Cases of equine proliferative enteropathy (EPE), an intestinal disease that principally affects weanlings between four and seven months, have been on the rise in recent years. As Nicola Pusterla, DVM, PhD, Dipl. ACVIM, of the University of California, Davis, explained, EPE, caused by the bacterium Lawsonia intracellularis, is an emerging disease; it remains unclear whether the disease is spreading rapidly or if it is being increasingly recognized. While EPE occurrences tend to be sporadic, outbreaks can occur in localized areas. Pusterla notes that on endemic farms, cases can increase yearly.

The impact of EPE ranges from foal mortality in worst case scenarios, to economic loss in the form of treatment cost and substantially decreased sale value of the affected horse as a yearling, noted Pusterla: "There’s a need for increased awareness. What we are trying to achieve is protection of the foal, prevention of the disease, and decrease spread of the organism by clinically and subclinically infected foals."

Recently, a team of researchers studied the protective effect of an intrarectal avirulent live L. intracellularis vaccine on weanling Thoroughbreds residing on three farms in Kentucky whose weanling population had been impacted by a 2008 EPE outbreak. In this randomized field trial, 202 weanlings free of L. intracellularis-specific antibodies were divided into vaccinated and nonvaccinated groups. The researchers administered the vaccine one month prior to the anticipated onset of the disease in September 2009, with a follow-up vaccine 30 days later. Subjects were monitored for clinical signs of EPE, serum total protein concentration, and total weight gain until yearling age.

No adverse reactions to the vaccine were noted, and study results showed that fewer than 2% of the weanlings exhibited clinical signs of EPE, which was less than the 10% anticipated by the researchers. The team believes this could be due to the vaccinated foals shedding avirulent vaccine organisms via feces, thereby triggering an immune response in nonvaccinated herdmates. Further, half of the herd population being vaccinated could have reduced the nonvaccinated horses’ exposure to the bacterium