Lyme Disease

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Lyme disease is an infectious disease caused by the bacteria Borrelia burgdorferi and affects horses, humans and many other animals. Although Lyme disease in horses is quite common for animals living in high-risk areas, equine Lyme disease is not easy to identify, since fewer than 10% of horses show any symptoms.

Symptoms vary from horse to horse, but an equine with Lyme disease may show an unexplained reluctance to move, as if sore all over, and/or a transitory lameness that cannot be ascribed to any specific cause. Joint swelling may also be apparent in some horses.

There is a striking variance in the symptoms suffered by individual horses infected with Lyme disease, with some able to tolerate infection without showing signs of illness, and others becoming severely ill. Poor recognition of the infection in horses means that there is little available research material to guide veterinarians.

Veterinarians usually apply a process of elimination when investigating symptoms common to Lyme disease and other illnesses in horses.

Diagnosis in horses is further complicated by problems in detecting the spirochaetal bacteria Borrelia burgdorferi, which is responsible for Lyme disease, in blood samples taken from horses.

Blood tests, such as ELISA and Western blot, are carried out to check for antibodies to the bacteria, although the accuracy of these tests has been questioned, especially in cases of early Lyme disease where insufficient antibodies have built up to trigger detection.

A study published in July 2011 looked at antibody profiling for Borrelia in horses, with a focus on those living in the New England and Mid-Atlantic regions of the US where Infection with Borrelia burgdorferi is common.

The researchers used these animals to assess the efficacy of Luciferase Immunoprecipitation Systems (LIPS) for detecting antibody responses to three antigens used in diagnosing equine Lyme disease. Results of the tests strongly suggest that the LIPS test shows promise in improving detection of the disease.

Horses with Lyme disease may experience problems with multiple organ systems, leading to both acute problems and possible permanent damage and chronic health issues, especially where the infection remains untreated. Lyme disease complications in horses can include liver damage and hepatitis, or severe neurological injury from encephalitis, resulting in ataxia, and/or behavioral changes.


The most common symptoms include:

  • Lameness
  • Swollen joints
  • Arthritis
  • Changes in behavior
  • Shifting from limb to limb
  • General stiffness
  • Irritability
  • Refusal to work or exercise


Lyme disease is caused by a bacterial spirochete, Borrelia burgdorferi, which is spread by ticks. The ticks belong to the Ixodes family and are primarily found on deer, but can also be found on rodents such as the white footed mouse, the kangaroo rat, and the wood rat. These animals serve as an intermediate host for ticks when then move on to infect horses, cattle, and pets, as well as humans.


At present, there is no licensed vaccine for equine Lyme disease, but, since vaccines have been developed for dogs and one also exists for humans, there are hopes that one for horses will soon be available.

The only prevention method currently available is tick control. The horse should be groomed frequently and all ticks removed immediately. On horses, the ticks are most likely to be found on the head, throat, stomach, or under the tail.

Tweezers are used to remove the ticks, pulling straight upwards to make sure the tick is completely removed; otherwise, mouth parts of the tick may remain embedded in the animal and infection is still possible.

Ticks need to be on the animal for 12 to 24 hours before they can transmit the infection. In tick-infested areas, veterinarians often recommend the use of tick repellants; those based on chemical permetherin are particularly effective.

Equine Lyme disease is not contagious, and one sick animal cannot infect others. However, an infected animal is a sign that there are ticks in the area, and that all animals, including humans, are at risk.


In delaying diagnosis of Lyme disease in horses while test results are determined, the primary window for treating the infection may be missed. Early symptoms are often attributed to exhaustion from work, arthritis, or muscle strain from over-activity. Horses out to pasture have lower detection rates for early infection compared to those in active duty. However, active horses, such as show-jumpers, dressage, or race horses, are more likely to experience misdiagnosis rather than late diagnosis.

Lyme disease in horses is diagnosed by checking the blood for specific antibodies and a clinical evaluation. The blood test has limitations because it does not differentiate between infection and mere exposure to the organism.

Lyme disease can be effectively treated with various antibiotics, such as tetracycline or doxycycline. Treatment takes several weeks. If equine Lyme disease is diagnosed correctly, the animal usually responds quickly, with the first signs of improvement seen in 2 to 5 days.

If there is no quick improvement, it is most likely that the diagnosis was not correct in the first place, and the horse has some other problem.

One study of Lyme disease using ponies deliberately infected with Borrelia bacteria showed that tetracycline treatment appeared most effective in eradicating infection. A four-week course of tetracycline, doxycycline, or Ceftiofur resulted in a drop in antibodies in all groups.

Use of anti-inflammatory drugs to minimize pain and stiffness and stomach medication to help the horse cope with the antibiotic treatment are helpful, but have no influence on the infection itself.

Considering the difficulties in diagnosing and treating Lyme disease, as well as the expense of veterinary care for horses, more accurate testing for Lyme disease in horses is being pursued by equine medical researchers.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.