Anoplocephala and Paranoplocephala are two genera of parasitic tapeworms that have horses, donkeys and other equids (mules, zebras, etc.) as final hosts. Prevalence shows regional and seasonal variations. Studies in Norway showed up to 27% and in Brazil up to 21% of the investigated horses being infected. Infections peaked usually during summer and early autumn. These tapeworms are particularly frequent in horse farms.
Both genera belong to the tapeworm family of Anoplocephalidae and are closely related to Moniezia spp, another tapeworm genus that affects mainly ruminants (cattle, sheep, goats, etc).
The species most important for horses are:
- Anoplocephala magna = Anoplocephala plicata, occurs worldwide, rather frequent in the USA but rare in many European countries.
- Anoplocephala perfoliata, occurs worldwide.
- Paranoplocephala mamillana, occurs worldwide.
These worms do not affect cattle, sheep, goats, pigs, dogs or cats.
The disease caused by these tapeworms is called anoplocephalosis.
Are horses infected with Anoplocephala tapeworms contagious for humans?
- NO: There is no evidence that these tapeworms can infect humans.
Final location of Anoplocephala and Paranoplocephala tapeworms
The predilection sites of adult Anoplocephala tapeworms in their final hosts are:
- the small intestine for Anoplocephala magna (occasionally in the stomach) and Paranoplocephala mamillana (mainly jejunum and ileum).
- the large intestine (ileocaecal valve) for Anoplocephala perfoliata.
Anatomy of Anoplocephala and Paranoplocephala tapeworms
Anoplocephala magna is the largest tapeworm infecting horses and can be up to 80 cm long and 2.5 cm wide. The head (scolex) is 4 to 6 mm long and has 4 suckers pointing forward, but no hooks. Adult Anoplocephala perfoliata are 8 to 30 cm long, 0.8-1.5 cm broad and has lappets (swellings) behind the 4 head suckers. Adult Paranoplocephala mamillana are smaller, only 1-5 cm long and ~0.5 broad, with slit-like rather than round suckers pointing sidewards.
The main body (or strobila) has hundreds of segments (called proglottids). The segments are much broader than long. As in all tapeworms, each segment has its own reproductive organs of both sexes (i.e. they are hermaphroditic) and excretory cells known as flame cells (protonephridia). The reproductive organs in each segment have a common opening called the genital pore. In young segments all these organs are still rudimentary. They develop progressively, which increases the size of the segment as it is pushed towards the tail. Mature gravid segments are full of eggs (several thousands) and detach from the strobila (i.e. the chain of segments) to be shed outside the host with its feces. Otherwise, as other tapeworms, they have neither a digestive tube, nor a circulatory or respiratory systems. They don't need them because each segment absorbs what it needs directly through its tegument. Individual gravid segments in the feces are visible by the naked eye.
Anoplocephala magna looks very much like a typical tapeworm, whereas Anoplocephala perfoliata looks rather like a fluke because the segments are crowded.
The eggs have a thick and rough envelope and an irregular, polygonal or semicircular shape. They have a characteristic pear-shaped structure, the pyriform apparatus that contains the hexacanth embryo. Those of Anoplocephala magna are ~50x60 micrometers, those of Anoplocephala perfoliata ~65-80 micrometers, and those of Paranoplocephala mamillana ~35x50 micrometeres.
Life cycle and biology of Anoplocephala and Paranoplocephala tapeworms
As all tapeworms, Anoplocephala and Paranoplocephala have an indirect life cycle with equids (horses, donkeys, mules, zebras, etc.) as final hosts, and oribatid mites (also called "moss mites" and "beetle mites") as intermediate hosts.
The adult worms lay eggs already in the intestine of their final hosts that are shed with the feces, or the gravid segments containing the eggs are shed out and release the eggs only outside the host. The eggs are sticky and adhere to the vegetation or soil particles. Depending on the species and the region they can survive for months in the environment and some may survive cold winters, but they are very sensitive to dessication.
The oribatid mites ingest the eggs, which hatch in their gut and develop to cysticercoids in the body cavity of the mites within 2 to 4 months. They are infective for the final hosts. Cysticercoids can survive for months inside the mites, which on their turn have a live span of up to 18 months.
The final host becomes infected after ingesting contaminated mites while grazing. The mites are digested and release the cysticercoids that attach to the gut's wall and develop to adult tapeworms within about 6 weeks.
The prepatent period (time between infection and first eggs shed) is about 6 to 10 weeks, suggesting that more than one generation can follow during a normal season in regions with a warm and humid climate.
Harm caused by Anoplocephala and Paranoplocephala tapeworms, symptoms and diagnosis
Anoplocephala infections are usually benign for adult horses and usually do not cause clinical signs. Nevertheless, they compete for nutrients with the host and their presence can negatively affect performance. Foals are more susceptible to suffer from infection. Attachment points to the mucosa are often eroded, irritated, abscessed or ulcerated and become easily infected with bacteria. Massive infections can cause colic, diarrhea, and weight losses. Intestinal perforation, peritonitis and anemia have also been reported. Anoplocephala magna can cause enteritis (hemorrhagic or ulcerative), and Anoplocephala perfoliata is known to cause fatal intestinal blockage when large number of tapeworms cluster in the ileocaecal area.
Paranoplocephala mamillana infections are seldom pathogenic.
Diagnosis is based on fecal examination. However, rather few eggs are shed with the feces, which may result in false negatives. Gravid segments (proglottids) may be also found in the feces. After necropsy the tapeworms are easily seen inside the gut and can be determined by their macroscopic characteristics.
Prevention and control of Anoplocephala and Paranoplocephala infections
It is not possible to eliminate the oribatid mites in the pastures. The use of insecticides for this purpose is not advisable, because it is more expensive than the potential economic loss due to the infections, and because it detrimental effect on the environment: it would kill not only the oribatid mites, but numerous beneficial insects as well.
In endemic zones with high incidence it is recommended to harvest the hay, to deeply plow the fields (the mites tend to burrow deeply in the soil) and to reseed them. This can reduce the mite population. Nevertheless, some mites will survive in the unplowed borders and will re-infect the pastures in a few years.
Since the mites prefer humid pastures and avoid light as well as dryness, they are more active early in the morning and at nightfall. This can be considered for deciding where and when to bring hoses for grazing.
So far no true vaccine is available against these tapeworms. To learn more about vaccines against parasites of livestock, horses, and pets click here. Very little is known regarding the immune response of horses to Anoplocephala or Paranoplocephala infections and whether horses acquire or not significant levels of immunity after repeated exposure.
Biological control of Anoplocephala or Paranoplocephala tapeworms (i.e. using its natural enemies) or their intermediate hosts (oribatid mites) is so far not feasible.
You may be interested in an article in this site on medicinal plants against external and internal parasites.
- Praziquantel, an isoquinoline is fully effective against Anoplocephala spp and Paranoplocephala mamillana at the usual dose. It is the most used cestodicide for horses and pets, often in mixtures with other anthelmintics (e.g. ivermectin, moxidectin, etc.)
- Pyrantel (mainly pamoate salt), a tetrahydropyrimidine is effective against Anoplocephala spp, but ~80% at the usual dose (6.6 mg/kg). At twice the usual dose efficacy is >90% and more reliable. It is not effective against Paranoplocephala mamillana.
- Broad-spectrum benzimidazoles (e.g. fenbendazole, oxfendazole) are also effective against tapeworms, but are not approved against Anoplocephala or Paranoplocephala tapeworms in many countries.
- Niclosamide, a salicylanilide is also effective against Anoplocephala at the usual dose, but this compound has been abandoned in numerous countries.
Macrocyclic lactones (mainly ivermectin, moxidectin) are not effective against any tapeworm species, but numerous equine anthelmintics contain mixtures of these compounds with a cestodicide (mainly praziquantel).
For use on horses most of these products are available for oral administration as pastes or gels.
Resistance of Anoplocephala and Paranoplocephala tapeworms to anthelmintics
So far there are no reports on resistance of these tapeworms to anthelmintics.
This means that if a product fails to achieve the expected efficacy, chance is very high that it is not due to resistance but to incorrect use, which is the most frequent cause of product failure. Or the product was unsuited for the control of Anoplocephala or Paranoplocephala tapeworms.
Ask your veterinary doctor! If available, follow more specific national or regional recommendations for Anoplocephala or Paranoplocephala control.