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Diagnosing Lyme Disease in Horses is Difficult

By: Louisa Shepard Date: Jul 28, 2015

Most horses in the Mid-Atlantic region show evidence of exposure to Borrelia burgdorferi, the bacterium that causes Lyme disease. The vast majority of those exposed horses do not develop clinical signs of disease. However, a small number of infected horses will develop disease of the nervous system, termed Lyme Neuroborreliosis.

Diagnosis of equine Lyme Neuroborreliosis is often elusive and the disease provokes more questions than answers: What signs do horses with Lyme disease show? What test can be used to diagnose the disease? How should test results be interpreted? 

New Bolton Center veterinarians, led by Dr. Laura Johnstone, a resident in Internal Medicine, sought to answer some of these questions by reviewing the medical records of horses that had histologic findings consistent with previous reports of Lyme Neuroborreliosis in both horses and humans. Dr. Amy Johnson, Assistant Professor of Large Animal Internal Medicine and Neurology, also participated in the research.

Horses seek shade under a tree.

The study included 16 horses that were patients at New Bolton Center from 2004 to 2014. Most of the horses were ataxic (lack of muscle coordination) and showed cranial nerve deficits such as difficulty eating, difficulty breathing, tremors of the muzzle, and poor tongue tone. Behavioral changes, muscle wasting, neck stiffness, and uveitis (eye inflammation) were also common. Only a couple of the horses were lame, and one horse had a heart rhythm abnormality.

Results of diagnostic tests were variable. Only about half of the horses were positive for Lyme on a blood test. The fluid that surrounds the spinal cord was analyzed in 13 of the 16 horses. Some of the horses had normal cerebrospinal fluid (CSF) while others showed evidence of inflammation. Testing the CSF using Polymerase Chain Reaction, a test that looks for Borrelia burgdorferi DNA, was negative in 7 of 7 horses that were tested.

This study demonstrates the poor reliability of diagnostic tests for Lyme Neuroborreliosis in horses. Whether negative or positive in blood or CSF tests, a horse may have Lyme disease. However, what appeared to be a frequent feature among these horses was the presence of cranial nerve deficits and uveitis.

Learn more about New Bolton Center's Comprehensive Equine Lyme Disease Evaluation Package.