Transmission dynamics of helminth parasites of pigs on continuous pasture: Oesophagostomum dentatum and Hyostrongylus rubidus

Abstract

An increase in alternative outdoor pig production systems is occuring in Denmark, and this study was designed to elucidate the transmission patterns of Oesophagostomum dentatum and Hyostrongylus rubidus in pigs allowed to graze continuously on a pasture. A group of pigs was turned out in May 1993 (Year 1 of the study) and subsequently inoculated with low numbers of both helminths. These pigs were followed parasitologically until October by serial necropsy and sampling of faeces, grass and soil. A non-inoculated group of pigs was similarly followed on the same pasture in Year 2 (1994). Pasture infectivity was measured using helminth-naïve tracer pigs during all seasons. The pasture vegetation was rapidly destroyed by the pigs, resulting in a dirt lot by the autumn of Year 2. The area was soon contaminated with eggs, resulting in heavy pasture infectivity and increasing worm burdens in late summer; then the numbers of larvae declined markedly. In May of Year 2, newly exposed pigs became only lightly infected (mostly O. dentatum), and no transmission was observed in July–August of Year 2, probably due to an unusually dry summer and a lack of protecting vegetation. The results indicate that both O. dentatum and H. rubidus are very sensitive to environmental factors, because significant transmission occurred only under the most favourable conditions (summer combined with protecting vegetation as in Year 1). Transmission was severely reduced during the low temperatures experienced in the winter between Years 1 and 2 and during the dry summer of Year 2, when vegetation was lacking. Continuous grazing actually reduced transmission of O. dentatum and H. rubidus because of the reduction in vegetation. This, however, is not a desirable alternative farming system, because of its adverse environmental effects. This environmental impact may be mitigated by employment of a pasture rotation system in place of continuous grazing.