Factors contributing to paddock contamination with worms

Minimum time before worm eggs can become infective larvae.

4–10 days

Short graze periods (less than 4 days) prevent ‘auto-infection’ (animals becoming infected by larvae arising from worm eggs the same mob have recently deposited onto the pasture).

Conditions required for significant numbers of worm eggs to hatch and become infective larvae.

4–10 days of:

Brown stomach worm
Temperature: daily maximum >8°C¹
Moisture in this time: >10–15 mm rainfall²

Black scour worm
Temperature: daily maximum >15°C for
T. colubriformis or >12°C for T. vitrinus
Moisture in this time: >10–15 mm rainfall³

Barber’s pole worm
Temperature: daily maximum >18°C¹
Moisture in this time: >10–15 mm rainfall³

¹Some hatching of worm eggs of all worm species can occur below these daily maximum levels, but this is usually at a small and insignificant rate.
²Brown stomach worm eggs can develop at low rates without rainfall even in a relatively dry faecal pellet.
³Development to infective larvae may occur without rainfall if soil moisture profile is high.

Unsuitable conditions prevent eggs hatching and developing into infective larvae.

Note: The eggs of the brown stomach worm are much more tolerant of cold and dry conditions, and in general, grazing management has less effect on its control.

Brown stomach worm: 21 days

Some brown stomach worm eggs may survive for longer periods. 

Black scour worm: 16 days

Barber's pole worm: 5 days

Prolonged periods without the right conditions (temperature/moisture) for egg development will result in the eggs dying. This lowers the worm-risk of paddocks.

However, once hatched, infective larvae of both black scour and brown stomach worm can remain in the faecal pellet until conditions are more suitable.

The time for about 90% of the barber’s pole worm infective larvae (L3s) to die (making paddocks low worm-risk).
Note: larvae of brown stomach worm and black scour worm can survive longer because they can remain in the faecal pellet for extended periods.

L3 larvae do not feed. While waiting to be eaten by animals, they wriggle randomly in drops of moisture, more so in warmer conditions. Increased activity in warm weather depletes their energy reserves faster, hastening death.

In extremely hot, dry and windy conditions the larvae dry out and die.

Minimum time for infective larvae eaten by animals to mature and lay eggs (the ‘pre-patent period’).

Sheep: minimum of 18 days for most sheep roundworms.

Goats: minimum of 14 (typically 21) days for barber's pole worm and 21 days for scour worms.

Worm larvae eaten by animals soon after an effective drench will take at least 18 days (in sheep) or 14–21 days (in goats) before they can lay eggs. During this period after administering an effective drench, animals are not re-infecting the pasture.