Lyme disease is named after the town of Old Lyme in Connecticut where an outbreak in 1975 led researchers to identify Borrelia burgdorferi as the bacterial infection responsible for the disease. It wasn’t until 2000 though when veterinarians began to describe cases of Lyme disease in horses, suggesting that cases were becoming more common.
By 2003 Cornelly University was carrying out research into the effects of equine Lyme disease, reporting that around half of all the horses in the Northeastern US tested positive for antibodies to the Lyme disease bacteria. As Lyme disease is only confirmed as transmitted through tick bite this suggests that many horses are being bitten by ticks carrying Borrelia burgdorferi.
Antibiotics for Lyme Disease in Horses
Horses that appeared symptomatic for Lyme disease were treated, for the most part successfully, with intravenous oxytetracycline and further research described how a Lyme disease vaccine used for dogs was also helpful in protecting some ponies from Lyme disease. More recently horses have begun to be treated presumptively, using doxycycline, tetracycline and ceftiofur, all of which can cause toxicity and undesirable side effects.
Symptoms and Signs of Lyme Disease in Horses
The nature of clinical signs of Lyme disease in horses makes it difficult to support the use of presumptive antibiotics. There are many other potential causes of fever, lameness, behavioural changes, and ataxia in horses. Some horses with Lyme disease experience muscle weakness and loss of coordination while others have joint swelling, fatigue, hyperaesthesia, oedema and a sudden aversion to being touched, groomed, or ridden.
Moonblindness in Horses with Lyme Disease
Uveitis has also been reported in horses with Lyme disease. Also known as moonblindness, this eye condition involves increased pressure in the eyes and horses may become anxious at dusk or be less willing to work early in the morning. There are other causes of uveitis in horses however and so it should not be assumed that Lyme disease is the root of the condition.
Equine Anaplasmosis and Lyme Disease
Horses that have fever and oedem may be suffering from Anaplasmosis, or both Anaplasmosis and Lyme disease. Joint swelling and reluctance to work may be caused by arthritis, while muscle tenderness and lethargy could simply be a result of being overworked. The most common symptom of Lyme disease in horses appears to be intermittent lameness. This can be in the front legs, the pelvis, the rear legs or throughout the spine and may be a symptom of Lyme disease or another infection, such as tick-borne encephalitis.
Prognosis for a Horse with Lyme Disease
Fortunately, Lyme disease is rarely fatal in horses, just as it is rarely fatal in humans, dogs, or cats. This is, of course, unless a horse’s guardian determines them to be no longer useful and euthanises the horse. Getting as accurate a diagnosis of Lyme disease in a horse as possible is, therefore, imperative as otherwise a horse that may respond well to antibiotics and return to full health may be killed for sudden lameness or other symptom.
Unfortunately, there is little to no research on the difference in clinical signs of Lyme disease in horses infected in North America compared to those infected in Europe, Asia, or elsewhere in the world. Just as with humans, the specific strain of Borrelia, be it burgdorferi, garinii, afzelii or otherwise may make some clinical signs of Lyme disease more likely.
Diagnosing Lyme Disease in Horses
Lyme disease may be diagnosed in a horse following a veterinarian’s assessment of their geographical location and travel history, their likely exposure to Borrelia burgdorferi, or a horse’s known exposure to the bacteria, such as when a tick that later tests positive for Lyme disease bacteria is removed from a horse or where a positive blood test or synovial fluid is present. In the absence of a positive test for Lyme disease, veterinarians will also consider whether symptoms are likely to be due to another cause. In some cases a horse may be deemed to have Lyme disease and a comorbidity making treatment for both conditions necessary to return the horse to health.