Influence of helminth parasite exposure and strategic application of anthelmintics on the development of immunity and growth of swine.

Infection of pigs with the intestinal roundworm parasite Ascaris suum and strategic application of anthelmintic drugs during the growing phase of development were observed for specific effects on 1) development of immunity in feeder pigs and 2) growth rate during the finishing phase. Management treatments included maintenance in a parasite-free concrete environment, maintenance in a concrete environment and inoculation with 1,000 infective A. suum eggs every other day over a 52-d period, and maintenance on a dirtlot contaminated with A. suum and Trichuris suis eggs. Within each management environment, pigs were either untreated, treated with ivermectin or treated with fenbenzadole at strategic times during parasite exposure. Protective immunity, assessed by a challenge inoculation with A. suum eggs following management treatments, was not affected by ivermectin or fenbenzadole treatment during exposure, but adult worm burdens were reduced and the pattern of A. suum larval antigen serum antibody responses were different from those in control pigs not treated with drugs. Exposure to A. suum and treatment with anthelmintics during the growing phase reduced adult worm burdens following the finishing phase of growth. Rate, but not efficiency, of gain was significantly improved by anthelmintic treatment following natural exposure to parasites. Strategic treatment of pigs with anthelmintics following inoculation with A. suum eggs in a concrete management environment had no effect on rate of gain. Results suggest that natural exposure to parasites during the growing phase without therapeutic treatment causes permanent damage to growth potential.