This is quite a long horse wormer article so, if you would like a summary, here it is:
  • Horse worms are becoming increasingly resistant to chemicals – as companies encourage us to “over worm” in pursuit of profit
  • Prevention is better than cure – active and diligent pasture management e.g. regular poo-picking is essential
  • There are natural wormers that are effective, especially when used at the right time (see below), and in conjunction with chemical wormers, if still effective – ensure that worm burden is monitored

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Horse worms are becoming increasingly resistant to chemicals

Resistance of parasitic resistance to wormers, resulting from the over use of chemical wormers,  is becoming an growing problem.

Increasing resistance in red worm populations, particularly the small red worms (Strongyles) to wormers of the Benzimadole group, has been confirmed by the Moredun Research Institute in the UK.

Pinworms (Oxyuris equi) are also a growing problem. UK livery yards, report many horses are carrying heavy burdens of pin worms immune to conventional wormers.

Whilst routinely deworming with chemicals kills all the worms susceptible to chemical & bacterial wormers, this leaves the resistant worms to lay eggs and multiply unchecked.

By concentrating high doses of chemical wormers on worms resident in the horse, a gene pool of super resistant worms has evolved that are resistant to all chemical and bacterial wormers. With every cycle of deworming their resistance  increases, leading to your pasture being infected by a strain of super worm resistant to chemical wormers.

This has severe health implications for horses such as anaemia, gut ulceration, colic etc.

Approach to deworming- Natural or Chemical?

Horse owners have been encouraged by the big pharmaceutical companies to regularly dose horses every 6 to 8 weeks all year round, ignoring the climate, worm biology and life cycles.

The consequence of this overselling and over dosing has been the development of super resistant horse worms. The simple truth is that horses do not need to be dosed all year round every 6 to 8 weeks to be worm free.

Rotational Deworming With Chemicals

In recent years, the idea that rotational deworming (by using drugs of different classes for each deworming) would overcome worm resistance was much promoted.

In America and Northern Europe, resistance to two of the three-dewormer drug classes – Benzimidazoles (i.e. Fenbenzadole and Oxibendazole) and Pyrimidines (i.e. Pyrantel) has been well documented. There have also been reports of resistance to macrocyclic lactones (Ivermectin, Moxidectin) in Europe and Brazil.

No longer can we rely on rotational worming to kill all horse worms. Alarmingly, these three classes of chemical dewormers are the only parasite killing drugs (anthelmintics), the pharmaceutical companies have to offer.

A New, Natural Approach to Effective Worm Control

Responsible horse owners worm their horse, seasonally (four times a year) and on well managed pasture significant worm burdens are not a problem.

Given the increasing immunity to chemicals, it is important to not only worm your horse but to follow deworming with a faecal egg worm count, in spring and winter. An ELISA blood test, done in autumn following deworming will determine the presence of tapeworm.

The ELISA Tapeworm blood test will determine whether your horse is likely to be infected with tapeworms. This test measures the level of antibody to a specific tapeworm antigen. Horses with significant tapeworm burdens have elevated levels of antibody. The level of antibody gives an indication of tapeworm infection intensity. About fifty per cent of all horses are infected, at any one time.

Faecal egg worm counts are very helpful when determining the worm burden of your horse and helping you plan your horse’s worming program. The faecal worm egg counts cover all horse worms except Tapeworms. To check the level of tapeworm infestation in your horse, you will need to ask your veterinary surgeon to do an ELISA (Tapeworm) Antibody blood test.

Focus on Big Worm Picture

Future measures to control the parasite burden on pasture will need to focus on the bigger picture, the factors outside the horse that influence horse worm populations

Worm Biology

Breaking the continuous life cycle by removing faeces, you destroy eggs waiting to hatch, effectively reducing the numbers of L3 infective larvae ingested by your horse. Yes, pooh picking weekly.

Climate & Temperature

Using climate & temperature to determine whether your horse may need deworming. The life cycle of horse worms, the climate requirements and conditions for successful hatching and survival will determine your Worming Schedule. See article When to Deworm Horses

Moon Cycles

Traditional wisdom is that deworming should occur during the full moon cycles. Red worms are heterosexual, it is believed that red worms detach from the mucous lining to mate during the full moon. At this time, they are more susceptible to wormers and expulsion.


Deworm for red worm larvae as soon as night temperatures dip under 10°C before ground frost becomes a reality. Expel them before they start to encyst and overwinter, Bots are also removed. Don’t bother to do a Faecal Egg Count, immature larvae don’t lay eggs.

Introducing Horses to the Herd

Isolation of all new horses until dewormed and FEC count results clear them. Care must be taken not to introduce horses from yards with known resistant worm strains.

Responsible Pasture Management

The removal of faeces, harrowing in hot weather and rotation of grazing, all help to break the red worm life cycle.

Improving Forage

Seeding pasture with tannin rich forage such as sainfoil, birdsfoot trefoil, yarrow, chamomile, wild marjoram. Don’t grub out old hedges full of natural dewormers such as bramble tips, rose hips, crab apples and elder shoots. Horses will instinctively pick out the shoots.

Herbal Wormers

Consider using traditional herbal wormers to stop the build up of resistant worm strains, usually referred to as “Internal Parasite Repellents”.

Herbal products can be useful if your horse will not accept the syringe three on delivery straight into the mouth that is required with some products. They can also be good if your horse has a sensitive digestion.

Vary the type of wormer used, alternating chemical wormers ( that you have proven effective after monitoring worm counts) with herbal parasite repellent products .Use anthelmintics proven effective when followed by a faecal egg count (FEC) 10 to 14 days after the last dose.

Carefully document all worm test results. Successive high FEC’s, indicate a resident population of super resistant worms.

Natural Worming Product – Verm-X

Verm-X is a natural product that has been on the market now for over 15 years. It has been formulated because of the resistance concerns raised in this article. Interestingly, the company reports no instances of resistance to Verm-X.

Some features are the product are:

  • It works over a 21 days cycle, rather than an immediate purge, as with chemical products. This is more gently on the horses stomach
  • Can be mixed in to food and water, so administering this has minimal impact – especially if you have a stressy horse
  • Provided with the product is a free Westgate Laboratory faecal egg counts kit, so worm burden can be monitored and managed in the ay we describe above