In this article from the University of Kentucky College of Agriculture, Linda Mittel, DVM, MSPH, of Cornell University, discusses Lyme disease in horses and humans.
Lyme disease is the most common human disease transmitted by arthropods in North America. Ixodes sp. ticks, the vector of Lyme disease, are being described in areas where they have not been reported before. States in the Northeast, mid-Atlantic, and the Great Lakes region have the highest number of reported human Lyme disease cases. Horses in these areas are at high risk of exposure to the Lyme organism, Borrelia burgdorferi.
Any outdoor horses are at risk due to increased exposure to ticks and the aggressive feeding of the Ixodes ticks. The small ticks are difficult to see in the hair coat. It is thought that ticks must be attached 12 to 24 hours before humans are infected, as is likely to be the case in horses.
Clinical signs associated with equine borreliosis are variable and include shifting leg lameness, myalgia (muscle pain), dermal hypersensitivity, behavior changes, weight loss, uveitis, and neurological signs.
B. burgdorferi diagnostics include culture, direct microscopic visualization and polymerase chain reaction (PCR), but the most often used diagnostics are serologic tests such as immunofluorescence assay (IFA), Western blot (WB) and enzyme-linked immunosorbent assay (ELISA). The new multiplex bead-based Lyme test combines testing by ELISA and WB proteins in a single, quantitative, bead-based assay.
Antibody-based diagnostics makes diagnosis difficult, since it is thought that 60-70% of horses in endemic areas may be seropositive. Often owners request Lyme testing for “baselines,” pre-purchase examinations, assistance in determining causes of poor performance, etc. In the equine borreliosis seropositive horse, it is estimated that a very low percentage actually develop or have signs that may be associated with Lyme disease.
The accepted treatment for equine borreliosis is the use of a member of the tetracycline family of drugs. Response to treatment has been seen by practitioners and owners as a confirmation of a Lyme disease diagnosis, but this response must be cautiously interpreted due to anti-inflammatory effects of tetracycline drugs. It is also difficult to determine response to treatment with serological testing, because B. burgdorferi antibody levels are known to persist for years in humans and are also apparently long-lived in horses. Retesting is suggested four to six months after treatment to see if there is a decrease in serological values. This retesting can be questionable in determining response to treatment because horses are likely to be exposed and possibly re-infected post-treatment if living in areas of high tick density. The new veterinary fluorescent bead-based multiplex test, however, may assist in determining response to treatment.
Horse owners should attempt to prevent exposure to ticks by using insect repellants and removing brush and dense undercover on pastures and close to trails, which provide habitats for host mammals infected with ticks. Attention to grooming also may help prevent tick attachment.
Further studies are needed on the effective diagnosis, treatment, immunity, and prevention of equine borreliosis.