Infection dynamics of Lawsonia intracellularis in pig herds.

Little information is known about the natural course and within-herd prevalence of porcine proliferative enteropathy caused by Lawsonia intracellularis. The objective of the study was to investigate the within-herd dynamics of naturally acquired L. intracellularis infection in pigs from weaning to slaughter. The study was designed as a longitudinal survey where 100 pigs from five herds were randomly selected at weaning (approximately 4 weeks of age). Every second week until slaughter (10-12 times, i.e. 20-24 weeks) the pigs were weighed and faecal as well as blood samples were collected. Faecal shedding of L. intracellularis was assessed by real time-PCR and sero-conversion by an indirect immunofluorescence antibody test (IFAT). Clinical disease was not reported but infection was present in all herds and the PCR assay indicated infection in 75% of pigs examined. Most L. intracellularis infected pigs were shedding at 10-12 weeks of age (22-29 kg) and shed for 2-6 successive weeks. After 18 weeks of age all shedding had ceased and re-infection at PCR detectable level was not seen. Variable L. intracellularis associated impact on growth rate was observed. Immediately before bacterial shedding and during early infection the average growth rate declined whereas a compensatory impact was observed during later infection and after bacterial shedding had ceased. The performance of the IFAT resembled the bacteriological test almost perfectly. Sero-conversion was first detected at 12-14 weeks of age. Relative to the bacterial shedding, the onset of sero-conversion was a little delayed, in general, most pigs had sero-converted 2 weeks after the first shedding. Once sero-converted, 92% of the pigs remained sero-positive over the entire survey period.