Equine proliferative enteropathy (EPE) caused by Lawsonia intracellularis has recently been recognized as an emerging disease in foals. The goal of this study was to document exposure to L. intracellularis in a population of Thoroughbred horses residing on a farm endemic for EPE.
The study population included 68 resident mare and foal pairs which were sampled over a period of 11 months (January-November). Serum samples from mares at delivery and foals pre- and post-colostrum ingestion and monthly thereafter were collected and tested for the presence of L. intracellularis antibodies by immunoperoxidase monolayer assay (IPMA). Serum collected from foals was also used to determine the concentration of total solids. Additionally, feces from mares at delivery and foals post-partum and monthly thereafter were collected, processed for nucleic acid purification and assayed for L. intracellularis using real-time PCR.
Thirty-seven mares (54.4%) had positive titers (>60) against L. intracellularis by IPMA at the time of foaling with titers ranging from 60 to 240 (mean titer 133). The seroprevalence in mares increased with each foaling month (33% January, 47% February, 55% March, 71% April, 100% May). All tested foals had negative antibody titers (<60) against L. intracellularis prior to colostrum ingestion. Passive transfer of colostral antibodies against L. intracellularis was documented in 36 foals (53.7%) with titers ranging from 60 to 240 (mean titer 123). Colostral antibodies remained detectable in the serum of foals for 1 (31 foals), 2 (4) and 3 (1) months. Exposure rates in foals by month ranged from 0 to 10% and the antibodies remained detectable for 1 to 2 months. Overall, 22 foals (33.3%) showed evidence of natural exposure to L. intracellularis throughout the study period. None of the study foals developed signs compatible with EPE and the concentration of total solids in the serum of all foals remained within normal limits. Only one single fecal sample collected from one foal in May tested PCR positive for L. intracellularis.
In conclusion, the results showed that colostral antibodies against L. intracellularis were passively transferred to foals born on a farm endemic for EPE. Further, the exposure rates in mares and foals varied with the month of collection. The measurable immunological response following passive transfer of colostral antibodies or following primary exposure appeared to be short-lived in foals. Despite high exposure to L. intracellularis in foals, no clinical EPE cases were recorded. It is possible that the high rate of foals with evidence of passive transfer of colostral antibodies against L. intracellularis may have reduced the incidence of clinical cases. Healthy resident mares and foals did not appear to shed L. intracellularis in feces.