HELMINTHS: CHARACTERISTICS, DISEASES, REPRODUCTION, FEEDING - BIOLOGY - 2025

The helminths are composed of animals of two edges: flatworms and nematodes. These are characterized by being worms, although some are flat and others cylindrical. The term "helminth" means parasitic worm, and nothing more appropriate than this definition, since the vast majority of helminths are parasitic species, both of vertebrates such as man, and of plants.

These parasites are widely distributed throughout the world, especially in developing countries, where hygiene and sanitation conditions are poor.

Helminths. Source: Гөлнур

Because of this, they are responsible for causing pathologies such as taeniasis, cysticercosis, hookworms, and lymphatic filariasis in their hosts. Many of these diseases are serious and can lead to fatal consequences if not treated promptly.

characteristics

Helminths are a group of worms that belong to organisms called eukaryotes. The main characteristic of these is that all their cells have a cell nucleus. This is a structure within which the genetic material (DNA) is found that forms the chromosomes.

Likewise, these organisms are considered triblastic, since during their embryonic development they present the three germ layers: mesoderm, endoderm and ectoderm. These layers are of great importance, since it is from them that the different tissues that make up adult individuals are formed.

With regard to the presence of coelom, flatworms are acellomed, that is, they do not have an internal cavity. On the other hand, nematodes are pseudocoelomed, since they have an internal cavity called a pseudocele. It should be noted that this does not have its origin in the mesoderm.

This is a quite varied group of animals, being able to observe dioecious species and hermaphrodite species. Likewise, there are those who reproduce sexually with internal fertilization, and those who reproduce asexually. They can be oviparous and viviparous.

With regard to lifestyle, although it is true that the vast majority are parasites, there are also a few species that are free-living, preferably limited to aquatic habitats.

Taxonomy

The taxonomic classification of helminths is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Filo: Platyelminthes and Nematoda

Class: The phylum platyhelminthes includes three classes: Trematoda, Cestoda, Monogenea, and Turbellaria. The nematode phylum is made up of two classes: Adenophorea and Secernentea.

Morphology

Taking into account that the group of helminths is made up of organisms with two different phyla, the morphology of each one will be specified separately below.

Nematodes

Nematode specimen. Source: See page for author

Nematodes, also known as roundworms, have a cylindrical body, which is not divided into segments, but rather has a smooth surface, the outer layer of which is a cuticle with a resistant texture.

In these animals a marked sexual dimorphism is observed, with the females being much larger than the males. Likewise, the body of females ends in a pointed way, while that of males ends in a characteristic curvature.

Likewise, the male has spike-like structures at its rear end called spicules. These are used during copulation.

Flatworms

Specimen of Fasciola hepatica. Source: Veronidae

Contrary to nematodes, flatworms are known as flatworms. In general, its body is not segmented, with the exception of the Cestoda class parasites (such as tapeworms) that do have a body divided into segments or proglottids.

The length of flatworms is variable and this determines the shape that the animal can have. In the case of those that are of short length, they adopt a follicular or leaf shape, while in the worms that are of great length, they acquire a ribbon shape.

In flatworms there is no sexual dimorphism because most of the species found in this group are hermaphrodites, that is, they present both types of reproductive organs, female and male.

Classification

Nematodes

Nematodes are made up of two classes: Adenophorea and Secernentea.

Adenophorea

They are nematodes that are exclusively found in aquatic environments, whether freshwater or marine. Most of the species included here are free-living and therefore do not pose any type of danger or threat to man or any other vertebrate.

Secernentea

It is mainly made up of worms from terrestrial habitats and parasitic habits. Most of them parasitize the intestinal tract of vertebrates such as humans. Members of this class are worms in which the sexual dimorphism already mentioned is markedly evident.

Flatworms

Flatworms are a very large group of living things, which are mainly classified into four classes: turbellaria, cestoda, trematoda, and monogene.

Turbellaria class

Here the free-living flatworms are located, that is, those that do not lead a parasitic lifestyle, so they do not need to live inside other living beings to feed on them.

They are characterized because they are small (between 1 and 4 cm in length approximately), flattened dorsoventrally and found in both freshwater and saltwater habitats.

Sometimes, the peat bogs exhibit, on a physical level, a series of very striking colorful patterns.

Cestoda class

This class includes approximately more than 3,500 species. It is mainly made up of endoparasites of the intestinal tract of certain mammals such as humans, dogs and cats.

Its body is covered by a protective cuticle and it is also divided into proglottids, each with both female and male reproductive organs. They present indirect development and do not have digestive organs.

Class Trematoda

They are the famous "staves". They are parasites of reduced size, compared to other flatworms. Its body is also covered by a cuticle that gives it protection. Likewise, the body is not divided into segments or zones.

They also have two suction cups, one at the ventral level and the other in the oral area. Among the most representative species of this class are: Fasciola hepatica, Schistosoma mansoni and Schistosoma mekongi, among other known human pathogens.

Monogeneous class

It comprises a total of approximately 1000 species. It is made up of ectoparasites that normally attach to the body of vertebrates such as fish, reptiles and amphibians.

They are very small parasites, barely reaching 2 cm in length. Its body, covered with a protective cuticle, is not divided. They also have fixing organs, mainly suction cups, although they can also have hooks. Through these they fixate and keep united to the body of their hosts. Monogeneous do not parasitize humans.

Reproduction

- Flatworms

In the group of flatworms you can see the two types of reproduction that exist: asexual and sexual.

- Asexual reproduction

In this type of reproduction, the union or fusion of gametes is not necessary, as well as the interaction of two individuals of different sexes. Here, from a single individual, other offspring can originate, which will be genetically and phenotypically identical to the parent.

Flatworms reproduce asexually through a process known as fragmentation. This is that, from a fragment of an organism, it is possible for a complete individual to regenerate. This as long as the environmental conditions are ideal to promote it.

This fragmentation process occurs thanks to the fact that some of the flatworm cells retain a property called totipotency. This is defined as the ability of certain cells to transform into any type of cell, to form various tissues.

Well, when a flatworm suffers trauma, the cells of the free fragment begin to undergo a process of cell division and differentiation, which allows the different tissues that make up an individual to form again, until a full adult individual develops.

Sexual reproduction

Sexual reproduction is the most frequent type of reproduction among these animals. Although they are hermaphrodites, selfing is not seen very often.

During the reproduction process, there will always be one animal that will play the role of male and another that will play the role of female. In the case of the latter, the eggs that have been produced by your uterus are brought into a chamber called the ootype.

Here also the yolk cells converge, which have a large amount of nutrients that will contribute to the successful development of new individuals.

Later they are taken to the uterus, which is where they will meet the sperm. Because fertilization in these animals is internal, it is necessary for the animal that acts as a male to introduce its copulatory organ or penis, also known as cirrus, in order to release the sperm there.

Once this happens, the sperm fertilize the eggs, thus forming the eggs. Each individual is capable of producing an impressive number of eggs.

Now, depending on the class, there are some that have direct development, such as the bog and monogeneous. While there are others, such as trematodes and cestodes that present an indirect development, that is, they go through one or more larval stages.

- Nematodes

In the group of nematodes, both types of reproduction also occur: asexual and sexual.

Asexual reproduction

Although it is the least common form of reproduction, it is present in some species. Of all the types of asexual reproduction that exist, parthenogenesis occurs in nematodes.

In this type of reproduction, what happens is that the unfertilized ovules undergo various mitotic divisions until they become an adult individual.

Sexual reproduction

It is the most common type of reproduction among nematodes. Fertilization is internal. For this to happen, the male makes use of the spicules that he has at the rear end of his body to be able to introduce the sperm inside.

Once fertilization occurs, each female is capable of laying approximately thousands of eggs, which are released outside the host in her feces.

Feeding

The diet of these animals depends on the class and lifestyle they have. In the case of those that lead a free life, that is, they are not parasites, they are carnivores. These feed on small animals such as crustaceans or mollusks. A few species are herbivorous and feed on algae remains that are found attached to the rocks in their habitat.

On the other hand, there is another group of helminths that are parasites of vertebrates such as humans. These parasites sometimes do not have a digestive tract with which to properly assimilate food. This is why they feed through a diffusion process in which they absorb the nutrients that are ingested by their host.

Finally, there are helminths that attach directly to the host's intestinal wall and feed on the blood they absorb there, which is why they are considered hematophagous.

Transmitted diseases

Taeniasis

It is one of the most common parasites in humans. It is caused by cestodes of the genus Taenia, especially Taenia solium and Taenia saginata. The human being ingests the eggs of the parasite, either in contaminated water or in food washed with this water.

Because the parasite is fixed in the human intestine, the symptoms have to do with the digestive tract. These include: abdominal pain, stomach upset, weight loss (caused by the parasite absorbing nutrients that the host ingests), loss of appetite, and general malaise.

Elephantiasis

It is also known as lymphatic filariasis. It is caused by the obstruction of the lymphatic vessels by a nematode parasite known as Wuchereria bancrofti. This parasite requires, in its life cycle, a definitive host (human) and a vector, which is the one who inoculates them into the host. Among the most frequent vectors are mosquitoes of the genera Aedes and Anopheles.

The most frequent symptoms of this pathology are: fever and swelling or edema of some parts of the body such as upper and lower limbs, breasts or genitals. The skin that lines the swollen sites thickens and loses its natural elasticity. Likewise, intense pain is another of its characteristic symptoms.

Trichuriasis

It is a pathology caused by the helminth Trichuris trichiuria. The infection is acquired by ingestion of food contaminated by the parasite or by contaminated water. This parasite lodges in the host's intestine, causing serious damage there.

Among the most prominent symptoms are: high fever, mucous or bloody diarrhea, decreased appetite, nutritional deficiency and consequent anemia.

Trichuriasis is a very common infection that is widely spread throughout the world, especially in places where sanitary conditions are poor.

Cysticercosis

This is an infection that is also caused by flatworms of the genus Taenia, specifically by the so-called swine tapeworm. Humans ingest the parasite's eggs by eating poorly cooked pork. These eggs form cysts that attach to different tissues such as the brain and muscles.

The symptoms of this infection depend on the site where the cysts have lodged. However, they can be mentioned: symptoms similar to those that occur when there is a brain tumor, loss of vision, heart failure and general weakness, among others.

Schistosomiasis

Schistosoma mansoni specimen. Source: Leonardo M. Lustosa

It is an infection caused by parasites of the Trematoda class, specifically those of the genus Schistosoma. During its life cycle, it requires an intermediate host, a snail, which releases the parasite's eggs to the ground.

Humans become contaminated when the larvae that are present in the soil cross the skin barrier and enter the bloodstream. In the blood vessels they become adults and begin to produce eggs.

Among the symptoms that a person infected by this parasite presents, the following can be highlighted: bloody diarrhea, blood in the urine and abdominal discomfort. If the infection progresses without treatment, enlargement of the liver or spleen may occur.

Fascioliasis

This is an infection whose causative agent is fluke parasites of the genus Fasciola, with Fasciola hepatica frequently being the main causative agent.

The parasites are mainly housed inside the bile ducts. The symptoms of this infection are: very high fever, diarrhea, nausea, anorexia, anemia, accumulation of fluid in the abdomen (ascites), enlargement of the liver (hepatomegaly) and spleen (splenomegaly), as well as jaundice.

Hookworm

It is an infection caused by the nematodes Ancylostoma duodenale and Necator americanus. Humans acquire it when the larvae of some of these parasites penetrate the skin and enter the bloodstream. These parasites can cause multiple damage to various human tissues, through which they circulate.

Among the most characteristic symptoms of this pathology are: skin problems (itching, rashes, edema, diarrhea, nausea, frequent dizziness, anemia, paleness and loss of appetite, among others.

References

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