SUMMARYEggs of Oesophagostomum dentatum in faeces deposited on grass plots from May to October developed into infective larvae. Warm wet weather facilitated development but weather conditions which caused the faeces to dry out rapidly resulted in a heavy mortality of eggs and pre-infective larvae. During the winter no development took place and the eggs died. In the early spring and late autumn a few eggs survived and developed into 1st-stage larvae but they died without developing further. In the laboratory no development took place at 4 °C but at temperatures ranging from 10 to 25 °C development into infective larvae was completed, the rate increasing as the temperature rose. Out-of-doors infective larvae survived in faeces and on herbage for 1 year under a wide range of climatic conditions including the exceptionally cold winter of 1978–79. In the laboratory, infective larvae suspended in water survived even longer at temperatures ranging from 4 to 27 °C. They were rapidly killed by continuous freezing and by desiccation when the relative humidity was less than 90%. The relationship between climatic conditions and the development and survival of the free-living stages is discussed.
The transmission of Oesophagostomum species in sows kept on pastureland on a commercial farm was studied over a period of almost two years. Worm eggs were excreted on to the pasture continuously but they gave rise to infective larvae during the summer and autumn only when climatic conditions were conducive to development. The output of worm eggs was maintained at a moderate level and the numbers of adult worms in culled sows rarely exceeded more than a few thousand but some sows carried heavy infestations of immature worms. No direct relationship between the level of the herbage infestation and the numbers of adult worms and the output of worm eggs was apparent. The use of anthelmintics had only a transitory effect on the level of infection and an alternative system of anthelmintic treatment is suggested.
Eggs of Trichostrongylus vitrinus in faecal pellets deposited on grass plots each month from April 1981 to March 1982 developed into infective larvae. From October to March development was slow and mortality of the pre-infective stages was very high. From April to September development was more rapid. The weather was generally dry and mortality of the pre-infective stages was high on plots with short herbage but was lower on most of the plots with long herbage, especially in July and August. In the laboratory, development of eggs into infective larvae was completed at temperatures ranging from 4°C to 27°C in faecal pellets which were either kept moist or dried out slowly, but not in faecal pellets which dried out rapidly. The rate of development increased as the temperature rose. Infective larvae survived for up to 16 months on the herbage of grass plots; some survived during very cold weather in the winter of 1981/82. In the laboratory, infective larvae suspended in tap water survived even longer at 4°C and 10°C but not at higher temperatures. They were rapidly killed by continuous freezing. They Survived for up to 8 weeks when subjected to desiccation, The relationship between climatic conditions and the development and survival of the free-living stages is discussed
Plots of pastureland with well grown herbage provided a more favourable environment for the survival and development of the free-living stages of Oesophagostomum dentatum in faeces during warm dry weather than did plots with short herbage. This was because well grown herbage gave better protection against the unfavourable effects of dryness than did short herbage. There was no significant difference between the plots during cool wet weather.
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