Winter Parasite Control

No matter how carefully grazing is managed, or how targeted a worming programme you have, there is one factor to consider which you can do nothing about – the weather!

Although you may think of spring and autumn as being optimal times for worms to thrive, a wet, mild winter can have a huge impact on parasite burden in horses.

Claire Shand, RAMA qualified Director of Marketing and Communication from Westgate Labs, explains all below.

Winter Parasite Control December – February
Here's what to consider for your parasite control this season.

  • Decide on your approach to encysted redworm; are you testing or treating?
  • Have you tested for tapeworm in the last 6 months? If not it’s time to EquiSal test.
  • Are you in an area where you need to consider bots?
  • If treating for redworm a reduction test is useful to monitor wormer efficacy.
  • There is one dangerous parasitic stage of the small redworm that we need to be particularly aware of at this time of year as it can have fatal consequences. Winter is the season to consider how we tackle the possibility of encysted redworm. This means either blood testing or treating with an appropriate wormer.

    Small Redworm Blood Test
    According to advice, horses with previous faecal egg count results of more than 200epg within the last year are considered high risk and may not be appropriate for this innovative new test, so should receive a routine treatment.

    If your horse is classed as low risk with counts of less than 200epg over the last year, is in a closed herd with good paddock management and there is frequent poo picking, then chat to your vet further about this test. Every horse that does not require a moxidectin dose is helping preserve to the chemical and delay resistance.

    When to Treat
    If you aren't doing the blood test for encysted redworm we advise putting one (or in exceptional cases two) worming dose(s) into your annual parasite control programme. This is best done in the winter months between December and February. Ideally we aim to wait until the weather has turned before giving this dose. Small redworm activity slows considerably on pasture below 6 degrees celcius so we are looking for the wintry weather and waiting for a few days where the daytime temperatures fall consecutively below this temperature. Worming after this time reduces the risk of reinfection and therefore many horses from needing a second treatment at the back end of the winter.

    This is why as our weather gets milder and more unpredictable our encysted redworm treatment has got later in the year. If we have a mild winter and the temperatures never dip, worm regardless before the end of winter.

    You may have heard the advice to do your winter worming after the first frost - this will kill off any lingering bot flies and prevents any further bot eggs from being laid on the horse's coat where they may be ingested.

    Choosing a Treatment for Encysted Redword
    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.

    Has the Treatment Been Effective?
    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 reduction test to your programme. If your winter worm egg count is positive. treat with the appropriate product and do a followup worm count 10-14 days after worming to ensure efficacy of the drug.

    Results from previous winter reduction 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.

    Until February 2021, Westgate are offering a reduction test for just £5.50.

    Westgate Labs and EquiToolz
    The team at Westgate Labs has created a short online course to help you learn a bit more about how to protect your horses from parasites. Sign up at and look for ‘An Introduction to Parasite Control’ on the course listings.