RESPONSE TO REPEATED INOCULATIONS WITH ASCARIS-SUUM EGGS IN PIGS DURING THE FATTENING PERIOD .1. STUDIES ON WORM POPULATION-KINETICS

This experimental study on pigs was designed to simulate natural, long -term exposure to Ascaris suum under modern management conditions. Par asite kinetics were followed in pigs receiving A. suum eggs as repeate d trickle inoculations at two dose levels beginning at a body weight o f 25 kg until their slaughter at 90 kg (baconers). In pigs inoculated twice weekly with 500 eggs, there was an initial marked rise in the nu mbers of hepatic milk spots, but as early as around week 6 after the s tart of inoculations and until week 16, at which time the last pigs we re slaughtered, the numbers of spots diminished drastically. In pigs r eceiving only 25 eggs twice weekly, low and moderately fluctuating num bers of spots were seen throughout the experiment. Larvae recoverable from the livers and lungs were observed mainly during the beginning of the experiment. Before patency, immature intestinal worms were found in moderate numbers that showed a rough positive correlation with the dose levels, but at the time at which adult worms started to appear, i mmature parasites could practically no longer be found. In all, only 1 0 of 40 pigs harbored adults, and 4 of these 10 pigs harbored 80% of t he total worm population. The results show that acquired dose-dependen t host responses to A. suum play an important role in regulating the w orm population along the migratory route of the parasite and that the final burden of worms in the small intestine is dose-independent and h ighly variable.