Dan Brown BVSc MRCVS

There is no doubt about it, worming horses effectively can be complicated. Horses play host to a number of different types of internal parasite and, unfortunately, there is no one wormer which will treat them all.

An effective, sensible worming program is one which controls all the potentially harmful parasites without overuse of drugs. To be able to create such a program, we need to understand what worms we are trying to control and which drugs to use for each.

The major internal parasites of horses can be divided into three broad groups – roundworms (nematodes), tapeworms (cestodes) and fly larvae (insects). Each group creates its own challenges for control.


This group contains most of the common parasites affecting horses in the UK.

Large Redworm (Strongyle) – historically the most feared group of worms, their larvae can block the arteries to the gut causing a potentially life-threatening colic. Thankfully, as most modern wormers control large strongyles, disease is now relatively uncommon.

Small Redworm (Cyathostomes) – this group has taken over as the commonest parasite. Part of the lifecycle involves the larvae becoming ‘encysted’ (forming a protective membrane around themselves) in the gut wall. They may remain in this state for months or even years before suddenly all emerging together and causing severe illness with diarrhoea, weight loss or colic. If this ‘larval cyathostominosis’ is not prevented, the success rate for treatment is only about 50%. Unfortunately, the encysted larvae are well protected and very few drugs are able to remove this most important stage of the worm.

Large Roundworm (Ascarid) – a very large worm (up to 40cm long) usually only found in animals under a year of age. Large numbers of ascarids in the small intestine of a foal may cause a partial, or even complete, blockage. Migrating ascarid larvae may travel to the liver and lungs and may cause respiratory disease in foals.

Threadworm – these worms cause dullness, ill-thrift and reduced weight gain in foals and are interesting in that they can be passed to the foal through the mare’s milk. This is prevented by worming the pregnant mare.

Pinworm (Oxyuris) – more of a nuisance worm than a serious health threat. Pinworms, also known as seatworms, glue their eggs onto the skin under the tail. This glue causes irritation and horses rub their hindquarters – often to the extent that they cause hair loss on the tail.

Lungworm – live in the airways and are common in donkeys. They are usually only a problem in horses which graze with donkeys. 


There are, in fact, 3 species of tapeworm which infect horses in the UK. They have been associated with several serious types of colic if present in large numbers. Contrary to popular opinion, equine tapeworms seldom reach over 5 cm in length.

Tapeworms are picked up when grazing horses inadvertently eat forage mites which contain the intermediate stage of the worm. Studies have shown that up to 69% of horses in the UK are infected with tapeworms.

Fly Larvae

Stomach bots are the larvae of the bot fly. This is the fly which lays the incredibly sticky small yellow eggs on the legs of horses in the summer. These eggs then enter the horse’s mouth and develop into reddish-brown ‘maggots’ in the stomach. They spend the winter inside the horse before being passed in the dung the following spring.

A well-planned worming program should aim to control all the parasites already mentioned. At first glance, the array of wormers to choose from appears mind-boggling.

However, by looking at the active ingredient it can be seen that many of the brands contain the same drug and perform the same function. Below is a summary of the active ingredients available in the commonly used wormers.


A 5 day course is effective against encysted small redworms. Resistance to this class of wormer is very common in small redworms in the UK. Routine use - repeat every 8 weeks. No activity against tapeworms or bots.


Frequently used as a double dose for tapeworms. As a routine wormer it has the limitation that it does not kill larval worms of most species. Additionally there is some resistance to pyrantel in the UK in both large and small redworms. Routine use - repeat every 4-8 weeks. No activity against bots or encysted small redworms.


Historically, a good general purpose wormer which treats most roundworms and bots, including those which are resistant to fenbendazole and pyrantel.

Routine use – repeat every 8-10 weeks. No activity against tapeworms or encysted small redworms.

Moxidectin (Equest)

The longest lasting wormer available – repeat every 13 weeks. No reported resistance in the UK. Kills approximately 80% of developing encysted small redworms with a single dose. Also, treats most roundworms and bots, including those which are resistant to fenbendazole and pyrantel. No activity against tapeworms.

 Praziquantel (Equitape)

Praziquantel is a specific tapeworm treatment and is effective against all three species of tapeworms found in the UK with a standard dose. Routine use – once or twice per year for tapeworms.

A worming program may be created using a combination of any of these drugs, but there are some general principles that should be adhered to:

1.      Treatment for encysted small redworms should be included at least once per year, ideally in the autumn/winter.

2.      Treatment for tapeworms should be included in the autumn and spring.

3.      Bot treatments should be timed for the autumn/winter, after the adult flies have died.

4.      Do not underdose. Use a weigh tape.

5.      All horses on a single yard should be treated at the same time, using the same type of wormer.

Well thought out use of wormers along with good pasture management, such as muck-picking and resting fields, allows for cost-effective worm control. Your veterinary surgeon will be able to advise you on a yearly program tailored for your needs.

Finally, an example of a worming programme. The example below effectively treats all the parasites mentioned above and provides year-round control of roundworm egg output. It also eliminates the need for complications such as double-dosing for tapeworm or five day courses for encysted small redworm.







(incl. larvae)











Dan Brown BVSc MRCVS


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