May 2021 

Horses are affected by a wide range of worms, many with varying life cycles. Having an effective worm control strategy in place is vital to help protect your horse from health problems such as weight loss and colic. The good news is there are many achievable steps you can take to help your horse stay healthy.

This page along with its partner pages aims to provide you with all the tools you need to create a tailored worming plan for your horse based on control methods, appropriate testing and targeted worming when necessary.


For worming advice specific to donkeys please visit The Donkey Sanctuary website.

Types of worms and their effects


Small Redworm

  • Up to 2.5cm long, thin and usually reddish in colour (the unfed worms appear white).
  • The most common threat to horses, with more than 50 different species in this parasite group1.
  • A short life span of 5-6 weeks


Small Redworm. Photo Credit: Ashbrook Equine Hospital, part of the VetPartners family


Large Redworm

  • Dark red in colour and up to 5cm long.
  • Has the ability to cause more damage and can cause blockages in the blood vessels, damaging vital organs.
  • Numbers have greatly reduced over the last 20 years due to the development of more effective worming regimes2.


  • Loss of condition / weight loss
  • Anaemia - commonly presented through, weakness and a lack of energy
  • Bloated stomach
  • Dull coat
  • Colic
  • Long term damage to the gut wall, can reduce the horse’s ability to absorb nutrients and therefore, the horse is likely to begin losing weight.

The Faecal Worm Egg Count (FWEC) can be used to detect redworm – See our guide to worm testing to find out more.


Redworm Life Cycle



  • Large creamy white worms, up to 40cm in length
  • Larvae ingested by the horse reach the small intestine, travel to the liver and then on to the lungs, feeding at each stage. The larvae then travel up the windpipe causing irritation and coughing, resulting in them being swallowed back into the small intestine. Here they reach maturity and are passed out in the horse’s droppings.
  • Young horses (under 4 years of age) are more susceptible to roundworms because their immune system has not fully developed.
  • Can affect older horses, potentially those grazed on pastures previously occupied by infected youngsters.


  • Foals suffering from a large worm burden, may display signs of exhaustion, poor growth, and even death.
  • Constipation or diarrhoea may be seen due to a stomach obstruction or inflammation.
  • Look out for lethargy, weight loss, coughing, a rough coat or pot belly.

The FWEC can be used to detect roundworm – See our guide to worm testing to find out more.



  • Eggs are ingested and travel through the intestines until they reach the rectum where they develop into adults. Once mature, the female adults travel to the horse’s anus and lay their eggs.


  • Unlike many worms, pinworm do not cause much internal damage, however, they often cause irritation expressed through itching and tail rubbing by the horse, as a result of the eggs being laid in this area.
  • This itching and tail rubbing can cause damage to the area which may lead to skin infections and further problems.

The Sellotape test can be used to detect pinworm- See our guide to worm testing to find out more.



  • Small, flat segments that are cream in colour and join together, they can be up to 20cm long.
  • Tapeworm eggs are passed in horse droppings, but the eggs are then ingested by a mite living in the soil, which acts as a host. The mites are then ingested by the horse while it grazes, the larvae are released, and they develop into adults.
  • Tapeworm live between the small intestine and large intestine and fix on to the horse’s gut wall and live off food that the horse eats.

Tapeworm in droppings. Photo Credit: Liphook Equine Hospital, part of the VetPartners family.


  • Damage can be caused to the gut tissue resulting in problems such as diarrhoea, weight loss and colic.

A saliva test or blood test is advised to detect tapeworm – See our guide to worm testing to find out more.



  • Bots aren't horse worms but rather flies common over the summer months that lay their yellow eggs onto the hairs of the horse (most commonly the forelegs, belly, flanks, shoulders and under the chin).
  • The horse then grooms or licks itself, ingesting the eggs, and causing them to hatch into larvae. The larvae then burrow into the horse’s gums and beneath the tongue.
  • The larvae stay inside the horse’s mouth for around one month until they are swallowed and then attach on to the stomach wall where they lie inactive for around 8-12 months.
  • They then release themselves and are passed in droppings where they then bury themselves into the soil and emerge as an adult fly 20-30 days later.
  • Horse owners can use a ‘Bot Fly Knife’ to scrape off any eggs laid on their horses to help break the cycle.


  • Inflammation can be seen at the mouth and gums.

Gastrophilus bots
Bot larvae. Photo Credit: Liphook Equine Hospital, part of the VetPartners family



  • A lung parasite predominantly affecting donkeys.
  • Larvae ingested by the horse or donkey travel from the intestine through the circulatory system to the lungs where they develop into adults.
  • Lungworms may grow up to 8cm long.
  • It has previously been thought that donkeys and horses cannot live together due to the lungworm parasite. However, The Donkey Sanctuary advise they can live together quite safely, provided a regular de-worming programme is followed.


  • Symptoms may include difficulty in breathing, severe coughing, and loss of appetite.



  • The threadworm parasite most commonly affects foals up to 4 months of age but often remain inactive in adult horses.
  • Mares may hold threadworm larvae in their mammary tissue (teat) which are then transferred to the foal when they suckle. See our guidance on worming the mare and foal.
  • A threadworm infestation may cause signs of weakness and diarrhoea.



1) Matthews, J, B. (2011) Facing the threat of equine parasitic disease. Equine Veterinary Journal.43(2) P. 126-132.
2) Peregrine, A, S. et al (2006) Larval cyathostominosis in horses in Ontario: An emerging disease? The Canadian Veterinary Journal. 47(1) P. 80–82.

Further Reading

1. XLVets. 2021. 112 Worming and Pasture Management Factsheet. Available from: https://xlvets-equine.co.uk/sites/xlvets-equine.co.uk/files/XLVets-Equine-Rebranded-112-Worming-%26-Pasture-Management-Factsheet_0.pdf
2. West Gate Lab. 2021. Parasites affecting horses. Available from: https://www.westgatelabs.co.uk/info-zone/parasites-affecting-horses/

horses grazing

Horse and foal

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