The number of human Lyme Disease cases is steadily increasing, with the CDC estimating there are at least 10X more cases than get reported. For every human case there are likely many more equine since they live outdoors and finding the tiny nymph stages responsible for most transmissions is extremely difficult on a horse even with meticulous grooming. Lyme Disease is most common in the Northeast (Maryland and Delaware and states north of there) and in the midwestern states adjoining the Great Lakes but has also been reported virtually anywhere except in the most arid desert regions.
The nymphal tick stages most likely to be transmitting Lyme Disease are much smaller than adults, about the size of a poppy seed.
Multiple studies have described antibody confirmation of equine infections with the Lyme organism, Borrelia burgdorferi. What we don’t know is how many infections actually produce symptoms. Researchers have been unable to experimentally produce symptoms even when it is confirmed several months later that the horse was successfully infected.
The most well documented consequence of Lyme in naturally occurring infections is neurological disease. Signs mimic other causes of meningitis or encephalitis and include anything from cranial nerve problems and sensitivity to touch to gait changes, ataxia and muscle loss. Borrelia can also invade the eye and produce a picture identical to “moonblindness”/periodic ophthalmia. The development of skin nodules has been reported at sites of bites from infected ticks. These are all issues that develop weeks to months after infection.
Signs of recent infection, fatigue and mild fever, are likely to be missed in horses and we can’t see the skin rash that is typical in people. Lyme may be treatable with a single dose of antibiotic in these early stages but horses are never diagnosed early. Additional signs that have been described in chronically infected horses include a shifting lameness, weight loss, depression, behavior changes and muscle pain.
Laminitis may also be seen in Lyme positive horses. It is often severe and when insulin elevations are detected they tend to be on the low end and disproportionate to the severity of the laminitis. These horses do not respond as expected to the usual dietary modifications.
The best diagnostic option available today is Cornell University’s Multiplex assay. This test can detect both early and late stage infections. Idexx Laboratories SNAP test is also accurate but may take up to 8 weeks after infection to become positive.
Treatment of Lyme disease in horses is complicated by the fact the infection is not detected until the late stages. It is known from human medicine this makes treatment more difficult. Oral doxycycline or minocycline is usually prescribed but treatment may need to be repeated. Research has shown the best results with 28 days of intravenous tetracycline.
Prevention is always preferable but options are limited. Ticks can be discouraged in areas where horses are kept by keeping them dry/well drained, mowing grass, eliminating brush and piles of trimmings, avoiding wooded areas and fencing out deer.
All common ingredients in equine fly sprays have some tick repellent activity, with permethrin best against the species which transmit Borellia burgdorferi. However, repellents and grounds keeping will never be able to protect the horse 100% from tick bites.
There is no vaccine commercially available for horses. It has been documented that horses develop antibodies in response to canine vaccines, and that antibody levels are better when the dose of vaccine is doubled, but antibody levels only persist for about 16 weeks. Horses would have to be vaccinated at least twice a year to keep their titers up and even then we don’t know for sure if those titers are protective. The safety of vaccinating a horse that is already infected is also unknown.
The threat of Lyme disease is likely to grow before it lessens. If you live in an area at risk for Lyme, always keep it on your radar. Positive antibody tests alone do not confirm the B. burdorferi infection is causing your horse’s issues but this puts it squarely on the list of possibilities for your veterinarian to consider.
Eleanor Kellon, VMD