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Larval survival of barber's pole worm

Problem:  Many producers are unaware how long is required to prepare low worm-risk paddocks, although surveys show most are in favour of using them.

Solution: Understanding the few conditions under which worm larvae will die is vital in creating low worm-risk paddocks.

Benefit: Knowing the ‘required time’ for your property to create low worm-risk paddocks.

Under what conditions do worm larvae die?

A common misconception, based on having fewer worm issues in winter, is that frosts kill worm larvae on the paddock. This is a myth. 

Worm larvae are quite tough and can easily survive cold, frost and snow in Australia under the protection of a pasture.


Many worm larvae will survive these conditions as they are protected by the pasture or topsoil. Source: Deb Maxwell.
Many worm larvae will survive these conditions as they are protected by the pasture or topsoil. Source: Deb Maxwell.

As larvae cannot feed, once their stored energy is consumed they will die. At higher temperatures they are more active and use more energy, so will die sooner. Generally over 90% die by 6 months, even during cool weather, though a very small proportion will live for over a year.

However, larvae can be killed by extremely hot, dry conditions. The graph below shows that daily maximums consistently at 35ºC will greatly shorten lifespan, with most larvae dying within 2 months. But for larvae to be killed rapidly requires temperatures to be above the 40ºC mark for a few days to a few weeks, at which temperature they rapidly desiccate (dry out) and die.

While some larvae die immediately and others live for more than a year, it is temperature that determines how long, on average, they survive on the pasture waiting to be eaten by a sheep or goat.

For a paddock to be considered low worm-risk, well over 90% of worm larvae from an already well-contaminated paddock must die. Months are usually then required for the remaining population to build up to harmful levels again once livestock are present and conditions are suitable.

If only 80–90% or less reduction occurs, this leaves enough larvae (particularly barber’s pole worm) for a reasonably rapid population build-up once livestock are present.

Typically, over 90% of larvae die in 2 to 6 months, depending on the temperature.

  • In warmer regions (e.g. coastal or far inland), 2 months is required during the hottest months and 5 months during the coldest months.
  • In colder regions (e.g. tablelands), 3 months is required during the hottest months and 6 months during the coldest months.

Source: Modeled from death rate of the L3 population in ‘Simulation of pasture larval populations of Haemonchus contortus’ by IA Barger, PR Benyon & WH Southcott. Proceedings of the Australian Society of Animal Production (1972) 9: 38
Source: Modeled from death rate of the L3 population in ‘Simulation of pasture larval populations of Haemonchus contortus’ by IA Barger, PR Benyon & WH Southcott. Proceedings of the Australian Society of Animal Production (1972) 9: 38

Why are there fewer worms in winter?

Worm eggs need specific conditions to hatch into infective larvae. Daily maximum temperatures need to be at least 12–18ºC  (depending on worm species), combined with sufficient moisture (usually about 10–15 mm rain over a few days). If these conditions are not met within 1–3 weeks (again depending on worm species), the eggs will die. >> More information on conditions for development of worms.

Sheep and goats can and will be affected by worms in winter

While it may be too cold in some areas for eggs to develop to larvae, the pasture may still be heavily contaminated with larvae that developed in the previous 6 months during warmer times. These infective larvae will be ingested by sheep and goats grazing the affected pastures during winter. Depending on the contamination levels, it is quite possible for the stock to acquire large worm burdens in winter resulting in illness and death. Yet the worm eggs these animals are depositing on the pasture may never be able to develop as it is too cold.

The same thing can happen during extended dry periods. The pasture can be contaminated with worm larvae that developed during previous months with favourable conditions, allowing the grazing animals to become infected, but any worm eggs laid on the pasture during the dry time will not be provided with suitable conditions to develop. Barber's pole worm eggs are most sensitive and will die within a week if their conditions for development ar not met.

Scour worm eggs/larvae are better able to withstand dry periods, hence why grazing management is most successful with barber's pole worm rather than scour worms. The scour worm eggs can live about 2–3 weeks, but partial development may occur—that is, there is just enough moisture for the eggs to hatch to larvae, then those larvae stay protected in the faecal pellet for weeks, or potentially a couple of months, awaiting sufficient moisture to soften the faeces for the infective larvae to move out onto the soil and pasture.


Using this knowledge to control worms

Preparing low worm-risk paddocks relies on the existing worm larvae dying, while no further contamination of the paddock occurs. The time required to create the low worm-risk paddocks generally takes 3–6 months depending on how the prevailing temperatures support or inhibit larval survival. However, in some environments, most worm larvae will die once temperatures have been extremely hot for a few weeks

In the colder districts of Australia, a longer period is required for preparation through winter (6 months) than in summer (3 months) for most larvae to die and a low worm-risk paddock to be created.

In warmer districts, a month can be taken of these periods. In very hot districts, a further month can be taken off;  extreme temperatures over 40°C for a couple of weeks will kill most larvae.

 

More information on preparing low worm-risk paddocks can be found here: Grazing management

or the specific recommendations for your region are found in your regional Worm Control Program.