[Repeated dehelminthization and its effect on the course of the natural infestation, of sheep by roundworms]

Long-continued use of levamizole for the control of gastrointestinal roundworms in sheep kept on pasture showed that the discrepancy in the results obtained in experiments and in the terrain is associated with the physiology of host and parasites. In practical use in the terrain, the preparation had an 85% effect in one case; otherwise the average effect was 61.1% and persisted five to six weeks in all flocks including those in which the animals were treated five times a year. Treatment performed later than in October did not destroy larvae in the mucous membrane because they already were in their anabiosis; only the imagoes were positively influenced. The larvae from the mucosa begin to maturate in February and eggs are produced hastily ("spring rise phenomenon"); the use of the drug in April mostly fails to reduce this abundant production of eggs. The roundworms Bunostomum phlebotomum, Oesophagostomum spp., Cooperia spp. and Trichostrongylus spp. almost disappeared from the flock after three-year treatment. Chabertia ovina, Ostertagia spp. and Strongyloides papillosus showed the highest resistance.