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Abstract
Male sheep (aged 15 months) from a Massey University flock selected for increased fleece weight for 37 years (FW, n = 13) and unselected controls (C, n = 13), were infected with larvae of Haemonchus contortus (n = 4000), Ostertagia circumcincta (n = 22750), and Trichostrongylus colubriformis (n ‐ 25000). Some FW sheep (n = 7) and C sheep (n ‐ 5) had previously been treated with an albendazole controlled release capsule (CRC), at 6 months of age. Faecal egg counts (FEC) were higher in FW than C sheep (4204 versus 300 eggs/g, P < 0.0001), as were the numbers of adult//, contortus (1151 versus 249, P < 0.01) and O. circumcincta (2268 versus 600 eggs/g, P < 0.05), when slaughtered at Day 28. Numbers of T. colubriformis (5838 versus 5266 eggs/g) did not differ between lines but were higher in previously CRC‐treated sheep than in untreated sheep (7585 versus 3810 eggs/g, P < 0.05). Thymus weights (corrected for liveweight) were higher in FW sheep than C sheep (P < 0.01) and lower in CRC‐treated sheep than in untreated sheep (P < 0.01). Numbers of mast cells were inversely related to number of parasites in FW sheep, but not C sheep. The level of circulating antibodies to H. contortus did not differ between groups. This study confirms that higher FEC in FW sheep are a result of decreased resistance to establishment of internal parasites (of some species).
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