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Worming and Resistance in Horses

Everyone knows how important it is to worm our horses regularly to make sure they are healthy and happy! This is especially important for those horses that are on pasture 24 hours a day, but is also important for stabled horses.

A few tips for worming horses:


  • Treat all horses grazing together at the same time and with the same product.
  • Take into account the horse’s age and type, local environment and climate.
  • Keep an accurate record of when you worm your horse and the product used.
  • Treat horses that are at grass during the winter, even if only for short periods.Do not overstock the paddocks.
  • When moving horses to new grazing worm them 48 hours before the move to help prevent the spread of parasites.
  • No single worming drug will kill all the horse’s internal parasites
  • Do not administer the drugs more frequently than recommended.


The key to worm control is to break the life cycle of the parasite, through drugs and pasture management.

Killing the four main groups (Large Redworms, Small Redworms, Tapeworms and Bots), will also kill all the other important worms. When you are moving from the warmer months and into autumn and winter, you may notice Bot flies hanging around your horse (mainly their legs). These annoying flies lay small white eggs on your horse in autumn, so using an effective boticide (ivermectin) when you worm at this time of year.

Worm Resistance

If the active ingredient in a worming product is used continuously on a property for a long time, there is an increased likelihood that the worms (especially Small Strongyles) may develop resistance to that family of drug, rendering the products no longer effective. Make sure to rotate wormers every year or so – this means getting a different wormer with a different active ingredient and not just a different brand – make sure you read the label or ask someone (such as your vet or the supplier of the worming paste) which active ingredients are different from the wormer you have been using.

The reason we do this is to ensure the worms within our horses are not building up a resistance to the type of drug in the wormers we are using – this can be a real problem as horses can carry heavy infestations of worms in their gut even though they are wormed on a regular basis, and start to lose condition and become unhealthy.

Ensure that your horse gets the correct dose for his weight. Don’t under-dose as this can lead to resistance in the worm population and the horse not being wormed effectively. This is particularly important to remember when you have a larger horse such as a friesian, as these horses often weight more than the highest dose level on the worming paste – often these pastes only go up to 550kg-500kg, and many friesians are over 650kg, with our heaviest horse weighing in at around 750-800kg!! The ‘mectin’ family of worming product is the most common class of wormer used in Australia currently – wormers such as Equimax (abamectin), Equest (moxidectin) and Equimec (ivermectin) are some examples. The ‘mectins’ are very good wormers, but there are growing concerns that over-use will lead to resistance in the horse industry and cause a loss of it’s effectiveness.

How to Avoid Resistance

Give the correct dose – especially do not under-dose

Use an effective worming product (not a product where resistance has developed, as it may make the problem worse)

Prevent the horse from spitting the paste back out and reducing the dose he gets – put the wormer all the way to the back of the tongue, and make sure to hold the horses head up for around 15 seconds after administering or until you can see that he swallows all of the paste. Rotate wormers (the drug not just the brand) – this can be done yearly.

Maintain good pasture hygiene – rotate paddocks and harrow in the summer months, or pick up manure in paddocks, or clean stables out daily. These measures avoid increasing the worm burden in the paddock/stable and hence also in the horse.

Remembering these simple steps and guidelines will ensure that your horse is happy, fit and healthy, on the inside and outside.

Natasha Althoff
Ebony Park Performance Friesians
where dreams are made reality

Equine Spring Care Notes

Daily InspectionSpring is officially here in Western Colorado, which is great for two reasons: grass is sprouting, and hay is finally starting to go down in price. With that being said anytime between now and first hay cutting is a great time to find some hay deals. Horse owners will need to anticipate hay prices continuing to be high as many areas in the west are already restricting water due to the drought. Buy your hay early this year during first cutting. In the event that producers are unable to get a second cutting this season or in the event of drought in the south and to the east of us in the Midwest hay prices will only go up.

My favorite time of the year to buy my hay is typically August. Waiting until August this year could be risky if droughts continue. The supply may not be available and the cost may rise higher than it already is. I plan to ramp up my fodder production this year, but more than likely will continue to offer hay to the horses to allow them something to chew on throughout the day. Currently I am feeding 75% fodder and 25% hay, more than likely after I buy hay this year I will go to 50% hay and 50% oat grain fodder. Cost wise it will cost more with the increased hay, but for maintaining the herd health and preventing cribbing I feel it will be a great balance for my horses.

As the grass is coming in remember to gradually allow access to pastures starting with a few hours per day allowing horses to graze freely and only allow horses out onto pastures during the late evening hours into the early morning hours of the day. This best management practice will reduce the likely hood of horses falling victim to grass founder.

With much of the region in drought, remember noxious weed mitigation is crucial right now to your pastures health. Noxious weeds often times spread the fastest in overgrazed, dry pastures. Rotate pastures frequently, keep up on your noxious weed mitigation, and be mindful of how many weeds are in your hay supply. If you cannot rotate pastures, fence off a pen, and feed your horses hay, or forage alternatives such as complete feed, grain fodder, hay pellets, or beet pulp. Keeping horses off a small pasture will save you more money in the end, than the increased feed bill that you initially incur. Preventing noxious weed problems in the first place is always the cheapest form of noxious weed mitigation. Allowing weeds to propagate can not only decrease the amount of forage your pasture can supply your horses, it can also affect your herds health due to ingestion of poisonous noxious weeds, and can get incredibly costly getting the problem under control, then re-seeding pasture areas.

Remember pest control for your horses such as fly spray, fly mask, and sunscreen for our fair-skinned equine friends. Break out the curry brushes, get outside, and enjoy the spring season!

By Jennifer_Hampson