TRANSMISSION DYNAMICS OF HELMINTH-PARASITES OF PIGS ON CONTINUOUS PASTURE - ASCARIS-SUUM AND TRICHURIS-SUIS

In Denmark, alternative outdoor production systems for pigs are becomi ng more frequent, and information on the transmission of Ascaris suum and Trichuris suis under continuously grazed pasture conditions is nee ded, A group of pigs was turned out on a pasture in May 1993 (Year 1 o f the study), inoculated with 200 eggs of A. suum and 1000 eggs of T. suis, and followed parasitologically. A non-experimentally infected gr oup of pigs was similarly turned out on the same pasture the following year (Year 2) and again followed parasitologically. Pasture infectivi ty was measured using helminth-naive tracer pigs. During the summer of Year 1, A. suum eggs became infective within 4-6 weeks on the pasture . However, transmission was moderate until August of Year 2, when a pr onounced increase in transmission occurred. After 2 months on the past ure, the continuously exposed pigs in summer seasons of both Years 1 a nd 2 harboured small overdispersed populations of adult A. suum, moder ate numbers of Liver white spots, and high specific IgG responses, The se parasitological measures on chronically exposed pigs did not, howev er, correlate well with pasture infectivity or with each other. In con trast, the liver inflammation and specific IgG responses (but not the intestinal A. suum burdens) of the tracer pigs were highly correlated (P = 0.0001) and appeared to better reflect pasture infectivity. The i noculated pigs excreted T. suis eggs by the late summer of Year 1, but no transmission took place before August of Year 2. Thus, the T. suis population of infective eggs built up very slowly. The results indica te that T. suis eggs may survive for a considerable time, however, The study results reveal that A. suum and T. suis eggs are much more resi stant to environmental factors than free-living infective Larvae of pi g parasites such as Oesophagostomum dentatum and Hyostrongylus rubidus . Control of these parasites in outdoor systems will present more diff icult challenges than that for parasites transmitted by free-living la rvae. (C) 1997 Australian Society for Parasitology.