Winter Parasite Control

Winter Parasite Control

Responsible horse owners aim to minimise the wormers they give their horses, basing their parasite control on the evidence of worms being present rather than treating routinely.

There is, however, one dangerous parasitic stage of the small redworm that we can’t test for (yet!) that can have fatal consequences.  Instead, we need to treat proactively for this – in most cases with a moxidectin wormer or alternatively a 5 day fenbendazole. We can also use this opportunity to test to ensure our worming has been effective, particularly if using the later chemical.


We advise putting one (or in exceptional cases two) doses into your worm control programme as a preventative against the possibility of encysted redworm (cyathastome) larvae. This is best done in the winter months between December and February and preferably after a few hard frosts.

Only two chemicals, moxidectin (in Equest and Equest Pramox) and five day courses of fenbendazole (Panacur Equine Guard) are licenced for the treatment of these encysted stages of small redworm. Moxidectin is the preferred treatment in most cases, particularly if you are also looking to target bots at the same time, because of widespread resistance to fenbendazole.

Equest contains only moxidectin while Equest Pramox has the addition of praziquantel meaning it also treats tapeworm. We would recommend testing for tapeworm first with the EquiSal tapeworm test and only giving the combination drug if you need to. More than 77% of horses won’t need that extra chemical!

Warnings: Moxidectin shouldn’t be given to underweight horses, foals under 4 months old (6.5 months if also combined with praziquantel) and isn’t licenced for donkeys. We also advise caution if using it in miniature Shetlands unless you can calculate dosage accurately. This is because the active ingredient is stored in body fat and these animals don’t have sufficient stores to take up the drug.

There are circumstances where a 5 day course of fenbendazole is more appropriate such as where moxidectin is contra-indicated and for young horses under four who are also susceptible to roundworm infection.


Just because you give a wormer it doesn’t mean it’s done the job we intended it to do. Resistance problems are rising across the five chemicals we have licenced to treat parasites in horses. Whichever drug you choose the winter dose is an ideal time to add in a resistance test to your programme (a followup worm count taken 10-14 days after worming) to ensure efficacy of the drug.

Results from previous winter resistance testing revealed that 40% of horses wormed for encysted redworm with fenbendazole and 2.8% of horses wormed with moxidectin had parasites that were showing some resistance to the wormer. Tests also showed 50% of horses with a positive redworm count that had been treated with moxidectin had been under-dosed, leaving those horses more susceptible to developing resistant strains of redworm.

Vet Carolyn Cummins commented: “It’s imperative that we get worming right for the health of our horses. A simple treat and test can help to pinpoint potential problems and offer peace of mind that these important issues are under control. Resistance and the challenge to keep horses disease free from parasites is a very real problem and one that we should all be vigilant for.

“Speak to your vet or SQP about which wormer to choose for your horse, know your horse’s weight and dose accordingly this winter. A worm count two weeks later is a very cost effective way to check efficacy and that your worm control programme is working.”


Wormers are very safe drugs and side effects are very rare. Some horses are however more sensitive than others. The very young, the very old and horses with immune systems compromised by conditions such as cushings, EMS and laminitis would be more at risk. If you are at all concerned then talk to your vet and consider these simple steps to minimise treatment risks:

  • Test first and only treat the worms you need to minimise dosing.
  • Avoid combination wormers; if you do need to give two chemicals separate out the different doses to give your horse’s system less to deal with and administer them at least two weeks apart.
  • Use a high strength probiotic such as Protexin alongside worming.

Vet Liam Gamble MA VetMB MRCVS, from Protexin Equine Premium says: “Worming your horse can cause a sudden and marked effect on the gut ecosystem. The gut can become inflamed, motility is affected, and the microflora can become imbalanced. This is especially so when there are large numbers of worms or when the encysted larvae of the small redworm are targeted.

“Increased probiotic supplementation by using a product such as Protexin Quick Fix to give alongside worming will help to reduce the negative side-effects of the drug by promoting healthy gut bacteria.”


Because we can’t test for the immature stages of small redworm and the consequences of encysted redworm colic are so serious, the advice in most circumstances is to worm proactively. However there are occasional situations where the risk of worming may outweigh the risk of not. Horses kept in well managed stable herds with three or more years of clear worm count histories are very unlikely to have any small redworm to encyst. In these circumstances it is up to the owner, in consultation with their vet to make an informed decision on whether to treat or not.


The majority of small redworm species go through a unique lifecycle stage that is particularly dangerous to the horse’s health. L3 larval stages ingested from the pasture burrow into the gut wall of the large intestine and become encysted. Some continue to develop here, re-emerging soon after to become egg laying adult worms. Others, especially during winter months, can encapsulate here lying in a dormant state known as inhibited encysted larvae where they can stay for lengths of up to three years. Tens of thousands of encysted larvae can line the intestine, where their presence can create a ‘leaky gut’ syndrome, impairing absorption of nutrients and resulting in possible weight loss and life-threatening illness.

As the season changes from winter to spring the increase in daylight hours and warmer temperatures can trigger a sudden ‘mass emergence’ of these encysted larvae from the gut wall. This activity can cause life-threatening bowel inflammation, known as colitis (larval cyathostominosis) in the horse, a very serious condition that requires urgent veterinary attention. 

This information has been kindly provided by Westgate Labs.

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