The right way to manage worms in horses *H&H VIP*

  • If you’ve been dutifully dosing your horse with regular anthelmintics (dewormers), you may feel that particular job is done.

    However, experts are urging owners to think again, as research reveals that certain internal parasites have developed resistance to most, if not all, of the deworming products licensed for use in horses. With no new treatments on the horizon, worm burdens may prove harder to eradicate, which could affect performance, and increase the risk of disease or even death.

    It became apparent some years ago that the most effective long-term solution involves a targeted approach using faecal egg count (FEC) results. Concern that owners are failing to take this message on board prompted action from key figures in the UK equine veterinary world, who met recently to discuss the implications of anthelmintic resistance and the way forward with worm control.

    We’ve outlined the deworming truths that you need to know:

    Gone are the days of reaching for a one-size-fits-all solution to wave goodbye to worms. An increasing resistance among worm populations to the drugs we have available to treat them calls for a smarter approach.

    Worms have outwitted us through a “survival of the fittest” process, where resistant worms mate and produce resistant offspring. The emphasis is now on strategies aimed at slowing the development of this resistance.

    Rather than trying to eradicate all worms within a group of horses, the key concept is maximising “refugia” — a percentage of parasites that are not exposed to anthelmintics and which keep the resistant worms in the minority. This means using the drugs at our disposal with more care, the report suggests, deworming selectively for the sustained health of the overall equine population.

    While guesswork is out, using an FEC to select appropriate anthelmintics is definitely in. The report authors recommend that most adult horses have three FECs during the grazing season, spaced equally between February and September. Research has shown that deworming on the basis of FEC results, rather than by date, can reduce deworming costs.

    Foals can develop worm burdens but have more complex deworming requirements, so ask your vet for advice.

    The report highlights that very few FECs are repeated after treatment — an important part of the process. An FEC reduction test can reveal whether treatment has worked and identify whether resistance is present.

    Ideally, a high shedder (see ‘superworms’ below) should be tested again 10-14 days after treatment. An annual FEC reduction test, at some point in the grazing season, is a good measure for all herd members as part of an overall worm management plan.

    Before being turned out, a new arrival should be dewormed to eliminate as many parasites as possible. The authors recommend doses of moxidectin and praziquantel, ideally once an FEC has determined the worm status.

    For maximum protection, the horse would not be turned out for 10-14 days until a reduction test can confirm that deworming had been successful. A more practical compromise is to stable the horse for at least three days, to allow any parasite eggs present in the gastrointestinal tract at the time of treatment to be passed.

    Well-managed pasture is home to fewer worms, which will reduce the amount of anthelmintics needed. The report includes various management recommendations, such as minimising the number of horses per acre and maintaining consistent herd groups.

    Poo-picking is encouraged, at least twice a week, particularly when weather is warm (above 10°C) and damp — the ideal conditions for strongyle larvae to emerge from their eggs and move from dung into pasture. Harrowing simply spreads parasites across the entire area, so should be avoided.

    The muck heap must be sited well away from grazing areas, as parasite larvae can migrate many metres.

    Another surprising fact is that a cold winter will not necessarily kill parasites lurking at grass. While fields do need grazing breaks, pasture is best rested in hot, dry conditions.

    Encysted small redworm — also called small strongyles or cyathostomins — present one of the greatest threats. These are the larval stages of the parasite that have buried in to the lining of the gut, where they can lie dormant for some time before emerging en masse. A condition called larval cyathostominosis, or parasitic colitis, can result, leading to diarrhoea, colic and often death.

    These larval stages will not show up on an FEC because they are not shedding eggs. Until a reliable detection test becomes available, owners are urged to implement a strategic end-of-season treatment plan under the guidance of a vet or an animal medicines advisor, known as a suitably qualified person (SQP). While most horses will receive moxidectin, this may be unnecessary in the well-managed horse. A vet may recommend an alternative if disease risk is low.

    Much depends on the level of exposure and risk, which should be determined on an individual basis, taking into account the animal’s age, management and previous FEC results. Youngsters aged between one and three years are particularly vulnerable.

    The traditional approach of routinely treating for tapeworm is now obsolete, according to the report. Instead, a serum or saliva antibody test should be performed every six to 12 months to identify horses at risk from tapeworm-associated disease.

    Experts point out that small numbers of this short, yellowish worm, which uses the suckers on its head to attach itself to the gut lining, rarely cause colic or disease, and add that fewer than half of adult horses in the UK are infected. Tapeworm-related complications are more common in youngstock, however, particularly on stud farms, where more regular monitoring and treatment is advised.


    Evidence-based responsible worm control is essential if we are to avoid the situation where “superworms” become resistant to all available anthelmintics.

    An FEC provides an internal “snapshot” of a horse’s worm burden by revealing the number and type of eggs in a measured sample of droppings. Depending on whether a horse is a “low”, “moderate” or “high” egg shedder, he can then be wormed appropriately — or not at all.

    Here’s how to standardise sample collection to minimise FEC test result variability:

    ● Stable or separate an individual and collect droppings from within 12 hours of excretion.
    ● Take one to three samples from at least three different balls of faeces, to make a sample the size of a table tennis ball (40-50g).
    ● Place the sample in a zip-lock bag and expel the air before sealing it and labelling it.
    ● Keep the bag refrigerated and make sure the sample is analysed within five days of collection.

    The authors

    Equine Deworming: A Consensus On Current Best Practice was commissioned by UK-Vet Equine to provide equine vets with up-to-date information, and produced from discussions between experts: David Rendle MRCVS (Rainbow Equine Hospital), Dr Corrine Austin (Austin Davis Biologics), Professor Mark Bowen (University of Nottingham), Ian Cameron MRCVS (Rossdales Equine Hospital), Tamzin Furtado and Professor Jane Hodgkinson (both of University of Liverpool), Professor Jacqui Matthews (Moredun Research Institute) and Professor Bruce McGorum (University of Edinburgh). The meeting was supported by The Horse Trust, Virbac and VetPartners.

    Ref Horse & Hound; 7 February 2019