Most roundworms share this basic roundworm life cycle.
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. 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.
The writhing movement of L3 results in them moving in moisture films onto the pasture and soil. L3 are carried in water films (from dew, mist or rain) onto the leaves and stems of pasture (and less commonly into the soil). Most L3 are concentrated near the base of the pasture, rarely higher than 10 cm. While the larvae do move toward light, it is not conclusively known whether larvae migrate up and down the grass blades according to light and temperature changes. Much of their movement is thought to be random or non-directional.
L3 do not feed but survive on energy reserves, dying when these have been used. L3 numbers on pasture decline very rapidly when temperatures exceed 40°C, as they die from desiccation.
When L3 on the pasture are eaten by sheep they develop into 4th stage larvae in the gut and then become adults to complete their life cycle. Adult male and female worms live and mate inside a sheep’s gut. After a minimum of 18 days developing to adults (pre-patent period), females lay worm eggs that are then passed onto the ground in the sheep’s dung.
The time for eggs to pass from the sheep after an effective drench
An effective drench will take some hours to kill all the worms present and therefore stop further egg-laying by female worms. Some viable worm eggs will already be in the sheep’s gut at the time of drenching and these won’t be affected by most drenches. It will take 3–4 days after an effective drench for the gut contents to carry most of the worm eggs out of the sheep. Bear this in mind if you wish to move sheep to a paddock that is to be kept uncontaminated.
The pre-patent period
This is the time taken for infective larvae, eaten by a sheep grazing pasture, to develop to adult worms in the gut, mate and start laying eggs, which appear in dung. The time depends on the worm species with barber’s pole worm completing this period in a minimum time of 18 days under ideal conditions. Most scour worms take about 21 days.
Therefore, little, if any, worm egg contamination of pastures will come from sheep in the pre-patent period from a few days after they have been given an effective drench that kills 98% or more of the worms present. Allow sheep to graze up to 21 days in barber’s pole worm areas and to 30 days in southern scour worm areas. This is a principle used in ‘Smart grazing’.
The auto-infection period
This is the time between eggs being deposited on pasture (in the sheep’s dung) and when the larvae that have developed from those eggs appear on pasture ready to re-infect sheep. This is 4–10 days; the shorter period when temperature and moisture conditions are ideal for the particular worm species.
This principle is used in fast-rotation grazing systems, where sheep are grazed on paddocks for a time shorter than the auto-infection period. They are then moved before they can become infected with larvae that have developed from eggs recently deposited by the same mob. This allows the sheep to continue through further paddocks, picking up infection at a slower rate from larvae already on the pasture.
Periods when the life cycle will be stopped
As worms require both warmth and moisture for eggs to develop to larvae (above 10–18°C depending on worm species, but ideally below 35°C, and with usually more than 15 mm rain over 4–7 days of rainy or overcast weather when the evaporation rate is low), there can be extended periods of the year in some locations when worms cannot successfully complete their life cycle. These include regions with particularly cold winters or hot summers or where there are lengthy dry periods.
Barber's pole worm eggs will die if these conditions are not met within about 10 days of them being deposited on the pasture. Scour worm eggs are able to survive a few more weeks awaiting suitable conditions for hatching.
During these conditions, sheep carrying worms can graze paddocks destined to be low worm-risk pastures for lambing ewes or weaners without contaminating them further. The deposited eggs won’t develop and will die within 1–2 weeks (1 week for barber’s pole worm, longer for other worms).
Note that during these periods the sheep will continue to be infected with any larvae surviving on the paddock from when conditions were suitable for development in the weeks or months beforehand.
The time for worm larvae to die
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.
This principle can be used when deciding how long it will take for paddocks previously contaminated with worms to become low worm-risk (i.e. when over 90% of the worm larvae have died).
Location of larvae on the pasture
As few larvae move higher than 10 cm up pasture plants, tall pastures or crops are a considerably lower worm-risk. As the pasture or crop is grazed lower, the sheep will consume more of the larvae that are present. Use this principle when choosing or preparing lower worm-risk pastures. Remember, however, that with crop there may be shorter grass around the edges of the paddock where larvae will be more available.