Size of inoculum dose regulates in part worm burdens, fecundity, and lengths in ovine Haemonchus contortus infections.

Density-dependent factors frequently have been shown to regulate population parameters of free-living and parasitic helminths. To test the effects of various infection levels of Haemonchus contortus on fecundity and worm size, lambs were inoculated with 3,000, 10,000, and 30,000 infective larvae. Daily eggs per gram (epg) and daily total fecal production per lamb were monitored continuously. Worms were collected from abomasa on 6, 15, 22, and 30 days postinfection (PI). Female worms were smaller on each day in the high-dose group when compared to female worms from the low-dose group; males in the high-dose group were smaller from days 15 through 30 PI. The high- and medium-dose groups had higher mortality rates on day 30 PI, and fecundity (eggs/female/day) was 78% lower. Daily epg and daily total eggs/lamb/day were lower in the high-dose group. Fecundity and worm size were correlated with the log-transformed dose level but not with adult worm number. Early parasite and/or host responses apparently exert long-term negative effects on growth and reproduction relative to the size of the establishing population of H. contortus.