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Seasonal Parasite Control Winter

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. This helps to slow down the process of them becoming resistant to the medication we have available.

Regular worm counts monitor adult redworm and roundworm burdens and the EquiSal saliva test detects tapeworm. There is however one dangerous parasitic stage of the small redworm that we can’t test for (yet!).  Instead we need to treat for this and we can also use this opportunity to test to ensure our worming has been effective.

Winter parasite control

Encysted stages of Redworm

Encysted Redworm

The majority of small redworm species go through a unique lifecycle stage that is particularly dangerous to the horse’s health. L3 larval stages of the worm burrow into the gut wall of the large intestine and become encysted. Some continue to develop within the gut wall, re-emerging soon after to become adult, egg laying worms residing in the large intestine while other larvae stay encapsulated for months or years within the horse’s gut wall, lying in a dormant state known as inhibited encysted larvae. Tens of thousands of these encysted larvae can line the intestine, where they impair absorption of nutrients, resulting in possible weight loss and life-threatening illness.

Here they wait until conditions, such as a change in season from winter to spring, triggers a ‘mass emergence’ from the gut wall. This activity can cause cause life-threatening bowel inflammation, known as colitis (larval cyathostominosis) in the horse and is a very serious condition.


Equest Pramox, Equest, Panacur 5 day

Vets and SQPs therefore advise putting one proactive dose per year into your worm control programme as a preventative treatment 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. Because there is documented resistance to fenbendazole, moxidectin is the preferred treatment for your winter dose in most cases particularly if you are also looking to target bots at the same time. However moxidectin shouldn't be given to underweight horses, foals under 6 months old and isn't licenced for donkeys. You should also be cautious about 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. 

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!

In some circumstances a 5 day course of fenbendazole is advised, particularly for equines in which moxidectin is contra-indicated and for young horses under four who are also susceptible to roundworm infection. 

Does your horse have resistant worms?

Targeted worming programme

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.”

December 2017 to February 2018 inclusive, Westgate are offering a resistance test for just £5.

> Shop Resistance tests
> More on Resistance testing


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 compromised immune systems 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: 

  1. Test first and only treat the worms you need to to minimise dosing.
  2. 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.
  3. 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." 

> Shop Protexin Quick Fix


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 worm or not.