The Sequential Development of Naturally Occurring Ostertagiasis in Calves

Summary The factors leading to the development of the different forms of bovine ostertagiasis were studied on 2 farms where the disease was endemic. The intake and fate of infective larval stages throughout the entire grazing season was assessed by a comparison of the worm burdens of ‘permanent’ and ‘tracer’ calves. The ‘permanent’ calves grazed from May until October unless severe ostertagiasis supervened. The ‘tracer’ calves were introduced each fortnight and allowed to graze for only 14 days. The results showed, firstly, that the onset of Type I ostertagiasis from July onwards depended on a process of cumulative infection from the beginning of the grazing season, apparently unaffected by any immune response. Autopsy of ‘permanent’ calves at this time showed that the worm burdens were predominantly adult and in excess of 20,000 Ostertagia ostertagi. Secondly, autopsy of those calves which survived after October revealed a marked increase in the proportion of O. ostertagi fourth stage larvae. This seasonal alteration was also apparent in the worm burdens of ‘tracer’ calves, and by November 90% of their worm populations were inhibited at the fourth larval stage. The worm burdens of these calves were typical of pre-Type II ostertagiasis; the ensuing development of the inhibited larval populations of these calves give rise to Type II disease.