CESTODES: CHARACTERISTICS, REPRODUCTION, DISEASES, SPECIES - BIOLOGY - 2025

The cestodes are a class of flatworms (phylum Plathelmynthes) exclusively endoparasites. Their body is divided into three regions: a scolex located anteriorly, followed by a neck and later a strobilus that is made up of numerous proglottids.

The proglottids are segment-like sections of the body that are arranged linearly to form the strobilus. Each of them has both male and female gonads and are formed from the neck, or the scolex in some species that lack a neck.

Proglottids of the cestode Taenia saginata. Taken and edited from: Public Domain, The cestodes are divided into two subclasses: Cestodaria and Eucestoda. The former contains few species that have a flattened body and lack the scolex and strobilus, while the latter is much more diverse and groups organisms that present strobili and generally also scolex.

As an adaptation to parasitic life, these organisms completely lack a digestive system, including the mouth, and are completely dependent on the integument for food through diffusion and probably pinocytosis.

Some researchers also argue that various cestodes can absorb nutrients through the scolex at the site of attachment of the parasite to the host.

These parasites are commonly known as tapeworms and have complex life cycles with diverse hosts, but almost all species are parasites of the digestive tract of vertebrates.

Some species are of sanitary interest, since they can parasitize humans and cause various diseases, such as sparganosis and cysticercosis.

General characteristics

Cestodes are all endoparasitic organisms with a life cycle that includes several intermediate hosts and a definitive host that is generally a vertebrate. In the latter, they almost exclusively parasitize the digestive tract or its accessory organs.

The body of a cestode is divided into three regions: the scolex, the neck, and the strobilus. The scolex constitutes the cephalic region and generally has suckers to adhere to the host. It may also have a rounded, retractable, hooked, apical projection called a rostellum.

The neck is narrower than the scolex and the strobilus and constitutes the proliferative part of the parasite, that is, divisions called strobilations occur in this area (mitotic divisions followed by transverse constrictions) that give rise to each of the proglottids that make up the strobilus.

The proglottids are sections similar to segments or body somites that are arranged linearly forming the strobilus, which is tape-shaped. Each of these sections features male and female sex organs.

The cestodes lack eyes and the main sensory organs are touch receptors that are located on the scolex.

They also lack a mouth, digestive system, and respiratory organs. The exchange of gases and the obtaining of nutrients occur through the skin. The cestode tegument presents projections from the plasma membrane, called microthric.

Taxonomy and classification

Cestodes are a class (Cestoda) of flatworms of the phylum Plathelmynthes, subphylum Rhapditophora, and the superclass Neodermata. This superclass was erected by Ehlers in 1985 to group flatworms that share characteristics related to the excretory apparatus and epidermal cilia, as well as a free-living larval stage.

The Cestoda class is subdivided into two classes, the Cestodaria and the Eucestoda. The former groups cestodes that lack scolex and strobilus, while suckers are only present in some species. They share a larva called lycophor that is characterized by having ten hooks.

It is a small group of parasites mainly of cartilaginous fish and bony fish, and exceptionally of turtles, which lodge in the digestive tract or in the cavity of the coelom of its host.

On the other hand, the Eucestoda have almost all scolex and strobilus and their first larva, called oncosphere or hexacanto, only has six hooks. In their adult stage they parasitize the digestive tract of different species of vertebrates and generally their life cycle involves one or more intermediate hosts.

Reproduction

Cestodes are all hermaphroditic organisms that can reproduce by cross-fertilization and in many cases also by self-fertilization. Each proglottid of the cestodes has its own complete reproductive system, including the male and female gonads.

In the case of cestodaries, which lack strobilus, there is only a complete reproductive system.

The proglottids are formed in the back of the neck by a series of mitotic divisions that are followed by the formation of a septum or septum by a transverse constriction. As new proglottids form, they cause the older ones to move towards the posterior end of the strobilus.

The latter gradually increase in size and degree of maturity as they move. When they reach maturity, cross-fertilization can occur with the proglottids of another individual (cross-fertilization), with others of the same strobilus, and it can even occur within the same proglottid (self-fertilization).

The testes are numerous, while the ovaries generally occur in pairs on each proglottid. The uterus, for its part, is a blind and branching sac.

During copulation, the copulatory organ (cirrus) evacuates and is introduced into the vaginal orifice of another proglottid and releases its sperm, which will be stored in the female seminal receptacle. Cross-fertilization can occur in more than one proglottid at a time, while self-fertilization occurs in only one.

The fertilized ovules and the resulting eggs are stored in the uterus, where their capsule hardens and development begins. The gravid proglottids separate from the strobilus and release the eggs inside the host, or once they are deposited outside with the feces.

Nutrition

Cestodes do not have a mouth and a digestive tract, so they depend almost completely on the digestive processes of their host to obtain predigested food substances.

Food enters your body through the skin through diffusion, active transport, and perhaps also pinocytosis. To increase the ability to exchange substances via the integument, it is provided with microthric.

Microvilli are projections towards the outside of the outer plasma membrane of the integument, forming a kind of microvilli that increase their exchange surface. The release of digestive enzymes by the parasite's tegument may also occur to reduce the size of the food particles.

According to some researchers, the scolex may or may not participate in the absorption of food, depending on the species, probably through a mechanism of pinocytosis at the point of attachment of this to the digestive wall of its host.

Histological section of the cestode Sparganum proliferum. Taken and edited from: Photo Credit: Content Providers (s): CDC /.

Diseases

Some species of cestodes have the human being as the natural definitive host during their life cycle, while on occasions it can also happen that other species accidentally parasitize it. Others, in turn, use it as an intermediate host. All these organisms can cause different diseases, being able to mention:

Sparganosis

It is a disease that is difficult to detect and difficult to treat, since the diagnosis is generally only achieved after the surgical removal of a tumor caused by the parasite. In addition, anthelmintic treatments are not successful in eliminating it.

The causal agents of the disease are tapeworms of the species Sparganum proliferum and by various representatives of the genus Spirometra. These organisms use copepods (procercoid larvae), fish, reptiles, amphibians, or mammals (espargano or plerocercoid larvae) as intermediate hosts. Occasionally these larvae can infest humans.

Infestation can occur by accidental ingestion of infested copepods present in contaminated waters, by consuming raw or poorly cooked meat from infested intermediate hosts, and even by contact with contaminated material (meat, feces).

These tapeworms cause tumors in various parts of the body, even the nervous system can be compromised, although this occurs very rarely. In the Far East it is common to infest the eye due to the custom of using frog plasters (frequent host of the parasite) in traditional medicine.

Cystocerciasis

Disease caused by the cysticercus larvae of Taenia solium. Generally the form of infestation is through the ingestion of food contaminated with feces of infested people. It can also be acquired by eating pork infested by poor cooking.

Self-infestation of an individual carrier can also occur through anus-hand-mouth contact. Symptoms of the disease can vary depending on numerous variables, including the number of parasites, their location, and the host's immune response.

When it affects the nervous system, it causes motor deficits, involuntary movements and even epilepsy. Hydrocephalus can also occur, when cysticerci are located in the cerebral hemispheres. In this case, the disease is generally fatal.

The most effective treatment to date is albendazole, however some forms of cystocerciasis cannot be treated with cestocides, because the death of the larvae can exacerbate the symptoms and lead to an extensive inflammatory reaction around the dead parasites.

Hydatidosis

Disease produced by the hydatid larva of organisms of the genus Echinococcus, mainly of the species E. granulosus. This species uses dogs and wolves as its definitive hosts.

The infestation generally occurs by consumption of water or food contaminated with feces from infested hosts. The parasite forms cysts inside the secondary host, whose location and dimensions are variable and condition the symptoms of the disease.

The most common hydatid cysts are liver cysts, which can press on the bile duct. Lung cysts can cause chest pain or trouble breathing.

The main treatment for the disease consists of the surgical removal of the cyst, and if it is inoperable, administration of drugs such as mebendazole and albendazole, which present variable results in the eradication of these cysts.

Intestinal taeniasis

Intestinal infestation produced by cestodes of the species Taenia saginata and T. solium, frequent parasites of cattle and pigs. The form of contagion is the ingestion of raw or undercooked meat of carrier organisms.

The parasite attaches itself to the mucous membranes of the small intestine using its scolex, generally only one parasite is attached per host, which can asymptomatically parasitize the host or cause abdominal pain and nausea. On some occasions complications can occur from accidental proglottid migrations that can trigger cholangitis or obstructive appendicitis.

Treatment with praziquantel can kill adult worms. Niclosamide has also been effective in treating the disease.

Featured species

Taenia solium

This species of worldwide distribution is characterized by presenting a scolex of a few millimeters, provided with four suction cups, a whitish coloration and a rostellum armed with a double crown of hooks. This species can reach several meters in length.

Each proglottid can contain between 50,000 and 60,000 spherical eggs that can reach up to 45 micrometers in diameter and have various membranes. They produce a hexacanto larva, which receives that name because it has six hooks.

The biological cycle of this species includes the pig as an intermediate host. When the pig ingests material contaminated with feces from a human carrier, the hexacanthus larva or oncosphere hatches into a cysticercus larva that will infest the musculature and central nervous system of its host.

If the man ingests infested pork raw or undercooked, the cysticercus larva attaches itself to the walls of the intestine and grows until it is sexually mature, thus completing its life cycle.

Hymenolepis nana

Hymenolepis nana adult organisms. Taken and edited from: Image contributed by the Georgia Division of Public Health..

It is the most prevalent cestode. It reaches a maximum of 40 mm and can present a complex biological cycle, with cockroaches and rodents as intermediate hosts, or directly infest humans.

When the cestode eggs are ingested by cockroaches they hatch into a larva known as cysticercoids. The feces of infested cockroaches, as well as the feces of humans or rodents, can contaminate the food, which if ingested by mice or humans is transformed into cysticerci that will mature and produce an adult tapeworm.

Among the symptoms of hymenolepiasis, which is how the disease produced by this tapeworm is known, are for example eosinophilia, agitation, insomnia, irritability and even epileptic seizures. The disease can be treated with niclosamide or praziquantel.

Echinococcus granulosus

Known as the dog tapeworm, this species is characterized by presenting a scolex with four suction cups and a rostellum crowned with a double row of hooks whose number can reach up to 50, although the common is that it has 30 to 36 hooks. The strobilus is made up of up to 5 proglottids and does not exceed 6 mm in length.

This species uses the dog as its definitive host, as well as sheep and goats as intermediate hosts. It can accidentally use other species such as cattle, pigs, deer, rodents and even humans as intermediaries.

In humans, it is the causal agent of hydatidosis or hydatid cyst.

References

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6. Taenia solium. On Wikipedia. Recovered from: en.wikipedia.org.
7. Hymenolepis nana. On Wikipedia. Recovered from: en.wikipedia.org.
8. Echinococcus granulosus. On Wikipedia. Recovered from: en.wikipedia.org.