Smart Worming and Wormer Resistance

Smart Worming and Wormer ResistanceSimply Monitor worm eggs Assess the Risks and only Treat if necessary


Why do we need to change?

  • Every year worm damage results in disease such as poor performance, weight loss ( or lack of weight gain) colic, diarrhoea and, regrettably, sometimes death
  • Studies have shown that most horses are being wormed too frequently and with inappropriate products
  • Horse with WormsThe number of worming products can be bewildering and advice on worming can be confusing and even misleading
  • Experts are all agreed that the traditional approach of worming at pre-determined intervals is misguided
  • Worming at pre-determined intervals has resulted in worms developing ‘resistance’ to wormers
  • ‘Resistance’ has already been identified to all the classes of wormer available in the UK
  • There are no new classes of wormer under development for horses
  • ‘Resistance’ has had devastating effects on sheep flocks and is likely to do the same to horses if nothing changes

If worming habits fail to change we will all have to accept the harsh realities of worm-related disease: poor performance, weight loss (or lack of weight gain) colic, diarrhoea and, regrettably, sometimes death.

What is wormer resistance? 

Small Roundworms passed in Faeces
Small Roundworms passed in Faeces

Wormer, or more correctly ‘anthelmintic’, resistance occurs when a proportion of the parasites picked up from a particular pasture are no longer killed by the wormer. The main factors that lead to wormer resistance are:

  • Using wormers too frequently
    – kills the susceptible worms before they can reach sexual maturity. This actively selects the worms that are genetically resistant leaving them to breed with one another producing offspring that are resistant.
  • Incorrectly estimating the weight of the horse and under-dosing
    – helps partially resistant worms to survive which also increases the proportion of resistant worms that are left to breed.
  • Repeatedly using wormers of the same type
    – applies the same selection pressure on a population and may hasten the development of resistance to that wormer.

Once resistance is present in a worm population in a horse or on a property it cannot be reversed. The health, welfare and performance of those horses infested with resistant worms may be compromised.

Does your pasture already have wormer resistance? Determining whether you have wormer resistance is straightforward and inexpensive

Horse in pasture
Horse in pasture

If you are on a pasture that has a lot of horses or have been using routine treatments without worm egg counts for many years then you would be strongly advised to check for resistance. Please contact your vet for more information on how to perform a ‘Faecal Worm Egg Count Reduction Test’.

Practical management tips 

Performing worm egg counts and targeted worming treatments will go a long way to reduce the threat of resistance but there are also certain management steps that will help to reduce the use of wormers, reduce the levels of infection and reduce costs of treatment.

  • Remove faeces from pasture (‘poo-pick’) twice weekly.
  • Avoid spreading horse manure on pasture grazed by horses.
  • Avoid overstocking and overgrazing.
  • Rotate grazing with sheep or cattle or rest pastures regularly.
  • Muck out and thoroughly clean stables regularly especially when they contain foals and weanlings.

What about younger horses? 

Foals in Pasture
Foals in Pasture

In youngstock more frequent worming is required as younger animals are more prone to infection. You should consult your vet for specific advice tailored to your circumstances.

Did you know?

  • For every ten adult horses grazing only one or two will need worming
  • Four out of five warmers are given unnecessarily
  • SMART worming is proven to be cheaper
  • Parasites aren’t all bad; they may help the immune system and it is beneficial to maintain a low level of sensitive worms called ‘refugia’ on the pasture
  • Overworming may reduce wormer efficacy
  • Resistance to warmers is already widespread

Every pasture is different and you should speak to your usual vet about your specific needs. For most premises with adult horses that are under consistently good management, a simple strategy of regular worm egg counts through the grazing season with occasional strategic doses of wormer is appropriate.

All you need to do is:

  • Collect a faeces sample when advised to do so by your vet. A good guide is to collect faeces in February, May. August and November but your vet may wish to modify this slightly according to your circumstances and the weather conditions. Where possible we would encourage yards to submit samples together so that all horses can be assessed simultaneously and managed appropriately
  • Once you have a Worm Egg Count kit, using the glove provided. take a large pinch of faeces from three different areas in the pile of fresh faeces and place all three pinches in the bag provided. Flatten the bag. expel air from it and seal it. The total volume should be no more than the size of a table tennis ball.
  • Complete your details where appropriate.
  • Place the faeces sample in the envelope provided. with the completed submission form and put it in the post within 24 hours. Store the envelope in the fridge or in a cool place prior to posting.
  • The sample wilt be delivered to the laboratory who will automatically inform your vet of the results. There is no need to send any payment, you will be charged by your vet in the usual way.
  • Your vet will contact you with the results within a few days and advise you on whether any treatment is necessary and. if so, what the most appropriate treatment will be.

This information has been brought to you by Merial, the manufacturers of Eqvalan® and Eqvalan® Duo.

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