Evaluation of Lawsonia intracellularis infection in a group of pigs in a subclinically affected herd from weaning to slaughter.

The purpose of this study was to follow the course of a subclinical Lawsonia (L.) intracellularis infection in a group of 60 pigs on a commercial farm from weaning to slaughter. From 6 to 16 and at 26 weeks of age, rectal faecal samples and blood samples were collected weekly from every pig for examination by PCR and blocking ELISA, respectively. At corresponding times starting at 8 weeks of age, pigs were randomly selected for necropsy (n=51). Intestinal tissues were examined histopathologically and by immunohistochemistry (IHC) for L. intracellularis antigen. Infection with L. intracellularis showed a mainly subclinical course. Shedding of L. intracellularis was detected by PCR in three pigs as early as 6 weeks of age and persisted up until 14 weeks of age. In most pigs shedding of L. intracellularis was seen only for 1-2 weeks followed by a rapid serum antibody response. More than 50% of pigs had seroconverted by week 10. At slaughter, 30.8% of investigated animals were still found to be seropositive by ELISA. Of the 60 study animals 39 were found positive by faeces PCR (65.0%), 49 animals were found positive by serology (81.7%), and 35 pigs (68.6%) had positive results by IHC at necropsy. All but one pig were found to be L. intracellularis infected by at least one of the three methods (98.3%). In conclusion, this is the first field study revealing the presence of prominent histological lesions characteristic for L. intracellularis infection and associated positive pathogen specific PCR and immunohistological results even in subclinically infected pigs. Although intestinal alterations disappeared after 3-4 weeks, L. intracellularis was detected by IHC for a longer time, especially in intestinal lymph nodes.