Adaptation and validation of a bacteria-specific enzyme-linked immunosorbent assay for determination of farm-specific Lawsonia intracellularis seroprevalence in central Kentucky Thoroughbreds

Reasons for performing study: Lawsonia intracellularis is the causative agent of equine proliferative enteropathy (EPE), a disease for which no large-scale seroprevalence studies have been conducted. Objectives: To validate and use an equine-specific enzyme-linked immunosorbent assay (ELISA) for L.intracellularis to determine the seroprevalence of L.intracellularis on numerous farms. Methods: An ELISA, in which purified antigen was used, was adapted from previous work in swine. A total of 337 Thoroughbreds from 25 central Kentucky farms were enrolled and monthly serum samples collected from August 2010 to January/February 2011. Samples were screened for L.intracellularis-specific antibodies using a modified ELISA. Farms were classified into one of 3 groups based on 3 year prior history with EPE. Results: The ELISA intra-assay coefficient of variation (CV) was 6.73 and inter-assay CV was 9.60. An overall seroprevalence of 68% was obtained, with farm-specific seroprevalances ranging from 14 to 100%. A significant difference was found in the average seroprevalence (P<0.05) on farms with a confirmed recent history of EPE cases. Additionally, both lower average ELISA unit (EU) values (P = 0.079) and maximum EU values (P = 0.056) were detected on farms with no recent EPE history when compared to the other groups. A bimodal exposure distribution to L.intracellularis was detected in the fall and winter months. Conclusions: Recent history of EPE was associated with higher average seroprevalence indicating increased exposure on farms with prior cases of EPE. Seasonally bimodal exposure was also observed. Potential relevance: The adapted ELISA appears to be useful for determination of L.intracellularis-specific antibody levels. The high farm-specific seroprevalences and bimodal distribution of exposure to L.intracellularis were unexpected and suggest that farms with a previous history of EPE remain at risk due to heightened exposure levels beyond early winter.