A female worm lays up to 10,000 eggs/day.They pass out in the dung, where they hatch if weather is warm and moist, go through 2 larval stages feeding on bacteria in the dung. In ideal conditions they can become the infective L3 larvae in 7 days. Rain & Dew wash these onto the grass where they are eaten by the sheep. Once in the sheep, they immediately moult and migrate to the abomasum (4th stomach).
IT TAKES 21-28 DAYS FROM LARVAL INGESTION TO EGGS PASSING IN DUNG.
(sorry for shouting, very important). This means an effective drench can help clean a paddock over this interval by taking up larvae but passing no eggs.
The 4th stage larvae & adult worm can remove 0.05 ml blood per worm per day ie: 1000 worms 50ml/day resulting in anaemia ie no stamina, pale gums and conjunctiva, even bottle jaw, constipation , and death.
Even with no visible signs of infestation, worms can reduce milk production and weight gain by 30%.