The distinct chemical groups in equine wormers in the UK

Posted by Wormers-direct on 7th Jan 2020

The distinct chemical groups in equine wormers in the UK

Equine wormers and their distinct chemical groups


View a worming calendar with tactical worming guidelines

View which brands fall in to which chemical group


Fenbendazole & Mebendazole based products are used less and less nowadays in routine worming as there is confirmed resistance to them. In some cases they still have a role to play when used as part of a rotational system, but it is best to take expert advice before relying on products within that group to control worm burden. Ascertain that your horse is infected with worms that are susceptible to it by taking a dung sample and having an egg count done. By using a 5-day course of a Fenbendazole-based wormer between October and March, small redworm encysted larvae acquired during the grazing season will be removed. If an early dosage is administered this treatment can be repeated again in February to remove larvae acquired during the winter months. This will also help reduce the levels of infective larvae on the pasture the following spring.


A Moxidectin based wormer has an ability to kill encysted developing cyathostome larvae (small redworm) in a single standard dose. Studies have indicated that the ingredient Moxidectin, suppresses egg reappearance and therefore leads to a long dosing interval for small redworm control. Moxidectin based Wormers are also effective against large redworm, pinworms, stomach worms, intestinal threadworms, Ascarids (adult and larval stages) and will also treat for bots.


Worms are an ever-present threat even during winter months. They can survive frost and snow, so even a short time on pasture during winter months can be enough for horses to pick up a new infection. Used routinely throughout the winter months, lvermectin based wormers will reduce the risk of infestation. The interval between treatments can be up to 12 weeks and in a single dose it will give effectiveness against both larval and adult stages of the more severe parasites of horses, including small redworms. lvermectin based wormers control parasites that are resistant to Benzimidazole-based wormers. Ivermectin based wormers control Pinworms, intestinal threadworms, large-mouthed stomach worms, lungworms, neck threadworms and ascarids.
lvermectin based Wormers will also control bots when given as a single dose.


Products based on Pyrantel are effective against most adult roundworms so they have a role to play in rotational worming. They are also effective for the treatment of tapeworm, but must be administered at twice the normal dosage amount to treat tapeworm. If your horse is stabled on straw or other products where mites can survive, then this double dose twice a year will ensure adequate control of tapeworm infestations. The tapeworm lifecycle is 6 months, so although treatment once a year (late September/early October) may well be adequate in some cases, it is recommended that you do treat twice per year to break the cycle.

A new study has shown that horses that have a tapeworm burden are up to 8 times more likely to suffer from colic. The timing of the tapeworm control doses is based on the life cycle of the worms.


Products using Praziquantel can be a very effective treatment for all three known species of equine tapeworm in a single dose, again being most effective when administered twice a year. Products that contain Ivermectin as well as Praziquantel can be used all year round as they are also an effective treatment against bots and round worms. Products that contain only Praziquantel as the active ingredient give owners the option of treating for tape worm without also treating for roundworm, thus helping to reduce the risk of equine anthelmintic resistance building up. This should still be as part of an annually rotated worming scheme with the other distinct chemical groups, but additional care should be taken with regard to encysted small redworm larvae where a treatment using a 5-day course of a Fenbendazole-based product or a Moxidectin based product should be considered.

The distinct chemical groups, which are:

MACROCYCLIC LACTONES lvermectin and Moxidectin
BENZIMIDAZOLES Fenbendazole, Mebendazole, Oxibendazole

Products based solely on Praziquantel only are effective for tapeworm control and must be used in conjunction with products based on the three main chemical groups.

There has been evidence of a resistance building to products in the Benzimidazole group in certain areas so it is best to take expert advice before relying on products within that group to control worm burden.