REGULATION OF EGG-PRODUCTION, WORM BURDEN, WORM LENGTH AND WORM FECUNDITY BY HOST RESPONSES IN SHEEP INFECTED WITH OSTERTAGIA-CIRCUMCINCTA

Following infection with Ostertagia circumcincta there was considerabl e variation in worm burdens, worm size and number of inhibited larvae even among sheep matched for age, sex, breed, farm of origin and histo ry of parasite exposure. There was also substantial variation among sh eep in the concentration of mast cells, globule leucocytes, eosinophil s, IgA-positive plasma cells and parasite-specific IgA in the abomasal mucosa. With the exception of faecal egg counts over time, the parasi tological and immunological traits were all continually distributed am ong animals and sheep did not fall into discrete high and low-responde r categories. The responses were correlated. Sheep with more mast cell s also had more globule leucocytes, more eosinophils, more IgA plasma cells and greater amounts of parasite-specific IgA in the abomasal muc osa. Female worm length was strongly and positively correlated with th e number of eggs in utero. Faecal egg counts were associated with vari ation in worm number and with variation in the number of eggs in utero . The worm burden was negatively correlated with the number of globule leucocytes in the abomasal mucosa, suggesting that worm numbers ave r egulated by immediate hypersensitivity reactions. Decreased female wor m length was associated with an increased local IgA response to fourth stage larvae. The number of inhibited larvae was positively associate d with the size of the local IgA response and positively associated wi th the size of the worm burden. The results suggest that variation amo ng mature sheep in faecal egg counts is due, at least in part, to vari ation in local IgA responses which regulate worm fecundity and to vari ation in local immediate hypersensitivity reactions which regulate wor m burdens.