Foal season will soon be upon us, and the timely subject of an age-specific equine disease should be on the minds of all breeders and foal owners. Equine proliferative enteropathy (EPE) is a transmissible intestinal disease caused by the bacteria Lawsonia intracellularis (LI), commonly known as “Lawsonia disease.” EPE usually affects horses under 1 year of age, and while it only affects approximately 10 percent of foals, the consequences well exceed the cost of vaccinating.
Equine medicine specialist Dr. Nicola Pusterla has been studying EPE for more than a decade. It still frustrates him that little is known about the organism that causes the disease, but he is relieved that the disease is now manageable – something in which he played a large part.
In the mid-2000s, Pusterla began working with Connie Gebhart, Ph.D., a professor at the University of Minnesota. Gebhart studied under G.H.K. Lawson, the discoverer of LI, and is considered one of the foremost authorities on Lawsonia disease in pigs, known as porcine proliferative enteropathy (PPE). Pusterla and Gebhart started comparing PPE to EPE and set out to develop strategies to screen foals, but mostly to prevent the disease in foals by understanding it better and by developing an equine-specific vaccine strategy.
Understanding more about LI proved to be difficult – the ecosystem where it comes from seems to be a bit of a chicken and egg scenario.
“We haven’t fully determined why or where the disease starts,” said Pusterla, who found the disease to exist in many wild animals – skunks, raccoons, rabbits, coyotes and others. “Are these wild animals infected by whatever is shedding in the area or are they the reservoir host?”
Pusterla believes that either there are some animal species in the environment that retain the disease (rodents, mice, rats, rabbits, etc.) or it comes with clinical or asymptomatic foals. Only about 10 percent of infected foals get sick. Pusterla speculates that all the other foals have a very mild (unrecognizable) clinical infection, and they shed LI in their feces without complications. However, in shedding it, they contaminate the environment, putting other foals at risk.
“I think that’s the way it happens,” Pusterla said. “A healthy looking foal sheds it, and then one of the other susceptible foals gets it.”
While most reported LI cases in foals are isolated events, outbreaks may occur at breeding facilities. Once diagnosed, it can be treated successfully with supportive care and antibiotics, with a survival rate in mid-90 percentile. That treatment can be costly, though, and can cause financial havoc for horse farms. Yearlings infected with LI will sell for 60 percent less than an unaffected foal from the same sire, due to a smaller stature.
With this in mind, Pusterla and colleagues sought a vaccine.
LI in pigs causes a wasting disease that can devastate entire herds. An oral vaccine for pigs was developed to prevent this and has been successfully used worldwide to reduce the clinical effects of PPE. The team wondered whether the same vaccine could work in horses and induce protection. However, the oral aspect to the pig vaccine was causing a roadblock for horses.
Horses have a low stomach pH, so an oral vaccine wouldn’t survive in a highly acidic environment. Researchers developed a process to premedicate the horses with a medication that increases pH level before administering the vaccine. They also tried vaccinating intrarectally to bypass the upper GI tract entirely. Premedication appeared to help the vaccine induce a stronger, earlier antibody response when given orally, but the best response was the intrarectal method.
The vaccine was tested on prominent horse farms in central Kentucky – the heart of horse country where large operations were calling for a vaccine in order to avoid the devastating financial implications the disease could have on their farms. Pusterla and former UC Davis resident Dr. Nathan Slovis of Hagyard Equine Medical Institute in Lexington, Kentucky proved the vaccine to be successful on these farms.
Once commercialized, Pusterla urges foal owners to have them vaccinated for Lawsonia. The relatively low cost, he explains, is a fraction of what treatment or devaluation will cost owners.