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Veterinarian shares development of equine leptospirosis vaccine

by Wyoming Livestock Roundup

Leptospirosis is a disease which has the ability to affect humans, dogs, livestock, horses and many other mammals, caused by spiral-shaped bacteria called spirochetes. Some types of leptospirosis are most common in certain species serving as the carrier host. 

Often found in wildlife populations, including deer and rodents, the disease can be spread through shed bacteria into the environment. Leptospires can survive in surface water, stagnant ponds, streams or moist soil for long periods of time at mild temperatures.  

Disease background  

The disease is spread to susceptible animals by discharges and secretions of sick and carrier animals, which often contaminate feed and water. Some infected animals appear to be healthy, yet harbor bacteria in their kidneys and reproductive tract and shed leptospires in urine or reproductive fluids.  

Bacteria 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 the pathogens enter the body, they multiply in the liver and migrate through the blood to the kidneys where toxins causing damage to red blood cells, liver and kidneys may enter. This sometimes can be a cause of acute kidney disease. 

Leptospirosis is a costly disease. In unvaccinated cattle, it is a cause of infertility, delayed breeding and early embryonic death, as well as abortions or birth of premature and weak calves.  

In horses, the initial infection is often mild and may go unnoticed, but can cause abortions, acute kidney disease and recurrent uveitis or moon blindness.  

History of vaccine 

There are effective vaccines available for cattle, swine and dogs, but until recently no approved vaccine was available for horses due to a lack of funding and interest from pharmaceutical companies. 

Professor of Epidemiology at the University of Kentucky Craig Carter worked for many years building a case for an equine vaccine.   

“When I was at Texas A&M, I spent a lot of time working on canine leptosporosis, for which there is a good vaccine,” he explained. “I came to Kentucky in 2005 and started looking at this disease in horses.” 

He continued, “I realized leptospirosis was a big problem, yet it was not being recognized anywhere else. Our research group in Kentucky kept looking into this.”  

In 2006, Kentucky experienced a bad year for leptospirosis abortions, opening the eyes of many horse producers, veterinarians and researchers. This all, of course, was underscored by the lack of an equine leptospirosis vaccine.  

“We looked at the economic data, even though we were only able to trace about half of the abortions we saw during the year,” Carter said. “Many folks didn’t want to talk about losses on their farms.” 

Part of the study in 2006 included a calculation of the estimated value of foals lost.   

“This was just the cases we saw here in the lab that we determined were leptospirosis abortions,” Carter explained. “We know what we diagnose in the laboratory is just the tip of the iceberg.” 

According to Carter, the value of foals lost in the 2006-07 reproductive season alone was $3.5 million. By taking 20 years of data on abortions from leptospirosis, the researchers were able to calculate very large economic losses to horse farms in Kentucky. The losses totaled well over $100 million and only accounted for 541 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 they thought leptospriosis abortion in horses wasn’t a problem anywhere except central Kentucky and maybe a little in New York and Florida,” Carter said. 

The researchers set out to see if the disease was more widespread. Through a nationwide study in 2010-11, results showed 45 percent of horses had been exposed and were positive for at least one leptospirosis serovar. 

“We shared our information with Zoetis and they took another look,” Carter shared. “After seeing our data, they duplicated our study with more animals. We looked at 1,500 horses across the country, and their study looked at 5,000 horses in 18 states.” 

Zoetis worked with several big equine hospitals dealing with a lot of horses.  “Their study mirrored ours, coming up with similar data. This gave them confidence to go ahead with a vaccine,” Carter said, noting the equine vaccine was released in October 2015.   

Importance of vaccine 

“There are three ways this disease can impact horses, through abortion, renal disease or recurrent blindness,” Carter explained, noting Zoetis felt there was a lot of economic value at stake due to the number of horses in the country.   

“Zoetis felt recurrent uveitis is actually a bigger economic problem than abortion, but for anyone raising horses, the abortion issue is a big one,” Carter said. “There are not very many regions in the country with a lab able to do leptospirosis testing, so most horsemen aren’t even thinking about it. They may see problems in their horses but are unable to get a diagnosis.” 

If a person is facing the challenge of trying to keep a horse from going blind or suffering bouts of recurrent uveitis, treatment can become quite expensive. 

  “When I was in practice, the farms I went to that had a horse with recurrent uveitis, were just heartbreaking.  By the time owners recognize the problem, there’s already been a lot of damage in the eye, and there’s not much veterinarians can do to successfully treat the animal,” Carter says. 

A vaccinated horse may still have issues, however, since L. pomona is the only serovar in the vaccine, and there are many serovars of leptospirosis, several of which can infect horses.   

Horsemen are glad to have 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 the cattle vaccine did help prevent abortion in mares, but there were no controlled studies to back up the theory. When a person uses a vaccine intended for another species, there is more risk for unwanted side effects and injection site reactions.  

“Many of the earlier vaccines for animals are produced with older technology,” he shared. “Zoetis used the most current methods, including micro-filtration, in building this vaccine to help remove extraneous proteins.” 

Carter continued, “Zoetis recommends vaccinating healthy horses over the age of six months prior to exposure with an initial dose, then a second dose three or four weeks later. With most vaccines, it is generally recommended pregnant mares should get a booster during the third trimester of pregnancy.” says Carter.  

An annual booster vaccination is recommended.  

Heather Smith Thomas is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to   

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