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Parasite Immunology Lab

Image Description

A circular diagram of the 3 life cycle stages of H. contortus.

Dung containing worm eggs is passed from the sheep onto the pasture

Dung Stage

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

Pasture Stage

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

Host Stage

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