Horse worming guide: Worming advice for horse owners

When it comes to your horse’s health, parasites like worms can cause all sorts of issues, and if left untreated they have the potential to cause life-threatening illness.. To help you prevent worm burden in your horse, this guide will take you through the basics of horse worms, as well as offering advice on treating and managing them.

We will cover:

What is a horse worm?
How do horses get worms?
What are the types of worms in horses?
What are the symptoms of worms in horses?
How do you test for worms in horses?
Which horse wormer should you use?
How often should you worm a horse?
Can you feed or ride a horse after worming?
How to reduce the risks of worms in your horse

What is a horse worm?

Worms that infest horses are a type of parasite that can live and feed within animals. They are able to survive by finding nourishment inside the body, typically in the gut, which disrupts the host’s ability to take in nutrients effectively, potentially causing weakness and disease as a result. All species of worm lay eggs, which hatch either inside the host or are passed outside.

There are a few types of parasitic worm that infest horses. They’re fairly common, to the point where they are present in nearly all horses — though at varying levels of infestation.

How do horses get worms?

Depending on the type of worm and its life cycle, the way that a horse can get worms can vary. One of the most common methods of transmission is when eggs are ingested from an infected pasture, after which they hatch and begin to live within the host. In a lot of species, an adult worm continues its lifecycle by laying eggs that become attached to faeces, which then make their way back to the field to widen the transmission of worms among other horses.

Worms — when present in low numbers — don’t necessarily cause any major issues with horses. It’s only when the infestation begins to grow that they become a major health risk, including a decline in body condition, ill health, and even colic. If left unchecked, high numbers of worms can damage the host’s internal organs, potentially causing death. Because of this risk, horses are treated with drugs known as wormers on a year-round schedule to reduce the chance of a serious infestation.

What are the types of worms in horses?

There are a number of different types of worm that can infest and cause problems in horses. Below, we will provide you with an overview of the main species that you may have to deal with.

Redworms

Redworms, sometimes known as bloodworms, are a type of parasite made up of three species that differ in size: S. vulgaris (up to 25mm), S. edentatus (up to 40mm) and S. equinus (up to 50mm). All three worms infest a horse when their larvae are consumed, transforming into their parasitic form as they enter the digestive system. Common effects of infestation include anaemia, weakness, weight loss, and diarrhoea. More serious problems can occur if left unchecked.

The S. vulgaris species is the most common parasite in horses and is known for damaging arteries in the intestines, which can lead to colic and other serious health issues. They also tend to burrow into the gut wall to hibernate through winter, which can cause problems in spring if lots of them emerge. The harm this can cause to the wall can also bring about colic, as well as weight loss and diarrhoea.

edentates, the largest redworm species, are among the most dangerous equine parasites as they migrate through blood vessels of the gut, causing damage and bleeding. Thankfully, they are also a lot less of a threat than they used to be thanks to effectiveness of wormer medication.

Roundworms

Roundworms are parasitic worms that can cause issues in the lungs, gut, and throat. They pose more of a problem for younger horses and foals, as they are not immunised against these worms.

Adult roundworms can become large, growing up to 50cm in length. A horse is infested when the larvae are ingested from pasture. When they reach the small intestine, they pass through the gut wall, before travelling to the liver and then the lungs. The next stage of the journey sees the larvae coughed up and swallowed again, taking them back to the small intestine to mature and reproduce.

A smaller infestation is typically fairly harmless for a young horse, but a serious infestation can be a problem. Respiratory issues can occur, such as a cough or nasal discharge, when larvae are present in the lungs and throat. Worms in the gut can cause weight loss and lethargy, as well as having the potential to stunt growth, impact the growth of the horse’s coat, and create a pot-belly. These worms can eventually bring on serious problems, like colic, if left untreated.

Tapeworms

Tapeworms differ slightly from other parasitic worms in that they infect a different host before they find their way into horses. This is because forage mites that live on grass eat the worm’s eggs and the larvae develop inside of them. The mites are then ingested by the horse when grazing, where they make their way to the horse’s intestine and develop to maturity — up to 20cm in length.

They adhere themselves to the gut wall at the junction between the small and large intestine in clusters, which can cause intestinal obstruction or rupture due to inflammation. Tapeworms can be a threat to a horse’s health as they’re capable of causing digestive disturbance, loss of body condition, potentially fatal blockages, and colic.

Bot flies

Bot flies are not purely worms, but the larval part of their life cycle takes place within the horse’s body. These flies are typically present in the late spring and summer grazing seasons, where they are an irritant to horses as they buzz around. Not only that, but they lay sticky yellow eggs on the horse’s coat, which are then ingested either by the horse itself or another horse during grooming.

Once eaten, the eggs hatch into larvae that reach the stomach and attach themselves to its lining as they continue to develop. This can cause irritation, obstruction, and digestive issues. When mature after 8–10 months, the larvae will detach and pass through the system until they are ejected via the horse’s faeces. At this point, the larvae burrows below the ground to pupate, before turning into a fly to begin the life cycle again.

Pinworms

Pinworms are not a true intestinal parasite and don’t really pose a serious threat to a horse’s health, but they can cause discomfort and be resilient to treatment.

The lifecycle of a pinworm begins when eggs are eaten from infested pasture. These eggs hatch and the worm matures as it lives in the intestines — this can take up to five months. Unlike other worms, the pinworm does not migrate through any other organs or tissue. However, the larval stages can be resistant to worming medicines, so they may still be present even after a horse has been treated. The worm will lay eggs around the rectal area but return to carry on living in the gut. These eggs hatch and the new pinworms will also enter the horse to mature.

Because pinworms don’t migrate through the body, they don’t cause the internal damage that other worms do. The only nuisance that they cause is an annoying itch around the tail head, where they lay their eggs. As a result, a horse can have a major infestation of pinworms and not show any of the other symptoms that other worms will often cause. Instead, it’s better to look out for skin irritation around the rectal area, rubbing of the tail, and biting and licking of hindquarters.

What are the symptoms of worms in horses?

There are a few signs that you can look out for that can indicate your horse has a serious infestation of worms. However, while these symptoms can be a warning sign, the best way to confirm whether your horse has worms is to carry out tests, like a faecal egg count and blood test.

It’s also worth remembering that worms are very common, to the point where they are present in most horses at some point, so some of these symptoms may not be present in less serious cases.

Some of the common signs of parasite worms in both younger and older horses include:

Lethargy: Is your horse being more lethargic in its movements? Does it seem tired or reluctant to work when it has been enthusiastic in the past?
Weight loss: Has you horse recently lost a lot of weight? Have you tried calculating their weight with a scale, weight tape, or formula to ensure accuracy?
Loss of body condition: Have you noticed that your horse’s body condition has declined? It may be worth carrying out a body condition scoring to determine how much of a change has occurred.
Loss of appetite: Is your horse eating less than before? You can read our feeding guide to get an idea of what a regular intake should be for your horse.
Dull coat: When you’re grooming your horse, have you noticed that their coat has lost its usual lustrous sheen? Is the growth of their coat uneven?
Diarrhoea: Are your horse’s stools liquid rather than solid? It could be a sign of disruption in their digestive system.
Colic: Has your horse started to suffer from colic? If so, worms could be the cause (or one of the causes) of the condition.
Bot fly eggs: Does your horse have sticky yellow bot fly eggs attached to its coat? It may have consumed some of these when self-grooming. Remove these during grooming and ensure your horse wears a fly rug.

How do you test for worms in horses?

Testing is the most reliable way to identify whether a horse has worms or not. One of these tests can determine what species is present, provide an idea of how many adult worms are present, and give you an idea of how badly infested a pasture is. There are three main tests that can be carried out:

A blood test: This test measures chemicals in the blood produced as part of inflammatory response when larvae migrate. Effective at detecting tapeworm and small redworms.
A faecal egg count (FEC): This test counts the number of worm eggs present in a sample of faeces from a horse (giving an eggs per gram result). Effective at detecting the presence of most adult worms, including roundworms and redworms, but not tapeworms.
A saliva test: A saliva test is a fairly new advancement that is capable of checking for the presence of tapeworm, offering an alternative to blood testing.

Please note: Always speak to a vet about testing your horse for worms and the appropriate course of action that’s required. They should be able to recommend the right test and wormer.

Generally speaking, a healthy adult horse can undergo a regular pattern of testing to find out if they need worming. Typically, a FEC is carried out three times per year and a saliva/blood test is carried out twice per year. Because worms can build up a resistance to worming treatments, the general advice is that the vast majority of worming should only be done if there is a positive test result. This allows the drugs to remain as effective as possible when they’re needed.

Encysted stages of redworm pose a problem as they are not yet mature, so won’t lay eggs that can be counted in a dung sample. Therefore, most horse owners choose to take a blanket approach to treating these worms without testing, typically by using an effective wormer in winter. An egg count can then be performed in the preceding months to see if any survived to maturity.

Foals and older horses will need more attention paid to their worming as they’re more vulnerable, so it will likely be necessary to carry out more frequent testing. Please take care to speak to your vet about any worming for a horse that may need extra care.

Which horse wormer should you use?

If you think your horse may have worms, it can be difficult to know what type of wormer to choose. There are a wide variety on the market, and the table below should give you a better idea of what is available and what type of treatment to use.

Please note: You should never diagnose and treat worms yourself — always speak to a vet to make sure that the treatment is right for your horse.

Type of drug	Generic name	Parasitic worm treated
Benzimidazoles	Fenbendazole	Large and small redworms, encysted redworms, pinworms, and roundworms.
Benzimidazoles	Oxibendazole	Large and small redworms, large roundworms, and pinworms.
Macrocyclic lactones	Ivermectin	Large and small redworms, roundworms, pinworms, bots, hairworms, lungworms, and largemouth stomach worms.
Macrocyclic lactones	Moxidectin	Large and small redworms, encysted redworms, hairworms, pinworms, and roundworms, largemouth stomach worms, bots, and tapeworms.
Tetrahydropurimidines	Pyrantel pamoate	Large and small redworms, large roundworms, and pinworms.
Tetrahydropurimidines	Pyrantel tatrate	Large and small redworms, pinworms, hairworms, stomach worms, lungworms, round worms, and bots.
Isquinoline-pyrozines	Praziquantel	Large and small redworms, roundworms, hairworms, stomach worms, lungworms, bots, pinworms, and tapeworms.

Worm resistance to worming treatment

It’s possible for a parasitic worm population to build up a resistance to some worming drugs. This is because a few can be born with genetic mutations that enable them to withstand the effects of the chemical, so when wormers are used indiscriminately in horses, only the resistant parasites are left.

As these survivors reproduce, a growing population of resistant worms begins to take hold. And, if a regime of regular worming continues, the number of these worms will increase each life cycle until they are the dominant variety. At this point, the wormers used become defective against the larger population, which can take decades to disappear in the local area.

This is why the equine community is moving towards surveillance worming, which means a testing-first approach is adopted, so horses only receive treatment as and when necessary. In theory, this limits the opportunity for a resistance to wormers to take hold.

How often should you worm a horse?

As we’ve just mentioned, the general outlook within equestrianism is that a testing-first approach works best for most horses — this involves performing regular tests and wormers as needed. This helps to ensure that worms aren’t able to build resistance to the treatment.

However, it’s also important to remember that every horse has their own needs, so it’s best to work with your vet to create a plan that works best for your horse, which may involve a more regular and rigid worming routine. They will help you come up with a worming schedule that encompasses both testing and treatment, like the example below.

Creating a horse worming schedule

As part of your horse’s health regime, you will need to create a worming schedule to ensure they’re always in great condition, well cared for, and as worm-free as possible. When you are discussing the plan with your vet, you will need to take into account a few factors that will determine what the best approach will be. You’ll need to consider:

Your horse’s age: Older horses may require greater attention when it comes to worming as their health tends to be less robust than younger horses. At the other end of the scale, foals have not yet developed a full immune system, so they will need extra attention.
Your horse’s weight: You will need to accurately weigh your horse in order to judge what dosage of wormer is required to be effective. Be sure to use a tape, bridge, or formula to do this rather than guessing, as you don’t want to under- or overdose them.
Test results: If your horse is currently worm-free according to their test results, there may be no need to go ahead with regular worming. Testing is also a good way to gauge whether past treatment has been a success.
Stabling your horse: While stabling your horse will mean they are not as exposed to the risk of an infected pasture or infestation via other horses, it does not mean that they will never come into contact with worms. They may come into contact with other horses or materials with eggs attached. This should be taken into account when planning a worming schedule.
Selectively choosing worming drugs: It’s usually best to limit worming drugs to those that target the worms detected in testing. This reduces the chances of overexposure to worming drugs, which can make them less effective in the future.
The weather in your region: It can be advantageous to be aware of the climate in your area and to keep track of the weather forecast. Wet and mild conditions are ideal for parasitic worms, so you can focus testing and treatment around times where the risk is highest. If you’re in a drought or experiencing extreme heat or cold, worming may not be necessary as parasites will struggle to breed.

With these considerations in mind, you can work with your vet to create a horse worming schedule to suit your horse that should provide a year-round plan for testing and treatment. Below, we’ve put together an example of such a plan, though you should always tailor your own to your horse’s needs.

Season	Autumn (Sept–Oct)	Winter (Nov–Feb)	Spring (Mar–Apr)	Summer (May–Aug)
Testing	Saliva/blood tests for tapeworm Faecal egg count for roundworms and other worms	Blood test for small encysted redworms	Saliva/blood tests for tapeworm Faecal egg count for roundworms and other worms	Faecal egg count for adult worms
Wormer	Praziquantel for tapeworm Benzimidazoles for other worms	Moxidectin for small encysted redworms and botfly larvae	Praziquantel for tapeworm Benzimidazoles for other worms	Benzimidazoles, ivermectin, or pyrantel for all worms

Can you feed or ride a horse after worming?

There are no hard and fast rules when it comes to feeding and riding after worming. The first thing you should do is check the instructions on the wormer to see if there are any guidelines that should be followed — be sure to check with your vet if you’re not sure.

Feeding after worming can depend on your horse. Most wormers don’t taste very nice, so they may want to have a break before eating, but on the other hand they may prefer to get rid of the taste right away. There are also some wormer products are designed to be mixed with feed, so you may need to provide food at the same time.

Riding your horse after worming is another area with no best practice. Some horses are completely fine to be ridden after treatment, while others can require some recovery time. If you want to follow a general rule, you could stick to giving your horse the day off from work to be on the safe side.

How to manage the risks of worms in your horse

Alongside an effective worming schedule of testing and treatment, it’s important that you take steps to effectively manage your pasture so that the risk of worms is reduced. There are a few things you can do to keep down the number of parasites in their grazing environment:

Regularly remove faeces from pastures: By taking away droppings from the pasture, you can reduce the build-up of worm eggs that could potentially be ingested. Aim to collect droppings on a daily basis — if this is not possible, it needs to be done at least twice a week in spring/summer and once a week in the autumn/winter.
Don’t overcrowd pastures: By keeping the number of horses per pasture to a sensible level, you can ensure that egg build up and social grooming is kept to a minimum. Generally, it’s recommended 1.5 acres per horse is allowed, though this may be more or less depending on your horse’s size.
Sharing pasture with other grazers can reduce egg count: Sheep and cows aren’t hospitable to worms that target equine animals, so eggs consumed by them cannot develop. Sharing a pasture with other grazers can ensure that more eggs are removed safely.
Rotate and rest pastures: Rotating and resting pastures ensures they have time to recover, and any infestations will be reduced. Fencing can even be introduced to single pastures to allow for areas to be rotated.
Use a rack for feeding: By putting hay and grain on a rack, rather than on the ground, you can reduce the risk of worm eggs contaminating your horse’s food.
Regularly grooming for bot fly eggs: Bot flies lay their eggs on horses’ coats, where they are consumed during mutual grooming. By regularly grooming your horse for these eggs, you can reduce the risk they pose. Pair this routine with a fly rug, which can be effective in preventing the flies from laying their eggs.
Cleaning stables and shelters: Worm eggs can survive for longer in stables and shelters than in the field, so it’s very important to keep a clean house and to remove all droppings from the vicinity.

Keeping your horse as healthy as possible is a priority, and effectively managing worms plays a big role in protecting their wellbeing. We hope this guide has given you an insight into how to test and treat worms, as well as how to keep the risk of parasites at a minimum.

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