A circular diagram of the 3 life cycle stages of H. contortus.
Dung containing worm eggs is passed from the sheep onto the pasture
Eggs develop through L1 and L2 stages to L3 ‘infective larvae’
- Time from egg to L3 is 4-10 days (slower when cooler, faster when warmer
- Generally, development to infective larvae is markedly reduced below 10 degrees C. above 35 degrees C. or when dry
- L1 and L2 feed on bacteria in the dung
Infective larvae wriggle out of the dung onto the ground and pasture
Third stage ‘infective larvae’(L3) move in moisture (rain/dew) and wriggle randomly, some onto the pasture to be eaten by sheep
- Quite resistant to cold and heat, but susceptible over 40 degrees C.
- Most L3 die within 3 (summer) to 6 (winter) months; some live over 1 year
- L3 do not feed; they die when energy reserves are used up (faster at higher temperature and humidity)
Infective larvae are eaten along with the pasture; Uneaten larvae die
Infective larvae become adults that live for many months within the sheep’s gut to reproduce and lay eggs
- Minimum time from L3 to egg laying is 18 days
- The sheep’s immunity can expel worms or suppress egg laying