Lyme Disease

Lyme Disease (Lyme Borreliosis) in Horses

Lyme disease, which is caused by the bacteria Borrelia burgdorferi and transmitted through the bite of a tick, affects domestic animals and humans. At least 3 known species of ticks can transmit Lyme disease. However, the great majority of Lyme disease transmissions are due to the bite of a very tiny tick commonly called the deer tick, or black-legged tick.

The best protection against Lyme disease is to avoid exposing your horses to the ticks that transmit the Borrelia burgdorferi bacteria. Keep pastures mowed and remove areas where rodents nest. Ask your veterinarian to recommend an appropriate tick repellent for your horse. Daily grooming away from stalls or exercise areas can remove ticks resting on your horse’s coat. If possible, capture and properly dispose of any ticks you find; otherwise they may reattach themselves to another horse or other animal. Remove any ticks by using fine-pointed tweezers to grasp the head of the tick (right where it enters the skin). Pull the tick straight off, making sure not to grasp or squeeze its body.

Not all infected horses develop clinical signs of Lyme disease. If clinical signs occur, they can include chronic weight loss, sporadic lameness, shifting leg lameness, low-grade fever, muscle tenderness, chronically poor performance, swollen joints, arthritis and diverse orthopedic problems. Changes in behavior and skin sensitivity (tactile hyper-aesthesia), both with rapid onset, are common clinical signs seen by vets in horses with potential Lyme disease. Neurological signs such as depression, dysphagia, head tilt and encephalitis were reported in chronic cases.

The diagnosis of Lyme disease in horses can be made based on:

  1. Horses living in an area where  burgdorferi infected ticks are endemic
  2. Horses that have a history of visiting an area with infected ticks
  3. Ticks that have been found on the horse
  4. Clinical signs compatible with Lyme disease
  5. Ruling out other causes of clinical signs that may have similar signs than those associated with Lyme disease, i.e. orthopedic disease, behavioral or training issues
  6. A high antibody to burgdorferi
Lyme Disease Treatment

The most common drugs used to treat equine Lyme disease are tetracycline antimicrobials, such as doxycycline, oxytetracycline or minocycline.

These drugs also have anti-inflammatory properties which may help alleviate other symptoms. Other antibiotics that are effective against B. burgdorferi include penicillin and cephalosporins.

In general, the earlier you start treatment after confirmation of Lyme disease, the better. Cases that are caught and treated early tend to have a better prognosis than chronic cases or cases with neuroborreliosis in which the prognosis is generally poor.

Treatment of equine Lyme disease is often difficult, especially if the infection is chronic. Overall, the treatment protocols are poorly defined because the disease is rare and there is a lack of clinical data in horses.

Treatment should be continued until the clinical signs resolve. Re-testing antibody levels may provide some valuable therapeutic information, but should not be used alone as an indicator of the resolution of the disease. Horses may have a positive test years after resolution of clinical symptoms.

Unfortunately, even after symptoms resolve a horse can contract Lyme disease again if bitten by another B. burgdorferi-carrying tick. Contracting Lyme disease once does not convey immunity.