Seasonal variation in development and survival of Ascaris suum and Trichuris suis eggs on pastures.

Pig faeces were deposited on experimental plots in the spring, summer, autumn and winter to study development and survival of Ascaris suum and Trichuris suis eggs under outdoor conditions. Faeces were placed either in short grass or 2 cm below the surface of bare soil, imitating pastures used by nose-ringed, grazing pigs or normally rooting pigs, respectively. The numbers and developmental stages of the eggs were recorded in faeces and soil for up to 50 weeks post-deposition. Embryonation took place only during the summer months and seemingly was independent of the microclimate. The majority of A. suum and T. suis eggs, which are generally considered to be extremely resistant and long-lived, seems to disappear rather fast. The disappearance rate for A. suum eggs was higher than for T. suis eggs, and both egg types disappeared significantly faster in the summer months than in the winter months, and when placed in short grass than when buried in soil (less exposed). We discuss how knowledge on egg development and survival may be used in the planning of pasture strategies for control of helminth infections in outdoor pigs.