Lyme's disease results from Borrelia burgdorferi bacterial infection. Individuals contract the infection through Ixodes tick bites. Horses, like humans and dogs, are incidental dead-end hosts and therefore not contagious for the disease.
Successful Borrelia burgdorferi transmission from tick-to-horse generally takes 16 to 24 hours to complete; a cursory bite usually won't precipitate the disease. Most horses begin to exhibit clinical signs of disease 2-5 months following bacterial transmission.
In addition to the more common manifestation of Lyme's disease, researchers have recently identified Borrelia-associated pseudolymphoma (a benign disorder of lymphoid cells) and uveitis (inflammation of the eye's uvea).
New Pathogen Linked to Lyme’s Disease
A novel bacteria named Borrelia mayonii causes Lyme disease, according to new research. Testing isolated the bacteria from two patients as well as in 3% of ticks collected in Wisconsin.
B. mayonii is genetically distinct from B. burgdorferi, previously the only pathogen known to cause Lyme's disease. The novel pathogen is associated with Lyme's disease symptoms but not the classic bull's-eye rash.
|READ MORE about Borrelia mayonii
Clinical signs associated with equine Lyme's disease are fairly nonspecific and include chronic weight loss, low-grade fever, intermittent and/or shifting limb lameness, muscle soreness and arthritis. Poor overall performance, increased tactile sensitivity, behavioral changes and abnormal neurologic signs are also infrequently observed.
Diagnosis is usually based on a combination of factors, which include
- The horse's potential for infection based on geographical location
- Clinical signs
- Absence of other causes of disease
- Positive B. burgdorferi antibody titer.
Historically, indirect fluorescent antibody (or IFA) testing, enzyme-linked immunosorbent assays (or ELISA) or Western blot titers have been implemented to detect B. burgdorferi antibodies. While these options remain effective, several newer tests have also proved useful.
The C6 SNAP test is a qualitative assay (meaning it only indicates the presence or absence of antibody as opposed to the amount of antibody) that detects B. burgdorferi antibodies. The test is attractive to many ambulatory veterinarians because it is fast, easy and can be performed on-site. That said, the test has been known to yield both false positive and negative results. Consequently a positive C6 SNAP test result should always be confirmed via quantitative analysis.
The Lyme multiplex assay is an effective option for serologic (blood) Lyme testing. This test, currently offered at Cornell, detects three different antibodies produced at varying stages of B. burgdorferi infection. The evaluation of three antibody patterns enables the equine Lyme multiplex assay to distinguish between early and chronic B. burgdorferi infections. Antibodies produced pursuant to vaccination are also characterized as such.
The multiplex is quantitative by design, thereby revealing the amount (quantity) of antibodies present. Quantitative antibody values allow for facilitated tracking of treatment success, vaccination stimulus and disease recurrence.
One of three antibiotics are typically utilized to treat Lyme's disease:
It is not uncommon to treat infected individuals with daily antimicrobial therapy for 30 or more days.
The multiplex test (see above) allows veterinarians to accurately tract the success of their therapy and determine a safe time to conclude treatment. A horse's antibody levels are expected to be about 50% of pre-treatment values within three months of initiating antimicrobial therapy. A decrease in bacterial numbers results in a commensurate decrease in antibody titer(s) based on quantitative analysis.
B. burgdorferi infection might be considered to be inactive if antibody values do not change significantly over time. In this case, infection can be successfully managed by the horse’s immune system and antibiotic therapy may not be necessary. Of course, treatment is always recommended for those individuals displaying clinical signs.
In high-endemic regions, chronically-infected horses are often monitored annually or biannually using the Lyme multiplex assay. Progressively-increasing antibody values might indicate bacterial reactivation and prompt antibiotic treatment, whereas stable antibody values would suggest inactive infection not automatically requiring medical therapy.
Despite treatment, some horses remain chronically-infected and exhibit persistent (lingering) clinical signs. Generally speaking, the longer that B. burgdorferi infection persists the is more challenging it is to treat.
Lyme's Disease is best prevented through successful tick control.
At the Atlanta Equine Clinic, we have had great success using topical Fipronil therapy during tick season. One (Frontline® Plus for Dogs, 89-132#) dose is applied along the base of the horse's mane and another dose is applied along the head of the tail once monthly to prevent tick bites. This practice also reduces fly bites by up to 85% in our experience.
Currently, no licensed vaccine targeted for equine Lyme's disease exists. Some veterinarians that practice in endemic regions administer dog vaccines (off-label) to horses in the hope of successfully preventing the disease. Results with regard to efficacy have been mixed.