The epizootiology of nematode parasites of sheep in the Border area

Abstract

Two experiments to determine the seasonal incidence of nematode parasites of sheep are described. These were based on differential egg counts and a comparison between critical slaughter trials and egg counts. Trichostrongylus, the dominant parasite, reached peak worm egg counts in May, Oesophagostomum in September, Ostertagia in March, Nematodirus in April and Trichuris in June. Haemonchus contortus reached peak egg counts in December, January or February and, in the critical slaughter trials, in May. Since there was close correlation between egg counts and worm burdens, the summer peak would probably have been noted if sheep had been killed during these months. Except for H. contortus, egg count data could not be correlated with worm burdens in the slaughtered sheep. Strategic dosing is recommended in December, March and June.