Worming can be a confusing subject, but one that is vital to ensure the health of your horse.

Worms are a major cause of colic, which is one of the largest causes of mortality in horses. Correct worming protocols are a vital part of keeping all horses and ponies in good health and top condition throughout the year.

Combining appropriate testing for parasite infection, with the correct use of wormers and good pasture management, can prevent internal parasite infections from reaching levels high enough to cause disease. This should be combined with monitoring of the effectiveness of worm control programmes.

Spasmodic colic is 8 times more likely in horses with a tapeworm infection

than those without and 81% of ileal impactions were associated with tapeworm infection.

The intensive use of wormers over previous years has led to the development of resistance to these important treatments. This means that the effectiveness of the drugs has reduced. To slow down the spread of resistance, it is critical that horse worming regimes are designed to identify and treat only the horses that specifically need worming.

In most cases, a diagnosis of parasite infection should be confirmed, before wormer treatments are administered. This ensures that only those horses that need worming are treated. The exceptions to this approach are foals, when regular treatments should be administered throughout the first 6 months of life.

Faecal egg count testing should form the framework of a horse’s parasite control programme, with specific testing for tapeworm and encysted small redworm included at specific times of the year.

Always seek professional advice when interpreting diagnostic test results; they should be interpreted taking in to account the individual horse’s situation. For example age, management, history of disease etc.

Horses can be infected by three groups of internal parasites; Roundworm, tapeworm and bots. Infections with these parasites can cause weight loss, colic, diarrhoea and poor body condition.

Small redworm larvae can encyst within the wall of the horse’s intestine and can remain there for up to 2 years.

Bots are the larval stage of the Bot fly (Gasterophilus spp.). Infection with bots can cause ulceration of the lips. Large numbers may lead to inflammation of the stomach Lining (gastritis), intestinal obstruction and rectal prolapse.

How horses become infected

Horses become infected with roundworms by ingesting worm larvae from the pasture. Adult worms live in the horse’s intestine and lay eggs that are passed out with the horse’s droppings. These eggs hatch into larvae on the pasture and the lifecycle starts over again

Diagnosing roundworm infection

Small and large redworm infections are diagnosed using faecal egg count testing. The encysted small redworm (ESWR) larvae will not be detected on faecal egg counts, however they can be diagnosed using a blood test that was launched in 2019.

Did you know?

Approximately two-thirds of worm infections involve tapeworms and the majority of these infections are accompanied by roundworm infections

Ask Advice - Horse wormers are a class of Prescription Only Medicines that can only be prescribed by Veterinary Surgeons, Pharmacists or SQPs (suitably qualified persons). All are trained to offer you the best advice.

All available horse wormers can be grouped into 4 different classes, according on the type of active ingredient in them that kills the worms. If there is resistance to a wormer in one category, there is a good chance the worms will be resistant to other products in that class too. This is why, with only 4 classes of wormers available and no new types of wormer on the horizon, it is very important that we do everything we can to maintain their effectiveness and delay the spread of resistance.

Currently, there are 4 groups of wormers available in the UK that treat equine parasite infections:

• Macrocyclic lactones (e.g. ivermectin and moxidectin)
• Benzimidazoles (e.g. fenbendazole)
• Pyrantel
• Praziquantel

Worming becomes complicated because there isn’t one type of wormer that kills all the different worms that infect horses. Each group of wormer has a different spectrum of activity against different types of worms, these are shown in the table above. This means that when a parasite infection is identified, the appropriate wormer to treat that type of worm must be selected. Also, not all wormers have equal levels of control, plus some have levels of resistance that compromise their efficacy. This means that despite your best intentions you may not be protecting your horse from the ill effects of a parasite infection if an inappropriate treatment is used. As moxidectin is the only effective treatment for encysted small redworm in many areas, using this product sparingly is recommended by experts.

Treat the right parasites at the right time

Different types of parasites need to be treated at different times during the year. This is known as strategic worming.

The number of different types of worms that affect horses and the many different types of wormer drugs available for horses can be confusing. A good worming programme for adult horses will include the following:

Spring and Autumn: test/treat for tapeworm

Tapeworm infestation will not be picked up by routine faecal egg counts, however there are specific tests available that can detect antibodies to tapeworm, in either blood or saliva samples. If testing is not performed then a wormer that is active against tapeworm should be given at these times of the year. Suitable treatments would contain praziquantel or pyrantel at twice the licensed roundworm dose.

Summer: test/treat for tapeworm

Faecal worm egg counts (FECs) should be used every 2-3 months From March through to September, to assess the roundworm burden. Your Vet or SQP will be able to advise you on whether your horse requires worming; usually horses with a FEC of >200 eggs per gram are treated with a product containing ivermectin or pyrantel.

Spring and Autumn: test/treat for tapeworm

An annual treatment for encysted small redworm larvae is usually given over winter. A blood test is now available to determine if this is necessary.

There are only 2 effective treatments for encysted redworm (moxidectin and a 5 day course of fenbendazole) and resistance is a big concern. Experts therefore recommend that the use of moxidectin should be reserved for a single annual treatment at this time. Autumn or winter treatment should also cover bots, which are the larval stages of Gasterophilus flies.

Foals need different worming protocols. Speak to your vet or SQP for advice on working your foal.

Where worming is deemed necessary, it is vital to ensure that horses receive the correct DOSE of wormer for their weight. Visual estimation of a horse’s weight could lead to inaccuracies and most often results in under-DOSING. Under-DOSING may render the treatment ineff ective and can increase the risk of worms developing resistance to wormers.

• Use a weighbridge or a weight-tape to accurately estimate your horse’s weight.
• Wormer syringes contain different amounts of wormer treatment, usually sufficient DOSE to treat 600 kg or 700 kg bodyweight. Make sure there is enough product in the syringe to provide all of the required DOSE.

Targeted worming is often perceived as an extra cost but it has actually been shown to reduce the cost of worming

Most wormer syringes contain approximately a teaspoon of wormer treatment.

Any ‘spit-out’ can represent a significant amount of the total dose and will result in under-dosing. This has several consequences; firstly, the product will not work as it should and secondly, it can contribute to the rapid development of resistant worms.

! Some wormer drugs are also highly toxic to other pets so be very careful when disposing of used syringes!