Lyme Disease

Lyme disease is a bacterial infection caused by Borrelia burgdorferi, a spirochete transmitted by Ixodes or blacklegged ticks.  Deer and mice are the normal host mammals in the Lyme disease cycle.  When the bacterium is introduced into aberrant hosts (unintentional hosts such as horses and humans) it produces multiple molecules that activate the host's immune system.  The immune response begins as a local reaction. If the organism is not cleared by this local response, but continues to spread through the system, a systemic reaction is mounted against it.  If the infection is still not cleared, the systemic immune reaction continues, damaging normal tissue as it attempts to eliminate the organisms. The tissues most commonly damaged in the horse are the joints.   

Transmission

An adult tick must have attached and fed on a deer or mouse infected with Borrelia burgdorferi.  The tick then carries the live bacteria in its "gut".   In order to transfer the bacteria to a horse, the infected tick must attach to the horse and feed for longer than 24 hours.  The live bacteria is transferred from the ticks gastrointestinal system into the horse’s bloodstream as the tick feeds and the horse then becomes exposed to Borrelia burgdorferi.  If the horses’ immune system cannot clear the infection, the organism will continue to spread and the horse will be infected with the bacterium resulting in a case of Lyme disease.

Clinical Signs

The presenting clinical signs are dependent upon where the bacteria settles.  Initially, a transient fever is seen. The fever often disappears within 24 hours. This is before the other clinical signs of diffuse (general) muscle soreness, generalized joint stiffness,  or a shifting lameness are seen. As the disease progresses, poor performance and weight loss may become evident. In rare cases, the bacteria can cross the blood brain barrier and infect the central nervous system.  If this occurs, neurologic signs like ataxia may appear.  

Many horses will not show clinical signs, even when infected.  Only 5-10% of infected animals will show clinical signs and those generally appear 2-5 months after infection, once the disease has entered the chronic stage.  Due to the vague nature of the presenting signs as well as the delayed onset of symptoms, Lyme disease may not be in the initial differential diagnosis of your horses’ issues. Instead, the clinical signs may be mistaken for age related decline, overwork, arthritis and orthopedic problems, or other infections.  

Diagnosis

Lyme disease is diagnosed via blood testing in the face of presenting clinical signs. The blood tests that may be used include the indirect fluorescent antibody test (IFAT), the Western blot test (WB), the whole cell enzyme-linked immunoassay  test (ELISA), or the multiple antigen ELISA test (Multiplex). Diagnosing Lyme disease is complicated by the fact that many horses have been exposed to Lyme disease and will have detectable antibodies to Borrelia burgdorferi but may not actually be infected.   

The "multiplex" test is recommended and widely used through Cornell Animal Health Diagnostic Lab. It identifies antibody titers to three separate Borrelia antigens and therefore can be used to distinguish between acute (new infections)-Osp C, chronic (or exposure to)-Osp F, and vaccine related- Osp A titers.

In conjunction with the blood test, the horse’s response to treatment is often used to diagnose Lyme disease.  However, the improvement with treatment is not a definitive diagnosis. Tetracycline class antibiotics, the ones used to treat lyme disease, also have anti-inflammatory properties.  In some cases, the clinical improvement may be due to these properties reducing muscle and joint inflammation and therefore, the resulting pain. This means, horses without an active infection may improve on these antibiotics due to the drug’s anti-inflammatory properties.  

For a definitive diagnosis of Lyme disease, a multiplex test should be performed before treatment.  A multiplex test should also be performed after treatment to confirm there has been a notable decrease (about 40%) in the level of antibodies.  If this is seen there is good evidence the disease is no longer active. At this time, there is no way to determine if the organism has gone dormant or is ‘hiding’. 

Treatment

Lyme disease can be treated with antibiotics from the tetracycline class .  This includes oxytetracycline, doxycycline, or minocycline. Oxytetracycline administered intravenously over several days is the gold standard for treatment. However, intravenous treatment requires catheterization or daily veterinarian visits.  Instead, oral doxycycline may be administered at home for 30 days and has provided good results.  

While this is a strong antibiotic class, the organism has at times been able to survive the initial treatment.  In these cases, clinical signs do not resolve, titers continue to rise, and longer treatment may be required or treatment with another drug must be tried.  Titers should be retested to document a 40% decrease below previous levels in 6-8 weeks.

In order to give your horse the best chance to overcome this infection, a good probiotic should be fed during antibiotic treatment.  In addition, the immune system should be supported with good nutrition and a good, balanced vitamin and mineral supplement. There is also some question as to the benefit of supplementing sulfur.  It can be easily added to the regime.

Due to the organism's ability to go dormant, “hide”,  a horse that has been diagnosed with and treated for Lyme disease should be monitored for the return of clinical symptoms, especially after stressful events.  

Prevention

Tick control is key to preventing Lyme disease. Pasture maintenance can significantly reduce the tick population around your horse. Keeping grass mowed in and around the pasture, removing brush, ensuring fences are “deer proof”, and setting fence lines away from the tree-line will all help to decrease the tick population near your horses. Some farms employ guinea hens to reduce the nearby tick population. 

Regular grooming and checking for ticks under mane/forelock, under chin, around sheaths/udders, and near the base of the tail, with quick removal of any ticks is the most important form of prevention, especially during tick season (mid spring through late fall). In some areas, a topical tick preventative may be required.  The only long-term tick preventative currently approved for use in horses is Equi-Spot.

Although a canine vaccine is available for Lyme disease, its use is extra-label in horses.  The vaccination will create elevated antibody levels against  Borrelia burgdorferi, but there is no research to support the effectiveness of the vaccine in horses.  In addition, the elevated antibodies may complicate diagnosing an infection.  This is less of a concern with Cornell Diagnostic Laboratories multiplex test.  Although there is not sufficient evidence based research proving that the vaccine is protective in horses, there is some clinical/anecdotal evidence that the vaccine may help.  If your horse lives in tick infested regions, vaccination may be warranted.

What to do if you find a Tick?

If you find a tick on your horse, please remove it.  It takes 24 hours of feeding to transmit a dose large enough to create Lyme disease.  The sooner you find and remove the tick, the better chance your horse has to clear the organism and stop the infection. 

When removing ticks, ensure you remove the head, as well as the body.  Tick removal scoops or forceps work well. The ‘wives tales’ of nail polish remover, petroleum jelly, or using heat or a match to make the tick retract from the skin are unreliable as well as  dangerous and should not be used. Using the forceps, grasp the tick as close to the skin as possible and pull upward with firm, steady, even pressure. Avoid twisting or jerking the tick. These abrupt actions can separate the ticks head from its body, allowing the mouth parts to remain embedded in your horses’ skin.  If this does occur, attempt to remove the head portions. Regardless of whether all of the tick parts have been removed, clean the skin with soap and water and monitor for signs of infection in the following days.  

Once you have removed the tick, place it in a clean dry plastic bag.  Do not crush or kill the tick. It can be submitted for diagnostic testing to the University of Massachusetts Laboratory of Medical Zoology (www.tickreport.com) where they can determine if the tick was carrying Borrelia Burgdorferi in its gastrointestinal tract.  Results are often back within 72 hours. From that information, your horses’ treatment options and the duration of treatment can be determined.