The Complete Guide to Horse Worming

To understand worm counts, you first need a basic understanding of the redworm lifecycle.  

While this is a general overview of the small redworm life cycle, a significant change occurs in winter, where the encysted redworms remain encysted and remain that way for up to 3 years.  

Typically, these encysted worms emerge in the following spring, a condition known as larval cyathostominosis. This can be detrimental to your horse's health if too many small redworms emerge at the same time, causing significant damage to your horse’s intestines.  

Fighting off small redworm infections can be tricky, since the encysted stage of the worm is burrowed into the intestine, allowing it to go undetected in most worm counts.  

Adult stages of small redworms can be treated with horse wormers containing fenbendazole, pyrantel, ivermectin and moxidectin. However, only moxidectin is an effective wormer for encysted redworms, but it should be saved for your winter horse worming to prevent resistance.  

Horse wormers containing moxidectin include:  

Large Redworms, also known as bloodworms in horses, are a type of parasite that horses are prone to picking up. Mature Large Redworms are typically a dark red colour and are bigger than Small Redworms, as the name suggests, and can reach up to 5cm long. 

There are three species of Large Strongyles in horses to be aware of, which are:  

Of these, Strongylus vulgaris is thought to be the most dangerous, but all of them can cause health problems in horses, such as:  

Worm counts are particularly important for detecting Large Strongyles, which can cause life-threatening damage to your horse during the migration stage of their lifecycle. Due to more horse owners adopting targeted horse wormer schedules, the risk of horses developing a high count of Large Strongyles has increased.  

Unlike their smaller counterparts, large redworms have migratory stages in their lifecycle, whereby the immature larvae travel through the horse's body via the blood vessels. This is what makes Large Redworms so dangerous, along with their longer lifecycle, which takes between 9 and 11 months to complete.  

As we've mentioned, there are three species of Large Strongyles your horse could potentially pick up, but the general lifecycle for Large Strongyles is as follows:  

The migration of these worms is what makes them so dangerous to your horse's health. The species of Large Strongyle will impact where they migrate, such as:  

While there are other species of L. Strongyle your horse can pick up, these are non-migratory and are not as dangerous as the Large Redworm.  

Unfortunately, egg counts cannot detect Large Redworm eggs because both Small and Large Strongyle eggs look the same under a microscope. This isn't so much a problem if you treat your horse routinely for worms, but if you opt for a more targeted approach, it can be challenging to obtain a definitive answer.  

This is why additional tests may be necessary to determine your horse's L. Strongyle load, particularly for horses that are at high risk, such as those used for breeding, competitions, or those that are moved frequently.  

The additional tests include: 

While PCR tests for L. Strongyles are not commercially available in the UK, larval cultures are and can be used to determine your horse's Large Strongyle load.  

As well as your normal egg counts, all horses over a year old should be tested annually at any point in the year. Foals, however, should be tested between 7 and 9 months old. For horses who've potentially been exposed, you should wait at least 7 months before testing. 

Luckily, it’s just the testing for Large Redworms that can be tricky. When it comes to treating them, there are plenty of options.  

Large Roedworm treatments should include one of the following active ingredients:  

There have been no recorded instances of resistance from Large Strongyles against any of these ingredients, so a broad-spectrum horse wormer like the Audevard Nexmectin® Oral Paste Wormer for Horses, or any wormer with the above-listed ingredients, should do the trick!  

Three species of tapeworm pose a risk to your horse:  

The flat, ribbon-like worms live in the ileocaecal junction, a space between the small and large intestine, where they use their sucker-like mouth to attach onto the gut wall to feed off the nutrients from the horse's diet.  

In horses, tapeworms can cause diarrhoea, damage to the intestinal wall, colic, and weight loss.  

According to Parasitipedia, tapeworm infections are hazardous in foals because the attachment points for the worms often become "eroded, irritated, abscessed or ulcerated and become easily infected with bacteria," making them more susceptible to bacterial infections and secondary infections. 

Severe tapeworm infections in foals can cause colic, diarrhoea, and weight loss, but may also cause: 

The Anoplocephala perfoliata species of tapeworms can also cause intestinal blockages in the ileocecal area, which can be fatal if not treated properly. 

Tapeworms are a type of parasitic worm that requires two hosts: an intermediate host (typically oribatid mites) to carry the eggs, and a final host. For horses and other equids (such as donkeys, mules, zebras, etc), the life cycle of a tapeworm is as follows:  

While most tapeworm infections in horses are benign, the tapeworms still consume the nutrients the horse needs to survive, which can have a negative impact on the horse’s performance.  

Horses should only be treated for tapeworms after a saliva test has been performed and the results indicate that an infection is present. This is essential as it prevents the overuse of certain chemicals in horses, and the resistance builds up to tapeworm treatments for horses.  

However, if you’d rather treat your horse for tapeworms routinely, it’s recommended you do so at least once a year, typically in Autumn. A second dose of treatment may be necessary for certain horses, such as:  

A good horse tapeworm treatment should contain praziquantel, like the Eqvalan® Duo Oral Paste Wormer for Horses. Still, it should be alternated with a double dose of pyrantel, such as this Strongid-P Oral Paste Wormer for Horses, to slow the development of resistance.  

Some horse owners choose to treat for tapeworms and redworms at the same time between December and February with a treatment like the Equest® Pramox Oral Gel Wormer, which contains both moxidectin to treat redworms and praziquantel to target tapeworms.  

Roundworms, also known as Parascaris equorum or ascarids, are more common in foals and young horses under four years old. These long, spaghetti-like worms can grow up to 40cm long and can also affect older horses that graze on pastures previously inhabited by younger horses.  

Each female roundworm is capable of laying 200,000 eggs a day, and it’s this rapid reproduction that makes them a threat to younger horses and foals, as they can quickly accumulate into massive worm burdens.  

Roundworms in foals can cause severe health decline, poor growth, or even death. What’s worse, roundworms can cause intestinal blockages in foals or inflammation, which can be detrimental as the foal suffers from constipation or diarrhoea while the worms themselves break into the lungs, which may cause haemorrhages.  

The key symptoms of roundworms in foals include:  

Adult horses tend to have a good resistance to roundworms; however, to keep numbers under control, it’s recommended that you:  

Continuing with regular worm counts is also essential in horses, regardless of age, so you can get a better idea of each horse’s worm burden and resistance and then treat them accordingly.  

The thing that makes roundworms unique is that they have migratory stages while inside the host that are a key part of their life cycle. The general life cycle of equine roundworms is as follows:  

Because of the parasite’s migratory patterns, roundworm infections can manifest as damage to the liver, although the organ’s regenerative capability is rarely permanently damaged. However, the lungs are a different story – the lungs heal by scarring, making the damage permanent and irreversible.  

Roundworm damage in the lungs can often cause further issues during performance activities, even after the horse has been treated. This can make foals in particular more predisposed to pneumonia or even pulmonary haemorrhage.

The most common roundworm treatment for horses is to administer a broad-spectrum benzimidazole anthelmintic like fenbendazole.  

There are various treatment options available, including Panacur® Equine Granules Wormer and Panacur® Equine Oral Paste Wormer, both of which contain fenbendazole as the active ingredient.  

Other, broad-spectrum horse wormers, such as Bimectin Oral Paste Wormer for Horses, are also effective against mature and 3rd and 4th stage roundworm larvae, as well as tackling lungworms, pinworms, hairworms, and more.  

Pinworms are unique in their lifecycle, which is what can make them so tricky to get rid of. Unlike other worms in horses, pinworms do not require an intermediate host (such as with tapeworms), nor do they migrate through other organs (like roundworms).  

The lifecycle also takes much longer, with it taking 5 months for a pinworm to develop from an egg into a mature pinworm, whereas other parasites complete their lifecycle within approximately 3 weeks.  

The general life cycle of a pinworm is as follows:  

Due to their unique nature, testing for pinworms is a little different. Since the eggs are laid outside of the body, they are not counted in faecal egg counts, nor will they be picked up on a saliva test.  

The common test for pinworms is a sellotape test. These are available commercially, but in essence involve taking a sellotape impression from the spot under your horse’s tail. This is examined under a microscope to look for pinworm eggs.  

Remember, just because your horse is showing symptoms of a pinworm infestation, it’s still essential to test your horse before treating them to prevent resistance to the treatment.  

Horse pinworm tests can be bought commercially or through your vet.  

The best horse wormer for pinworm treatments is any licensed wormer that contains pyrantel or fenbendazole, like the Panacur® Equine Guard Oral Suspension Wormer.

However, another key part of treating pinworms and ensuring they stay gone is maintaining a high standard of stable and field hygiene through regular cleaning and the routine disinfecting of food and water bins, grooming equipment, stable doors, covers, etc.