Acquired resistance to migrating larvae of Ascaris suum in young pigs by repeated drug-abbreviated infections.

Cross-bred 3- and 8-wk-old pigs were used to test whether drug-abbreviated infections with Ascaris suum can stimulate acquired resistance to challenge. During the immunization period, both age groups of animals were infected with increasing numbers of A. suum eggs (500, 1,000, 2,000, 5,000, 10,000, and 20,000) at 7-day intervals while the pigs were receiving pyrantel tartrate in the feed. Two days after the last infective dose, animals were placed on unmedicated feed for 8 days and then challenged with 10,000 eggs. All pigs were killed 7 days after challenge, and milk spots on the livers and larvae recovered from the lungs were counted. Larval recoveries from lungs of the immunized animals were significantly smaller than those from the unimmunized animals in both age groups, suggesting that the pigs were capable of acquiring strong resistance to parasitic infections. In immunized animals, challenge infection did not contribute significantly to milk spot formation. The number of milk spots was significantly greater in the older animals, indicating that milk spot formation may be age related.