Transmission dynamics of helminth parasites of pigs on continuous pasture: Ascaris suum and Trichuris suis.

In Denmark, alternative outdoor production systems for pigs are becoming more frequent, and information on the transmission of Ascaris suum and Trichuris suis under continuously grazed pasture conditions is needed. A group of pigs was turned out on a pasture in May 1993 (Year 1 of the study), inoculated with 200 eggs of A. suum and 1000 eggs of T. suis, and followed parasitologically. A non-experimentally infected group of pigs was similarly turned out on the same pasture the following year (Year 2) and again followed parasitologically. Pasture infectivity was measured using helminth-naïve 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 pronounced increase in transmission occurred. After 2 months on the pasture, the continuously exposed pigs in summer seasons of both Years 1 and 2 harboured small overdispersed populations of adult A. suum, moderate numbers of liver white spots, and high specific IgG responses. These parasitological measures on chronically exposed pigs did not, however, correlate well with pasture infectivity or with each other. In contrast, 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 inoculated 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 indicate 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 resistant to environmental factors than free-living infective larvae of pig parasites such as Oesophagostomum dentatum and Hyostrongylus rubidus. Control of these parasites in outdoor systems will present more difficult challenges than that for parasites transmitted by free-living larvae.