Causative Agent: The spirochete (spiral shaped) bacterium, Borrelia burgdorferi, causes Lyme disease in horses and other mammals, including humans. The spirochetes infect ticks, which in turn infect mammals when they feed off their blood. The disease most often occurs in the Northeast, mid-Atlantic states, Minnesota, Wisconsin, and northern California, where Borrelia burgdorferi is most prevalent.
Clinical Signs: Victims of Lyme disease suffer from fever, decreased appetite, and arthritis, most often in the joints of the front limbs. Stiffness, lameness or frequent shifting from one limb to another are also common. Upon closer examination, one will note swelling, heat, and pain in the joints. Occasionally, laminitis has been associated with Lyme disease. Problems with the kidneys, heart, and nervous system have also been reported.
It is common for domestic animals, including horses living outdoors in a region where Lyme disease is common, to be bitten by infected ticks and only develop subclinical infection (e.g. infection without the development of clinical disease). Horses develop symptoms of Lyme disease less than 10% of the time.
Disease Transmission: Lyme disease is spread by ticks in Ixode species, namely the "deer tick" or "black legged" tick. Larval stages of these ticks contract the disease when they take a blood meal from a mammal (usually a rodent) already infected with the bacteria.
Ticks have 3 developmental stages (larvae, nymph and adult) and must have a blood meal before they can molt into the next stage. They live for 2 years, even in cold climates. They may transmit the infection to new hosts when taking subsequent blood meals. The ticks must attach to the new animal host and feed for 12-24 hours before they can transmit the bacteria to infect the new host. Rodents, especially the white-footed mouse, serve as the natural wildlife hosts of the tiny larval Ixodes ticks. Middle stage ticks (nymphs) feed on a variety of wild and domestic species, including humans, rodents, horses, cats, birds, etc. Deer are the predominant wildlife host for adult ticks, but domestic animals, such as horses, may serve as substitute hosts. Adult ticks, which are present in the fall and early spring, are most likely to feed on horses.
Diagnosis: Diagnosis can be difficult because common blood tests for Borrelia burgdorferi infection measure antibodies (part of the body's response to infection) and would be positive in an animal which had been exposed to Borrelia burgdorferi regardless of whether it became clinically ill or not. Furthermore, symptoms of Lyme disease are similar to symptoms of other musculoskeletal injuries. However, a serology test from a blood sample is the first step. It is also important to note the horse's history of exposure to ticks or tick infested areas. Ultimately, the horse's response to treatment and ruling out other diseases are the best indicators that the disease has been properly diagnosed.
Treatment: Antibiotics can be used at standard doses to treat the bacteria. Symptoms may be relieved with pain relievers, rest, and fluids, depending on the severity of the disease. Note however, that laminitis may result in smaller horses due to toxins released by dying Borrelia burgdorferi during the first few days of treatment. If a reluctance to move, tender stepping, or increased heat in the horses' hooves are noticed, laminitis should be suspected and a veterinarian should be contacted immediately.
Prevention: Currently, there is no USDA approved Lyme disease vaccine for use in horses. There are some reported instances where owners are giving dog Lyme vaccines to their horses. There is no scientific evidence that this is effective or safe. Giving dog Lyme vaccines to horses may also interfere with future blood testing for Lyme disease in the vaccinated horses.
The best prevention for Lyme disease is tick control. Any animal taken out of a tick-free environment should be inspected daily for ticks. Any ticks that are noticed should be removed immediately. Ticks are most often found around the head, throatlatch area, belly, and under the tail of horses. To remove a tick, grasp its mouth parts adjacent to the skin of the horse with tweezers and pull gently back to remove the whole tick.
Keep pastures mown to make the environment less hospitable for ticks. Remove brush and wood piles from pasture areas to decrease rodent nesting areas, which in turn helps to decrease tick populations. Routinely using topical anti-tick products containing the chemical permethrin can also be very helpful. Because the disease tends to vary from area to area, a local veterinarian should be consulted for the best information on prevention.