Researchers might not know everything about horse health conditions, but they often know enough about ailments to render some type of treatment. However, there are some conditions that remain difficult to treat. And diagnose. And study. One of those conditions is equine Lyme neuroborreliosis (NB).
At the 2014 American College of Veterinary Internal Medicine Forum, held June 4-7 in Nashville, Tennessee, Amy Johnson, DVM, Dipl. ACVIM, reviewed Lyme NB, what researchers know about the condition, and what they're still trying to decipher.
Lyme NB, a rare condition that occurs when Borrelia burgdorferi infects a horse's nervous system, can cause a variety of clinical signs and is very challenging to diagnose, said Johnson, an assistant professor of large animal medicine and neurology at the University of Pennsylvania School of Veterinary Medicine (Penn Vet) New Bolton Center .
Six cases have been reported in the literature, and Johnson has dealt with several other cases at Penn Vet. In the latter group of horses, presenting complaints varied and included:
- Weight loss;
- Gait abnormalities;
- Neck stiffness;
- Behavior changes;
- Uveitis (a common cause of vision loss);
- Dysphagia (difficulty swallowing);
- Atrial fibrillation (an abnormal heart rhythm);
- Dyspnea (difficulty or labored breathing);
- Muscle atrophy (wasting);
- Muscle tremors or fasciculations (twitching); and
- Cranial nerve deficits.
Affected horses also had a variety of histopathologic (identified on microscopic evaluation) lesions in the central nervous and other body systems, and all of the Penn Vet horses either died or were euthanized, despite varying degrees of improvement and deterioration of their conditions.
Horses infected with B. burgdorferi experimentally, however, have had very different experiences, Johnson said.
Researchers from Cornell University conducted three B. burgdorferi studies, ultimately hoping to establish an infection model in horses and assess different treatment and vaccination efficacies.
In all the experiments, the team placed B. burgdorferi-infected ticks onto the clipped sides of ponies and allowed them to feed, attempting to mimic natural exposure.
Johnson said 27 ponies from all three studies became infected, and all but one seroconverted (produced detectable antibodies). The researchers identified B. burgdorferi in the skin and a variety of internal organs, and three ponies had histologic lesions in their nervous systems.
However, Johnson relayed, none of the 27 affected ponies showed outward clinical signs of infection, indicating that subclinical infection is likely common. Thus, it’s been very difficult to extrapolate information from these studies to naturally occurring cases, where the real challenge is determining whether an infection is clinical or subclinical, she said.
Currently diagnostic options for Lyme NB are limited, but Johnson suggested using criteria based on human medicine, including:
- Potential exposure to the organism (such as time spent in an endemic area);
- The presence of neurologic signs; and
- Evidence of nervous system infection.
Johnson said serology (blood testing) alone doesn't have much value, as positive results only indicate exposure to the organism—be it from a past or current infection. She said cerebrospinal fluid (CSF) analysis of affected horses often yields abnormalities, but horses with little or no central nervous system involvement (e.g., horses with peripheral nerve involvement) might have normal CSF samples.
Johnson noted that the most useful tests for Lyme NB include identifying the organism in CSF, identifying intrathecal antibody production (antibodies produced in the central nervous system), seropositivity, and identifying B. burgdorferi from other body sites, such as the eyes or skin.
Johnson cautioned, however, that "antemortem diagnosis of Lyme NB is rarely accomplished, with post-mortem examinations frequently required."
Like diagnostic options, the ideal treatment options for Lyme NB remain unclear. While antibiotics such as tetracycline, doxycycline, and ceftiofur all eliminated Lyme NB infection in experimentally infected ponies, similar results in the field are uncommon, Johnson said.
"Successful treatment of equine Lyme NB is difficult," she said. "This assertion is certainly biased as definitive diagnosis is often based on post-mortem findings. However, the horses that have succumbed to NB have often been treated for long periods of time (months, sometimes) prior to death or euthanasia with drugs that, in theory, should be effective, including oxytetracycline, doxycycline, and minocycline."
Johnson recommended the following treatment protocols for horses suspected of having Lyme NB:
- If finances allow, parenteral (intravenous) penicillin, cefotaxime, ceftriaxone, or ceftazidime;
- If the aforementioned options are cost-prohibitive, minocycline might be more effective than doxycycline or oxytetracycline.
"Of the six cases reported in the literature, only one responded to treatment with long-term success," Johnson said. "This poor long-term prognosis was reflected in the clinical progression of horses diagnosed at post-mortem with Lyme NB at the University of Pennsylvania."
She added that there are also anecdotal reports of affected horses responding well to treatment.
Clearly, there is much to learn about equine Lyme NB, why experimentally and naturally affected horses appear to handle the disease differently, and how to best diagnose and treat the condition.
"As always, owners of horses that might be showing signs of neurologic disease should contact their veterinarians immediately," Johnson said. "Lyme NB can mimic many other neurologic conditions, so diagnostic testing must be performed to rule in or out other potential causes of neurologic disease, such as equine protozoal myeloencephalitis or cervical vertebral stenotic myelopathy (commonly called wobblers syndrome). If infection with Borrelia burgdorferi is confirmed and other neurologic diseases are ruled out, treatment for Lyme NB is warranted."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.