Worm eggs that have passed from the sheep in dung hatch and develop through first (L1) and second (L2) larval stages to become infective larvae (L3). The success and speed of this development depends on weather conditions, specifically warmth and moisture, and require a minimum of 4 days and rarely more than 10 days.

2. What temperature and moisture conditions does this development require?

Temperature requirements vary for each worm type, but most require about 15 mm of rain over a few days (but also depends on evaporation rates) to provide sufficient moisture for development. The L3 leave the dung moving onto pasture and soil, rarely more than 25 cm from where they were deposited in the dung.

3. How long do infective larvae live?

Infective larvae are relatively tough and can withstand dry, cold and moderately hot conditions. All populations of living things vary in their life expectancy and worms are no different; some larvae will die within days, but some will live to around a year or more. Generally, over 90% of larvae will be dead within 6 months under cooler conditions and as little as 3 months when temperatures are ideal (about 25–30°C). Under extremely hot, dry conditions larvae will be desiccated and can die in a few days to weeks of these conditions, explaining why worms are rarely a problem in the arid zone.

4. How long does it take to make a paddock low worm-risk in winter versus summer after wormy sheep have grazed it?

Paddocks are considered low worm-risk when over 90% of the worm larvae have died. The graph (right) shows the decline in larvae over time under different temperature conditions.

Paddocks prepared for spring use need 5 or 6 months of preparation, whereas paddocks prepared for summer or autumn use need about 3 months of preparation time.

5. About how many eggs are produced per day for barber’s pole worms and black scour worms?

Barber’s pole worms are quite long (20 to 30 mm) and clearly visible. Only adult females have the characteristic ‘barber’s pole’ appearance due to the pink (blood-filled) intestinal tract of the worm twisted around the paler reproductive tract; whereas the males are smaller (around 15 mm) and pale pink. Females are prolific egg layers, laying up to 10,000 eggs per day, as such, higher worm egg counts are usually seen with these worms.

Adult female black scour worms lay 100–200 eggs per day. Black scour worms live in the first three metres of the small intestine of the sheep and cause damage to the lining of the gut. The adult female in the small intestine lays eggs, which are passed out in the dung.

6. Aside from leaving a paddock destocked, what are two ways that you could use a paddock while it is being made low worm-risk?

The following practices or a combination of these can create paddocks with less worm contamination and lower worm-risk:

• Rotational grazing with sheep
  Compared to set-stocking, this typically involves creating a higher stocking rate with larger mobs (at least twice the set-stocking rate) and introducing them to the paddocks when the pasture is about 7 cm high and grazing down to 3 cm high. Aim to have a non-grazing rest period of at least 2 months in winter and 3 weeks during the active pasture growth phase.
• A non-sheep use in at least the 6–8 weeks prior to use with weaners or hoggets
  • Grazing paddocks with cattle
  • Cropping
  • Haymaking
  • New pasture establishment
• Grazing with adult sheep that have a tested low worm egg count (less than 50 epg)
• Grazing with sheep only in the 30 days after they have received an effective drench (also see ‘Smart grazing’).

7. Aside from sheep, name 2 other farm livestock species that can carry sheep worms?

Goats and alpacas can carry sheep worms. While young cattle/calves also carry some sheep worms, adult cattle tend to have very low burdens of sheep worms and contribute very little to contamination of pastures with worms affecting sheep.