Studies on the ex vivo survival of Lawsonia intracellularis

Objective: To examine the ex vivo survival of Lawsonia intracellularis in conditions relevant to cleaning procedures on pig farms.

Methods: Two co-cultured strains of L. intracellularis were suspended in vitro for 30 minutes at 20 degrees C in various disinfectants according to label recommendations for usage, then washed in saline. Disinfectant-treated and Control bacteria were then added to fresh cells for co-culture for a standard 5-day incubation. Further cell-free suspensions were exposed to air at 5 degrees C, 20 degrees C, or 37 degrees C for up to 8 days, then added to fresh cells for co-culture for a standard 5-day incubation. Lawsonia intracellularis within co-cultures were determined to be viable by visualization after indirect immunoperoxidase staining that incorporated specific monoclonal antibody. Groups of weaned pigs were inoculated orally with L. intracellularis-infected feces, either on the day the feces were collected, or after storage for 1, 2, or 5 weeks at temperatures between 5 degrees C and 15 degrees C.

Results: Mixing of suspensions of L. intracellularis with the quaternary ammonium compound, cetrimide (3.3% wt per vol), for 30 minutes, produced no detectable Lawsonia on reculture; mixing with 1% povidone-iodine for 30 minutes produced no or few (< 1% of controls) detectable Lawsonia on re-culture. Mixing of suspensions with either 1% potassium peroxymonosulfate or a 0.33% phenolic mixture for 30 minutes were less effective at the recommended concentrations tested, as small numbers of L. intracellularis (1%-18% of controls) were detected on reculture of some strains. Detectable numbers of viable L. intracellularis were present after exposure of cell-free suspensions to air for up to 6 days at 5 degrees C. Colonization of the intestine with L. intracellularis was detected in pigs inoculated orally with feces stored for up to 2 weeks at temperatures between 5 degrees C and 15 degrees C.

Implications: Lawsonia intracellularis can probably survive in extracellular conditions for 1 to 2 weeks at 5 degrees-15 degrees C. Pure cultures of L. intracellularis were fully susceptible to a quaternary ammonium disinfectant (3% cetrimide), less so to 1% povidone-iodine, but: not ro 1% potassium peroxymonosulfate or a 0.33% phenolic mixture.