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serovar,” he explains. (A serovar is a distinct variation within a species of bacteria, virus, or immune cells)
“We promoted that information and spoke at conferences. We shared our information with Zoetis and they took another look. At that point in time they had equine lepto vaccine
on their potential product list, but it was way down the line in priority—so it wasn’t going
to get touched. But after seeing our data, they went ahead and duplicated our study, with more animals. We had looked at 1,500 horses across the country, and their study looked at 5,000 horses in 18 states. They covered fewer states, but examined more horses,” says Carter.
Zoetis contracted with several big equine hospitals around the country that deal with a lot of horses. “Their study mirrored ours, and came up with similar data. This gave them confidence to go ahead with a vaccine. They kicked that one right up into development mode,” Carter says. The equine vaccine was released in October of 2015.
All of the research for an equine vaccine had already been done; the company just needed to know that there would be a market for the vac- cine. “Zoetis wanted to find out the level of the problem. And, of course, there are three ways that this disease can impact the horse - either through abortion, renal disease, or recurrent blindness,” Carter explains.
Looking at the number of horses in the U.S. (something between 5 and 7 million), Zoetis felt there was a lot of economic value at stake. “They feel that recurrent uveitis is actually a bigger economic problem than abortion. But for anyone raising horses, the abortion issue is
a big one. The interesting thing is that there are not very many regions in the country that have a lab that can do leptospirosis testing, so a lot of horsemen aren’t even thinking about it. They may see problems in their horses, but are unable to get a diagnosis,” he says.
It can become quite expensive for a person facing the challenge of a horse suffering bouts of uveitis and trying to keep it from going
After entering the body, leptospirosis bacteria will multiply in the liver (a leptospirosis-positive liver is pictured here) and then migrate through the blood to the kidneys.
blind. Treatments can be very costly. If the vaccine can help pre- vent this problem, it will be a major mile- stone. “When I was in practice, the farms I went out on that had a horse with recurrent uveitis, it was just heartbreaking. By the time you recognize the problem, there has already been a
Craig Carter, DVM, Professor of Epidemiology at the University of Kentucky
that have had a history of lepto abortions. We strongly encourage horse owners and veterinar- ians to be sure to report if horses have been vaccinated when sending specimens to the lab for testing.
“This is very exciting, to finally have a vaccine, and the efficacy data was very good. Zoetis tested it on a much smaller number
of animals than they used for the safety
test, but in their efficacy test the vaccine
did protect 100% of the vaccinated animals against infection in those animals challenged with high doses of live lepto organisms of the Pomona variety. If it truly does protect horses when they get exposed, that could be the
end of many problems. If a horse doesn’t get circulating leptospires after exposure, then that horse shouldn’t get uveitis and shouldn’t abort, and also won’t get kidney infection. We are very hopeful.”
The situation is so much better now in having an effective, safe vaccine that’s legal to use on horses, rather than having to use the cattle vaccine off-label as some horsemen and veterinarians were doing in earlier years. There was a lot of anecdotal data indicating that the cattle vaccine did help prevent abortion in mares, but there were no scientific controlled studies to prove it. When a person uses a vac- cine intended for another species, there is more risk for unwanted side effects and injection site reactions because it wasn’t tested on horses.
“Some people thought that the cattle vac- cine actually caused uveitis in a proportion of the vaccinated horses. There are some similari- ties between some of the antigens in lepto and some of the antigens in the eye,” says Carter. Since recurrent uveitis is an immune-mediated disease (inflammation in the eye causing the body to attack its own tissues), this might occasionally be a problem.
One way this disease can impact the horse is recurrent blindness. Other ways are abortion or renal disease. The infection is often mild and may go unnoticed, but can cause uveitis, or moon blindness.
lot of damage in the
eye—and there’s just
not much in the vet-
erinary toolbox to successfully deal with this. People generally hang onto these horses until they go blind, and it can be a tough situation,” Carter says.
Zoetis did the required safety and efficacy studies on their vaccine. For the safety study, they vaccinated 1,808 horses and only had reactions in 0.02% of them. “These were just local swellings at the injection sight. These results were very encouraging, and now the vaccine is beginning to be used by horse owners and large breeding farms. I talked with Zoetis representatives at the AAEP meeting in Las Vegas, only about 2 months after the vaccine became available, and they said sales were going very well,” he says.
“We are already testing many horses here in Kentucky that have been vaccinated. This cre- ates a whole new ball game because vaccination will confuse the issue on what we will see on titers, for diagnosis. It will be important to be able to discern whether a high titer is from vac- cination or from exposure to the disease—espe- cially on the L. pomona because that’s the only serovar in the vaccine,” Carter explains.
This is also a bit of an issue because of
the fact that there are many serovars of lepto, several of which can infect horses, and only one of those is covered in the vaccine. We don’t know which strains might be most important around the country, and it could vary from one region to another. It might also make a differ- ence if the horse is on a farm with cattle, dogs, or various species of wildlife that might carry other strains.
“We did see some statistical differences in the serovars and titers around the country in our study,” Carter says. “Right here in Kentucky, pomona is the major one, though we do occasionally see grippotyphosa abor- tions. We’ve been seeing a lot of higher titers to L. grippotyphosa, so we are watching this, too. We will be looking closely at the farms that vaccinated their mares, especially farms
SPEEDHORSE, August 2016 71
equine health
Matt Barton