The spread of Lyme disease in horses: Is it time for a vaccine?

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An adult deer tick, a known vector for Borrelia burgdorferi, which causes Lyme disease. Photo: Scott Bauer (Public domain), via Wikimedia Commons
An adult deer tick, a known vector for Borrelia burgdorferi, which causes Lyme disease. Photo: Scott Bauer (Public domain), via Wikimedia Commons

Experts who investigated Lyme disease in horses have asked whether it is time for an approved equine vaccine against the troublesome infection.

Cornell University veterinary professor Tom Divers and his colleagues raised the question in a consensus statement on the tick-borne infection in North American horses published in the Journal Of Veterinary Internal Medicine.

The eight-strong team, which prepared the statement on behalf of the American College of Veterinary Internal Medicine, considered 124 scientific papers in coming to their consensus view on the disease, which can infect horses, humans and small animals.

Lyme disease is caused by the bacterium Borrelia burgdorferi and is spread by Ixodid ticks.

Borrelia burgdorferi infection is common in horses living in Lyme endemic areas, and the geographic range for exposure is increasing, the team said.

Documented, naturally occurring syndromes attributed to B. burgdorferi infection in horses include neurological problems, eye inflammation and skin problems. Other clinical signs such as lameness and stiffness are reported in horses, though are often not well documented.

Treatment in horses is similar to that for humans or small animals but success might not be the same because of species differences in antimicrobial bioavailability and the length of the infection before treatment is started.

The team said the high seroprevalence of B. burgdorferi in adult horses in some areas of North America and the lack of documented cases of Lyme disease have made B. burgdorferi infection and Lyme disease a controversial topic in equine practice.

The seroprevalence of B. burgdorferi in dogs, horses and humans is increasing nationwide, they noted, as is the range of the Ixodid tick.

Evidence shows that the regional spread of Ixodes ticks is linked to increased animal and human exposure to B. burgdorferi in the Midwest, Pacific, and East Coasts of the United States.

Tick spread has predominantly occurred in the Northeast from increased numbers in New York and northward into Vermont, New Hampshire, Maine, and southern Ontario, Canada.

Their distribution has also expanded into Western Pennsylvania and Ohio. In the South, Ixodes has increased in West Virginia, Virginia, and North Carolina.

In the Midwest, spread is predominantly thought to have begun in Wisconsin, and then spread to Illinois and Minnesota, and from Indiana to both Illinois and Michigan.

“The greatest factors thought to affect spread were suitable climate and habitats that support white-tailed deer and mice, and included waterways, river valleys and forests.

“There is strong evidence that equine B. burgdorferi exposure is common in several geographic areas of the United States. There is also strong evidence that the geographic range for exposure is increasing in horses.”

Surveys have demonstrated that clinically normal horses living in endemic areas often have detectable antibody levels against B. burgdorferi.

The researchers described definitive diagnosis of clinical Lyme disease in horses as challenging.

“Positive serology merely confirms past exposure or present infection but does not confirm clinical disease.

“Many other equine diseases share similar clinical signs to Lyme disease, making the diagnosis difficult and raising the strong possibility that Lyme disease is over-diagnosed in areas with a high seroprevalence.

“Ruling out other diseases that might cause the clinical signs in the horse being examined should be a high priority before making a diagnosis of Lyme disease.”

In addition, antigen detection when possible should be used to help confirm a diagnosis, they said.

Response to treatment can rarely be used as a diagnostic method, they said.

Turning to treatment, they said that dealing with the disease in horses is complicated by the difficulty in confirming the diagnosis, as well as the poor bioavailability of oral antibiotics commonly used for treating Lyme disease in horses, and the tendency toward a longer duration of infection in horses before beginning antimicrobial treatment.

Tetracycline antibiotics and a select number of β-lactam antibiotics are the consensus recommendation for treatment of horses with confirmed Lyme disease.

“Duration of treatment is not well-defined in the horse but should be based upon clinical response and to a lesser degree decline in serum antibody level. Treatment should not be based solely on positive serology.”

They advised against treating nonclinical, seropositive horses, saying it would result in unnecessary expense, increased risk of adverse events and inappropriate use of antimicrobials.

They noted that studies evaluating the effectiveness of environmental management of ticks in horses have been relatively sparse.

“A great deal of tick control measures are not equine-specific, nor are they specific to the Borrelia vectored species of tick.”

Well maintained, dry, sunlit, regularly disturbed, and clean areas tend to have fewer ticks. Reduced exposure to woodland and woodland edges is recommended.

Pasture mowing, leaf and debris clearing from pastures and, if possible, exclusion of deer from the surrounding area may be helpful.

“However, a major reduction in risk of exposure is not expected from environmental control. Ticks can survive in stalls and in pastures regardless of winter conditions.”

Chemical protection of horses against tick attachment can provide at least several hours of protection.

Better prophylactic treatments for tick control in horses were needed, they said.

The team noted that there were no approved equine label Lyme vaccines, but there is strong evidence that proper vaccination could prevent infection in horses.

“The conclusion drawn from the results of one vaccination study, using a challenge model, was that ponies could be protected from B. burgdorferi infection when using an aluminum adjuvanted recombinant outer-surface protein A (rOspA) vaccine.”

They continued: “The high number of confirmed cases of Lyme disease in humans and the strong evidence for geographic expansion of B. burgdorferi infection in both humans and horses, in addition to the low success of tick control measures, have led to the question: is it time for an approved Lyme vaccine?

“An OspA vaccine was previously shown to be effective in experimental ponies and a similar vaccine was approved in the United States for use in humans between 1998 and 2002; canine approved vaccines are currently used off-label in some horse populations.

“There will undoubtedly be many questions, some controversial, that arise regarding the need, efficacy, safety, antigen preference, duration of immunity, and cost of an approved human or equine Lyme vaccine.”

Divers, T.J., Gardner, R.B., Madigan, J.E., Witonsky, S.G., Bertone, J.J., Swinebroad, E.L., Schutzer, S.E. and Johnson, A.L. (2018), Borrelia burgdorferi Infection and Lyme Disease in North American Horses: A Consensus Statement. J Vet Intern Med. doi:10.1111/jvim.15042

The consensus statement, published under a Creative Commons License, can be read here

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