Page 72 - August 2016

Leptospirosis can affect humans, dogs, livestock, horses, and many other mammals.
Lepto Vaccine
for HorSeS
by Heather Smith Thomas
Spiral-shaped bacteria called spirochetes cause leptospirosis. This disease can affect humans, dogs, livestock, horses, and many other mam- mals. Some types of lepto are most common in certain species of animals that serve as the carrier host. The disease is often found in wildlife popula- tions, including deer, raccoons and rodents, that shed the organism into the environment. The lepto-
spires can survive in surface water, stagnant ponds, streams, or moist soil for long periods of time at mild temperatures.
Lepto is spread to susceptible animals by dis- charges and secretions (especially urine) of sick and carrier animals, which often contaminate feed and water. Some infected animals, particularly with strains of lepto that are adapted to that species of host, appear to be healthy, yet harbor bacteria in their kidneys and reproductive tract, shedding leptospires in urine or reproductive fluids.
These pathogens may enter a susceptible animal via nose, mouth or eyes by contact with contaminated feed, water or urine, or through breaks in the skin on feet and legs when walking through contaminated water. Urine or contaminated water splashing into the eyes of susceptible animals can spread the disease, as can breeding. Once leptospires enter the body, they multiply in the liver and migrate through the blood to the kidneys. They release toxins that damage red blood cells, liver and kidneys, sometimes causing acute kidney disease.
In horses, the infection is often mild and may
go unnoticed, but it can cause abortions, acute kidney disease, and recurrent uveitis (moon blind- ness). There have been effective vaccines available for cattle, swine and dogs for many years, but until recently there was no approved vaccine for horses. One of the roadblocks was funding and lack of interest from pharmaceutical companies.
Craig Carter, DVM, Professor of Epidemiology at the University of Kentucky, has been working for many years to build a case for an equine vac- cine. “When I was at Texas A&M, I spent a lot of time working on canine leptosporosis, for which there is a good vaccine. I came to Kentucky in 2005 and started looking at this disease in horses. I realized lepto was a big problem here, yet it
was not being recognized as a problem anywhere else—which I thought was strange. Our research group here in Kentucky just kept looking into this,” he says.
“Then in 2006, Kentucky had a serious issue with lepto; it was one of the worst years they’d
had in a long time for lepto abortion. This was an eye-opener for me, underscored by the fact that there was no equine vaccine. We looked at the economic data even though we were only able to trace back about half of the abortions we saw that year. Understandably, many folks didn’t want to talk about losses on their farms,” says Carter.
As part of the study that year, the estimated value of the foals that were lost was calculated. “These were just the cases we saw here in the lab that we deter- mined were lepto abortions. We always know that what we diagnose in the laboratory is just the tip of the iceberg. The value of foals lost in the 2006-2007 reproductive season alone was $3.5 million. Then, we took 20 years of data that we had on abortions from lepto. With simple math, calculating that out for a 20-year period in which 541 cases were confirmed at this lab, we showed that lepto represents a very large economic loss to horse farms in Kentucky,” he says. It was well over 100 million dollars and only accounted for cases confirmed by the laboratory.
Carter talked to other veterinarians in Kentucky, including Dr. Stuart Brown at Hagyard Equine Medical Institute. “We started talking to drug companies to see if any of them would be interested in producing a vaccine, but their perception was that lepto abortion in horses isn’t a problem anywhere except in central Kentucky and maybe a little bit in New York and Florida,” says Carter.
“So, we set out to find out if this disease was more widespread. We did a nationwide sero-epidemiologic study in 2010-2011 that involved 30 laboratories—29 states were represented in the U.S. and one Canadian province. In that study, we found that about 45% of horses—anywhere you looked—had been exposed and were sero-positive for at least one leptospirosis
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SPEEDHORSE, August 2016
equine health