Lyme disease, also known as Equine Borreliosis, is caused by the bacteria Borrelia burgdorferi. It got the name Lyme disease because William Burgderfer in Lyme, Connecticut initially diagnosed it in 1982 as the cause of inflammatory arthritis in humans.
Several different ticks nationwide carry Borrelia burgdorferi. On the East Coast, and here in New England, it is carried by the deer tick (Ixodes dammini) and to some extent by a relative of the deer tick (Ixodes scapularis). In the western half of the U.S. it is carried by the western lack-legged tick (Ixodes pacificus). It is reported that anywhere from 12-99% of deer ticks may carry the Burgdorferi. Only 4-5% of western black-legged ticks may be infected.
The ticks go through several life cycle stages on different animals. It goes through larval and nymph stage (immature) stages while feeding on the white footed mouse. As an adult, the tick feeds on the white tail deer. The tick can pick up B.burgdorferi from either the mouse or the deer. It is the adult form of the tick that feeds and transmits it to the horse. As many as 35% of horses in the eastern half of the U.S. may have been exposed to Lyme disease. There are other animals that can be infected with Lyme disease by the same kind of tick, including dogs, cows and humans.
Once an infected tick feeds long enough on a horse the organism multiplies in the skin at the site of the tick bite. It becomes numerous enough to spread through the body via the blood stream. The most common signs of Lyme Disease that are initially observed are fever, lameness due to joint disease (it can shift from one joint to another), irritability form being handled or tacked up, or having swollen lymph nodes. There have also been reports of heart disease, founder, reproduction failure, eye problems and death of foals due to Lyme disease but these are unusual cases.
So how do we diagnose a horse with Lyme Disease? A diagnosis needs to be based on exposure to ticks showing clinical signs and blood work. Sometimes it is hard to know if a horse has had exposure to ticks so that sometimes has to be assumed. The bloodwork test that is run measures the level of antibodies in a horse’s bloodstream (titers) to B burgdorferi. Currently Cornell University offers the best quantitative test for Lyme Disease called the Lyme multiplex Test. Unfortunately horses will produce antibodies to the organism after exposure to it (but they may not necessarily have an infection) or even after vaccination for it. For this reason, it may be recommended to measure the titers twice about 3-4 weeks apart. If the titer increases on the second test, and there are clinical signs of Lyme Disease there is a presumptive diagnosis can be made, and treatment should be started.
Avoiding exposure to the ticks that carry B burgdorferi can prevent Lyme disease. Unfortunately this can be tough to do in some places, such as Lyme County in Connecticut. There are also a few vaccines that can be ordered to vaccinate against Lyme disease. Although these vaccines are relatively new and designed for dogs they are considered safe and do offer some potential for protection. So even if preventative measures are taken, a horse can still become infected with Lyme disease.
If a horse is diagnosed with Lyme disease it can be treated. Early infections (which are hard to catch) can be treated with the antibiotic Doxycycline for 4 weeks. Later stage infections we see, can be treated with the same antibiotic for longer time periods, ranging from several months to several years. It is also recommended to measure titers at 3-4 months after going through the course of treatments. Using these titers, we can evaluate the effectiveness of the treatment, and get a base line for further tests. Once clinical signs of Lyme disease diminish and the titers decrease or stay the same, therapy can be stopped. Therapy usually can get rid of clinical signs, but if the horse has had the disease for a while sometimes the changes (such as joint problems) can have permanent effects.
In summary, Lyme disease here on the East Coast is becoming more commonly diagnosed and can be difficult to prevent in some areas. We are fortunate to have antibiotics that are effective against the organism. If Lyme disease is diagnosed early enough it is less likely to produce permanent problems.