Roundworms, tape worms and flukes are all internal parasites of sheep. Roundworms, or nematodes, are of most concern, and there are 29 species in New Zealand. They are estimated to cost about $300 million per year in lost production and drench use. Adult roundworms live in the sheep’s gut and produce eggs, which are passed out with dung onto pasture. Over the next few days to several weeks, depending on moisture and temperature, the eggs hatch and develop through three larval stages. The larvae climb up moist grass and are eaten by grazing sheep.
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Source: D. M. West, A. N. Bruere, and A. L. Ridler, The sheep: health, disease and production. 2nd ed. Palmerston North: Veterinary Continuing Education, Massey University, 2002, fig 8.1