[The effect of dehelminthizations performed during the year on the seasonal dynamics of natural nematode infections in sheep].

In four sheep flocks of two age categories dynamics of natural infections by pulmonary and gastrointestinal nematodes was studied; sheep were kept on a farm in Western Bohemia. Dehelminthizations were performed in different intervals during the grazing period on the basis of the results of quantitative coprologic examinations. Total effectiveness of 80--100% intenseffectiveness (IE) was obtained as a result of single peroral or intraruminal dehelminthization with the following preparations: pyrantel hydrochloricum (Spofa), helmatac (SKF) and nilverm (ICI); the effectiveness concerned gastrointestinal nematodes of the genus Haemonchus, Cooperia, Ostertagia, Trichostrongylus, Bunostomum, Chabertia, Nematodirus, Strongyloides, Oesophagostomum and Trichocephalus. The effectiveness of nilverm on lungworms of D. filaria and P. kochi reached 100%; the preparation was less effective and ineffective on M. capillaris. Dehelminthization practices during three years were more successful as to lowering of incidence of lungworm infections of D. filaria and P. kochi than in gastrointestinal nematodes. If sping dehelminthizations had been postponed till the second half of May or June, the climax of the elimination of ova from summer re-infection was put off till November, with an initial significant increase in September. The third dehelminthization, applied in August, did not result in the increased elimination of ova in autumn, while there was no usual autumnal climax following September dehelminthization. Effective dehelminthization performed at the end of November in all three years maintained low levels of infections during winter housing and significantly influenced the health conditions of ewes before lambing. Dynamics of the elimination of ova after dehelminthization was affected by nematodes with the migration phase in organs and tissues -- S. papillosus, Oesophagostomum sp. and Ostertagia sp.; the same effect was observed, during pasture, in nematodes with relatively short exogenous development of the genus Haemonchus, Ostertagia, Trichostrongylus and Nematodirus. In spite of these dehelminthization practices, the elimination of ova and larvae corresponded to the course of temperatures and rainfall in spring and autumn climaxes every year.