Anoplocephala perfoliata is a common horse tapeworm found worldwide. It is spread by an oribatid mite host that lives on paddocks. These tapeworms can cause damage to the intestinal lining, leading to colic. It is important to treat horses to prevent disease but, to avoid drug resistance, horses should only be wormed if testing indicates the presence of a tapeworm burden.

Equine Tapeworm

Three species of tapeworm are known to infect horses:

- Anoplocephala perfoliata
- Anoplocephala magna
- Anoplocephaloides mamillana

A. perfoliata is the most prevalent tapeworm in the UK. It can infect horses of any age or breed. This parasite is distinguishable from other tapeworm species by its size and the distinctive flaps on its head. It is also the only tapeworm associated with equine disease.

A. perfoliata possesses a flattened body that can reach up to 8 cm in length and 1.5 cm in width. Its body consists of segments called proglottids, and its head, or scolex, has four suckers that it uses to adhere to the intestinal wall. The attachment of these suckers can cause damage by pulling out plugs from the gut wall, leading to inflammation and, potentially, colic. Studies indicate that the greater the number of tapeworms a horse has, the more severe the intestinal damage. Even relatively low burdens of tapeworms (more than 20 worms) can lead to significant lesions, which may result in thickening of the gut wall and motility problems that cause colic.

A. magna and A. mamillana typically reside in the small intestine, while A. perfoliata is commonly located at the junction where the small intestine meets the large intestine, near the caecal connection. This narrow area is prone to a build-up of A. perfoliata, which, aside from causing damage and thickening of the gut wall, may lead to a blockage manifesting as impaction colic.

Tapeworm head containing large suckers

Tapeworms in the intestine

The Tapeworm Life Cycle

Unlike common equine worms like small redworms or ascarid roundworms, tapeworms require oribatid mite intermediate hosts for the development of their infective stages, known as cysticercoids. These mites, which live in the environment, are ingested by horses, after which the mites' bodies are digested to release tapeworm larvae. These larvae then mature into hermaphroditic adults within the horse's intestine, attaching to the gut wall and feeding on its contents during maturation. The process from larva to mature adult worm takes 6-10 weeks.

The body of an adult tapeworm consists of a series of proglottids that mature progressively, each equipped with male and female reproductive organs. Behind the scolex, new segments continually form and mature as they move towards the end of the worm. The most mature proglottids, typically containing fertilized eggs, detach and may burst as they travel down the intestine, releasing eggs in dung. These eggs can then be consumed by oribatid mites. Inside the mite, eggs hatch into larvae, which require 2-4 months to develop into infective cysticercoids.

The release of egg-containing tapeworm segments is intermittent, meaning that eggs may not be continuously released into dung. A horse may harbour tapeworm infections comprising a range of different stages, including larvae, mature worms without egg-laden segments, and mature worms with segments containing eggs. This means that a significant burden may exist even though it may not be picked up by looking for eggs in dung samples. As a result, conventional faecal egg count tests are not suitable for identifying tapeworm infections requiring treatment.

Oribatid mite: intermediate host of tapeworm

Tapeworm eggs are released in dung

Although tapeworm eggs can be identified in dung, it is difficult to accurately diagnose infection, especially in horses with burdens of 1-20 tapeworms.

Standard Faecal Egg Count (FEC) methods are not reliable for detecting tapeworm infections for the reasons mentioned above.

Furthermore, several studies have demonstrated that the uneven distribution of small amounts of tapeworm eggs in dung makes it difficult to detect these parasite stages in FEC tests. This can result in the high occurrence of false negative results if tapeworm infection is assessed using FEC methods. This has been demonstrated by several independent research studies.

This means that a negative FEC result for tapeworm does not mean that the horse does not have a tapeworm infection.

The EquiSal Tapeworm Test detects tapeworm burdens by measuring specific antibodies in horse saliva instead of counting eggs. These antibodies are generated when the immune system recognises a tapeworm infection.

The test quantifies worm-specific antibodies to assess the presence of tapeworms. The worm-specific antibody levels have been demonstrated to correspond with the degree of tapeworm infection in individual horses (Lightbody et al., 2016. Vet. Clin. Pathol. 45:335-346). Peer-reviewed research verified the test's high sensitivity, particularly when over 20 tapeworms are present, indicating that no horses were misdiagnosed with this potentially harmful level of infection.

Diagnosing Tapeworm Infection

Saliva sampling

Tapeworms pose a significant health risk to horses as they are linked to various types of colic, with the disease risk correlating to the horse's tapeworm burden level.

Tapeworms are implicated in several colic forms:

- Intussusception, where a segment of the intestine slides into an adjoining part, causing colic
- Caecal perforation or rupture, which leads to peritonitis as the intestinal contents leak and infect the abdominal cavity
- Ileo-caecal junction and mucosal thickening, resulting in intestinal obstruction and abnormal gut motility. Intestinal blockages may also occur due to tapeworm clusters at the small and large intestine's narrow junction
- Ileal or caecal torsion, where the intestines twist, leading to extreme pain and blockage
- Spasmodic colic, characterized by horses experiencing recurrent episodes that may increase in frequency and severity.

Some of these conditions can lead to the sudden onset of colic with intense pain, which may necessitate surgical intervention. More commonly, mild colic results from decreased flow of gut contents due to tapeworm-associated obstructions.

Research indicates that a greater tapeworm burden increases the likelihood of horses developing colic. Even relatively small numbers of worms can cause intestinal damage. It has been observed that as few as 20 tapeworms can harm the intestinal lining and have clinical significance. The ileo-caecal junction, a common habitat for A. perfoliata, is a narrow passage where clusters of worms are more prone to cause damage or blockages, potentially leading to disease. Given that the burden level and clinical symptoms are linked, and considering the growing concern over wormer resistance, it is crucial to use testing to guide treatments. This ensures that horses with significant burdens receive treatment, while those with minimal or no burdens avoid unnecessary treatment. In the UK, hundreds of thousands of EquiSal tests have been conducted, with approximately two-thirds of horses not requiring treatment. This reduces the use of wormers and preserves their effectiveness.

If you are concerned that your horse has colic, consult your veterinary surgeon.

Tapeworm and Horse Health

Heavy burdens of tapeworms in intestine cause damage

Horses with tapeworm can show signs of colic

Anthelmintics, commonly known as wormers, are frequently overused by equestrians. Traditionally, horses received treatment for tapeworms once or twice annually, during spring and/or autumn. This practice of treating all horses in a group has led to the development of drug-resistant worms.

Resistance refers to the worms' ability to survive the killing effects of wormers. The escalation of resistance is alarming; resulting in horses with high burdens of tapeworms that are resistant to anthelmintics and not responding to treatment, placing them at risk of serious illness.

Reports have emerged of resistance to the two available anti-tapeworm wormers, praziquantel and pyrantel, which is alarming given the pharmaceutical sector's lack of development of new equine anti-tapeworm treatments (Nielsen, 2023. Int J Parasitol Drugs Drug Resist. 22;96-101).

Special attention should be paid to the management of tapeworm infections in horses to minimise the risk of drug resistance occurring

Best practice guidelines recommend controlling equine worm infections through a combination of effective pasture management to minimize infection risk and diagnostic testing to determine which horses require, or importantly, do not require worming. This approach identifies horses with potentially harmful tapeworm loads, and reduces the use of wormers.

Tapeworm antibody tests categorise results as 'low', 'borderline', and 'moderate/high'. Treatment for tapeworm is advised only for horses with 'borderline' or 'moderate/high' diagnoses. Published research and analysis of current EquiSal Tapeworm test data indicate that fewer than one-third of horses tested require treatment for tapeworms, thereby minimizing unnecessary worming and preserving the effectiveness of anthelmintics (Lightbody et al., 2018. Eq Vet Int J. 50:213-219; Matthews et al., 2024. In Pract. 46:34-41) .

The key message is to carry out routine testing and not routine worming, reserving tapeworm wormers for when they are recommended following diagnostic testing

Many veterinary surgeons, SQPs, and horse owners in the UK follow the industry's Best Practice guidelines and have now incorporated the EquiSal Tapeworm test into their sustainable worm control programmes.

For more information on how to integrate tapeworm testing within an overall parasite control programme please see the Using The Test page.

Modern Approach to Tapeworm Control

Good pasture management is key to worm control

ELISA testing: only one third of tested horses is advised for worming based on EquiSal Tapeworm test results.

The British Equine Veterinary Association recently published guidelines (the ProtectMeToo Toolkit) for worm control. These recommend that horses be tested for tapeworm as part of a sustainable worm control programme

Reducing Risk of Infection

Assess risk

In implementing tapeworm control, assessing the risk of infection is the initial step. Horses at a higher risk are those grazing paddocks without proper hygiene measures, those on more densely populated pastures, young horses under five years-old, and those in groups with a history of high infection levels as determined by testing. For these groups, it is crucial to implement strategies that lower the risk of disease by reducing pasture infection levels. These strategies may involve enhanced pasture management, frequent testing to pinpoint horses that are sources of contamination or those at greater risk of disease, as well as the judicious use of wormers.

Pasture management procedures to reduce tapeworm infection levels on paddocks

Tapeworms are common parasites of horses; however, animals on well-managed pastures typically do not carry significant tapeworm burdens. Conversely, horses on poorly managed paddocks are likely to have greater burdens, increasing their risk of disease. Since tapeworms are transmitted through oribatid mites that live on paddocks, management practices that lower parasite infectivity of these areas can significantly impact tapeworm loads in grazing horses.

The life cycle of the equine tapeworm can be disrupted effectively by regularly removing dung from paddocks, thus preventing mites from consuming tapeworm eggs present in the dung. The period from late spring to late summer is optimal for the development of larval stages within mites, and mites are more active and develop faster during these times. It is advised to clear dung from paddocks at least weekly, particularly in spring and summer, to eliminate egg-contaminated dung. Regular removal of dung prevents the development of infective stages where horses graze, thus reducing contamination and ensuring safer pastures. Always dispose of dung well away from paddocks and watercourses. In the UK, harrowing is not advised for parasite control as it will merely serve to spread worms across the pasture.

Maintaining low stocking densities is beneficial in reducing the risk of infection on paddocks. The fewer horses grazing an area, the less likely it is to become contaminated with worm stages. It is advisable to keep stocking densities below one horse per acre.

Additionally, alternating or cross-grazing with ruminants can aid in lowering worm infection levels in the environment as ruminants do not serve as hosts for equine tapeworm infections.

Testing to identify horses that are sources of contamination and are at most risk of colic

Regular tapeworm testing provides crucial information to identify horses that are likely sources of pasture contamination as well those animals that are at higher risk of tapeworm-associated colic. Conducting tests in spring helps pinpoint horses that may significantly contribute to paddock contamination during the peak transmission phase in oribatid mites in summer. Autumn testing detects horses that may have developed significant tapeworm burdens during the grazing season, and treating those that test positive can reduce disease risk.

Whenever feasible, test all horses in a group or herd simultaneously. Tapeworms tend to be unevenly distributed among hosts, with a few animals often harboring large burdens that lead to higher infection levels on paddocks. Without testing, these horses remain unidentified.

By testing, the frequency of treatments is reduced, thereby lowering the chance of developing drug-resistant tapeworm populations.

Implement good practices when worming horses

It is crucial to use the correct dosage of anti-tapeworm anthelmintic when worming. The administered dose should be based on an accurate estimation of the horse's weight, ideally determined using weigh scales. Ensure that the entire dose is swallowed. Administering less than the optimal dose can lead to higher selection for drug-resistant tapeworms.

Consider infections in new horses to the group

To mitigate the risk of introducing new tapeworm infections into a herd, including those resistant to wormers, appropriate quarantine measures are necessary. Horses should be denied access to paddocks and undergo testing to determine the presence of tapeworms. The results of these tests will dictate whether anti-tapeworm treatment is needed for new horses. Should treatment be necessary, horses must be kept off paddocks for a minimum of 48 hours post-worming. Subsequent testing, conducted three months after treatment, will help evaluate the exposure or infection level of an individual once they are part of the herd.

Consider the risk of infection from new environments

Limit grazing when away from home, as the prevalence of tapeworm-infected oribatid mites at various locations is uncertain.

Consider the risk of wormer resistance

In cases of suspected anthelmintic resistance, it is advisable to consult a veterinary surgeon. They can perform a comprehensive risk assessment and consider methods to evaluate the effectiveness of wormers.