Why you need to worm a horse in winter and which wormer you need to give now!

When it comes to worming your horse it can feel literally like a can of worms.   And while we advise never to just reach for the wormer as the season changes. You may have heard you need to worm your horse in winter, but don’t give just any wormer!  

Read on to discover why winter is the one time you must give your horse a wormer, even if you regularly do worm count tests and you horse has a low burden.

worm a horse in winter the one time you must worm and why

Why should you worm your horse in Winter? 

So you have been vigilant and had worm counts all year,  why do you need to give a wormer now? 

Surely the point of worm count testing is to avoid giving wormers?

Totally, we don’t want you giving all year round wormers.  However, there is one dangerous parasite stage that we cannot see with testing; inhibited encysted small redworm.

Inhibited encysted small redworm are the larvae of small redworm also known as cyathastome. They bury themselves into the lining of the gut, they then lie dormant over winter before they emerge in the spring. This is often referred to as ‘mass emergence’ due to the levels of red worm hatching.

As they are larvae and not egg producing adults, they do not show up on equine faecal egg counts / worm counts.

Although it is a normal part of the life cycle throughout the year for the small red worm larvae to encyst into the gut wall, it is the fact that they become dormant or ‘inhibited’ over the winter months. That allows levels of encysted larvae to build up in the gut, leading to associated problems if left untreated.

They are extremely dangerous for horses as they cause colic, diarrhoea and even death if left untreated. 

So even if your horses worm count has been low all year, you will still need to treat.

worm a horse in winter even if worm count low

How horses pick up Small Redworm

  • The infective larvae started as eggs passed out in dung, they then hatch and contaminate the grass.
  • As the horse grazes he ingests the infective larvae, which are on the contaminated grass..
  • When the larvae reach the gut, they lay dormant until the temperature increases in the spring.
  • In spring they then emerge in their masses, and this is when they cause significant damage to the intestinal wall, cause internal bleeding, anaemia, weight loss, colic, diarrhoea, shock and death. 

Therefore they must be treated before they mature and lay eggs and they can lay dormant for up to 2 years!

What wormer should you give and when for encysted redworm?

Between November and February, with the optimal time being in December – January to treat encysted redworm you need to give a wormer with moxidectin (Equest) as its active ingredient.

Traditionally, we used to treat with a 5 day course of a fenbendazole based wormer (Panacur Equine Guard), but there is widespread small redworm resistance to bendimidazole now so this is not recommended if resistance is present.

Some moxidectin based wormers (Equest Pramox) contain praziquantel so will also treat tapeworm, but we recommend that you do not use this unless you have tested for tapeworm (get a tapeworm test here) and need to treat.  As doing so you could be contributing to resistance. And chances are you don’t need to give this extra chemical (See our blog post on why here)

Moxidectin will also treat bots.

Please Note: If your horse is underweight or under 4 months old do not give them Moxidectin (6 ½ months if treating with Moxidectin & Praziquantel / Equest Pramox).  Moxidectin is also not licensed for donkeys and should be used cautiously with miniature ponies. Please calculate dosages accurately before treating. If your horse is young or in poor condition a 5 day course of fenbendazole may be better suited.  Please seek veterinary advice if you are unsure.

when to worm a horse

How do you know that you have successfully treated inhibited encysted small redworm?

As we know resistance is a real problem in worming horses and sadly simply giving wormers doesn’t always means that it worked, especially if the dose was incorrect. (this is why if is essential to use a weight tape or weighbridge to calculate the dose.  Too little will contribute to resistance.

Performing a worm count a few weeks after worming will help to remain vigilant.

Any Questions?

We hope this blog has been helpful to you.  Do you have any questions we can help you with?

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