Worm burdens in cows. II. An analysis of the population of nematodes in the abomasa of adult dairy cows.

Over a period of one year, from February 1979-February 1980, the abomasa of four dairy cows were examined each week for the presence of parasitic nematodes. Based on the identification of adult male worms, Ostertagia ostertagi (94%) and Trichostrongylus axei (75%) were the most prevalent species, and Haemonchus contortus was absent. Total worm burdens varied from zero (one cow) to 100,890. The geometric mean total number was 3011. The geometric mean number of arrested larval stages was 685. Of the adult worm burden, more than 85% consisted of O. ostertagi, about 12% of T. axei, the remaining 3% was other species. No relationship could be found between worm burdens and the age of cow, the weight of the abomasum or the milk yield. The adult Ostertagia burden was highest in winter, lowest in summer. Besides those of O. ostertagi, small numbers of arrested larvae of T. axei were observed from September onwards. Cows from farms with a zero-grazing system had significantly fewer worms than cows from other farms independently of the time of the year.