Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavity, bilaterally symmetrical hair-shedding animals.
Don't spread. Nematodes are one of the most widespread species of animals that can colonize different habitats - from the interstitium (the space between sand grains) and moss communities to arctic ice (for example, nematodes).Theristis Melnikovi and Cryonema crissum, found in the thickness of perennial ice in the central part of the Arctic Ocean).Parasitic nematodes are of particular interest to researchers, including host diversity.
Construction plan. Slender fusiform stem, tapering towards the tips, round in cross-section.The mouth is at the front, and the anus is at the back.The outer part of the body is covered with a multi-layered elastic cuticle - a non-cellular formation secreted by the hypodermis.The hypodermis or epidermis is located under the cuticle.Muscles are represented by a layer of longitudinally curved striated muscle fibers.The primary body cavity (schizocoel), devoid of its own epithelial lining, is filled with fluid.
Digestive system. The oral cavity at the front end of the body is surrounded by protrusions - lips (usually three) and leads to a muscular ectodermal pharynx with a triangular lumen.The pharynx leads from a single layer of columnar epithelial cells to the endodermal midgut.Then comes the short ectodermal hindgut that opens to the anus.
Excretory system. The excretory organs are unicellular glands that replace the protonephridia.In the front part of the body, there is usually a cervical gland, from which a short excretory duct arises.There are also "storage kidneys", which are phagocytic organs that collect insoluble metabolic products that are not removed from the body.
Circulatory and respiratory systems. These systems do not exist.Respiration occurs through the skin.Anaerobic metabolism is also possible (anaerobic breakdown of glycogen into butyric and valeric acids in parasites).
Nervous system. The nervous system is scalariform type.It is represented by one nerve ring and six longitudinal trunks.The two nerve trunks running along the ventral and dorsal lines are stronger and are connected by semicircular nerve bridges (commissures).
Sense organs. There are tactile organs located around the mouth - papillae and setae.Some marine representatives have primitive eyes - pigment spots.Chemical sense organs, amphids, usually have a pocket, spiral or slit shape.They are located on the sides of the tip of the head and are especially well developed in males, because they help in finding females.
Reproduction and development. Nematodes are dioecious animals.Internal genital organs are paired and tubular.Reproduction is only sexual.Sexual dimorphism is pronounced: females are larger, males have a curved back end of the body.Fertilization is internal and vitality occurs.In development, nematodes go through four larval stages, separated by molting, which is accompanied by shedding of the cuticle.In some species, the third stage (including the famous Caenorhabditis elegans) under unfavorable conditions, it passes into the so-called dauer stage - a resting larva.
Parasitism. Of the more than 24,000 nematode species currently described, about half are parasitic.They can affect almost all tissues and organs: connective tissues, muscles, blood and lymphatic vessels, sex glands, sensory organs, as well as the body cavity, etc.Among them are both ecto- and endoparasites of plants, vertebrates and invertebrates, including other nematodes, and even protozoa.
Below is a description of the most important representatives of roundworms from the point of view of medical parasitology.
Human roundworm(Ascaris lumbricoides)
Appearance.The body is pinkish-white, marked at the ends.Sizes: men - 15-25 cm, women - 20-40 cm.The body is covered with a ten-layer flexible cuticle that protects it from mechanical stress and the host's digestive enzymes.
Don't spread. The species is cosmopolitan - distributed everywhere, but different countries have different percentages of infected people.For example, in Japan, more than 90% of the population is infected with roundworms due to the use of human feces as fertilizer.Roundworms are less common in areas with hot, dry climates.
Life cycle.Development continues without changing owners.Adult worms parasitize the small intestine and cause ascariasis.A person is usually affected by several dozen roundworms (the record is 900).Its life in the intestines is about one year.Roundworms, like other nematodes, are dioecious.A sexually mature female lays about 200,000 oval-shaped eggs per day and is released into the external environment with feces.Roundworms are classified as geohelminths - they require larval stage development in soil.When exposed to favorable conditions (at a temperature of about 25 ° C and moist soil with sufficient access to oxygen), the larva develops in the egg.The development period varies from 16 days to several months and depends on the air temperature.Such eggs with larvae can be considered invasive.
Infection occurs when eggs are ingested with food or water;transmission does not occur directly from person to person.In the intestine, the larvae pass through the intestinal wall, enter the blood vessels and liver, then migrate through the inferior vena cava to the right atrium and right ventricle.From the latter, the larvae move through the pulmonary circulation to the lungs, where they pass from the blood to the pulmonary vesicles, bronchi, trachea and oral cavity.Secondary infection occurs in the oral cavity: the larvae are swallowed, enter the intestines and become sexually mature after three months.In nematodes, the "growth" process is associated with molting (usually four of them).
Clinical picture of ascariasis. During the migration stage of ascariasis, cough (helps the larvae enter the throat), chest pain, allergic reactions, and fever are observed.
In the intestinal stage, the intestinal mucosa is damaged and the body is poisoned with toxic metabolic products.Symptoms: nausea, vomiting, stool disorder, loss of appetite.
Long-term effects of infection: general decrease in performance, sleep disturbance.When worms crawl into the bile ducts and respiratory tract, the result is fatal.Also, roundworm larvae can enter the brain (for example, from the inferior vena cava to the superior vena cava, then along the brachiocephalic vein), causing meningoencephalitis with migraines.
Prevention. Wash hands before eating and preparing food.Washing vegetables and fruits.Eggs are also carried by flies, so control of these dipterans, using, for example, Velcro, also prevents ascariasis.
Interesting fact. There are studies showing positive effects of roundworm infection in relieving symptoms of autoimmune diseases and increasing fertility in women.Scientists attribute this to the parasites influencing the immune system by affecting the level of T cells in the body, but the mechanism is currently too poorly understood to draw reliable conclusions.
pinworm(Enterobius vermicularis)
Appearance. Grayish-white nematode, length of males 2-5 mm, length of females 8-14 mm.The tail is pointed (hence the name).A characteristic swelling of the esophagus is noticeable at the front end of the body.
Life cycle.Pinworms cause enterobiasis by parasitizing the lower part of the small intestine and large intestine.Life time is 1-2 months.The front end of the pinworm attaches to the intestinal wall.A sexually mature female crawls through the large intestine through the anus, lays 5-15 thousand eggs on the skin near the anus, after which she dies.
Female crawling is accompanied by itching.When scratching the skin, the eggs are transferred to the hands and more.Flies also participate in the transfer of eggs.Infection occurs by ingestion.Larvae hatch from eggs that enter the intestines.
Epidemiology and clinical picture of enterobiosis. Enterobiosis is widespread, especially in children, due to non-compliance with the rules of personal hygiene, "overcrowding" in kindergartens and schools.It is transmitted from person to person without an intermediate host.Reduces the effect of vaccines.
Symptoms: abdominal pain, loss of appetite, headaches, allergic manifestations, perianal itching (leads to sleep disturbance, increases irritability).
Trichinella(Trichinella spiralis)
Description.A small nematode 2-4 mm long.It parasitizes the mucous membrane of the small intestine.It is distributed in Eurasia and North America.
Life cycle. A change of host is necessary for the development of Trichinella.Usually these are wild animals (fox, wolf, bear, boar), as well as people and livestock.Females attach to the intestinal epithelium with the front end of the body and give birth to 1-2 thousand larvae.Ovoviviparity is typical: hatching of larvae occurs in the female genital organs.Larvae spread throughout the body through blood and lymphatic vessels and reside in striated muscles.At this stage, there is a stylet, which they use to destroy muscle tissue, which causes the host to form a capsule in which they are located in the future.After several months, the capsule is soaked with lime.Such muscle trichina can exist for several years and can survive even after the death of the owner and the dismemberment of his body.
Once in the stomach of the new host (after eating the previous corpse), the larvae are released from the capsule, penetrate the mucous membrane and undergo four molts over several days to become adult worms.
Clinical picture of trichinosis. Temperature increase, facial swelling, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted through food through infected meat.Therefore, to prevent the disease, the meat must undergo a veterinary examination and be properly prepared - it must be boiled for 2-3 hours.Cooking methods such as smoking and salting do not destroy Trichinella.
He built a whip(Trichocephalus trichurus)
Appearance.The worm is whitish in color, about 4 cm long.The front end is thin, reminiscent of hair (hence the name).
Don't spread.They prefer countries with a humid and warm climate.
Life cycle.The worm is a parasite in the primary part of the large intestine, only in humans.It causes trichuria.A person's life span is several years.The thin tip penetrates the thickness of the mucous membrane of the intestinal wall.It feeds on tissue fluid and blood.
The female lays 1-3 thousand eggs, which are released into the environment with feces.Like the roundworm, the whipworm is related to geohelminths: the eggs need to remain in the soil at a certain humidity and temperature (25-30 ° C) for a month to become invasive.After that, infection occurs when the eggs are swallowed;the larvae emerge from them in the host's intestines, penetrate the intestinal villi and grow within them for about a week.Then, destroying the villi, it goes out into the lumen of the intestine, reaches the large intestine, settles there and matures within a month.
Clinical picture of trichocephalosis. The worm damages the lining of the large intestine and causes poisoning of the host by its waste products.Whipworm is a hematophagous, so it can cause anemia.Trichocephalosis is accompanied by abdominal pain, headache and dizziness.Because the whipworm sticks to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
Appearance.A slender whitish nematode, the length of females is 30-120 cm, the length of males is no more than 4 cm.There is a small spine on the tail.
Distribution: Tropical countries of Asia and Africa.
Life cycle.Infection occurs when drinking unboiled water containing copepods.Crustaceans in the stomach die under the influence of hydrochloric acid, but the larvae of the guinea worm survive and spread throughout the body through the lymphatic system.Then they penetrate the body cavity, where they molt and reach sexual maturity.After mating, the male dies, and the female moves to the subcutaneous tissue, where a purulent abscess is formed, accompanied by burning and pain.Cold water is best for pain relief.
The development of eggs forces the woman to move her "head" towards the surface of the skin, leaving an inflammatory process along the way, which turns into a purulent abscess and then bursts.When the female's uterus enters the water, it splits open and the hatched larvae emerge.To ensure that development is not interrupted, the larvae must infect the intermediate host cyclops crabs.The larvae that remain in the water die.After the crustaceans are swallowed by the definitive host, the crustaceans dissolve under the influence of stomach acid and the larvae easily enter the intestine, pass through its walls and fall into the lymph nodes, and the development cycle continues.The disease caused by the guinea worm is called dracunculosis.
Dracunculosis.The incubation period lasts up to nine months and ends when the female reaches puberty.At this time, purulent abscesses begin to form in a person already ill with dracunculosis.The only escape from pain is a pond.Relief is immediate, but on contact with water the bubbles burst and release the guinea worm larvae into the water.Crustaceans eat them and the life cycle begins again.
When treating dracunculosis, an incision is often made at the blister site, and the worm is gradually removed by wrapping it with a stick.This takes days, sometimes weeks (the worm must be removed slowly and carefully so as not to break).It is suggested that the appearance of a guinea worm wound around a stick becomes a kind of prototype for the symbol of medicine - the staff of Asclepius entwined with a snake.
Bancroft thread (filaria) or Bancroft string(Wuchereria bancrofti)
Appearance.White thread nematode, females 10 cm long, males 4 cm long.
Distribution. Tropics, subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually occur in the lymph nodes and vessels, obstructing lymphatic drainage and causing persistent swelling.Female larvae - produce nocturnal microfilariae that appear in the peripheral blood at night and enter the body deep (pulmonary vessels and kidneys) during the day.This is due to the fact that the intermediate host is usually blood-sucking mosquitoes in the evening and at night.The larvae enter the mosquito's stomach, then the body cavity, where they grow, after which they collect near the proboscis and pass to humans by sucking blood.Bancroft's filaments lead to sera, or sera, or sera.It should be noted that this disease can be caused by other nematodes.
Clinical picture and treatment of the elephant. The growth of any part of the body occurs due to hyperplasia (painful growth) of the skin and subcutaneous tissue, which is caused by the inflammatory thickening of the walls of the lymphatic vessels and lymphatic stagnation caused by the blockage of the lymphatic vessels by individual filaments of Bancroft.The skin of the diseased part of the body is covered with ulcers.
Treatment of the elephant is aimed at improving fluid flow.The use of anthelmintic drugs is effective.Later stages may require surgery.
