December 5, 2017
Lyme disease is a frustrating problem confronting veterinarians and horse owners in areas where ticks are prevalent. The disease, caused by a spirochete bacterium called Borrelia burgdorferi, is transmitted through the bite of infected deer, or blacklegged ticks. As ticks are virtually ubiquitous, the carrier of the disease is essentially impossible to eradicate.
Lyme disease usually causes vague clinical signs. Common signs may include generalized stiffness, joint swelling (less common), hyperesthesia, shifting leg lameness, loss of muscle tone, slight fever (less common), weight loss, and poor performance. Serious cases can develop nervous system disease, serious eye problems, or both.
The lack of specific clinical signs is problematic because horses can show vague clinical signs from many conditions, and for many of reasons; even normal horses may show “typical” signs on an off day. As such, some normal horses are diagnosed with the disease, get treated, and get “better,” even though they never had Lyme. This makes confirming the true scope of the problem difficult.
If Lyme disease is a possible concern, antibody titers are often run. There are several tests, for example, one developed at Cornell University (bit.ly/2zDPwN8). However, while the tests certainly are useful, they are not without problems. For example:
Recent work at the University of California, Davis indicates that the bacteria causing Lyme disease may be able to fool a horse’s immune system into not launching a full-blown immune response or developing lasting immunity to the disease. This may explain why some patients are infected repeatedly. It also suggests that blood tests may not be an effective method for detecting previous exposure to Lyme disease.
Because of the above problems, a positive test for Lyme disease is likely to be meaningless in an otherwise normal horse. As such, there is no reason to run Lyme antibody tests during the course of a prepurchase examination.
Antibiotics from the tetracycline class, e.g, doxycycline and minocycline, are most commonly used in treatment, although ceftiofur sodium also can be used. Treatment recommendations are usually to medicate the horse for 30 to 90 days, depending on the response. The medications themselves may not be without problems—horses may not eat them (powder, pills, or capsules), daily IV dosing is often inconvenient and expensive, and the availability of oral tetracylines has been spotty over the past several years.
Compounded medications appear to be relatively unstable, losing potency after seven days,1 and compounded liquid doxycycline or chews do not have acceptable drug content compared to the labelled dosages at Day 1 or Day 21.2
Tetracyclines have anti-inflammatory effects that may help certain conditions that aren’t Lyme disease, confounding an evaluation of the response to treatment. Horses that do not have Lyme Disease and are treated may appear to get better, further compounding evaluation.
Treatment is usually continued until the horse seems better. Titers are not a good way to monitor treatment response; in fact, titers in some horses increase with treatment. Unfortunately, in serious cases, e.g., if the eyes or nervous system is affected, treatment probably is unlikely to help due to bioavailability issues. Some cases of chronic can only be diagnosed post-mortem.
There is no vaccination against Lyme disease that has been approved for horses. There are at least two for dogs, however, and one of them has been tested in horses (Recombitek). When two vials are given under the skin, three weeks apart, a good immune response has been demonstrated. However, the vaccine is relatively expensive, what constitutes a protective titer against Lyme disease is unknown, and after the initial series, the vaccine must be repeated every six months.
The most effective prevention methods would appear to be those that minimize or repel the tick population. In forested areas, or in areas with stalls and warm bedding—for example, the entire Northeastern US—it’s essentially impossible to eradiate ticks. Various canine tick sprays or repellants may be recommended, including dilute solutions of malathion. Stall cleanliness is important, as is brush control around horse pastures and barns. Still, the fact is that these are very hardy parasites.
Experienced practitioners have strategies that they may find effective in endemic areas, and internal medicine specialists at universities in endemic areas can be an invaluable source of information about Lyme disease. However, regardless of one’s level of expertise, the diagnosis and treatment of equine Lyme is frustratingly difficult, much as in human medicine.
1 Papich MG, Davidson GS, Fortier LA. “Doxycycline concentration over time after storage in a compounded veterinary preparation.” JAVMA. 2013; 242(12): 1674-8.
2 KuKanich K, KuKanich B, Slead T, Warner M. “Evaluation of drug content (potency) for compounded and FDA-approved formulations of doxycycline on receipt and after 21 days of storage.” JAVMA. 2017; 251(7): 835-842.
Dr. David W. Ramey is a Southern California equine practitioner who specializes in the care and treatment of pleasure horses. Visit his website at doctorramey.com. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.
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