THE KEY COMPONENTS OF RESISTANCE TO OSTERTAGIA-CIRCUMCINCTA IN LAMBS

One of the most remarkable features of parasitic injections in general , and Ostertagia circumcincta infection of sheep in particular, is the extensive variation among hosts in resistance to infection, as assess ed by parasite burdens and production of eggs or infective larvae. Her e Mike Stear, Michael Park and Stephen Bishop describe the factors tha t account for the variation among animals within a pock, including cla m, sire, sex, date of birth and history of exposure to infection. Ther e are no detectable genetic effects in lambs less than three months ol d. Genetic control of an acquired response develops in two stages: fir st, a reduction in the average egg production per worm, which is assoc iated with the development of a parasite-specific local IgA response; and second, control of worm burden, which is associated with the produ ction of globule leukocytes in the abomasal mucosa.