore, cartilage protective agents (chondroprotection) are often used simultaneously for the best outcome in many cases.
Prevention should be aimed at reducing the tick burden on your property. Keep area of pastures where ticks are likely to live mowed and trimmed. Clean up leaves in pastures and yards, ticks like to live here. Protect pets (dogs and cats) from ticks so that they are not transporting ticks into your equine environment.
Check your horse daily for ticks. Look closely in and around ears, the mane, tail, and neck/chest. Topical fly sprays, spray on or spot on tick preventatives can help to decrease the number to ticks that attach. Fly sheets, fly leg wraps and fly masks (particularly those with ears) can also help to keep ticks from your horse. If your horse does have ticks attached, grasp the tick as close to the mouthparts as possible and pull straight out. Tweezers can be helpful. DO NOT try to burn a tick off with a match.
There is no vaccine for Lyme disease in horses. Vaccination is currently being done off-label using the canine vaccines. The vaccines have proven to be safe in anecdotal trials, but the effectiveness of the vaccine in horses has not yet been published. Vaccination of at risk populations of horses should be based on a veterinarian’s recommendation.
Registered 2014 Equestrian Collections; Author: Sallie S. Hyman, VMD, DACVIM, CVA
Information in this article is for educational purposes only and is not a substitute for evaluation by an equine professional. In particular, all horse owners should seek advice and treatment from a licensed veterinarian for their horses’ medical care.