Worming horses – parasites, prevention and treatment

As horse owners, we’re all aware of the importance of worming. But, how much do you really know about equine parasites?

We’re going to explore the types of parasites horses can get, symptoms of a worm burden, worm prevention, and the active ingredients used in wormers throughout the year…

What parasites can horses pick up and what are the symptoms?

Despite being aware that horses need to be wormed, it can be confusing to keep track of the various worms and how our horses are burdened with them.

Here are the types of worms your horse may encounter, and how they’re most likely to pick them up:

Tapeworm

These white and flattened worms can grow to around 20cm in length (or longer, in some cases!). 

This worm develops in the digestive system, and your horse may get tapeworm if they eat harvest mites in their forage – since harvest mites ingest tapeworm eggs from manure. 

Symptoms of tapeworm in horses include:

• Diarrhoea.
• Weight loss.
• Colic.
• Damage to gut tissue.

Small redworm

They may be just 2.5cm in length, but small redworm are the most commonly found parasite in horses.

Also known as ‘encysted small redworm’ at certain stages, this parasite’s larvae can stay dormant and bury themselves (become encysted) in your horse’s digestive system for years at a time. Then, when small redworm all emerge at once, they cause serious damage to your horse’s intestines.

Horses are most likely to pick up small redworm larvae while grazing. Redworm larvae typically emerge in horse poo, and transfer onto the grass to begin their life cycle again.

Symptoms of small redworm in horses include:

• Loss of condition.
• Dull coat.
• Weight loss.
• Anaemia.
• Bloated stomach (appearance of a ‘pot belly’).
• Diarrhoea.
• Colic.
• Damage to intestinal wall.
• Inability to absorb nutrients.

Large redworm

Although they’re darker and longer than small redworm, at 5cm in length, large redworm live in your horse’s large intestine and follow the same life cycle as small redworm.

To find out more, check out this redworm lifecycle video by the British Horse Society (BHS).

Symptoms of large redworm in horses include:

• Loss of condition.
• Dull coat.
• Weight loss.
• Anaemia.
• Bloated stomach (appearance of a ‘pot belly’).
• Diarrhoea.
• Colic.

Roundworm

This parasite is a cream-coloured worm who can reach up to 40cm in length.

Luckily, most adult horses naturally develop a resistance to roundworm. However, foals, and equines less than four years old, don’t have strong immune systems, which puts them at greater risk of contracting roundworm through grazing pasture containing egg-contaminated droppings. 

Additionally, roundworm eggs can stick to a mare’s udders and might be ingested by her foal while she’s feeding them. 

Symptoms of roundworm in horses include:

• Severe weakness.
• Lethargy.
• Weight loss.
• Coughing.
• Rough coat.
• Pot belly.
• Poor growth.
• Intestinal blockage.
• Intestinal inflammation.
• Constipation.
• Diarrhoea.
• Lung haemorrhages (loss of blood from sudden break of a blood vessel).

Lungworm

A lungworm infection damages a horse’s lower respiratory tract (windpipe and lungs). It’s more common for donkeys to get lungworm infections, though horses who share fields with donkeys are thought to be at higher risk than those who don’t. 

Lungworm are spread when the host equine (e.g. a donkey) coughs and ends up swallowing eggs that go on to hatch in the equine’s droppings. Larvae then remain in piles of droppings on the grass, where they wait to be ingested by horses who are grazing nearby. Once a horse has swallowed lungworm larvae, they’ll become infected. 

Unfortunately, lungworm infections can be difficult to detect because foals and donkeys tend to experience few (if any) visible symptoms.

Symptoms of lungworm in horses include:

• Coughing.
• Increased respiratory rate.
• Loss of condition (in older horses).
• Generally unhealthy (in older horses).

Pinworm

Tiny parasites that take five months to develop from egg to adult, pinworms enter your horse’s body when ingested from their stable or field. 

The life cycle of a pinworm involves being eaten by a horse, settling in the colon or small intestine, then crawling out to lay eggs around the anus. Pinworm eggs will drop off to irritate your horse’s skin or stick to nearby surfaces, where they wait to be ingested.

Symptoms of pinworm in horses include:

• Intense itching around their tail area.
• Inflammation due to a skin infection around their tail.
• Hairless patches of skin where they’ve been scratching.
• Restless behaviour.
• Loss of condition.
• Sticky substance around their anus.
• Eggs or fully-grown worms visible in their droppings. 

Neck threadworm

While neck threadworm are unusual parasites in the UK, these tropical visitors can be found here from time to time. 

These long, thin worms reach up to 30cm in length, and like to live in your horse’s nuchal ligament (around their withers) that supports your horse’s head and neck. Also, neck threadworm have been known to live up to 10 years!

Using midges to transport their larvae, neck threadworm have a complex lifecycle that involves entering a horse’s bloodstream through insect bites.  

Symptoms of neck threadworm in horses include:

• Intense itching around their head, neck, and shoulders.
• Hotspot at the base of their mane, in front of their withers.
• Hair loss, skin damage, crusty skin, and swelling due to scratching.
• Small lumps along the underside of their neck and head.
• Blue cloudiness in their eye.

Bots – a parasite, but not a worm! 

Many of us may have mistaken bot eggs for being associated with equine worms, though bots are actually a type of flying insect. Bots require a horse as their host to fulfil their entire life cycle. 

To begin their life cycle, a ‘common bot fly’ lays pale yellow eggs onto your horse’s body, and those eggs are ingested as they scratch. Then, pinhead-sized larvae hatch in your horse’s mouth, before making their way to the digestive system, where they attach to the stomach. After 12 months, the larvae will pass out through your horse’s droppings and onto grass, where they pupate in soil, before emerging a few weeks later as bot flies.

The lesser known ‘throat bot’ lays yellow eggs on a horse’s chin, throat, jaw, and face – while a ‘nose bot’ lays black eggs around a horse’s lips.

While wormer might manage bot larvae when they’re in your horse’s stomach, the best way to tackle bots is to protect your horse from bot flies using insect repellent. It’s also important to remove any bot eggs from your horse’s coat using a bot knife.

Symptoms of bots in horses, depending on bot life cycle stage include:

• Eggs – yellow eggs stuck to their coat.
• Eggs – mild irritation where eggs have been laid.
• Larvae – can cause inflammation of their mouth and gums.
• Larvae – damage to stomach lining.
• Larvae – stomach ulcers and abscesses, triggering colic.

Ringworm – not a parasite!

Thanks to its confusing name, ringworm is often mistaken for being a type of worm or parasite – however, ringworm is actually a fungal infection. Another common assumption is that ringworm appears in the shape of a ring, though that’s not true either!

Ringworm is caused by types of fungi that thrive in wet weather. It spreads through direct contact (e.g. horses grooming each other) or contact with infected items (e.g. tack, grooming brushes, stables, etc.). The time between infection and appearance of symptoms can range from six days to six weeks.

Wormers will not be effective in the prevention or treatment of ringworm. To treat ringworm, you’ll need to act quickly to prevent it from spreading to other horses, the stable yard environment, and people (yes, you can catch ringworm, too!). After your vet has confirmed that your horse has ringworm, they’ll advise as to which anti-fungal products (e.g. creams) and washing routines should be most effective.

Those most at risk of being infected by ringworm include:

• Young horses.
• Elderly horses.
• Horses with weakened immune systems.
• Horses who have damaged skin.

Symptoms of ringworm in horses include:

• Rash or bald spot (usually in the saddle or girth area).
• Tufts of hair sticking out from the surrounding coat.
• Grey, flaky patches where tufts of hair fall out.
• Mild irritation or soreness where they have a rash or bald spot.

What times of year are horses most at risk of getting worms?

Each type of parasite can become active, and pose a higher risk to your horse, at specific times of the year, for example:

• Tapeworm = spring and autumn.
• Small redworm = summer and winter.
• Encysted small redworm = autumn.
• Roundworm = summer.
• Pinworm = winter.
• Bots = autumn and winter.

However, appropriate year-round management is essential to lessen the risk of your horse suffering from a harmful worm burden.

What wormers are available and which parasites do they target?

‘Wormer’ is the umbrella term used for a wide range of medications developed to prevent and treat parasite infections in horses.

Before we delve into the types of wormers available, it’s important to note that parasites can become resistant to the active ingredient in a wormer. So, it’s vital that you rotate the wormers you use throughout the year, according to your vet’s advice.

Here are some of the active ingredients used in equine wormers to target certain types of parasites:

• Praziquantel targets tapeworm.
• Pyrantel targets tapeworm, roundworm, and pinworm.
• Ivermectin targets small redworm, lungworm, and bot larvae.
• Moxidectin targets small redworm, lungworm, and bot larvae.
• Fenbendazole targets encysted small redworm, roundworm, and pinworm.

Setting up a worming programme

Your equine vet can help set up an effective worming programme for your horse. 

A worming programme works best when all horses, ponies, and donkeys living on the same premises follow an identical monitoring schedule and parasite management routine.

Owing to the way parasites are developing resistance to the active ingredients in wormers, testing your horse’s worm count may remove the requirement to give them a wormer. 

It’s normal for horses following a yard worming schedule to need wormer at different times of year to their stablemates/field friends – whereas some horses won’t require worming treatment at all!

Remember: If your horse needs worming regularly, it’s important to rotate the type/brand of wormer based on their active ingredients; always ask your vet for advice about this.
 
Every three months, you should carry out something called a ‘faecal worm egg count’, or ‘FWEC’, which allows vets to test your horse’s poo for the presence of worm eggs.

At six-monthly intervals (starting in the autumn/winter), you’ll need to perform a tapeworm saliva test for your horse. A tapeworm saliva test involves collecting a sample of saliva from your horse, so your vet can test for a tapeworm infection. 

Please note: There is a blood test available for diagnosing small redworm burden in horses with low FWEC results, though any benefits of further testing should be discussed with your vet.

Other ways to prevent horses from getting worms

Thankfully, there are lots of ways to prevent your horse from being struck with a harmful worm burden!

Examples of preventative stable and field management routines:

• Ensure there’s plenty of field space for each horse.
• Poo-pick paddocks and fields every day (or at least twice a week!).
• Cross-graze fields with other animals (e.g. sheep) who clear parasites safely.
• Rotate grazing, to allow fields to ‘rest’ (have a break from grazing animals).
• Avoid using horse manure to fertilise fields used for grazing. 
• Quarantine new horses arriving at the yard and test/treat for worms.
• Only turn foals out onto fields that have been rested for six months.

If you need any advice about pasture management and equine parasite prevention strategies, please speak to your vet.

Have you found an unknown word, term, or phrase anywhere in this article? Check out our glossary to discover definitions for many equestrian words, terms, and phrases!