Winter worming horses is no longer about automatically giving every horse a wormer as the weather gets colder. This vet-reviewed guide explains whether you should worm horses in winter, when treatment may be appropriate, and what to worm with.
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Key Takeaways
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Should You Worm Horses in Winter? - The Shift from Blanket Worming to a Risk-Based Approach
Not necessarily. Winter has traditionally been associated with routine worming, particularly treatment with moxidectin to target encysted redworm. However, current guidance has moved away from blanket worming.
The reason is simple: wormer resistance is increasing, while there are very few treatment options available and no new equine wormer classes on the immediate horizon. Using wormers when they aren't needed can contribute to resistance and make treatments less effective in the future.
The CANTER (Controlling ANTiparasitic resistance in Equines Responsibly) guidelines recommend assessing each horse's individual risk and using testing wherever possible before deciding whether treatment is necessary.
For a low-risk adult horse, routine winter worming may not be required. For a young or higher-risk horse, further testing or treatment may be appropriate.
Why Is Encysted Redworm a Concern?
One of the main reasons winter worming requires careful consideration is encysted redworm. Unlike adult redworm, encysted larvae can remain dormant within the lining of the horse's large intestine. They are not producing eggs, which means they cannot be detected through a standard worm egg count.
When large numbers of larvae emerge from the gut wall, they can cause a serious condition known as larval cyathostomiasis. Historically, this risk led to routine winter treatment with moxidectin. However, the current approach is more targeted because larval cyathostomiasis is considered very rare in adult horses that are at low risk.
When to Worm Horses in Winter
There is no single date when every horse should be wormed. Instead, worming in winter depends on a few factors.
These factors include:
- Your horse's age
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Previous worming history
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Previous worm egg count results
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Grazing and pasture management
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The number of horses sharing the pasture
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Whether new horses have recently joined the group
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Your horse's overall risk profile
Low-risk adult horses
A healthy adult horse with good pasture management and a history of low worm burdens may not require routine winter treatment. Rather than automatically reaching for a wormer, review recent test results and your horse's risk profile with your vet or SQP.
Young or higher-risk horses
Young horses and those considered higher risk may need a different approach. Your vet may recommend additional testing or targeted treatment for encysted redworm.
A blood test can provide information about encysted redworm and is considered the preferred testing option where appropriate, although cost can be a limiting factor, particularly when managing larger groups of horses.
You can use the CANTER guidelines to assess your horse’s parasite risk:
What to Worm Horses With in Winter
If your horse needs treatment, the choice of wormer should be based on the parasite you are targeting, your horse's risk profile and previous treatment history.
Moxidectin
Moxidectin has traditionally been used during winter because it’s effective against encysted redworm. However, moxidectin should not be routinely used in low-risk horses. Unnecessary use contributes to selection for resistant parasites and can reduce the effectiveness of this important treatment option.
Where a higher-risk horse requires treatment, your vet or SQP can advise whether moxidectin is appropriate.
Fenbendazole
A five-day course of fenbendazole may also be considered in certain circumstances for treating encysted redworm. However, resistance to fenbendazole is widespread, so it should not be assumed to be effective without appropriate veterinary or SQP guidance.
The important point is that what to worm with should be determined by the individual horse and the reason for treatment, rather than simply following a seasonal routine.
Winter Worming Checklist
If you're wondering what your horse needs this winter, start with these steps:
1. Review your horse's risk - Consider age, previous test results, grazing conditions and management.
2. Check recent test results - Review your horse's WEC and tapeworm testing history rather than automatically treating based on the season.
3. Consider encysted redworm - Remember that encysted redworm cannot be detected by a standard WEC. Discuss blood testing with your vet if your horse is considered at risk.
4. Treat only when appropriate - If treatment is required, your vet or SQP can advise on the most appropriate active ingredient.
5. Record the outcome - Keep track of treatments and test results to help guide future decisions and monitor treatment effectiveness.
What Can You Do to Reduce Worm Risk Over Winter?
Worming is only one part of parasite control. Good pasture management can reduce the number of infective larvae your horse encounters.
Consider:
- Regular poo picking to remove faecal contamination.
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Avoiding overcrowding and excessive stocking densities.
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Managing grazing to reduce pasture contamination.
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Quarantining and testing new horses before introducing them to an established group.
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Keeping accurate records of WEC results and wormer treatments.
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Reviewing your horse's risk profile regularly.
Harrowing should not be relied upon as a replacement for poo picking in the UK. Our climate is generally not harsh enough to reliably destroy all stages of the parasite lifecycle.
Keep Your Horse Protected
Worming your horse in winter should be based on risk, testing, and veterinary advice rather than the calendar. While encysted redworm is an important consideration, routine moxidectin treatment is not necessary for every horse, particularly those at low-risk.
By testing where appropriate, managing pasture carefully and reserving wormers for horses that need them, owners can protect their horses while helping to preserve the effectiveness of the limited worming treatments available.
Looking for ways to test before you treat? Explore VioVet's horse worm testing and worm control range.
FAQs
VioVet vet, Dr Sophie Meers BVetMed MRCVS, answers your common questions about winter worming for horses.
Can a worm egg count detect winter worms?
During winter red worm larvae stop developing and burrow into the gut lining ready to re-emerge in Spring when temperatures become warmer again. These ‘encysted larvae’ don’t lay eggs and therefore worm burden cannot be detected by WECs. Instead your vet can perform a blood test to more accurately determine encysted red worm burden.
Why shouldn’t you blanket worm in winter?
Only moxidectin and fenbendazole are effective against encysted stages of small red worm. Dosing low risk horses unnecessarily selects for only the strongest, most resistant worms accelerating resistance to these wormers. The risk is that eventually there will be no effective treatments.
Do horses still get worms in winter?
As temperatures drop below 6 degrees red worm activity slows and horses are less likely to be infected with new worms from the pasture. However they can carry encysted worm larvae in the gut lining from the previous grazing season which are ready to emerge as the season changes to spring.
Written by: VioVet
