Roundworm: Gastro-Intestinal Nematode Life Cycle

An effective drench will take some hours to kill all the worms present and therefore stop further egg-production by female worms. Some worm eggs will already be on their way down the gut during drenching and won’t be affected by most drenches.

It will take 2–4 days after an effective drench for the gut contents to carry most of the worm eggs out of cattle. Bear this in mind if you wish to move cattle to a paddock that is to be kept totally uncontaminated.

This is the time taken for infective larvae, eaten by cattle grazing pasture, to develop to adult worms in the gut, mate and start laying eggs back into dung.

The time depends on the worm species, with barber’s pole worm (Haemonchus placei) completing this period in a minimum time of 26 days under ideal conditions. Most small intestinal worms (Cooperia species) take around 21 days but can be as short as 12 days for C. punctata and C. pectinata.

Little or no worm egg contamination of pastures will come within the pre-patent period from cattle who are given an effective drench that kills 98% or more of the worms present.

Cattle can graze up to 25 days in barber’s pole worm areas and nearly 20 days in southern areas with small intestinal worms without shedding eggs.

This is the time between eggs being deposited in dung onto the pasture and when the larvae that have developed from those eggs appear on pasture, ready to re-infect cattle. It varies between about 4 and 10 days, depending on temperature (which governs the rate of egg development) and the availability of moisture (essential for larval development and movement out of the dung pat).

This short time period means that permanent weaning paddocks can become heavily contaminated with worms, if the conditions of temperature and moisture favour this.

The development of worm larvae on pasture requires both warmth and moisture, with requirements varying between species. In most environments, there are extended periods of the year when worms cannot successfully complete their life cycle, when eggs die before they can develop to infective larvae, and larvae on the pasture also die.

Generally, ideal temperatures are in the range of 10–18°C, but development can occur up to around 35°C if there is abundant moisture. Moisture is critical, and a useful index is the presence of green pasture, which indicates there is sufficient moisture at ground level for development within the dung pat.

In winter rainfall regions, where small brown stomach worm (Ostertagia) is the most important species, egg development occurs from autumn to late spring or early summer, when hot and dry conditions quickly kill eggs as they are deposited. Development begins again in autumn once there is sufficient rainfall to promote pasture growth. (Note, some larvae survive through summer in the dung pat, where summer temperatures are relatively mild.)

Barber’s pole worm (Haemonchus placei) differs from small brown stomach worm, with a higher requirement for moisture and better tolerance of hot conditions, but a poor cold tolerance. Rainfall of least 15 mm rain and up to 50 mm over several days, with a low evaporation rate, provide ideal conditions for development. Periods where it is too dry or cold (and occasionally, too hot) on the pasture often occur sporadically even where the climate is generally favourable, explaining the large variation in risk of barber’s pole worm infection between paddocks and over time.

The worm eggs of most species will die if suitable conditions are not met within about 10 days of being deposited on the pasture. During such conditions, wormy cattle can graze paddocks without significant further contamination with larvae. As the eggs deposited won’t develop, and will die within a short period, they may be suitable for grazing with calves or weaners.

However, during these periods young cattle may continue to be infected with any larvae surviving on the paddock from when conditions were previously suitable for egg hatch and larval development (as explained below).

Infective larvae are relatively tough and can withstand dry, cold and moderately hot conditions. The life expectancy of larvae varies depending on the environmental conditions at the time: some will die within days, while others will live for a year or even more.

Generally, almost all larvae will be dead within 6 months of development 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.

These principles can be used when deciding how long it will take for paddocks previously contaminated with worms to become low worm-risk (i.e. when most of the worm larvae have died).

As few larvae move higher than 10 cm on pasture plants, cattle grazing on pastures or crops taller than 10 cm are at a considerably lower risk of picking up larvae.

As the pasture or crop is grazed closer down to the ground, cattle will consume more larvae, as they are present in greater numbers.

Use this principle when choosing or preparing lower worm-risk pastures.

Remember, however, that with crops there may be shorter grass around the edges of the paddock where larvae will be more available.