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Parasites affecting horses

SMALL REDWORM (Cyathostomins) Strongyles

The most common parasite affecting horses. Approx. 95% of eggs detected in regular worm counts are small redworm eggs. These parasites can be up to 2.5cm long, red in colour (white when unfed), and reproduce in large numbers very quickly with a life cycle of around 5- 6 weeks. Problems caused by infestation include distended stomach, dull coat, anaemia and colic.

These can also be dangerous as they have an unusual lifestage where they burrow and encyst in the horses gut wall, causing problems when emerging en masse in spring (encysted redworm). In its adult egg laying form, small redworm is detected in our worm counts.







Large redworm is far less common than the small redworm, as it has been largly controlled by modern worming regimes in recent years. Potentially it can be more damaging than the small version, causing blockages in blood vessels, damaging organs and causing internal bleeding. Large redworm is detected in our worm counts (The eggs cannot be differentiated from the small redworm but the treatment is the same, you will see the collective term "strongyles" on the results meaning redworm)

ROUNDWORM (ascarids)


(c) Researchgate

Roundworms are large creamy white worms up to 40cm long. They are not as common as redworm, and are more usually found in young horses, or old horses who have grazed pasture previously used for youngsters or who had a poor start in life. There has been an increasing number of ascarids seen in worm counts so far in 2020, possibly related to the changing weather patterns in the uk. Infection can have terrible concequences, with huge worm burdens leading to intestinal blockages, poor growth and even death. Symptoms of infestation include a pot belly "wormy" look, poor coat, poor growth and weight loss. Our worm count test detects roundworms.


TAPEWORM (moniezia)

There are 3 species of tapeworm that can infect horses in the UK. Tapeworms live inside the horses gut, both in the ileocaecal junction between the small and large intestine and in the small intestine. They attach to the gut wall and live off the food a horse digests. Tapeworms rely on an intermediate host in order to infect equines, a tiny orbital mite that lives on the pasture.  Research has shown only 23% of horses in the UK carry a tapeworm burden, but it is important to take the threat seriously as tapeworm infestation can cause damage to gut tissue, diarrhoea, weight loss and colic. We recommend using the Equisal saliva test once or twice a year depending on risk factors, in order to check for tapeworm as it cannot be detected in normal worm counts. 

PINWORM (Oxyuris equi)

Pinworm are not a true intestinal parasitic worm but can be a very pesky problem. Horses injest pinworm eggs and they travel to the intestine where they hatch and live. The worms wiggle out the horses anus at night to lay eggs around the hairless anus area instead of being passed in the droppings like other eggs. This can cause intense itching in horses causing them to rub their tailheads on trees, fences and stable walls. Because the eggs are not shed in the poo, its unlikely a worm count will show pinworm eggs, so instead a sticky tape impression is taken of the horses anus area and viewed under a microscope to show the eggs. Good stable hygiene is key in preventing and threating a pinworm infestation. Our sticky tape pinworm test is available in the online shop.



(c) Researchgate

Image by Erwan Hesry

LUNGWORM(Dictyocaulus arnfieldi

Lungworm is a lung parasite that affects primarily donkeys but can also affect other equines. The larvae burrow through the intestine walls once eaten by the horse or donkey, travelling through the body to the lungs where they develop into adult lungworm. It can take around 6 weeks to reach maturity, can be up to 8cm long and reside inside the lungs as adults. Infection irritates the lungs, causing coughing, breathing difficulties and bronchitis. It is harder to isolate a positive sample in horses, so is often better to test the donkeys that horses live with, as these are thought to be the natural host. Although it is commonly believed that up to 70% of UK donkeys are infected, research by the Donkey Sanctuary has shown that only around 4% actually carry the burden. Nevertheless, where donkeys and other equines live together, regular testing of the donkeys is advised. Our lungworm test kit can be used to check your donkey or horse for the presence of larvae, available in the online shop.

Liver Fluke (Fasciola hepatica)

Liver fluke is a leaf shaped flat parasite found in grazing animals inside their livers. Usually it infects sheep or cattle, but can also infect horses too. Weather conditions can affect the likelihood of fluke infestation, with warm wet conditions encouraging the parasites growth. Liver fluke have an intermediate host, a mud snail, which is infected by the fluke first. The flukes leave the snail in a cyst form, which is then ingested by the grazing horse. Inside the body the fluke tunnels into the liver, where it matures and feeds on the horses blood, producing eggs in around 8 weeks. Symptoms include anaemia, liver damage, weight loss, poor growth and possibly diarrhoea. There are currently no treatments licensed for use against fluke in horses, so if an infection is confirmed by the test, your vet will be able to prescribe a product intended for sheep or cattle "off license". Liver fluke tests are available in our online shop.



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