B+LNZ Wormwise vet and farmer Mary Bowron busts the myth that hard frosts will kill internal parasite larvae on pastures.
Yes - worms can survive winter. They haven’t persisted for millions of years without evolving ways to cope with cold snaps.
The reality is that worms are a year-round concern. You can make smarter management decisions heading into spring, if you understand what is REALLY happening in winter.
It’s best not to assume a few frosts will wipe out larvae on pasture.
While cold conditions do slow parts of the worm life cycle down, cold is far from a silver bullet. The worm challenge on your pastures in winter is largely set by the level of contamination that has built up in the autumn - and that contamination doesn't simply disappear when the temperature drops.
It can take months, not weeks, for worm larvae on pasture to die off. So, if your animals were shedding eggs heavily in autumn, there's a good chance those pastures are still carrying a significant larval burden well into winter. Which carries through into the following spring.
What Happens to Worms in Cold Weather?
The worm life cycle has several stages, and winter affects them differently:
- Egg hatching and early larval development slows right down in cold conditions. Bit like a bear going into hibernation. Variations to the life cycle depend on the worm species, with Teladorsagia (sheep) and Ostertagia (cattle) adapted to hatch eggs at cooler temperatures, so often worm burdens of these species show up earlier in spring.
- Third-stage larvae (L3), however, are a different story. This is the stage that infects your animals, and they are remarkably hardy. They have a protective double skin that helps them survive cold conditions - and some species can even survive several freeze/thaw cycles.
A few frosts simply aren't enough to wipe out large numbers of L3 larvae. While the overall worm challenge on pasture does gradually reduce as winter progresses, it is never fully eliminated during the cold season.
Not all worm species behave the same
The variation between species drives the seasonal differences we see with certain worms becoming more dominant at different times of the year.
Haemonchus in sheep prefers warmer climates so not generally a problem through the winter (except in the winterless North!).
Ostertagia in cattle and Teladorsagia in sheep prefer cooler climates and can survive cold, including freeze/thaw conditions. Massey parasitologists have shown that Teladorsagia L3 larvae can survive for up to four weeks at -4 degrees and then defrost and wiggle away. Trichostrongylus are also pretty hardy, they migrate up herbage as temperatures drop and can tolerate ice crystals forming within their bodies.
Nematodirus filicollis deserves a special mention as it has a unique winter survival strategy. For farmers in the north, you may not have heard of this worm species but for farmers in the south, it can be a problem.
The trick with Nematodirus is that larvae develop to the infective third stage (L3) inside the egg. That eggshell plus the L3 sheath gives them exceptional ability to survive cold, dry conditions. This means Nematodirus can survive winter in large numbers, with infection carrying straight from one season’s lambs to the next. This can result in sudden, early outbreaks in lambs before weaning.
Practical Steps to Reduce Winter Worm Challenge
The good news is that there are practical grazing management tools that can help reduce the worm burden your animals face over winter. Some options to consider are:
- Graze alternative forages and crops – these typically carry a much lower worm challenge than resident pastures
- Renew areas of pasture in autumn – replacing old sward removes much of the resident larval population
- Sell trade lambs earlier, so they aren’t contributing to larval contamination on pasture. This will free up more feed for your capital stock.
- Cross graze with another species, (e.g. sheep on calf areas, or cattle on lamb areas)– to help to break the worm lifecycle, generally cattle worms don’t infect sheep and vice versa.
- Graze calf areas with well-fed undrenched R2 or older cattle, or lamb areas with well-fed undrenched ewes – these will vacuum up worm larvae and lower overall challenge. They also provide refugia.
- Keeping grazing residuals high for young stock – most worm larvae are found in the bottom third of the grass sward, so avoiding close grazing reduces the dose ingested.
- Prioritise early-life feeding and growth of lambs and calves; well-grown lambs and calves will start to develop a functional immune response to worms earlier and will contribute less to autumn pasture contamination because of this.
The Bottom Line
Winter is not a worm-free period. L3 larvae are hardy, some species are specifically adapted to overwinter, and autumn pasture contamination carries forward into the colder months. Being aware of this - and making smart grazing and management decisions - is the best way to protect your stock and reduce the worm burden heading into spring.