Worm Disease in Horses

A wake-up call in preventing parasitic worm disease in horses

Parasite control, for a little while, has been taken for granted by many owners and veterinary surgeons alike. Many people trust that regular worming with different brands of anthelmintics is enough to control the problem.

There is growing evidence that this is leading to significant build-up of worms that are resistant to nearly all wormers. There are no new generation wormers coming onto the market, nor are there likely to be. So, unless our approach changes we are looking at a serious problem re-emerging. We will have increased incidence of colic, ill thrift, and diarrhoea, as well as liver and lung disease.

Some of the mistakes made by horse owners are:

1. Treating all horses the same. Different ages and types of horses should have different programmes.

2. Treating too frequently. Inappropriate worming is the single biggest factor in building up a resistant worm population. It is advisable to leave horses with a low worm burden untreated so as to leave a percentage of non-resistant worms in the environment. These worms will out compete and suppress the development of resistant worms.

3. Linking treatments to suit other management events.

4. Moving horses to clean pasture after worming.

This is a sure way of contaminating clean pasture only with eggs from resistant worms.

Therefore, a new approach is needed and is based under three headings:


Although 150 different species of worms have been identified in horses there are really five types of main concern.

A) Large red worms (Strongylus vulgarus and Strongylus edentatus), which are not very common now.

B) Small redworm (cyathostomes), which are currently the most important. They can encyst in the lining of the large intestine for long periods and can suddenly re-emerge in springtime or sometimes after a worm dose with devastating consequences.

C) Round worm (Parascaris equorom), which effects young animals under a year old but whose eggs are very resilient and can last for years.

D) Tapeworms (Anoplocephala spp), which are becoming more prevalent and reside in the distal ileum where they can cause blockages. They are not eliminated by routine wormers and can only be identified by a blood test.

E) Thread worms (strongyloides) affect foals and can cause fatalaties. The foal ingests larvae through mare’s milk or the larvae can burrow through the skin in a foal’s mouth.


A) Benzimidazoles: e.g. Panacur. Will kill only adult redworm and not immature or encysted worms. Widespread resistance reported. Worm eggs will appear again four weeks after worming. A five-day course will kill some of encysted larvae in gut wall.

B) Pyrimidines: e.g. Strongid-P, Embotape, Pyratape. Will kill adult redworm but resistance reported. A double dose will kill tapeworms. Eggs will reappear four weeks after dosing.

C) Macrocyclic lactones; Ivermectin: e.g. Eqvalan, Bimectin, Equimax. Will kill adult redworms and round worms but will not kill encysted larvae or tapeworms. Resistance developing and eggs will reappear six to eight weeks after dosing. Moxidectin: e.g. Equest. Very effective against adult and larval encysted redworm but not as effective against round worms and no effect against tapeworms. Eggs will reappear after 12 weeks from dosing.

Praziquantel: e.g. Equitape. Only effective against tapeworms but with no activity against any other worms.


A) Base treatments on needs of individual animal:Foals and younger horses under the age of three are more susceptible to developing high worm burdens and, consequently, require a more rigorous programme. Some 80% of older horses have excellent natural immunity and require a minimum of worming during the year. However, there are 20% that are potentially high shedders and, therefore, require more frequent monitoring and worming. Mares should be wormed just after foaling to protect foals from threadworms. Foals should be treated at two months of age with an ivermectin against round worm.

B) Only give treatments from spring through autumn: There is no need to worm between the end of October and the end of March, unless there is a very mild winter or a problem identified from worm egg counts.

C) Administer wormers at optimal intervals determined by worm egg counts: Taking worm egg counts is absolutely vital in the control of worms in horses. All horses should have a worm egg count done at end of March to establish which horses have a high (above 500epg), medium (200-500epg) or low (less than 200epg) worm burden. Horses with a low burden would be best not treated. The high- and medium-burden horses should be treated then. If there is a concern regarding resistance to wormers on a farm then a repeat worm egg count should be done 10 to 14 days after worming to establish effectiveness of wormers used. Otherwise it would be sufficient to repeat worm egg count in six to eight weeks to see which horses need worming again. The Moxidectins suppress egg re-emergence for longer than the other types of athelmintic. The high-burden horses should be wormed every four to 12 weeks (depending on type of wormer used) during spring and summer. The moderate-burden horses may just be wormed again in autumn. Regular worm egg counts will save money in the long run and will help prevent the development of worm resistance to anthelmintics. It is possible to do pooled worm egg counts on groups of up to 10 horses, which will further cut down on costs.

D) Leave as many horses as possible untreated (based on low worm egg count) apart from an anti tape worm treatment in the autumn.

E) Good management policies: Removing droppings from pasture regularly (preferably weekly) during summer months is five times more effective for worm control than the use of anthelmintics. Harrowing fields during a hot, dry spell and leaving fields ungrazed for at least two weeks is also effective. Letting sheep and cattle graze pastures reduces worm-egg burdens.

F) Check all new horses with worm-egg count before turn out onto pasture.


A) Routinely worm your horse at regular intervals without knowing their worm burden status.

B) Swap anthelmintics randomly or use most economical availability.

C) If a wormer is required, using too small a dose in adults.

D) Giving too large a dose to foals, especially with moxidectin, which can be toxic.


Reference: Elizabeth O’Flynn, MVB CertWEL, MRCVS. Published Veterinary Ireland Journal