Name. It's named after the town of Old Lyme and Lyme in Connecticut, where the first human cases were recognized.
Organism. Borrelia burgdorferi, a spirochete bacterium.
Transmission. By tick bite. Although many species have been found to carry the infection, it is the Ixodes ticks (black-legged tick, common deer tick) that are most likely to feed on a horse and transmit it.
Hosts. All mammals could be infected, although they vary widely in how quickly they clear the infection and whether they maintain enough circulating organism to pass the disease on to feeding ticks. Reservoir hosts are animals that infect ticks during feeding. Rodents are the most important known reservoir host, showing high levels of circulating organisms in their blood for several months after an infection. Research is looking into a possible role for some birds as a reservoir of infection, including the common robin.
Risk. We still don't know what percentage of horses exposed to the Lyme spirochete will develop signifi cant symptoms, but risk begins with exposure. Human statistics show reports of Lyme disease from almost all states, but the heaviest concentration by far is in the Northeast (Delaware and further north), Minnesota, and Wisconsin. Approximately 25,000 cases of human Lyme disease are reported each year.
Immunity. Immunity to reinfection is short-lived (no longer than a year) in other species, and likely in the horse as well.
Fever can be an important clue, since horses with things like osteoarthritis or EPSM aren't going to be running a fever. Persistent and/or recurrent fevers coupled with stiff or painful muscles and joints will raise the index of suspicion for Lyme before blood work changes.
The two most widely available diagnostic tests are ELISA tests for antibodies and Western Blot. Most ELISA tests use cultured organisms as their antigen source. The horse's blood is mixed with the antigen bound to an enzyme. If there are antibodies that bind to the test, a color change occurs. Problems with this test include the inability to diagnose early infections and false positives from cross-reactions. All equivocal or positive reactions on this type of ELISA need to be confirmed by Western Blot.
Western Blot is a technique that binds antibody to antigen then uses electrophoresis to separate out the antigen-antibody complexes into specific bands, each corresponding to an antibody to a specific component of the organism. A minimum of three bands is required to call a sample positive. The more bands there are, the more strongly positive the result and likely the more chronic the infection is, since the Lyme organism is known to change its outer coat multiple times during the course of an infection.
A newcomer to ELISA testing is directed to a Lyme antigen called C6. This test was first available for human testing seven years ago. It uses a synthetic antigen that is derived from an outer surface protein called C6.
Researchers found that the C6 antigen is present in all Lyme organisms from around the world and doesn't change when organism goes through the various mutations inside the body. The C6 antigen can also distinguish between vaccinated and actively infected animals.
Data from confirmed human Lyme infections suggests it may become positive earlier in the course of an infection that other antibody tests. There are virtually no false positives with the C6 ELISA (no cross-reactions), 37% false negatives in early disease but 0 false negatives in later stages.
This test is currently being offered by Idexx Laboratories as either an on-the-farm quick Lyme test (the SNAP 3Dx or SNAP 4 Dx-developed for dogs originally but work, for equine samples as well), or as an in laboratory C6 ELISA, which also gives a titer. The in-laboratory test is useful since it allows your veterinarian to follow the titers to see if an infection is active (rising titer) and/or if the horse is responding to treatment (dropping titer).
The IgenX Laboratory in California offers testing for Lyme such as:
• Lyme Dot-Blot Assay (LDA) - Uses antibodies against the organism to check for antigens in the urine. False positives possible because of cross-reactions with other organisms.
• Reverse Western Blot - Tests for antigen as above, but separates out the complexes into specific bands for confirmation that it is truly Lyme.
• Multiplex PCR - Very sensitive test that can detect either whole organisms or pieces of them by detecting bits of DNA. This test can be run on tissues, blood, urine, cerebrospinal fluid or joint fluid.