Ecological influences on transmission rates of Ascaris suum to pigs on pastures

A study was conducted to determine the distribution and transmission rate o f Ascaris suum eggs and Oesophagostomum dentatum larvae in a pasture/pig ho use facility, which during the preceding summer was contaminated with helmi nth eggs by infected pigs. In May, four groups of 10 helminth naive tracer pigs were exposed to fenced sections of the facility for 7 days and necrops ied for parasite recovery 9-10 days later (trial 1). The highest rate of A. suum transmission (201 eggs per day) occurred in the pig house (A). On the pasture, egg transmission decreased with the distance from the house: 8 eg gs per day in the feeding/dunging area (B); 1 egg per day on the nearest pa sture (C); <1 egg per day on the distant pasture (D). Only a few O. dentatu m infections were detected, indicating a poor ability of the infective larv ae to overwinter. Soil analyses revealed that the highest percentage (5.8%) of embryonated A. suum eggs were in the house (A). Subsequently, the facil ity was recontaminated with A. suum eggs by infected pigs. A replicate tria l 2 was conducted in the following May. A major finding was the complete re versal of egg distribution between the 2 years (trials 1 and 2). In contras t to previous results, the highest rates of transmission (569 and 480 eggs per day) occurred in pasture sections C and D, and the lowest transmission rates (192 and 64 eggs per day) were associated with the feeding/dunging se ctions and the house (B and A). Soil analyses again supported the tracer pi g results, as the pasture sections had the highest concentrations of embryo nated eggs. Detailed soil analysis also revealed a non-random, aggregated e gg distribution pattern. The different results of the two trials may be due to the seasonal timing of egg deposition and tracer pig exposure. Many egg s deposited during the summer prior to trial I may have died rapidly due to , high temperatures and dessication, especially when they were not protecte d by the house, while. deposition in the autumn may have favored egg surviv al through lower temperatures, more moisture, and greater sequestration of eggs in the soil by rain and earthworms. The latter eggs may, however, not have become embryonated until turnout the next yean The results demonstrate that yearly rotations may not be sufficient in the control of parasites wi th long-lived eggs, such as A. suum, and that a pasture rotation scheme mus t include all areas, including housing. (C) 2001 Elsevier Science B.V. All rights reserved.