equine health

Pasture, Soil & Grasses

Horse Worms and Worming
When to Deworm Horses
Controlling Horse Worms on Pasture
Worm Resistance to Wormers

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Wormer Resistance: The sensible way to avoiding anthelmintic resistance in horse worms

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. Resistance is already well established in Europe, due to the over use of chemical wormers.

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.

The Wrong Approach to De-Worming

By concentrating high doses of chemical wormers on worms resident in the horse, a gene pool of super resistant worms has evolved, resistant to all chemical & bacterial wormers. Horse owners have been encouraged by the big pharmaceutical companies to regularly dose horses every 6 to 8 weeks, all year round. In their greedy quest for big profits they have chosen to ignore 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.

Resistant Worms

Routinely deworming all the horses every 2 months kills all the worms susceptible to chemical & bacterial wormers, leaving the resistant worms to lay eggs and multiply unchecked. The gene pool of resistant super worms develops with every deworming, 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.

Rotational Deworming

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 anthelmintics, the pharmaceutical companies have to offer.

A New Approach to Effective Worm Control

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
The life cycle of worms from egg to laying adult. Breaking the continous 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 of deworming 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.
Herbal Wormers
Consider using traditional herbal wormers to stop the build up of resistant worm strains, usually referred to as “Internal Parasite Repellants”.
Test Your Wormer
Use anthelmintics proven effective when followed by a faecal egg count (FEC) 10 to 14 days after the last dose. Careful documentation of all worm test results. Successive high FEC’s, indicate a resident population of super resistant worms.
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.


A blend of traditional herbs added to the feed to maintain the overall health of coat and gut whilst repelling unwanted internal parasites. Added to the feed over 5 consecutive days. One litre is sufficient for an average sized (500kgs) horse for 2 parasite repel applications 8 to 12 weeks apart. Two litres is enough for one average sized horse, to cover the entire year's parasite repel program, 4 seasonal applications in all.

N.B. Not suitable for pregnant/lactating mares or foals under 6 months old.

Contains: Extracts of traditional vermicide & demulcent herbs. One litre self dispensing bottle

Free Faecal Worm Count Kit & Moon Calendar is included and full directions on using Zilch Verm

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