Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavities, bilaterally symmetrical moulting animals.
Expansion. Nematodes are one of the most widespread species of animals that have been able to colonize different habitats – from interstitium (the space between sand grains) and moss communities to arctic ice (e.g.Theristis Melnikovi and Cryonema crissum, found in the thickness of multi-year ice in the central part of the Arctic Ocean).Parasitic nematodes are of particular interest to researchers, including because of the wide range of their hosts.
Construction plan. Thin spindle-shaped body, tapering towards the ends, with a round cross-section.The mouth is at the front end, and the anus at the back.The outside 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 below the cuticle.Muscles are represented by a layer of longitudinal obliquely striated muscle fibers.The primary body cavity (schizocoel), devoid of its own epithelial lining, is filled with liquid.
Digestive system. The oral opening at the front end of the body is surrounded by protrusions - lips (usually three) and leads into the muscular ectodermal pharynx with a triangular lumen.The pharynx leads to the endodermal midgut from a single layer of columnar epithelial cells.Then comes the short ectodermal hindgut, which opens into the anus.
Excretory system. The excretory organs are unicellular glands that replaced the protonephridia.In the front part of the body, there is usually one cervical gland from which a short excretory duct emerges.There are also "storage kidneys" - phagocytic organs that accumulate insoluble metabolic products that are not removed from the body.
Circulatory and respiratory systems. These systems are missing.Breathing takes place through the skin.Anaerobic metabolism is also possible (anaerobic breakdown of glycogen into butyric and valeric acid in parasites).
Nervous system. The nervous system is of the scalariform type.Represented by a nerve ring and six longitudinal trees.The two nerve trees that run along the ventral and dorsal lines are stronger and are connected by semicircular nerve bridges (commissures).
Sense organs. There are papillae and setae - touch organs located around the mouth.Some marine representatives have primitive eyes - pigment spots.The chemical sense organs, amphids, are usually pocket, spiral or slot shaped.They are located on the side of the head and are especially well developed in males, as they help in finding females.
Reproduction and development. Nematodes are dioecious animals.The internal genital organs are paired and have a tubular structure.Reproduction is only sexual.Sexual dimorphism is pronounced: females are larger, males have a curved back end of the body.Fertilization is internal and viviparity occurs.In development, nematodes go through four larval stages, separated by molting, which is followed by shedding of the cuticle.The third stage in some species (including the well-known Caenorhabditis elegans) under unfavorable conditions turns into the so-calleddauer stage - a dormant larva.
Parasitism. Currently, of the more than 24,000 described species of nematodes, about half are parasitic.They can affect almost all tissues and organs: connective tissue, muscles, blood and lymphatic vessels, gonads, sensory organs, as well as body cavities, etc.Among them are ecto- and endoparasites of plants, vertebrates and invertebrates, including other nematodes and even protozoa.
The following are descriptions of the most important representatives of roundworms from the point of view of medical parasitology.
Human roundworm(Ascaris lumbricoides)
Appearance.The body, pointed at the ends, is pinkish-white.Dimensions: males - 15-25 cm, females - 20-40 cm.The body is covered with a ten-layer flexible cuticle that protects against mechanical stress and digestive enzymes of the host.
Expansion. The species is cosmopolitan - it is distributed everywhere, but different countries have different percentages of infected people.In Japan, for example, more than 90% of the population is infected with roundworms due to the use of human feces as fertilizer.In areas with a warm and dry climate, roundworms are less common.
Life cycle.Development takes place without changing owners.Adult worms parasitize in the small intestine, causing ascariasis.A person is usually affected by several dozen roundworms (the record is 900 pieces).The life span in the intestines is about one year.Roundworms are dioecious, like other nematodes.A sexually mature female lays about 200,000 oval-shaped eggs per day, which are released into the external environment with feces.Roundworms are classified as geohelminths - they require larval development in the soil.When it is exposed to favorable conditions (moist soil at a temperature of about 25 °C and with sufficient access to oxygen), a larva develops in the egg.The development period varies from 16 days to several months and depends on the air temperature.Such eggs containing a larva can be considered invasive.
Infection occurs when the eggs are ingested in food or water;transmission does not occur directly from person to person.In the intestines, the larvae burrow through the intestinal wall, enter the blood vessels and liver, and 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 move 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.The process of "growing up" in nematodes is associated with molting (usually four of them).
Clinical picture of ascariasis. In the migratory stage of ascariasis, cough (it helps the larvae to enter the throat), chest pain, allergic reactions and fever are observed.
In the intestinal phase, there is damage to the intestinal mucosa and poisoning of the body with toxic products of metabolism.Symptoms: nausea, vomiting, stool disorder, loss of appetite.
Long-term effects of infection: general decrease in performance, sleep disturbances.When worms enter the bile ducts and respiratory tract, the outcome 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, accompanied by migraines.
Prevention. Washing hands before eating and preparing food.Washing vegetables and fruits.Eggs are also carried by flies, so fighting against these dipterans using, for example, Velcro also helps prevent ascariasis.
Interesting fact. There are studies showing the positive effects of roundworm infection on alleviating symptoms of autoimmune diseases and increasing fertility in women.Scientists attribute this to the parasite's effect on the immune system by affecting the level of T cells in the body, but currently the mechanism is too poorly understood to draw firm conclusions.
pinworm(Enterobius vermicularis)
Appearance. Greyish-white nematode, males 2-5 mm long, females 8-14 mm.The end of 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 parasitize in the lower part of the small and large intestine, causing enterobia.The lifespan is 1-2 months.The front end of the pinworm attaches to the intestinal wall.A sexually mature female crawls out of the large intestine through the anus and lays 5 to 15 thousand eggs on the skin near the anus, after which she dies.
The crawling of females is accompanied by itching.When scratching the skin, the eggs are transferred to the hands and other things.Flies are also involved in egg transfer.Infection occurs by ingestion.Larvae hatch from eggs that enter the intestines.
Epidemiology and clinical picture of enterobiosis. Enterobiasis is widespread, especially common among children due to non-observance of personal hygiene rules and "crowding" in kindergartens and schools.It is transmitted from person to person without an intermediary.It reduces the effect of vaccination.
Symptoms: abdominal pain, loss of appetite, headache, allergic manifestations, perianal itching (leads to sleep disturbances, increases irritability).
Trichinella(trichinella spiralis)
Description.Small nematode 2-4 mm long.It parasitizes the mucosa of the small intestine.Widespread in Eurasia and North America.
Life cycle. A change of host is necessary for the development of trichinella.These are usually wild animals (foxes, wolves, bears, wild boars), as well as people and livestock.Females anchor themselves in the intestinal epithelium with the front end of their body and give birth to 1-2 thousand larvae.Ovoviviparity is typical: hatching of larvae from eggs occurs in the female's genital tract.The larvae are spread throughout the body through the blood and lymphatic vessels and settle in the striated muscles.At this stage they have a stylet, which they use to destroy muscle tissue, causing the host to form a capsule in which, curled up, they reside in the future.After several months, the capsule is soaked in lime.Such muscular trichina can exist for several years and survive even after the death of the owner and the decomposition of his corpse.
Once in the stomach of the new host (after eating the corpse of the previous one), the larvae are released from the capsule, penetrate the mucous membrane and within a few days, after four litters, turn into adult worms.
Clinical picture of trichinosis. Increased temperature, facial swelling, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted through food via contaminated meat.Therefore, in order to prevent the disease, the meat must pass a veterinary examination and be properly prepared - boiled for 2-3 hours.Cooking methods such as smoking and salting do not destroy trichinella.
Whipworm(Trichocephalus trichurus)
Appearance.The worm is whitish in color, about 4 cm long.The front end is thin, reminiscent of hair (hence the name).
Expansion.They prefer countries with a humid and warm climate.
Life cycle.The worm parasitizes in the initial part of the large intestine, only in humans.It causes trichuriasis.The lifespan of a person is several years.The thin end penetrates the thickness of the mucosa of the intestinal wall.It feeds on tissue fluid and blood.
The female lays 1-3 thousand eggs, which are released into the external environment with feces.Like the roundworm, the whipworm is related to geohelminths: in order for the eggs to become invasive, they must remain in the soil at a certain humidity and temperature (25-30°C) for a month.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 in them for about a week.Then, after the destruction of the villi, they exit into the lumen of the intestine, reach the large intestine, establish themselves there and reach maturity within a month.
Clinical picture of trichocephalosis. The worm damages the mucous membrane of the large intestine and causes poisoning of the host with waste products.The whip is hematophagous, so it can lead to anemia.Trichocephalosis is accompanied by abdominal pain, headache and dizziness.Because the whipworm binds to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
Appearance.Thin whitish nematode, females 30-120 cm long, males 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.Cancers in the stomach die under the influence of hydrochloric acid, but the guinea pig larvae survive and spread throughout the body through the lymphatic system.Then they penetrate into the body cavity, where they molt and reach sexual maturity.After mating, the male dies, and the female moves into the subcutaneous tissue, where a purulent abscess forms, accompanied by burning and pain.Cold water is best for pain relief.
The development of the egg forces the female to start moving her "head" forward towards the surface of the skin, leaving in its path an inflammatory process, which turns into a purulent abscess, which then bursts.When the female's uterus enters the water, it ruptures, and the larvae that hatch from the eggs come out.To ensure that development is not interrupted, the larvae must infect the cyclops crab, which is an intermediate host.Those larvae that remain in the water die.After the crabs swallow the final host, under the influence of stomach acid, the crabs dissolve, and the larvae easily enter the intestine, break through its walls and end up in the lymph nodes, where the development cycle continues.The disease caused by the guinea pig is called dracunculiasis.
Dracunculiasis.The incubation period lasts up to nine months and ends when the female reaches sexual maturity.And in a person who has already suffered from dracunculia, purulent abscesses begin to form at this moment.The only escape from the pain is the pool.The relief is immediate, but on contact with water the bubbles burst and the guinea worm throws the larvae into the water.The crabs consume them and the life cycle begins again.
When treating dracunculosis, an incision is often made at the site of the bladder and the worm is gradually pulled out, wrapping it around a stick.This goes on for days, sometimes weeks (you have to pull the worm out slowly and carefully so it doesn't break).It is assumed that the appearance of a guinea worm coiled around a stick became a kind of prototype for the symbol of medicine - the staff of Asclepius entwined with a snake.
Bancroft's filament (filaria), or Bancroft's wire(Wuchereria bancrofti)
Appearance.White thread nematode, females 10 cm long, males 4 cm.
Distribution. Tropics, subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually occur in the lymph glands and vessels, obstructing lymph drainage and causing persistent swelling.Females produce larvae - nocturnal microfilariae, which appear in the peripheral blood at night, and during the day they go deep into the body (into the pulmonary vessels and kidneys).This is due to the fact that the intermediate hosts are mosquitoes, which usually suck blood in the evening and at night.The larvae enter the mosquito's stomach, then into the body cavity, where they grow, after which they accumulate near the proboscis, from which they are transmitted to humans by sucking blood.Bancroft's filaments cause elephantiasis, or elephantiasis, or elephantiasis.It is worth noting that this disease can also be caused by other nematodes.
Clinical presentation and treatment of elephantiasis. The enlargement 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 the stagnation of lymph, which occurs due to the blockage of the lymphatic vessels by adult Bancroft's phyllodes.The skin on the affected part of the body becomes covered with ulcers.
Treatment of elephantiasis is aimed at improving the outflow of fluid.The use of anthelmintic drugs is effective.In later stages, surgery may be required.
