Experimental Oesophagostomum dentatum infections in the pig: worm populations resulting from trickle infections with three dose levels of larvae.

This paper describes the worm populations in pigs experimentally infected by trickle infections with different dose levels of the nodular worm, Oesophagostomum dentatum. Four groups each of 20 helminth naïve pigs, 10-12 weeks old, were inoculated with 0 (group 1), 100 (group 2), 1000 (group 3), or 10,000 (group 4) infective larvae twice weekly, and the pigs were killed after 10-13 weeks. No overt clinical signs were observed, and only group 4 had slightly lower food conversion rate (P < 0.05) than the controls. Faecal egg counts revealed that the nodular worms in pigs of groups 2 and 3 had a short prepatent period (3-4 1/2 weeks) and a fairly stable egg output, while the worms in the pigs of group 4 had prepatent periods of 3-10 weeks and low, unstable egg excretion. The mean worm burdens increased with the dose rate (group 2: 929 worms; group 3: 7467 worms; group 4: 19,847 worms), but detailed analyses of the worm populations from 10 pigs from each of the infected groups revealed a clear dose-dependency in worm recovery, percentage adult worms, worm lengths and female fecundity, as all these measures declined significantly with increasing dose level. The adult worms seemed to be shorter and less fertile when they were located posteriorly to their predilection site, and especially in group 4 many stunted infertile adults measuring only 2-5 mm were found in the posterior half of the colon, but there were no indications of worm expulsion. Superimposed on the main experiment was a cohort study in which 4 pigs of group 3 were given a single dose of 1000 pyrantel resistant larvae at day 56 (all other larvae were pyrantel sensitive), treated with 28 mg pyrantel per kg body weight at day 85 and killed at day 90. Appropriate control groups were included. The mean establishment of the cohort was similar to previously uninfected controls, but between-animal variation was much higher in the trickle infected group.