Worm Resistance In Horses

What is worm resistance?

Worm resistance in horses is a growing concern within Australia and around the world. It is when worms develop the ability to survive certain chemicals that once would have been effective in killing them. Studies are showing that the effectiveness of some wormers are significantly diminished or completely ineffective in managing some worm populations. Worms may develop a resistance to one type of wormer or active ingredient but then they are likely to be resistant to all chemicals from that same group. As there are only a limited number of chemicals that are effective in de-worming horses the issue is that once worms develop the ability to be unaffected by these chemicals it will lead to increased worm burdens which will be detrimental to horse health and potentially fatal.

What are they resistant to?

Horse wormers will contain one or a combination of different active ingredients to kill worms. There are four chemical groups that are currently used to manage intestinal worms in horses. The groups of chemicals used for horse deworming are;

Macrocylic Lactones (MLs) are “mectin” based wormers with active ingredients that end in “ectin” including ivermectin, abamectin or moxidectin. These wormers are the most widely used anthelmintics as they are effective against all worms including tapeworm and bots.

Benzimidazoles (BZ’s) are wormers which contain active ingredients ending in “azole” including oxfendazole, fenbendazole and oxibendazole. BZ group wormers are effective against most worms except bots and tapeworm. Although there is some evidence that small strongyles are becoming resistant to BZ’s.

Praziquantel is only effective in the management of tapeworms, having no effect on other worms.

Tetrahydropyrimidines (THPs) group of wormers have the active ingredients of pyrantel or morantel. They are effective in treating most worm species except for bots.

Why is resistance becoming an issue?

For years the general recommendation to keep your horse worm free has been to worm at set intervals, as per product instructions. Rotating the active ingredients at least once a year or using a different active ingredient year on year off has been recommended to help prevent a worm resistance and to ensure that all worms are being treated. This worming protocol has been recommended regardless of whether worms were present or even suspected.

This consistent high exposure to active ingredients is the main reason that worm resistance is occurring. Worms have become so used to these chemicals that they have been able to genetically modify themselves to become unaffected. Overuse of anthelmintics, incorrect use or under-dosing, are the major contributors to the wormer resistance problem now being faced.

What now?

It looks like the way we have always wormed our horses is no longer going to be a viable solution for long term worm management. The risk of worms developing a resistance to certain chemicals means that the way intestinal worms in horses are managed will need to change. Current research suggests that a more targeted approach to worming is required. This means only treating/worming for worms that are present..

To determine what worms are present, a simple test on the horse’s manure can be conducted. A faecal egg count can determine if any worms are present, what type and how severe the burden is. This test is where a manure sample is examined under a microscope to count the number of eggs present. A faecal egg count can be run by most veterinarians or independent laboratories. By conducting faecal egg counts and determining the worm burden a targeted approach to treatment can be undertaken.

By moving to a targeted worming approach the reliance on anthelmintics can be reduced. With less use of anthelmintics hopefully the effectiveness of current worming treatments will be extended and remain effective for when required.

With more research and studies being conducted it is likely that in the coming years we will see a change in the “best practice” for worming horses. Although changes may need to be made in how to manage worms, keeping your horse worm free is essential to their overall health and well being.

Originally published in MyPet Magazine Spring/Summer 2021.

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