The most commonly heard misconception about worms has to be “frost kills worms”, and therefore sheep don’t get infected after the frosts.
Unfortunately, more than a few sheep producers each year are caught out with very wormy sheep in winter—even for barber’s pole worm—after having frosts.
While it is true that some types of worm larvae directly exposed to frost can die, many larvae are very well protected by pasture and soil. The fact remains that a pasture covered in frost still has the potential to be a significant worm infection risk for grazing stock.
The misconception stems from confusion about what affects the different pasture stages of worms. Worm eggs are deposited on pasture in dung and can develop to infective larvae. Larvae can then be consumed on the pasture by livestock and become adult worms within the animal.
But worm eggs and worm larvae are quite different in their ability to survive tough conditions.
While eggs, particularly those of barber’s pole worms, don’t like cold, the larvae survive on pasture very well during cold, frosty conditions, taking about 6 months for most of them to die during the colder months of the year.
Development of worm eggs to infective larvae
Generally, the eggs of roundworms that affect sheep and goats are not particularly tough: brown stomach worm has the greatest ability to develop after unfavourable conditions, whereas barber’s pole worms need very favourable conditions within a week of being deposited on the pasture to successfully hatch to infective larvae.
Development requires moisture—around about 10–15 mm of rain over a couple of days or so, or similar conditions caused by heavy dews or waterlogged ground combined with high humidity.
But temperatures must also be warm enough. When daily maximum temperatures consistently fall below 18°C, few barber’s pole worm eggs develop to infective larvae, and these eggs generally die within a week.
Scour worms handle colder conditions better—hence why they are more common in southern Australia. Black scour worm eggs (Trichostrongylus colubriformis) can hatch when daily maximums are down to about 15°C, whereas Trichostrongylus vitrinus find their limit around 12°C. Most brown stomach worm eggs stop hatching when daily maximums fall to about 8°C.
Scour worm eggs can also wait longer for the right conditions than barbers’ pole worm eggs before they die; about 2–3 weeks.
So progressively colder weather slows and finally stops the development of worm eggs to infective larvae on pasture.
So how can sheep and goats still get wormy in winter?
Unlike the eggs, larvae can survive for months. The colder it is, generally the longer they survive, as it’s rarely so cold in Australia to act as a killing event, even in the tablelands areas.
In the frosty northern tablelands of NSW, the beginning of May typically sees barber’s pole worm eggs stop developing, but the eggs that developed to larvae in autumn—particularly in March and April—will mostly live on through the frosts.
While the number of larvae on pasture does decline over winter as they gradually run out of energy and die, remaining larvae continue to infect sheep grazing those pastures during winter and early spring.
The knowledge that development of eggs to larvae can be halted by cold can be exploited in a strategy to prepare low worm-risk lambing/kidding or weaning paddocks in summer rainfall areas. However, the larvae that developed in autumn are still a risk to the stock during winter, and regular WormTests should be carried out.
Larvae also don’t like very hot conditions. The warmer it is, the faster they die. They are more active in warmer conditions, so run out of their finite energy reserves faster.
In summer, in cooler districts, most larvae die in about 3 months, but in hotter areas that time drops. Again, this knowledge is used with grazing strategies to create lower worm-risk paddocks.
In the extremely hot inland areas during the peak of summer, when conditions exceed 40°C, larvae only survive a few weeks. In hot, dry conditions they become desiccated (dry out) and die.
You can apply these strategies on your property. Go to Your Program in WormBoss to see the grazing management strategies that reduce the exposure of livestock to worms and help to take the reliance for worm control off drenches.