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, autu mn and winter to study development and survival of Ascaris suum and Trichur is suis eggs under outdoor conditions. Faeces were placed either in short g rass or 2 cm below the surface of bare soil, imitating pastures used by nos e-ringed, grazing pigs or normally rooting pigs, respectively. The numbers and developmental stages of the eggs were recorded in faeces and soil for u p to 50 weeks post-deposition. Embryonation took place only during the summ er months and seemingly was independent of the microclimate. The majority o f 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 m onths, and when placed in short grass than when buried in soil (less expose d). We discuss how knowledge on egg development and survival may be used in the planning of pasture strategies for control of helminth infections in o utdoor pigs.