PARASITOLOGICAL CHARACTERISTICS OF FLEECE-WEIGHT-SELECTED AND CONTROLSHEEP

Male sheep (aged 15 months) from a Massey University flock selected fo r increased fleece weight for 37 years (FW, n = 13) and unselected con trols (C, n = 13), were infected with larvae of Haemonchus contortus ( n = 4000), Ostertagia circumcincta (n = 22750), and Trichostrongylus c olubriformis (n = 25000). Some FW sheep (n = 7) and C sheep (n = 5) ha d previously been treated with an albendazole controlled release capsu le (CRC), at 6 months of age. Faecal egg counts (FEC) were higher in F W than C sheep (4204 versus 300 eggs/g, P < 0.0001), as were the numbe rs of adult H. contortus (1151 versus 249, P < 0.01) and O. circumcinc ta (2268 versus 600 eggs/g, P < 0.05), when slaughtered at Day 28. Num bers of T. colubriformis (5838 versus 5266 eggs/g) did not differ betw een lines but were higher in previously CRC-treated sheep than in untr eated sheep (7585 versus 3810 eggs/g, P < 0.05). Thymus weights (corre cted for liveweight) were higher in FW sheep than C sheep (P < 0.01) a nd lower in CRC-treated sheep than in untreated sheep (P < 0.01). Numb ers of mast cells were inversely related to number of parasites in FW sheep, but not C sheep. The level of circulating antibodies to H. cont ortus did not differ between groups. This study confirms that higher F EC in FW sheep are a result of decreased resistance to establishment o f internal parasites (of some species).