What worms can infect horses?

Why are they a problem?

There are many parasites that can infect your horse, and they can cause a wide variety of symptoms ranging from skin irritation to colic, weight loss and even death.

The descriptions below explain which worms infect the different parts of a horse’s body, and the damage they can potentially cause.

The worms which are most important to monitor for and treat if necessary, are redworms, especially small redworms, and tapeworms, plus Parascararis equorum (roundworms) in foals and youngstock. This is because of the risk of serious symptoms such as colic and severe weight loss which these worms can cause.

These worms produce eggs whilst they are living in the horse, and the eggs are shed into the environment via the horse’s droppings. The eggs develop into larvae that live on the grass, which are swallowed when the horse grazes, allowing them to develop into new adult worms inside the horse. This lifecycle means that:

We can measure how many eggs each horse is passing into the environment
We can reduce the risk of horses getting infected by picking up their droppings before the larvae develop and migrate onto the grass (poo-picking).

The other types of worm are less common and tend to only be treated if they are causing a problem¹.

SMALL REDWORM

Also known as redworm, small strongyles, cyathostomins, cyathostominosis, cyathostominiasis

3-6 generations a year
In mild, warm, and wet weather, there is fast development from eggs to the infective stage of the larvae that horses consume whilst grazing. It is assumed that in the mild autumns and winters we often have in the UK, that infective stages of small redworms will be present throughout most, if not all, of the year.
In (mainly) young horses, it may be an important cause of ill-health, diarrhoea, low blood albumin levels, usually in vulnerable young horses with poor management (e.g. high stocking density).
Can be fatal in some of these horses.
Resistance is reported to most of the dewormer groups, particularly fenbendazole.
It is recommended to use an FECRT once a year to check that the dewormer used was effective, as resistance is hard to predict from previous dewormer user.

TAPEWORM

Anocplocephala perfoliata

3-4 generations a year
As with all tapeworms, there is an intermediate host (a forage mite) which consumes tapeworm eggs in the horses' faeces and these mites are then eaten by horses as they graze.
Considered as common in grazing horses.
Horses may be at risk from 9 to 12 months and throughout adulthood.
Most horses infected with tapeworm tend to have small numbers of worms, that have minimal consequences on their intestinal tract, thus causing no overt problem for the horse.
Several studies have shown the presence of tapeworms in certain types of colic in the ileocaecal region (a junction in the bowel where the tapeworms attach).
Medicines that control tapeworm infections include praziquantel or a double-dose of pyrantel.
Resistance to tapeworm dewormers is challenging to determine.

LARGE REDWORM

Strongylus vulgaris, also known as large strongyles or bloodworm

1-2 generations a year
Used to be considered the most problematic internal parasite in horses, but many years of using dewormers means this is rarely encountered in the UK. Tapeworm and small redworm are probably now more of a problem in certain groups of horses.
The parasite migrates from the small intestine to abdominal blood vessels, causing blood clots that can damage the intestines and cause colic.
If dewormer treatments decrease, without good pasture management, this parasite could re-emerge as a clinical problem.
No major resistance reports, so should be susceptible to dewormers such as fenbendazole, ivermectin, moxidectin or pyrantel.

PINWORM

Oxyuris equi

2-3 generations per year
The female lays eggs underneath the tail before dropping off.
Signs vary in intensity, but some horses become increasingly itchy, rubbing their tails and perineum (dock area). The skin can become damaged from itching behaviour and the rubbing spreads eggs around the horse’s environment.
Only some horses appear to be hypersensitive to the parasite, most horses with pinworm show no signs whatsoever.
Treatment for pinworm is challenging and should be combined with decontamination of eggs from the environment.
Ivermectin, fenbendazole, pyrantel and moxidectin are all licensed for treatment of pinworm in horses in the UK.

LUNGWORM

2-6 generations a year
Commonly caught from shared grazing with infected donkeys, who are often carriers without symptoms.
Larvae burrow through the intestine and are carried in the bloodstream into the lungs, blocking the airways. This can lead to coughing and laboured breathing, especially on exercise.
Diagnosed through specific changes in airway samples (tracheal wash) collected by your vet.
Ivermectin is the only medicine licensed for the treatment of lungworm in horses in the UK.
No resistance issues reported.

ROUNDWORM

Parascaris equorum, also known as parascaris spp, ascarids

4-5 generations a year
These are the largest worms to affect horses – white and up to 50cm long.
Mainly a problem in foals under 18 months of age.
The larvae are eaten on pastures (grazed by other youngstock), and migrate through the liver, and lungs, before being coughed up, swallowed and attaching to the small intestine.
Clinical signs are associated with the partial or complete obstruction of the bowel with the parasite, sometimes after dewormer has been given where there was a high burden.
An FEC will detect these eggs, and show which foals are shedding the highest amounts onto their pasture but does not estimate the size of the worm burden. Abdominal ultrasound (non-invasive and quick) can predict the size of a burden with accuracy.
Resistance is reported to each of the dewormer medicines, so it is important to check a FEC 2 weeks after deworming foals (e.g. at 9 or 12 months), to be sure the foal is no longer shedding Parascaris eggs and that there was no resistance to the dewormer medicine.

THREADWORM

Strongyloides, also known as strongyloides westeri

1 - 2 generations a fortnight
Common in young foals, but is rarely associated with disease and is not considered an important problem even in foals with diarrhoea in this age group.
This parasite can complete the lifecycle in the environment, so treating all foals/mares with dewormer is not a suitable means of control.
Intestinal damage can lead to diarrhoea, loss of appetite, dull coat and poor growth but rarely causes disease. Larvae can also penetrate skin causing dermatitis.
Larvae infect young foals from their mare’s milk. They migrate to the lungs, are coughed up and then swallowed and mature in the small intestine.
Ivermectin, fenbendazole and moxidectin are licensed for treatment of this worm and are assumed to remain effective.

BOTS

Gastrophilus spp

One generation per year
Botfly larvae are rarely associated with clinical disease and are generally not considered to be a problem in the horse.
The adult flies can worry horses when they lay eggs on their legs, and the larvae can cause inflammation in the mouth and gums where they burrow in.
They are a common incidental finding during gastroscopy (when vets check a horse’s stomach for gastric ulcers).
Ivermectin and moxidectin are both licensed for treatment of bots, with no resistance issues reported, but horses should not be dewormed just because of botfly infection.