Lyme Disease in Horses

Summary

Lyme disease is caused by the tick-borne bacteria Borrelia burgdorferi. Horses become infected with the bacteria when an infected tick feeds on them. Equine Lyme disease most commonly affects the brain, eyes, and skin. Symptoms such as lameness, muscle stiffness, swollen joints, and behavioral changes have also been reported, however, the link between Lyme disease and these symptoms in horses is unconfirmed.   Diagnosis of Lyme disease requires identification of the pathogenic organism in blood or cerebrospinal fluid along with a thorough diagnostic workup to rule out other potential conditions.  Treatment of Lyme disease requires long-term antibiotic therapy, and has a moderate success rate. Many horses with mild symptoms show reduced or resolved symptoms with appropriate treatment. More severe syndromes, such as the syndrome affecting the brain, have an extremely poor prognosis.

Risk Factors

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.

Possible Causes

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.

Main Symptoms

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. Symptoms of neuroborreliosis include:

• Severe loss of muscle • Difficulty swallowing (dysphagia) • Difficulty breathing (dyspnea)

• Uncoordinated gait (ataxia) • Weakness and paralysis • Painful muscle stiffness

• Muscle tremors

The other well-defined Lyme-associated syndromes are uveitis (inflammation of the eye) and dermatitis (inflammation of the skin). Horses with uveitis show symptoms such as:

• Squinting (blepharospasm) • Cloudy eye (corneal edema) • Small pupils (miosis)

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

Detailed Characterization

Other symptoms associated with Lyme disease in horses include:

• Lower limb and joint swelling • Lethargy • Behavioral changes

• Intermittent lameness or rotating-leg lameness

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.

Testing and Diagnosis

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.

Steps to Recovery

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.

Prevention

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.

Is Lyme Disease Common in Horses?

Lyme disease appears to be rare in horses, particularly when compared to dogs and humans.

Typical Treatment

Treatment of Lyme disease requires long-term antibiotic therapy.

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