Distribution of Ascaris suum in experimentally and naturally infected pigsand comparison with Ascaris lumbricoides infections in humans

This paper describes the distribution of Ascaris strum in experimentally an d naturally infected pigs, and offers a comparison with A. lumbricoides inf ections in humans. In the first study, worms were recovered post-mortem fro m a group of 38 pigs that had been trickle inoculated with 10 000 infective A. suum eggs twice weekly for 12 weeks. In the second study, worms were co llected from a group of 49 pigs that had been kept on a pasture contaminate d with infective A. suum eggs for 10 weeks, after which they received treat ment with an anthelmintic; they then were turned out on the same pasture fo r a second 10-week period before slaughter. The worm burdens of the natural ly infected pigs were recorded both at treatment and post-mortem. Mean worm counts were similar at all occasions but the prevalence of infection was h igher in the trickle infected and naturally reinfected pigs. Furthermore, t he prevalence in naturally infected pigs increased significantly over the s tudy period. Worm burden distributions in all groups were heavily overdispe rsed, but the distribution patterns differed significantly between groups: lower exposure (initial natural infection) gave a low prevalence and an alm ost uniform distribution of worm burdens among infected hosts. Continued or higher exposure (trickle and natural reinfection) resulted in increased pr evalence and a reduction in the proportion of hosts with increasing worm lo ad. A positive correlation was found between initial and reinfection worm b urdens in the naturally infected pig population, suggesting that individual pigs are predisposed to a high or low intensity of infection. The prevalen ce and intensity as well as the distribution observed for A. suum infection in pigs were comparable to those reported for A. lumbricoides in endemic a reas, and there is evidence for predisposition to A. suum in pigs, with an estimated correlation coefficient similar to that found in humans. It is co ncluded that A. suum infections in pigs are a suitable model to study the p opulation dynamics of A. lumbricoides in human populations.