The influence of stocking rate on transmission of helminth parasites in pigs on permanent pasture during two consecutive summers

This study was made to elucidate the transmission of nematode infections in outdoor pigs at different stocking rates during two consecutive seasons. F ive pigs (Group IA) inoculated with low doses of Oesophagostomum dentatum, Ascaris suum, and Trichuris suis and five helminth-naive pigs (Group 1B) we re turned out together in June 1996 on each of four pastures at stocking ra tes of 100, 240 (two pastures) and 576 m(2) per pig, respectively. The pigs were slaughtered in early October, and pasture infectivity was subsequentl y measured using helminth-naive tracer pigs (Tracer). In 1997, 10 helminth- naive pigs were turned out on each pasture in May (Group 2) and again in Au gust (Group 3), and allowed to graze for 12 weeks. The percentage of grass cover was reduced considerably at the high stocking rate in comparison to t he other stocking rates. Transmission of all three helminths was observed o n all pastures. In 1996, the O. dentatum faecal egg counts and worm burdens were significantly higher in pigs at the high stocking rate compared to pi gs at the other stocking rates. O. dentatum did not survive the winter and pigs of Group 2 were inoculated with 3000 larvae each to reintroduce this p arasite. Ascaris suum ELISA values and worm counts were highest at the high stocking rate in 1997 (Group 3). Transmission of T suis was not significan tly influenced by stocking rate. The results indicate that transmission of O. dentatum, and to some extent A. suum is influenced by stocking rate. How ever, both A. suum and T suis eggs are still expected to constitute a high risk of infection on intensively used pastures where eggs may accumulate fo r years. The relationship between host density and helminth transmission se ems more complex for grazing/rooting pigs than for grazing ruminants. This may be due to the differences in behaviour of the animals and the resulting differences in microclimate of the developing eggs/larvae. (C) 2001 Elsevi er Science B.V. All rights reserved.