Lawsonia Prevalence Patterns Investigated
Yearly variability in exposure to a severe disease-causing bacterium of young horses appears to be different than previously thought. Despite the common belief that the incidence of equine proliferative enteropathy (EPE), a severe gastrointestinal disease of foals and long yearlings, spikes higher in some years than in others, researchers have recently found that perception might not reflect reality.
EPE is caused by Lawsonia intracellularis, a Gram-negative bacterium. The condition appears seasonally in Central Kentucky with spikes of cases in October and November and in January and February. At the 2012 American Association of Equine Practitioners Convention, held Dec. 1-5 in Anaheim, Calif., Allen Page, DVM, of the University of Kentucky's Gluck Equine Research Center, presented a retrospective study on the "Incidence of Yearly Lawsonia intracellularis Assay Variations from Horses in Kentucky."
He compiled fecal and post-mortem (necropsy) polymerase chain reaction (PCR, a type of DNA test) results and serum immunoperoxidase monolayer assay results from cases across Kentucky logged from July1, 2002, to June 30, 2010. He and his colleagues also reviewed enzyme-linked immunosorbent assay (ELISA) test results of weanlings from three farms where EPE is endemic.
According to Page, veterinarians and scientist haven't confirmed how horses contract L. intracellularis, but they think it is likely through ingesting contaminated feces. While post-mortem examination is the gold standard for diagnosing EPE, the fecal PCR test is highly specific; this means that a positive result has a high correlation with infection. However, the PCR is not as sensitive, meaning that some PCR-negative horses might still be infected. Researchers also believe infected horses shed the organism intermittently in feces, so it might be missed with one-time testing. Some at-risk horses without clinical signs were positive on serological assay, meaning that the horses had likely been exposed to L. intracellularis but did not develop clinical EPE.
Veterinarians also use serum total protein and albumin concentrations to screen horses for EPE; Page pointed out that low albumin and protein are not very specific for L. intracellularis infection, however.
In this study Page and his co-authors found that L. intracellularis incidence did not vary significantly from year to year across Kentucky, although they noted October and February spikes in each year. However, on the three endemic farms, seropositivity to L. intracellularis was significantly lower from 2011 to 2012 than it was from 2010 to 2011.
Page and his colleagues aren't certain whether the seasonal spikes are the result of increased numbers of susceptible animals (weanlings) during this time or specific seasonal environmental factors. They did find that although the statewide incidence of disease does not appear to change from year to year, specific farms might see variation in L. intracellularis exposure rates from one year to the next.
This study helps shed light on the patterns of EPE development and on the relative value of the various forms of L. intracellularis screening.
Christy Corp-Minamiji, DVM, is a writer and equine practitioner from Northern California.
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