Worm egg counts for horses for effective equine worming strategies


Protect your horse

20% of horses carry 80% of the total worm burden. So if you own a horse, you should make sure WECs are an essential part of your routine. They will help you:

  • Only treat your horse when necessary
  • Use a targeted treatment
  • Predict future heavy worm burdens
  • Monitor the effectiveness of your worm control programme
  • Slow the rate of Anthelmintic Resistance (AR)

Effective worm control

The aim of effective worm control is to stop worms completing their lifecycle and prevent future contamination.

A WEC tells you about your horse’s parasite burden at a single point in time. However, the lifecycle of a worm includes several larval stages that do not show in WECs. So even if the test results show no evidence of active adult worms, your horse may still carry a worm burden.

Therefore, to build an accurate picture of your animal’s internal health, you should repeat the test regularly.


Worms commonly found in horses

There are several internal worm parasites that affect horses. Left untreated, they can cause significant harm to the animal’s health, particularly in foals.

Large roundworms (ascarids)

Notes The largest parasite found in horses. Larvae migrate through the liver and lungs and return to the stomach to mature.
Most commonly affect Foals up to 18 months of age, after which they develop immunity.
Signs and symptoms Coughing; poor growth; dull coat.
Can lead to Lung or liver damage; blockage or rupture in the gut. May be fatal.
Control The eggs are robust and can survive for years in stables and soil. Disinfect stables between foal seasons and regularly remove droppings during the summer months.

Stomach worms (Trichostrongylus axei)

Notes Ingested by eating contaminated pasture.
Most commonly affect All horses, although foals are particularly susceptible.
Signs and symptoms Weight loss; loss of appetite; diarrhoea.
Can lead to Irritation and damage to the stomach lining.
Control Regularly remove droppings during the summer months.

Large redworm (strongyles)

Notes Ingested by eating contaminated pasture. Larvae migrate through the body of the horse, including through blood vessels. Adults lay their eggs in the large intestine and feed off the intestinal lining. Less significant now due to modern worming strategies.
Migrating larvae will not be detected on a WEC.
Most commonly affect All horses.
Signs and symptoms Depression; colic; dull coat; poor growth.
Can lead to Blood clots; obstruction of the blood vessels; bleeding; anaemia. May be fatal.
Control Regularly remove droppings during the summer months.

Small redworm (cyathostomes)

Notes The most common internal horse parasite. Once ingested the larvae lie dormant in small cysts in the gut wall and emerge in the spring. At the encysted stage can account for up to 90% of a horse’s worm burden. Some have developed resistance to wormers.
Will not show up in a WEC at the larval encysted stage.
Most commonly affect Can effect horses at any age but are more likely to affect younger or older horses.
Signs and symptoms Gradual or rapid weight loss; diarrhoea; lethargy; recurrent colic.
Can lead to Severe damage to the gut lining. May be fatal.
Control Have regular WECs performed and monitored. Treat with anthelmintic as indicated by WEC. Treat for inhibited larvae in late autumn/winter as necessary. Pick up dung at least twice a week.

Intestinal threadworm (Strongyloides westeri)

Notes Ingested via milk or by penetrating the skin. Once inside larvae migrate to the lungs from where they are coughed up and swallowed to mature in the intestines.
Most commonly affect Mainly a problem in foals. Horses usually develop immunity by the age of 6 months.
Signs and symptoms Diarrhoea; dull coat; loss of appetite; loss of condition.
Can lead to Damage to the lining of the gut.
Control Clean and change bedding regularly as larvae need damp, warm conditions to survive. Treat foals with suitable anthelmintic.

Pinworm (Oxyuris equi)

Notes Ingested by eating contaminated pasture, food, bedding and stable matter, through drinking water and by biting at the eggs around the anus. Females live in the colon and migrate out of the anus to lay eggs.
Most commonly affect All horses.
Signs and symptoms Itching and rubbing of tail; loss of hair around tail.
Can lead to Sores; open wounds; infection.
Control Check the underlying cause when tail rubbing is observed and treat with an appropriate anthelmintic. Clean around the anus regularly where horses are infected to remove eggs. Maintain good stable hygiene to prevent eggs remaining on the areas where they have been dispersed by tail rubbing.

Tapeworm (most commonly Anoplocephala perfoliata)

Notes Tapeworm eggs are eaten by forage mites, where they develop to an immature stage, which in turn are eaten by horses. Adult tapeworms attach themselves to the junction of the small and large intestines.
A WEC is not a reliable test for tapeworms.
Most commonly affect All horses.
Signs and symptoms Digestive disturbances; ileal impaction; spasmodic colic.
Can lead to Spasmodic colic; colic obstruction and intestinal perforation in heavy infection. May be fatal.
Control Worm with anthelmintic with tapeworm indication in late autumn or early winter. Dose high risk horses again in the spring.

Lungworms (Dictyocaulus arnfieldi)

Notes Ingested by eating contaminated pasture. Likely to occur where horses and donkeys share grazing.
Most commonly affect All horses.
Signs and symptoms Loss of appetite; coughing; difficulty breathing.
Can lead to Permanent lung damage.
Control Donkeys do not show any clinical signs of lungworm infection. Therefore, horses that share grazing with donkeys should be tested and treated for lungworm regularly.

Liver fluke (Fasciola hepatica)

Notes Most commonly seen in sheep and cattle but can also infect horses. Liver fluke are trematodes and go through a snail intermediate host. Ingested by eating contaminated pasture. Adult worms lay their eggs in the bile ducts of the liver, which then pass out in the faeces.
Ask your vet for advice as there are no licensed treatments for liver fluke in horses.
Most commonly affect All horses.
Signs and symptoms Anaemia; weight loss; loss of appetite; diarrhoea; colic.
Can lead to Liver damage.
Control Don’t graze horses on pasture with a history of liver fluke in cattle or sheep. Suck pastures are likely to have marshy areas or ponds.

Bots (Gasterophilus spp.)

Notes Bots are the insect larvae of the bot fly, which are a common irritant to horses in the summer. Adult flies lay their eggs on the horse’s coat. Horses then ingest the eggs during grooming. Larvae migrate from the mouth to the stomach where they develop into mature larvae.
Most commonly affect All horses.
Signs and symptoms Irritation to the base of the tongue and the gums.
Can lead to Stomach ulcers.
Prevention Remove eggs from your horse’s coat daily in summer.