Winter often feels like a welcome break from the heavy parasite pressure that challenges small ruminants during the grazing season. While cold temperatures do slow parasite development, internal parasites, especially Haemonchus contortus (the barber pole worm), have developed strategies that help them survive winter conditions and reemerge in spring.
The Mill’s small ruminant experts frequently remind producers that winter is not a parasite-free season. Understanding how worms behave during the colder months helps sheep and goat owners make informed management decisions and protect their animals year round.
Winter Does Not Kill Worms. It Delays Their Activity.
When the environment becomes too cold or dry for worm eggs and larvae to develop on pasture, parasites enter a state known as hypobiosis, or arrested development. In this phase, larvae pause their growth inside the host until conditions improve. Once temperatures warm and moisture increases, these larvae resume their lifecycle, develop into adults, and begin egg production. This shift can lead to significant pasture contamination early in the grazing season.
Winter Activity Highlights
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Very few worm eggs hatch in winter because they require soil temperatures of about 56 to 60 degrees and adequate moisture.
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Most larvae on pasture become dormant to survive the cold and reemerge in spring.
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Snow cover or plant residue can insulate larvae and help them overwinter. Milder winters often result in greater survival.
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Larvae acquired in late fall frequently arrest in the abomasum rather than maturing. These larvae do not feed or reproduce until temperatures rise.
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When spring arrives, these previously dormant larvae resume development and can rapidly increase infection pressure.
This explains why many flocks and herds experience a spike in parasite activity early in the warm season.
Hypobiosis and the Periparturient Egg Rise
Hypobiosis is closely linked to the periparturient egg rise, a temporary reduction in immunity that occurs in ewes and does from two weeks before through eight weeks after birthing. As immunity decreases, hypobiotic larvae resume development and begin producing eggs.
This natural process leads to significant early-season contamination of pastures at a time when young lambs and kids are most vulnerable.
Climate Plays a Major Role
Hypobiosis occurs most often in temperate climates, such as the Mid Atlantic, where winter temperatures limit larval development on pasture. In tropical and subtropical regions, hypobiosis is less common because conditions allow near year-round larval activity.
Dry conditions can also trigger hypobiosis, making prolonged drought another factor that influences parasite behavior.
Regardless of the trigger, hypobiosis is an evolutionary survival strategy that enables parasites to persist from one grazing season to the next.
The Importance of Winter Parasite Management
Although parasite activity slows in winter, small ruminant owners should still monitor their animals and prepare for spring. Hypobiotic larvae do not respond the same way to all dewormers, and no product eliminates every worm stage. Many larvae survive the winter despite treatment.
Winter Management Considerations
Body Condition Monitoring
Animals losing weight or failing to maintain condition may be harboring parasite burdens that require attention.
Fecal Egg Counts
Even in winter, mild weather can allow some egg shedding. FECs help determine whether intervention is needed.
Preparing for Lambing and Kidding
Nutritional support and timely parasite management reduce the impact of the periparturient egg rise.
Pasture Management
Rotating pastures, avoiding overgrazing, and resting high-risk areas help reduce spring contamination.
Planning Ahead
Winter is an ideal time to review grazing plans, assess deworming strategies, and prepare for the upcoming parasite season.
Can Worms Be Eliminated Completely?
Complete eradication is not realistic. Resistance to dewormers continues to increase, and hypobiotic larvae are particularly resilient. The goal is not elimination but effective, sustainable parasite management that protects animal health and minimizes resistance.
Final Thoughts
Winter does not end the parasite cycle; it simply changes it. By understanding how worms survive and behave during the cold months, producers can better prepare for spring, reduce parasite pressure, and support healthier animals.
The Mill’s livestock team is available to assist with fecal egg count interpretation, deworming recommendations, and pasture management planning for sheep and goat owners.
