Parasites in Horses: Symptoms, Treatment, and Prevention

What are parasites in horses?

Parasites in horses are organisms that settle on or inside the horse’s body and feed off their host (the horse). These parasites can include various types of worms, such as strongyles and roundworms, as well as other organisms like lungworms and skin parasites such as mites and fly larvae. They can cause a wide range of health problems—from weight loss and a dull coat to serious illness. That’s why it’s important to recognise and treat parasite infestations early in order to maintain the horse’s health and performance.

Types of parasites in horses

The list of equine parasites is long but manageable. Generally, parasites are divided into endoparasites and ectoparasites. The key difference is where they live: endoparasites live inside the horse’s body—typically in the digestive tract or other organs—while ectoparasites live on the horse’s outer surface or in its immediate environment.

Both types can cause serious health problems in horses, so it’s important to take preventive measures and apply appropriate treatments to minimise the risk of infestation and protect the horse’s health. Regular checks for signs of parasites, along with suitable prevention and treatment strategies, can help maintain wellbeing and performance.

What are endoparasites in horses?

Endoparasites are organisms that live inside their host and feed on its tissues or body fluids. In horses, endoparasites mainly include different types of worms that live in the gastrointestinal tract. These include, for example, strongyles, roundworms, and tapeworms.

Below you’ll find an overview of the worm species that can infest your horse, how horses can become infected, which symptoms may occur, and how these parasites are diagnosed:

These parasites can multiply and establish themselves in the horse’s digestive system, which can lead to a range of health problems. If the endoparasite population increases unchecked inside the horse, it can damage the digestive tract, cause digestive upsets, impair nutrient absorption, and even lead to serious illness.

Some endoparasites can also migrate to other organs and cause damage there. For example, lungworms can infect the horse’s respiratory system and lead to breathing problems.

Endoparasites in Horses You Should Know About

Small strongyles

As soon as the grazing season starts in late April or early May, horses are increasingly exposed to small strongyles. Horses become infected by ingesting infective worm larvae on pasture. The larvae penetrate the intestinal lining in the large colon and can migrate there for several months.

Encystment of the larvae: a survival mechanism

A special characteristic of small strongyles is their ability to encyst in the intestinal wall. This process is controlled by communication between adult worms and larvae:

• If there are enough egg-laying adult worms, they signal the larvae to encyst.
• Inside the cyst, larvae can remain inactive for up to three years.
• Only under favourable conditions do they continue developing into adult worms.

Symptoms of an infection with small strongyles

An infestation with small strongyles can cause serious symptoms, including:

• Diarrhoea
• Weight loss
• Colic
• Fever
• Oedema (fluid retention along the lower abdomen and limbs)

Risk of larval cyathostominosis

Larval cyathostominosis occurs when encysted larvae are suddenly released from the intestinal wall. This condition causes severe damage to the intestinal lining and occurs especially in spring.

Main risk groups:

• Young horses up to around 6 years of age are particularly affected.
• This form of worm infestation can be fatal.

Diagnosis and treatment

Detecting the infestation

Small strongyle infestation is detected through repeated faecal egg counts as part of a modern selective deworming strategy. Treatment is usually recommended from 200 EPG (eggs per gram of faeces).

Active ingredient moxidectin: key to control

Treating encysted larvae is especially challenging due to resistance issues with active ingredients such as ivermectin. Moxidectin, a fat-soluble active ingredient, is the only approved product that:

• Penetrates the connective tissue cyst that surrounds the larvae
• Effectively kills larvae in their dormant stage
• Provides a sustained reduction of the main worm burden in the horse

Limited effect of other products

Other approved products only target the roughly 10% of larvae present in the intestinal lumen. They do not reliably prevent larval cyathostominosis and therefore do not provide comprehensive protection.

Large strongyles

Large strongyles—also known as bloodworms (Strongylus vulgaris)—pose a significant risk to a horse’s health. Adult worms live in the caecum and large colon, while their larvae migrate through the vessel walls of the intestinal arteries. This migration can cause mucosal bleeding and inflammation, which may lead to colic and lameness. In severe cases, the larvae can cause vessel blockages or aneurysms; if a vessel ruptures, it can be life-threatening.

Route of infection and life cycle

Infection occurs through ingestion of larvae on contaminated pasture or in the stable. Foals are especially at risk during their first grazing season.

• Egg laying: Females lay eggs in the intestine, which are passed in the manure.
• Development into larvae: Within a few days, the eggs develop into infective larvae that are ingested by the horse.
• Larval migration: After ingestion, the larvae go through several developmental stages within the vessel walls before returning to the intestine and maturing into adult worms.

Symptoms of a bloodworm infestation

An infection with large strongyles may show the following signs:

• Fever
• Loss of appetite
• Rough, dull coat
• Colic

Prevention and treatment

• Diagnosis: A faecal egg count can detect an infestation, although migrating larvae cannot be detected directly.
• Hygiene: Regular manure removal, keeping stable walls dry, and avoiding feeding on the ground are essential.
• Pasture management: Measures such as pasture rotation, topping/mowing, and draining marshy areas reduce the risk of infection.
• Deworming: Regular deworming treatments are necessary to combat infections and help prevent future infestations.

Pinworm (Oxyuris equi)

Pinworm—also known as the “anus worm”—is a parasite that particularly affects stabled horses. Adult worms live in the caecum and large colon. Their activity can irritate the intestinal lining and, in some cases, contribute to colic. Older horses are considered especially susceptible.

Life cycle

Female pinworms leave the intestine and lay their eggs around the horse’s anus in a tough, sticky substance—often referred to as “egg strings.” These eggs end up in the bedding and are then taken up again by the horse through feed or contact.

Inside the horse, the larvae develop through several stages into mature worms, restarting the cycle. If hygiene is poor, this cycle can quickly lead to the parasite spreading.

Signs of infestation

• Intense itching: Egg laying causes severe itching around the anus.
• Behaviour changes: Horses rub their hindquarters more frequently, which can lead to skin damage.
• Skin lesions: Rubbing can create raw, open areas that are prone to bacterial infection and further complications.

Diagnosis

Diagnosis is made by checking the anal area for egg strings. Because eggs are often not detectable in manure, a microscopic tape test or swab from the anal region is commonly used to confirm the diagnosis.

Treatment and prevention

Deworming medication:

• Benzimidazoles such as fenbendazole and pyrantel are considered effective, although pyrantel’s effectiveness can vary.

Hygiene:

• Thoroughly washing the anal area with soapy water removes the sticky egg strings and helps reduce spread.
• Regular bedding changes and deep stable cleaning are essential to remove eggs from the environment.

Prevention:

• Avoid feeding on the ground to reduce ingestion of contaminated material.
• Ensure regular deworming and ongoing monitoring of the horses.

Botflies – the gastric bot (Gasterophilus)

What are gastric bots in horses?

Gastric bots (often called botfly larvae) are parasites belonging to the botfly genus. The species Gasterophilus intestinalis is the one most commonly found in horses. The females lay their eggs on the horse’s coat. The larvae are then taken up by the horse and can cause damage in the stomach.

What do gastric bots do in horses?

Gastric bot larvae can cause inflammation of the stomach lining, leading to symptoms such as lethargy, weight loss, a dull coat, and digestive upsets. A severe infestation can cause significant damage to the gastric mucosa and may even contribute to gastric ulcers.

Which active ingredient works against gastric bots in horses?

To treat gastric bots, dewormers containing active ingredients such as ivermectin (or similar substances) are used. These are effective against botfly larvae and can help reduce the infestation.

Which dewormer should be used against gastric bots?

Choosing the right dewormer is crucial for effective treatment. Your veterinarian can help select the most suitable product based on the individual needs of the horse.

When should a horse be treated for gastric bots?

Deworming against gastric bots is usually carried out in late autumn to winter, ideally between November and December. This is the period when parasite activity is highest and horses are at increased risk of infection. Some experts also recommend a second treatment in January to ensure the last remaining larvae are eliminated.

What are ectoparasites in horses?

Ectoparasites in horses are organisms that live outside the host’s body and feed on its outer tissues or body fluids. Unlike endoparasites, which live inside the body, ectoparasites live on the surface of the horse’s body or in its immediate environment. These ectoparasites can take various forms, including fleas, mites, ticks, and certain types of fly larvae. They can attach to different parts of the horse’s body, including the skin, coat, mane, and tail. Some ectoparasites can also live in open wounds or skin lesions.

But beyond the typical symptoms that help us recognise ectoparasites in horses, there are also serious secondary diseases that can be transmitted to the horse—for example through the parasites’ saliva.

So which parasite transmits which secondary disease to our beloved horses?

Ectoparasites often cause a range of problems in horses. They can trigger itching and skin irritation, leading to excessive scratching, rubbing, or biting. This can result in skin damage, wounds, and infections. In addition, some ectoparasites can transmit diseases—for example, ticks can spread Lyme disease.

Common Ectoparasites in Horses: What You Need to Know!

Ticks, horseflies & more

Ticks, horseflies, and other pests can be especially annoying during the summer months. A lovely hack through woods, meadows, or down to the lake can quickly turn into an ordeal when bloodsuckers are on the hunt. But you can take quick action.Fly sheets and fly sprays can help you and your horse a lot—so horseflies and flies will give you a wide berth. Fly repellents are available as sprays or as gels, which are a great option for horses that are afraid of spray bottles.

🎥 Video tip: No fear of spray bottles! In this video, we’ll show you practical tips to help your horse overcome its fear of being sprayed—for stress-free application of fly sprays and more calm throughout the summer.

The deer ked (Lipoptena cervi)

The deer ked is an ectoparasite that can also infest horses. It is often called a “flying tick” because it looks like a mix between a fly and a tick. Deer keds are mainly active from July to October, during the warm season when vegetation is dense. They are most common in rural areas with forests and tall grass, where the insect can easily come into contact with potential hosts.

How does the deer ked infest a horse?

During a hack or a walk, a deer ked can drop onto the horse. It then moves to typical infestation sites such as the mane crest or the anal area. Many horses already become nervous when the insects are nearby—even before they land.

Symptoms of deer keds in horses

• Painful bites: The bite can be painful and cause skin irritation.
• Allergic reactions: Some horses react allergically to the deer ked’s saliva, leading to intense itching and skin rashes.
• Restlessness and nervousness: Many horses show noticeable discomfort and unusual behaviour because the parasites are extremely irritating.

Can deer keds transmit diseases?

Although deer keds can carry bacteria such as Bartonella schoenbuchensis, it is not yet clear whether these are actually transmitted to horses or humans through a bite. Nevertheless, caution is advised, as infestations can have significant consequences.

What helps against deer keds in horses?

Prevention

• Insect repellents: Special products aimed at deer keds can help prevent infestation.
• Fly sheets: These provide effective protection against bites.
• Natural home remedies: Products such as coconut oil are often used, as their scent may deter the insects.

Control

• Rinsing off: If you suspect infestation, thoroughly hosing the horse can help remove keds from the coat.
• Thorough grooming: Regular brushing helps remove insects and allows you to spot skin reactions early.
• Stable hygiene: Cleanliness and regular maintenance of the environment can reduce the risk of infestation.

Which remedy helps best?

In addition to chemical insect repellents, natural approaches such as coconut oil are popular home remedies due to their deterrent effect. Combining both approaches can provide more comprehensive protection.

Mites in horses: chorioptic mites, mange mites & more

Mange in horses—what is it, and what do chorioptic mites, “skin-scab-eating” mites, and burrowing mites have to do with it? How can you tell if a horse is infested?

Burrowing mites and other mite species are ectoparasites that can infest horses. These tiny, wingless parasites feed on skin debris and can live in the coat or on the horse’s skin. The three most common types of mites in horses are Chorioptes equi (chorioptic mites), Sarcoptes scabiei (burrowing mite), and Psoroptes equi (sucking mites).

A mite infestation is a serious problem that can affect the health and wellbeing of our horses. To minimise this risk and provide our beloved four-legged partners with a healthy environment, cleanliness and hygiene in the stables are essential.

Regular grooming and cleaning of horses are crucial for preventing mite infestations. This includes brushing and washing horses routinely to remove dirt and potential mites. Pay particular attention to areas around the ears, under the mane, and on the legs, as mites often settle there.

Once a mite infestation is detected, quick action is necessary because mites can multiply rapidly. It’s important to consult a veterinarian to get the appropriate treatment. Various medications are available that work effectively against mites, and some home remedies have also proven helpful. To prevent reinfestation, it’s important to treat the entire herd, even if only one horse is affected. In addition, all equipment such as saddles, bridles, and rugs should be thoroughly cleaned and disinfected to prevent the mites from spreading.

Biting lice in horses

What are biting lice, and what do they look like on horses?

Biting lice (Werneckiella equi) are ectoparasites that commonly occur in horses. They multiply particularly strongly in warm, humid conditions in autumn and spring and can infest entire groups of horses. It’s normal for horses to carry a small number of biting lice, but if there is intense itching and the parasites are visible to the naked eye, this indicates an infestation that is too severe and needs treatment.

Biting lice are around 1.5 mm long (sometimes slightly larger) and have a rice-grain-shaped body with yellow and brown stripes.

A biting louse lives for about 40 to 60 days and can produce up to 100 eggs. These eggs are laid in clusters that form small whitish patches, usually at the hair roots. After about 10 days, young nymphs hatch. They already look like adult lice, but are smaller. They become capable of reproduction quickly, which can lead to rapid spread.

Biting lice feed on skin flakes and hair, which can cause hair loss that often looks “moth-eaten.” They bite and chew the hair off just above the skin, leading to a patchy, holey coat. Biting lice can be transmitted to humans through contact with infected horses, but this is rare. Infected horses can also pass lice to other stable animals such as dogs and cats, so it’s important to contain an infestation promptly.

Does my horse have biting lice?

In addition to intense itching and a patchy coat, signs of a biting-lice infestation include restlessness and irritability. Some horses may become aggressive under saddle, buck, or rub themselves raw, which can lead to further skin issues such as swelling, scabs, and pastern dermatitis. Treatment is therefore essential to protect the horses’ wellbeing and prevent further spread.

What kills biting lice in horses?

To control biting lice, you need to make the parasites’ environment as unfavourable as possible. In severe infestations, clipping the horse and trimming feathering on the legs may be necessary. A veterinarian should be consulted. Ivermectin can be effective, but the hygiene management plan must be adjusted as well. Alternatively, spray-on or wash lotions can be used—however, caution is required due to substances that may be toxic to aquatic environments.

Home remedies for biting lice

Coconut oil, neem oil, tea tree oil, and black seed oil may be used, but they can be irritating and their effectiveness is not confirmed. Diatomaceous earth can harm lice, but it should not be inhaled. Thorough cleaning of the stable environment and changing the bedding are important. Rugs, grooming tools, and equipment should be removed and cleaned if an infestation occurs. Repeated treatments every 14 days are necessary to prevent reinfestation.

Deworming: the miracle cure for every type of worm in horses?

To minimise the risk of endoparasite infestation in horses, regular deworming is essential. Veterinarians can recommend deworming programmes tailored to the individual needs of each horse. It’s important to follow these programmes consistently to ensure effective control of the worm population and maintain your horse’s health.

🎥 Video tip: Give dewormer stress-free! Learn how to make administering dewormer or oral medication easier for your horse—almost as simple as picking out hooves!

Deworming strategy 1: Blanket deworming

For many years, blanket deworming every three months was the standard approach to parasite control in horses. This method was introduced primarily to combat the large strongyles that were widespread at the time. Today, however, these parasites are hardly present in many herds, and this strategy is increasingly viewed critically.

Approach

• All horses are dewormed quarterly, without prior diagnostics to confirm whether a parasite infestation is actually present.
• A fixed deworming schedule made this strategy easy to implement and widely used.

Problems

Resistance development:
Because dewormers were used regularly and often unnecessarily, parasites were able to adapt to the active ingredients and develop resistance. As a result, dewormers lose effectiveness and parasites can survive inside the horse.

No individual adjustment:
Without diagnostics, there is no distinction between horses that are actually infected and those that are not. This leads to unnecessary medication exposure for the horses and the environment.

Conclusion

Blanket deworming is considered outdated. If you want to protect your horse effectively, it’s better to use targeted, diagnosis-based methods to avoid resistance and support your horse’s long-term health.

Deworming strategy 2: Strategic deworming

Strategic deworming is a more modern alternative to the blanket approach. It combines regular testing with targeted, needs-based deworming. This strategy sits between blanket and fully selective deworming and therefore offers a more effective and sustainable solution.

Approach

Regular faecal testing:
Manure samples are analysed at set intervals to determine whether a parasite burden is present.

Targeted treatment:
Deworming is only carried out when an infestation is confirmed. The active ingredient is selected in consultation with the veterinarian and rotated to help prevent resistance.

Benefits

Better control:
Parasite levels are monitored, and treatments are only given when necessary. This protects the horses’ health and reduces exposure to unnecessary medication.

Reduced resistance risk:
Rotating active ingredients can significantly lower the risk of resistance developing.

Sustainability:
Targeted use of dewormers supports environmental protection because fewer medications end up in nature.

Challenges

• Higher costs, since faecal samples need to be analysed
• More effort, because treatment plans must be adapted to each horse individually

Conclusion

Despite the extra effort, the advantages of strategic deworming clearly outweigh the downsides. It’s a recommended approach for horse owners who want sustainable and effective parasite control.

Deworming strategy 3: Selective deworming

Selective deworming is one of the most effective strategies—especially for controlling small strongyles, which horses commonly pick up on pasture. This method is primarily used in adult horses.

But how do you switch from blanket deworming to selective deworming—and is it the best approach for your horse?

Approach

Faecal monitoring:
In the first year, faecal samples should be tested four times per year for parasites such as small strongyles, roundworms, tapeworms, and pinworms.
This schedule starts in April/May and continues through October/November.

Deworming as needed:
Treatment is only given if the faecal egg count is over 200 eggs per gram (EPG) or if other parasites are detected.

Benefits

• Reduces unnecessary use of dewormers and helps prevent resistance.
• Regular faecal testing provides clear information about the horse’s parasite burden.
• An annual deworming treatment is still recommended regardless of faecal results to cover certain parasites such as tapeworms or pinworms.

Challenges

• Inaccuracy: The number of eggs in manure does not always reflect the true number of parasites in the intestine. A horse can shed few eggs but still carry a significant parasite burden.
• Cost: Faecal testing typically costs between €9 and €40 per test, depending on the provider.

Conclusion

Selective deworming is particularly well suited to adult horses and offers individual, effective parasite control. The ongoing effort pays off through targeted dewormer use and better long-term health for the horses.

Deworming strategy 4: Modern selective deworming

The main difference between selective deworming and modern selective deworming lies in the diagnostics and the treatment strategy. Modern selective deworming is a further development of selective deworming and is based on more comprehensive diagnostics.

Approach

• In addition to analysing faecal samples, further factors are considered, such as the presence of specific parasite species within the herd.
• The treatment strategy is based not only on faecal test results, but also on an overall assessment of the individual circumstances.

Benefits

• ‍Holistic approach: This method combines scientific findings with practical experience and takes each horse’s specific needs into account.
• ‍Efficacy monitoring: Regular monitoring of parasite burden makes it possible to assess treatment effectiveness accurately.
• ‍Sustainability: Targeted use of dewormers reduces medication load and supports environmental friendliness.

Conclusion

This strategy offers a modern, sustainable approach based on scientific principles. It ensures effective control of endoparasites and supports the long-term health of horses.

FAQ: Worms and parasites in horses

How long does a horse pass worms after deworming?

After successful deworming, it’s important to know that your horse may pass increased numbers of worms over the next three days. Deworming is always carried out on a set date, and in the days following treatment, horses may shed higher amounts of larvae, eggs, and adult worms. For this reason, it’s advisable not to turn horses out together immediately after deworming, and stables/boxes should be cleaned thoroughly to minimise the spread of parasites.

Are worms killed after deworming?

Whether a dewormer kills all worms in a horse depends on several factors. As mentioned under “Deworming: the miracle cure for every type of worm in horses?”, resistant worms may survive a treatment, while susceptible worms are eliminated. A complete eradication of all worms is therefore not guaranteed—especially if resistance is present.

To maximise effectiveness and help prevent resistance, several measures are necessary. These include selective deworming based on faecal egg counts (treating only horses with a high burden) and avoiding excessive deworming to maintain a refugium of non-resistant worms.

It’s important to follow your veterinarian’s instructions closely and monitor your horse regularly for parasites in order to establish appropriate deworming strategies and reduce the risk of resistance.

Can worms cause itching in horses?

Yes, worms can cause itching in horses. If a horse has a worm infestation, various symptoms may occur, including itching. This can be caused by worm movement in the intestine, eggs laid around the anus, or allergic reactions to larvae or dying worms. Regular deworming programmes and routine veterinary checks are important to prevent and treat worm infestations.

How can I tell if my horse has worms?

There are several signs that may indicate a worm infestation. Common symptoms include:

• Weight loss or poor coat condition despite adequate feeding
• Loss of appetite or sudden changes in eating behaviour
• Abdominal pain or colic
• Diarrhoea or changes in manure consistency
• Anaemia, leading to tiredness and reduced performance
• Itching around the anus or tail head
• Coughing, especially with lungworm infections

It’s important to test your horse’s manure regularly and consult a veterinarian if you suspect worms. Your vet can perform faecal testing to determine whether worms are present and which type, and then recommend appropriate deworming.

What can you feed a horse against worms?

There are no specific foods that directly eliminate worms in horses. The best way to control worms is regular deworming combined with good management to reduce infection risk.

Helpful management practices include:

• Regular deworming according to your veterinarian’s recommendations
• Pasture hygiene: Keep paddocks clean and remove manure regularly to reduce worm egg spread
• Rotational grazing: Avoid overgrazing by rotating horses between pastures
• Remove droppings regularly—both on pasture and in stables/paddocks
• Avoid feeding hay or straw on dirty ground to minimise contamination with worm eggs
• Maintain good hygiene during feeding and handling to reduce transmission

A healthy, fibre-rich and nutrient-balanced diet can support the horse’s overall health, which may help its ability to cope with infections. Talk to your veterinarian about optimising your horse’s diet to ensure all essential nutrients are covered.

What helps against deer keds?

It can help to remove these annoying parasites with a fine-toothed comb. However, the fastest and most effective method is to rinse the horse (or dog) thoroughly with water. Water makes deer keds lose their grip and they get washed out of the coat. For humans, it’s also recommended to shower as soon as possible, as deer ked bites can be very unpleasant to painful.

Why do horses get mites?

Horses can get mites for several reasons. Mites are natural inhabitants of the environment around horses and are often widespread. They feed on skin flakes and, in some cases, the horse’s blood. Under certain conditions—such as a weakened immune system or warm, damp environments—mites can multiply rapidly and cause an infestation. Factors such as heavy feathering and long manes (which provide ideal hiding spots), poor hygiene, immune suppression, infections, or malnutrition can also increase a horse’s susceptibility to mites.