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Is it ESSENTIAL to use a chemical ‘wormer’ on a horse?

Jan 29, 2021

The problems with using chemical 'worming' pastes on horses

Many horse owners think using a chemical ‘wormer’ (an anthelmintic) is enough to control worms (parasites) in their horse. However, there are many reasons why you may need to think differently:

  • Due to overuse, chemical worming products (like antibiotics) are becoming less effective. The parasites are becoming resistant to the chemicals in them – which means they are not as effective. Parasites that survive after a chemical wormer has been used on a horse are RESISTANT to the chemicals. These parasites then pass on this resistance to future generations, creating larger populations of resistant parasites.

Due to overuse, chemical worming products (like antibiotics) are becoming less effective over time.

  • It has been many years since new chemicals for parasites have been released. There are no new chemicals on the horizon to combat parasites. ‘New’ chemical worming products are simply the same old chemicals rebranded.
  • Chemical wormers are expensive. Especially if you use them as often as recommended, that recommendation mainly comes from chemical worming product manufacturers so, of course, they suggest that you use them often. Reputable companies are now recognising the damage this over-recommendation/use has caused. However, it is now part of the ‘culture’ of horse keeping that you should chemically worm your horse regularly – so the belief is difficult to change.
  • Chemical wormers are dangerous and even deadly for other animals such as dogs, dung beetles and other wildlife (if they eat the paste that drops on the ground when a horse is being wormed or if they eat horse manure that contains it). It also affects micro-organisms in the soil.


Should you never use chemical worming products?

This does not necessarily mean that chemical wormers should never be used. They still have a place (although, in time, reliable alternatives may be found). Still, they should ONLY be used when necessary rather than as a routine (i.e. every few weeks as recommended by many manufacturers). As already mentioned, they are no longer as effective because they have been overused in the past.

Current thinking is that it probably benefits a horse to have a LOW worm count (rather than zero) as it helps build up their natural resistance.

The good news is there is a lot you can do to vastly reduce parasites from getting into your horse in the first place. This involves good pasture and horse management. Much of the advice that follows may go against what you have been taught in the past so bear with us!

You do not usually have to do all these things to reduce parasites, just do the ones you can, and you WILL see a reduction. This reduction can be tested by doing faecal worm egg counts (more on that later).


What horse pasture management can you carry out to reduce parasitic worms in horses?

  • Rotate pastures – in conjunction with other good land management practices, is one of the most critical steps for parasite reduction. It has so many other benefits – there is not space to go into them here, but you can sign up for this free mini-course Horses, Pasture and Grazing for more information www.equiculture.net/equiculture-free-mini-course - the course is just one hour long and consists of several short videos. By resting your pasture, some of the parasites will die before the horses return. They cannot survive without access to a horse to complete their natural lifecycle. They will also be more opportunities for the parasites to be eaten by predators or killed by inclement weather - wind (which dries them out), heat and cold.
  • Composting – when done correctly – kills parasites and parasite eggs. Collect and compost droppings in small paddocks and loafing (surfaced) yards. This composted manure can then be used back on the land.
  • Harrow/spread manure on larger rotated paddocks (when the horses have been moved to the next paddock). Yes, this does spread them around the land, but it also helps to kill them as they are exposed to the weather. Spreading manure also has the added advantage of reducing or eliminating ‘roughs’ (toilet areas), which means more pasture is available for grazing. On the other hand, if you do not manage your land correctly, you can lose the use of more than 50% of it to this ‘dunging behaviour’ which is peculiar to horses.

Spreading manure also has the added advantage of reducing or eliminating ‘roughs’ (toilet areas), which means more pasture is available for grazing.

  • Cross graze (mixed graze) whenever possible. This means letting other animal species graze the same land as the horses (usually after grazing the paddock). Parasites are by large ‘host-specific’, and most horse parasites are killed when picked up by other animals such as cows or sheep, for example.

Cross graze (mixed graze) whenever possible.


How do dung beetles play a part in reducing worm burdens in horses?

  • Encourage and nurture dung beetles. These creatures are the world's manure management experts! – NOTHING you do in an attempt to manage horse manure comes close to what they do and gives the benefits they do. They are a whole subject in themselves, and they go a long way to helping you control parasites in your horse. Remember that some worming chemicals can kill dung beetles. Make sure you read the article or watch the video here - Horses and dung beetles - they are just incredible! 


The best management to reduce worms (parasites) in horses

  • Use an effective worming paste only when necessary. It is better to use a broad-spectrum (and usually more expensive) product less often than a cheaper narrow-spectrum product more frequently.


Are faecal worm egg tests a good idea for horses?

  • Test your horse/s using a faecal worm egg test before you use a chemical wormer because you will not need one in many cases. It is not difficult to do a test; many videos online show you how to do this. Although the test does not pick up on all the parasites, it does indicate the current level of your horse’s parasite burden. If you have a closed herd (no new horses joining the herd), you will not need to do this as often as you learn more. Again, look out for more info about this subject in one of our other articles. Generally, 20% of horses carry 80% of the parasites (in a given herd), usually very young, very old or unhealthy horses. This means that not all of the horses in a herd will require the same chemical worming regime. Identifying which horses need it and when can dramatically cut your use of chemical wormers.

  • Avoid feeding on the ground if there is manure present. Feed in large feeders but keep the area around feeders clean.

  • Do not overstock horse paddocks as this leads to high levels of manure and more parasites.



What should you do about worming a new horse?

  • Chemically worm any new horses before they are released into a paddock/herd. If you can, keep them in a surfaced yard and collect the manure for three days afterwards. Bag this manure up and take it to the tip rather than add it to your compost heap. Do not use it to fertilise your garden as it will kill the earthworms etc.
  • If you acquire a new horse that looks particularly ‘wormy’ (pot-bellied, poor, scurfy skin etc.), get veterinary advice before using a chemical wormer on them as the sudden death of large numbers of parasites can be very dangerous for a horse (they can block arteries etc.). It is a good idea to get veterinary advice before worming any horse when you do not know their history.



If you enjoyed this article, check out the others on the blog. Also, make sure you sign up for the free mini-course called Horses, Pasture and Grazing and the free 23-page PDF called 10 common POSITION and BALANCE checks for riders


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