[The natural pattern of Ostertagia-infections in cattle (author's transl)]

In experiments carried out under field conditions with calves reared free from parasites and kept on pastures contaminated with Ostertagia larvae, the natural pattern of infestation with this parasite in calves was studied. It was shown that calves turned out early (late in April) developed a much higher primary (overwintered) infestation than calves turned out six weeks later on mown pasture. During the grazing season the serum pepsinogen levels were more indicative of the severity of the infestation than was the egg output. There also was a correlation between the serum pepsinogen level and loss of weight. During the housing period following the first grazing season, adult worms were eliminated more rapidly than inhibited fourth-stage larvae, resulting in a worm population consisting of up to 90 per cent EL-4 at the end of the housing period in April.