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To worm or not to worm?

Author: Katherine Murray BVMedSci BVMBVS CertAVP MRCVS

Hang in there, worming is a huge topic so this is a long read…

It’s really useful to understand how all of the different worms affect your horse and why diagnosis and treatment varies between them. You'll also find a section on management of pasture to reduce worm burdens as well as an explanation of the term ‘resistance to wormers’. Grab a cup of tea and read on.

However, if you’re short on time scroll to the bottom for an overview of when to treat with what!


What are worms?

Worms are parasites. Eggs usually hatch on grass and the larvae make their way through the equine digestive system as part of their life cycle. They mature to adults inside the horse and produce eggs which are released back onto the grass.

Lifecycle of small strongyle worms in horses

What’s the problem?

In small numbers they usually cause no issue to your horse. However, in older or younger horses or when present in large numbers they can cause a number of clinical signs such as:

  • Colic

  • Diarrhoea

  • Weight loss

  • Pot-bellied appearance

  • Tail scratching

Types of worms

Different types of equine worms

Large Strongyles
Large strongyles
Large strongyles

Larvae hatch from eggs on the grass (see graphic above) and are ingested by the horse. They can reach 4-5cm in length. They then migrate through the digestive tract and can cause damage to blood vessels; this can lead to blood clots and damage to the intestinal walls.

Consequently, infection with these worms causes a variety of clinical signs but most often diarrhoea and colic.

Large strongyles can be problematic all year round. Their eggs are visible on a faecal worm egg count. So the best management for this type of worm is to submit a faecal sample approximately every 2-3 months and then if eggs are present in the sample your vet or suitably qualified person (SQP) will be able to advise on the best type of wormer for treatment.

Treatment options include: ivermectin, pyrantel, 5 consecutive days of fenbendazole (high levels of resistance), moxidectin (save for the cold weather).

Small strongyles (encysted redworm)

These worms may be small (<1cm) but they are problematic! The larvae are ingested and then encyst in the lining of the small intestine. They remain encysted during the colder winter months and then emerge in the spring time when the temperature begins to increase. This causes severe damage to the intestinal lining and can result in colic signs.

As these worms cause clinical signs in the larval stage a worm egg count is no good for detecting large burdens of encysted redworm. A blood sample can be taken to test for antibodies to small redworm, however this doesn’t always indicate current infection levels. Therefore, ALL horses should be treated for encysted redworm once a year ‘after the first frost’ with a moxidectin wormer to kill larvae after they have encysted once the temperature has dropped.

These worms are becoming more common and more unpredictable to treat as our climate becomes milder. Treatment options are limited: moxidectin after the first frost, fenbendazole (5 days course in horses with unknown worming history – kills worms slowly rather than all in one go).

Large roundworms (Ascarids)
Life cycle of ascarids in foals

These worms look a bit like noodles and adults can reach up to 40cm in length

Horses develop immunity to these worms once they reach about 2 years of age, so they usually only cause a problem in youngsters.

Larvae are ingested and migrate to the lungs where they mature, this can cause some scarring in the lung tissue (see life cycle image above). Adults are then coughed up and swallowed. Due to their size ascarids can cause blockages within the gut leading to colic. Alternatively, they can reduce nutrient uptake leading to poor condition and diarrhoea.

Diagnosis with WEC is difficult as eggs are shed intermittently so alternative testing such as regular ultrasound scanning and systematic treatment of youngsters is needed on yards or stud farms where there is a known resistance problem.

Treatment options are limited due to high levels of resistance. A 5 day course of fenbendazole is the best option for young horses. If you want further information on this topic in youngsters please contact the practice to speak to one of the vets.


The image shows immature tapeworms, as they mature, they can grow up to 8cm and have a ‘segmented’ appearance as seen in the image below.

The life cycle is different to other worms as tapeworms need an ‘intermediate host’ in order to develop from egg to the larval stage. For the tapeworm species commonly found in horses the orbatid mite is the intermediate host, this mite can be found on pasture as well as in hay and straw. The larvae take up to 15 weeks to develop within the mite and then it takes up to another 10 weeks for the adult to develop inside the horses gut before eggs are released and the cycle starts again!


In low numbers tapeworm is unlikely to cause too many issues in the horse. However equine tapeworm tends to congregate at the ileocaecal valve – the junction between the end of the ileum (small intestine) and the caecum / large intestine. The tapeworm causes damage in this area which can lead to spasmodic type colic and decreased intestinal motility. Heavy infection may also cause anaemia in some cases.

Diagnosis of tapeworm infection is difficult due to intermittent shedding of eggs. A blood sample, or more recently a saliva sample (EquiSal) can be taken to measure the level of antibodies to tapeworm. This test may be useful to use in the Spring but historical infection can alter results so they may be difficult to interpret. Therefore, it may be prudent to treat your horse for tapeworm once every 6 months, in the late autumn and spring.

Gasterophilus (bots)
Bot fly eggs

Eggs are laid on horses’ coats during the summer as seen in the image on the right. They can be removed with a ‘bot knife’ if they are present in large numbers. Eggs may be ingested when the horse grooms itself and they mature to larvae in the stomach. They will sometimes be found incidentally ‘hanging out’ along the margo plicatus in the stomach when horses undergo gastroscopy to diagnose gastric ulcers.

These rarely cause clinical disease and adult worms are either excreted at the end of the cold season or will be killed during treatment for other more clinically relevant worms.

Pinworms horses

Pinworm is slightly different! This worm doesn’t cause any problems as it passes through the horses’ intestine. Eggs are laid in a sticky residue around the anus and tail head. This causes irritation in this area, leading to the horse rubbing its bottom and tail head on stable doors and fencing.

A worm egg count is no good for diagnosing pinworm, instead if an infection is suspected a ‘Sellotape test’ can be useful to detect eggs. A length of Sellotape around 10cm should be pressed firmly onto the bare skin around the anus, this can then be analysed in the lab to check for the presence of pinworm eggs.

Treatment with pyrantel or 5 days of fenbendazole should kill the pinworm. Stable hygiene is also really important to kill off any eggs in the environment.



This is different for each type of worm and has been discussed above… but to recap:

Worm egg counts (WEC)
Microscopic view of worm eggs
  • A faecal sample is taken (ideally <12 hours old) and then analysed in the lab. Eggs are counted in a small sample and then a total worm egg count is calculated from this.

  • To treat or not to treat? This will be based on the WEC, time of year and the health status of the horse. Your vet will be able to advise on this.

  • If the count is very high it may be advisable to give an anti-inflammatory at the time of worming to reduce the chance of colic when the worms die and irritate the intestinal lining. Your vet will advise if this is necessary.

  • Remember WEC are not diagnostic for all types of worms.

Blood test
  • A blood sample can be tested using an ELISA to detect antibodies to tapeworm and small strongyles (redworm)

  • This can be useful in some cases for ongoing monitoring BUT historical infection may make results difficult to interpret

Saliva Test
  • EquiSal is used to test antibody levels to tapeworm in the horses’ saliva

  • It has the same shortcomings as the ELISA blood test from an interpretation point of view, however it can be taken by owners which reduces the cost


Worm prevention

Prevention is better than cure! The risk of worm infection can be lowered following some simple management steps:

  • Poo picking! A boring task especially in the rain and cold but very helpful for reducing the worm burden on the pasture. This should ideally be done at least twice a week.

  • Pasture rotation. Allowing fields to rest especially during frosts and dry / hot weather will reduce the number of live larvae on pasture.

  • Mixed grazing. The worms outlined above are all specific to the horse. Grazing cattle or sheep on equine pasture will mean that larvae are ingested by the cattle/sheep and removed from the pasture without affecting the animals.

Resistance – why and what to do about it!

In recent years resistance to wormers has become more of a problem. This is partly just evolution of pesky, clever worms but has been accelerated by over use of wormers as a substitute for good planning and management.

What is resistance?

Worms are constantly evolving and adapting. When we treat our horse with a worming product it will kill all of the worms which are ‘susceptible’ to the wormer leaving behind the ‘resistant’ worms to live and breed more worms with resistant genes. This problem is accelerated by overuse of wormers as the resistant populations are selected for more quickly. Pasture that has been grazed by horses for a long time is more likely to have a population of resistant worms.

Weigh tape horse

What can we do about it?

  • The management described above helps to reduce worm burdens on pasture

  • Worm according to weight (use a weigh bridge or weight tape to estimate) – do not over or under dose

  • Strategic worming and WECs in place of ‘over worming’ when it is not necessary

  • Rotation of wormers – rotate between different types of wormer each year (other than the mandatory moxidectin and praziquantel treatment after the first frost)

Worm egg count reduction tests

These can be really helpful to determine if you have a problem with resistant worms on your pasture. How does it work?

  • Submit a baseline faecal sample for a WEC BEFORE you worm your horse.

  • Worm your horse with the recommended product

  • Two weeks later submit a post-worming faecal sample for another WEC.

  • The number of eggs should have reduced by at least 80% if the wormer has been effective

  • If it hasn’t reduced by 80% then it is likely that the worms on your pasture are resistant to the type of wormer you used. In this case it is advisable to switch to a different type of wormer in order to treat your horse effectively.


The bottom line…

When to treat with what?

Worming plans should be specific to your horse and your environment as they may be affected by the pasture contamination, number of horses grazing, age of the horse etc.

However, a rough guide for the ADULT horse is outlined below:

Year round:

Poo pick!! Ideally rest and rotate grazing – either leave paddocks empty or graze alternative livestock.

Autumn / Winter (after the first frost):

Worm ALL horses for tapeworm, encysted redworm (and large strongyles) with a combination wormer containing moxidectin and praziquantel.


Submit a faecal sample for a worm egg count (WEC) Consult your veterinary surgeon or SQP (suitably qualified person) regarding the results and they will recommend either a combination wormer to treat roundworms and tapeworm or a tapeworm only treatment if the WEC is low or zero.

Summer (and Winter):

Submit faecal samples for a WEC every 8-12 weeks between worming treatments. Timing depends on previous treatment, WECs, age and health of the horse. Worm only according to veterinary advice DO NOT OVER TREAT.

Less worming treatment = less cost and less resistance in the future!

If you have a young, old or sick horse the above rough guide may not be completely relevant. Donkey’s also have a different set of rules for worming!

If you have any questions please get in touch with the practice and one of the vets will be happy to speak to you about a worming plan specific to your horse, pony, foal or donkey!


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