Lyme Disease in Horses

Lyme disease is a tick-borne illness associated with generalized illness and long-term neurological and kidney effects in humans and dogs. Although horses are exposed to ticks with the same level of risk as other outdoor animals, Lyme disease in horses is thought to be rare and not well understood.

Diagnosing Lyme disease is an extensive process, with numerous tests required to rule out other similar conditions. Most horses with Lyme disease have a good prognosis, with symptoms resolving or reducing after treatment. Lyme disease affecting the brain has an extremely poor prognosis.

While humans can contract Lyme disease with the appropriate tick vector and necessary attachment and incubation times, the disease is not transmitted directly from horses to humans.

Lyme disease appears to be rare in horses, particularly when compared to dogs and humans. Horses in endemic areas such as the Midwest, Pacific, and eastern United States are more likely to become infected with the bacteria that causes Lyme disease.

Having antibodies against Borrelia is a common finding in horses in areas where Lyme disease is prevalent. Some reports from endemic areas show up to 75% of horses have antibodies. Of these horses, the exact proportion that develop symptomatic Lyme disease is unknown.

The deer tick (blacklegged tick) transmits the bacteria that causes Lyme disease. These ticks feed on an infected animal, such as a wild rodent. The tick then feeds on a horse and transmits the bacteria into the horse’s bloodstream. Transmission does not occur immediately after attachment, and the odds for transmission go up the longer the tick is attached to the horse.

Symptoms of Lyme disease in horses are poorly defined. Further research is needed to better characterize this disease in horses.

The most well-established form of Lyme disease in horses is neuroborreliosis, a condition where the bacteria affects the brain.

The other well-defined Lyme-associated syndromes are uveitis (inflammation of the eye) and dermatitis (inflammation of the skin).

The dermatitis form of Lyme disease typically results in firm nodules at the site of a tick bite.

Other symptoms associated with Lyme disease in horses include: Intermittent lameness or rotating-leg lameness Lower limb and joint swelling Lethargy Behavioral changes

Since little is known about Lyme disease in horses, diagnosis is often challenging. Ruling out other possible causes of the symptoms is the first step. Other conditions that cause similar symptoms include neurologic conditions and any condition associated with eye inflammation, lameness, or muscle stiffness.

A confirmed diagnosis requires sampling blood or cerebrospinal fluid then testing for evidence of the bacteria and the bacteria’s DNA. Testing for the bacterial DNA is important, since a positive antibody result only indicates the horse has been exposed, not necessarily that the bacteria is actively infecting the horse. Direct testing for the bacteria is the only method of confirming an active infection.

Therapeutic trials are an option for suggesting a diagnosis of Lyme disease. If a horse responds favorably to treatment for Lyme disease, the conclusion is drawn that the horse probably had it.

Treatment protocols for Lyme disease in horses are largely based on protocols for dogs and humans. A long-term course of antibiotics is the most common treatment recommendation. In some cases, antibiotic therapy continues for months.

Most horses with Lyme disease have a good prognosis with symptoms either improving or fully resolving. Some horses will continue to have symptoms despite treatment, and it is unknown whether these cases are truly cases of Lyme disease or if there is another condition occurring. Symptoms such as joint swelling occasionally prevent horses from returning to athletic performance.

Neuroborreliosis is the most difficult Lyme disease-associated syndrome to treat, as many antibiotics are unable to cross the blood-brain barrier to effectively treat the infection. There has only been one successfully treated case of neuroborreliosis reported, making the prognosis for this form of Lyme disease extremely poor.

Lyme disease primarily affects dogs, horses, and humans. The bacteria that causes the disease, Borrelia burgdorferi, is exclusively transmitted by ticks. Taking appropriate precautions to reduce horse exposure to ticks is the best method of prevention. Strategies to reduce exposure include cutting long grass, removing fallen leaves, and reducing wildlife movement through pastures or common areas. Grooming horses daily to identify and remove ticks is recommended because transmission of the pathogenic bacteria is not immediate following attachment. Using veterinarian-approved insecticides for horses to reduce tick bites is also an effective preventative measure.

Lyme disease appears to be rare in horses.

Treatment of Lyme disease requires long-term antibiotic therapy.