Adult tick, Ixodes scapularis
Lyme disease is caused by a bacteria called Borrelia burgdorferi, a spirochete that is transmitted by ticks. It was first diagnosed in humans around 1980 in Old Lyme, Connecticut. When humans are infected with the Lyme spirochete, Borrelia burgdoferi, the clinical signs include rash, flu-like symptoms, joint pain, and rarely arrythmias of the heart and neurologic disease. Dogs usually present with high fevers, joint pain and swelling, and in some breeds, glomerulonephritis, a disease of the kidneys. The disease in horses is much more nebulous and controversial than it is in dogs and humans. Researchers debate whether or not horses develop clinical signs of Lyme disease. However, it has been shown that horses do become infected with the organism and that the infection can be transmitted to and from them. It is the opinion of the doctors in this practice that some horses do develop clinical signs when they are infected with the Lyme spirochete.
Unlike humans and dogs, where severe signs may accompany infection, signs of disease in the horse are usually more subtle. The main clinical signs that horses show are the following:
1. Intermittent lameness that shifts from leg to leg.
2. Severe back pain.
3. Skin sensitivity-usually an increase in skin sensitivity is observed when currying the horse.
4. A change in attitude- a docile horse may become fractious, an energetic horse may become lethargic.
5. A subtle change in the way the horse is moving-usually noticeable to an experienced rider.
Less commonly there are reports of uveitis, laminitis, neurologic problems and rheumatoid arthritis.
There are four tests available to diagnose Lyme disease in the horse. The first test is an in house ELISA snap test that tests for the antibody against the C6 peptide, a unique section of one of the Borrelia borgdorferi surface antigens. This test will test positive within 3-5 weeks of infection. Although the test is labelled for dogs, studies have shown that the test can detect antibodies to Borrelia burgdoferi in infected horses. The second test is the Lyme disease multiplex assay. The third test is an equine ELISA test that measures the level of antibodies against the disease, but is not completely specific for the Lyme organism, nor is it a good indicator of current infection. The fourth test , the Western Blot, looks for specific antibodies against the DNA within the nucleus of the Lyme spirochete. This test is much more specific for the organism and a better indicator of current infection. The Lyme multiplex assay combines the advantages of the previous ELISA and Western blot testing. It is a fully quantitative assay that simultaneously detects antibodies to different specific antigens of B. burgdorferi as indicators of acute or chronic infection. The assay also distinguishes between antibodies induced by infection with the spirochetes or vaccination against Lyme disease. The equine Lyme multiplex assay is sent to an outside lab where the results take 3-7 days. If the in house snap test is positive, treatment can start immediately.
If a horse tests positive for Lyme disease and has clinical signs, we recommend treatment. There are four treatment protocols available. The Lyme bacteria is sensitive to ceftiofur, doxycycline, minocycline and oxytetracycline. We will discuss the treatment options at the time of diagnosis and select the option that is best for the individual patient .It is important that the horse does not miss any doses of antibiotic therapy. If a dose is missed, the bacteria can form a biofilm rendering further treatment ineffective. We also treat the horse with probiotics while they are on the antibiotic, and an anti-endotoxic dose of flunixin meglumine for the first three days of antibiotic treatment. The treatment protocol is tailored to the indivdual horse's circumstances and needs. The horse is re-tested at least 8 weeks after treatment to monitor changes in the ELISA and Western Blot.
The tick that serves to transport and deliver the Borrelia burgdorferi organism in the northeastern United States is the deer tick, Ixodes scapularis.The pictures to the right demonstrate how small these ticks are during the various stages of their life cycle, and why they can go undetected as they feed on your horse. The female tick lays about 2000 eggs in the spring. The eggs hatch into a larva which attaches to a host as soon as possible. The host is usually a white-footed mouse. If the mouse is carrying the Lyme disease spirochete, Borrelia burgdorferi, the larva can become infected at this point.
The larva are dormant until the following spring. At this time( usually one year later), the larva molts into the next stage of life-the nymph. The nymph is slightly larger than the larval stage and now looks for another host. The host can be the mouse, a dog or a human. The nymph attaches itself to a host and feeds for 3-5 days. If the nymph is carrying the Lyme spirochete, it can infect the host at this time. If it is not infected with the Lyme spirochete, it can be infected at this time if the host it's feeding on is infected with the Lyme spirochete.
Once the nymph is done feeding, it drops off the host and becomes dormant until the late summer. At this time it molts into an adult tick. The adult tick seeks a larger host, usually a deer, however it will attack any host it comes across. Horses and humans are exposed because of the shared enviornment they have with the ticks. (i.e. trails, wooded areas surrounding paddocks, etc.)
Once the adult ticks finds their host, they mate, feed, and transmit disease-if they are carrying it. Once the female tick is engorged with blood, she drops off the host and hides under leaves through the winter. She lays her eggs in the spring and the 2 year cycle starts again. The male tick usually remains on the host through the winter.
In order for the tick to transmit the Lyme spirochete, it must remain attached to the host for 48 hours. As the tick feeds, by sucking the hosts blood, it regurgitates enzymes from it's midgut to keep the blood from clotting. This regurgitation process brings the Lyme spirochete from the ticks midgut to it's mouthparts, where it is passed into the host.
Borrelia burgdoferi has a unique way of avoiding destruction once inside it's host. It changes it's outer surface protein depending on it's location. When it resides in the ticks midgut, it's outer surface protien is OspA. Once it passes through the ticks mouthparts into the host, it travels to the connective tissue, where it changes it's outer surface protein to OspC. ( One of the earliest signs of Lyme disease in the horse is severe back and shoulder pain. This is because the organism is attached to the connective tissue in the back and shoulder muscles.) Later on in infection, the outer surface protein changes to Osp E and Osp F where it travels to the joints. The lyme spirochete can also change shape into different forms where it can hide in cellular folds.
By changing it's form and shape, the spirochete is able to avoid detection by the host's immune system. For example, by the time the host makes antibodies to destroy the outer surface protein that comes from the midgut of the tick, outersurface protein A or Osp A, it has changed into Osp C.
Think of the immune system as an army defending it's country. The soldiers are produced when an enemy is recognized. The body's enemies include bacteria, viruses and parasites. The coat that the enemy wears is how the soldier recognizes it's enemy. If the body produces a soldier that recognizes a coat with a OspA on it, it will destroy any bacteria with OspA. The Lyme bacteria changes that coat before the body has time to send out the OspA soldiers. When the OspA soldiers arrive, they can't find the bacteria , because they are now wearing OspC coats. The Borrelia burgdorferi organism is a master of disguise and camouflage!
Methods to control tick infestation can help in preventing disease. Products containing fipronil can be used on a weekly basis to decrease the feeding time of the tick on the host. Most products do not repell the ticks, but kill the ticks when they start sucking the hosts blood. Remember that the tick needs 48 hours to transmit the Lyme spirochete. Also, the Damminix system can reduce tick numbers when placed in proximity to houses and barns.