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Nematodeases: general characteristics of nematodes
Last reviewed: 23.04.2024
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Nematodoses are diseases caused by parasitic roundworms of nematodes. They are common on all continents. About 3 billion people are invaded by nematodes in the world.
Nematodes have an elongated, cylindrical body shape. The cross section of the body is round. Nematode sizes range from 1 mm to 1 m or more.
Outside, the nematodes are covered with a skin-muscular sac formed by the cuticle, the hypodermis and one layer of longitudinal muscles. The cuticle is multilayered, it serves as an outer skeleton, protects the body of nematodes from mechanical damage and chemical influences. Under the cuticle lies the hypodermis, which is a symplast and consists of a layer underlying the cuticle - subcuticum and longitudinal ridges, the number of which varies from 4 to 16 or more. In the hypoderm, metabolic processes are active and intensive biosynthesis takes place. Under the hypodermis is a single layer of longitudinal muscles, separated by hypodermic beads on several longitudinal bands. Movement of nematodes is limited. The body flexes only in the dorsoventral plane due to the fact that the abdominal and spinal muscle bands act as antagonists. Inside the skin-muscular sac is the primary body cavity, which does not have a special lining, containing the cavity fluid and internal organs. The cavity fluid is under great pressure, which creates a support for the musculature (hydroskeleton) and plays an important role in metabolic processes. In some nematodes this liquid is toxic.
Digestive, excretory, nervous and reproductive systems are well developed. Respiratory and circulatory system absent.
[Digestive system of nematodes
The digestive system is represented by a straight tube, which is divided into three sections - anterior, middle and posterior. It begins with a mouth opening located at the anterior end of the body. In most nematodes, the mouth is surrounded by three lips. Some species have a mouth capsule, armed with teeth, plates or other cutting elements. The mouth follows the pharynx and the cylindrical esophagus, which in some species has one or two extensions (bulbuses). Behind the esophagus is the middle gut, which passes into the back, ending with the anus. In some species of nematodes there is no anus.
Excretory system of nematodes
The excretory system is represented by 1-2 unicellular cutaneous glands replacing protonephridia. From the gland 2 long lateral canals extend along the entire body of the nematode in the lateral ridges of the hypodermis. Behind the canals end blindly, and in the front part they join into one unpaired canal, which opens outward, sometimes near the front end of the body. Nematodes have special phagocytic cells (1-2 pairs) in which various insoluble metabolic products are retained and accumulated. They are located in the body cavity along the lateral excretory canals in the anterior third of the body.
Nervous system of nematodes
The nervous system is represented by the peripheral nerve ring surrounding the anterior part of the esophagus. From the ring, the nerve trunks move forward and backward. Forward are 6 short nerve branches. Backward, too, are 6 trunks, among which the most powerful dorsal and ventral, the hypoderms passing in the ridges. The two main nerve trunks are joined together by numerous commissures, which look like thin semirings that encircle the body alternately on the right and then on the left. The sensory organs are poorly developed. There are organs of touch and chemical sensation.
Sexual system of nematodes
Nematodes are dioecious and have an external sexual dimorphism. Females are larger than males. In some males, the posterior end is twisted to the ventral side. The male has one tubular testis, which passes into the vas deferens, followed by an ejaculatory duct opening into the posterior part of the intestine. The males have a cloak. Near the cloaca in the males are the copulatory spicules. In some nematodes, males have, in addition to the spicules, a copulatory bursa, which is an enlarged and flattened wing-shaped lateral part of the posterior end of the body.
In females, the reproductive system is paired, tubular, consisting of ovaries, oviducts, queens and vagina. The narrowest, blindly closed sections of the tube are the ovaries. They gradually move into the broader departments that perform the functions of the oviducts. The widest sections of the uterus - join together and form an unpaired vagina, which opens outward on the ventral side in the anterior third of the body of the nematode. Nematodes are characterized by sexual reproduction and internal fertilization.
The development of nematodes
Most nematodes lay eggs, but there are viviparous species. The formation and maturation of larvae occurs most often in the external environment. In some species, the development cycle can end in one host organism. In most species, the larva develops in the egg before the invasive stage in the external environment and leaves it in the intestine of the host that swallowed the egg. In the process of development, the larvae moult several times.
Larvae, from a number of nematodes, leaving the egg in the environment, are able to lead a free life in the soil. There are rhabdite-shaped and filariform larvae. The rhabdite larvae have two extensions (bulbus) in the esophagus, and in the filarial larvae they have a cylindrical esophagus. Larvae can actively penetrate the skin of the host, and not just get through the mouth.
Cycles of nematode development are diverse. Most nematodes are geogelmints. Their development occurs in a direct way, without a change of owners. For the larvae of many geogelminthes, migra- tion along the organs and tissues of the host to the site of final localization, where they reach puberty, is characteristic. Some geohelminths develop without migration of larvae. Geohelminths that infect humans can not parasitize animals. Nematodoses caused by these helminths are referred to as anthropogenic diseases. Other types of nematodes are classified as biogelminthes. Their development takes place indirectly. They need an intermediate host. They can be blood-sucking insects, crustaceans, or the same organism serves consistently first final, and then an intermediate host.
Infection of a person with nematodes-biohelminths occurs both alimentary by eating an intermediate host, and as a result of their transfer by a carrier.
The majority of nematodes parasitizing in humans live sexually in the human digestive system. Some have localization in the lymph nodes and vessels, in the connective tissue, under the skin of the extremities, in the subcutaneous fatty tissue.