Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavity, bilaterally symmetrical animals.
The spread. Nematodes are one of the most widespread types of animals that have been able to colonize a variety of habitats - from interstitium (the space between sand grains) and moss communities to Arctic ice (such asTheristis 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 variety of their hosts.
Construction plan. Body slender, fusiform, tapering towards the ends, round in cross-section.The mouth is in the front, and the anus is in the back.The outer part of the body is covered with a multilayered 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 oblique muscle fibers.The primary body cavity (schizocoel), without its own epithelial lining, is filled with fluid.
Digestive system. The oral opening 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 to the endodermal midgut by a single layer of columnar epithelial cells.Next comes a short ectodermal hindgut, which opens into the anus.
Excretory system. The excretory organs are unicellular glands that replace the protonephridia.There is usually a cervical gland in the front of the body, from which a short excretory duct arises.There are also "storage kidneys" - phagocytic organs that accumulate indigestible metabolic products that are not removed from the body.
Circulatory and respiratory systems. These systems are missing.Breathing 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 of the scalariform type.It is represented by a nerve ring and six longitudinal trunks.The two nerve trunks that pass along the ventral and dorsal lines are more powerful and are connected by semicircular nerve bridges (commissures).
Sensory organs. There are papillae and setae - touch organs located around the mouth.Some marine representatives have primitive eyes - pigment spots.The chemical sense organs of amphids are usually pocket, spiral or slit shaped.They are located on the sides of the edge 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, in males the back end of the body is curved.Fertilization is internal and viviparity occurs.In development, nematodes pass through four larval stages, separated by molting, which are accompanied by shedding of the cuticle.The third stage in some species (including the famous Caenorhabditis elegans) under unfavorable conditions, it passes into the so-called dauer stage - a resting larva.
Parasitism. Currently, of the more than 24,000 species of nematodes described, about half are parasitic.They can affect almost all tissues and organs: connective tissues, muscles, blood and lymphatic vessels, gonads, sensory organs, as well as the body cavity, etc.Among them there are ecto- and endoparasites of plants, vertebrate and invertebrate animals, including other nematodes and even protozoa.
Below are descriptions of the most significant representatives of roundworms from the point of view of medical parasitology.
The human roundworm(Ascaris lumbricoides)
The exterior.The body, pointed at the edges, is pinkish-white.Size: male - 15-25 cm, female - 20-40 cm.The body is covered with a ten-layer flexible cuticle that protects against mechanical stress and the digestive enzymes of the host.
The spread. The species is cosmopolitan - 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 hot, dry climates, roundworms are less common.
Life cycle.Development continues without changing owners.Adult worms parasitize the small intestine, causing ascariasis.A person is usually affected by several dozen roundworms (the record is 900 pieces).The life expectancy in the intestines is about one year.Roundworms are dioecious, like other nematodes.A sexually mature female releases about 200,000 oval-shaped eggs per day, which are released into the external environment with feces.Roundworms are classified as geohelminths - they require the development of a larval stage in the soil.When exposed to favorable conditions (moist soil at a temperature of about 25 °C and with sufficient access to oxygen), a larva develops into an 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 eggs are introduced into food or water;transmission does not occur directly from person to person.In the intestine, 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, throat 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 "growth" in nematodes is accompanied by molting (usually four of them).
Clinical picture of ascariasis. In the migratory phase of ascariasis, a cough (it helps the larvae enter the throat), chest pain, allergic reactions and fever is observed.
In the intestinal phase, there is damage to the intestinal mucosa and poisoning of the body with toxic metabolic products.Symptoms: nausea, vomiting, stool disorders, loss of appetite.
Long-term effects of infection: general decrease in performance, sleep disturbances.When the 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, accompanied by migraine.
Prevention. Washing hands before eating and preparing food.Washing vegetables and fruits.Eggs are also carried by flies, so the fight against these dipterans using, for example, Velcro also helps prevent ascariasis.
Interesting fact. There are studies showing the positive effects of roundworm infection in relieving 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 at the moment the mechanism is too poorly understood to draw reliable conclusions.
pink worm(Enterobius vermicularis)
The exterior. White to gray nematode, males 2-5 mm long, females 8-14 mm.The end of the tail is pointed (hence the name).At the front end of the body, a characteristic swelling of the esophagus is observed.
Life cycle.The worms parasitize the lower part of the small intestine and large intestine, causing enterobiasis.Shelf life is 1-2 months.The front end of the tapeworm attaches to the intestinal wall.A sexually mature female crawls from the large intestine through the anus and lays 5 to 15 thousand eggs on the skin near the anus, after which she dies.
Female procrastination is accompanied by itching.When scratching the skin, the eggs are transferred to the hands and more.Flies are also involved in egg transfer.Infection occurs through ingestion.Larvae hatch from eggs that enter the intestines.
Epidemiology and clinical presentation of enterobiasis. Enterobiasis is widespread, especially frequent among children due to non-observance of personal hygiene rules and "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, 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.Distributed in Eurasia and North America.
Life cycle. For the development of Trichinella, a change of hosts is necessary.Usually these are wild animals (foxes, wolves, bears, wild boars), as well as people and livestock.Females anchor from the front end of the body to the intestinal epithelium and give birth to 1-2 thousand larvae.Ovoviviparity is typical: hatching of larvae from eggs occurs in the female genital tract.The larvae are carried throughout the body through the blood and lymphatic vessels and settle in the striated muscles.At this stage, they have a stylet, they use it to destroy the muscle tissue, causing the host to form a capsule in which, wrapped, they stay in the future.After several months, the capsule is soaked in lime.Such muscle trichina can exist for several years and survive even after the death of the owner and the decomposition of his corpse.
After entering 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 two days, after undergoing four molts, turn into adult worms.
Clinical picture of trichinosis. Fever, swelling of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted through food through contaminated meat.Therefore, to prevent the disease, the meat must undergo a veterinary examination and be properly prepared - boiled for 2-3 hours.Cooking methods such as smoking and salting do not destroy Trichinella.
The whipworm(Trichocephalus trichurus)
The exterior.The worm is white, about 4 cm long.The front part is thin, reminiscent of hair (hence the name).
Spreading.They prefer places 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 intestines of the host, penetrate the intestinal villi and grow in them for about a week.Then, after destroying the villi, they exit the intestinal lumen, reach the large intestine, settle there and reach maturity within a month.
Clinical picture of trichocephalosis. The worm damages the mucosa of the large intestine and causes poisoning of the host by waste.The whipworm is a hematophagous, so it can lead to anemia.Trichocephalosis is associated with abdominal pain, headache and dizziness.Because the whipworm attaches to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
The exterior.A slender white 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.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 into the body cavity, where they fuse and reach sexual maturity.After mating, the male dies and the female moves to the subcutaneous tissue, where a purulent abscess forms, accompanied by burning and pain.Cold water is best for pain relief.
The development of the eggs forces the female to start moving the "head" forward towards the surface of the skin, leaving an inflammatory process along its path, turning into a purulent abscess, which then bursts.When the female's uterus enters the water, it ruptures and the larvae emerge from the eggs.To ensure that development is not interrupted, the larvae must infect the cyclops crustacean, which is an intermediate host.Those larvae that remain in the water die.After the crustaceans are swallowed by the final host, under the influence of stomach acid, the crustaceans are digested and the larvae easily enter the intestine, make their way through its walls and end up in the lymph nodes, where the cycle of development continues.The disease caused by the guinea worm 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 fallen ill with dracunculiasis, purulent abscesses begin to form at this time.The only escape from pain is a pond.The relief is immediate, but upon contact with water the bubbles burst and the guinea worm throws the larvae into the water.Crustaceans consume them and the life cycle begins again.
When treating dracunculiasis, an incision is often made at the site of the blister and the worm is gradually extracted, wrapping it around a stick.This takes days, sometimes weeks (you have to pull the worm slowly and carefully so it doesn't break).It has been suggested that the appearance of a guinea worm wound around a stick became a kind of prototype for the symbol of medicine - the rod of Asclepius entwined with a snake.
Bancroft's filament (filaria), or Bancroft's string(Wuchereria bancrofti)
The exterior.White thread nematode, female 10 cm long, male 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 the drainage of lymph and causing persistent swelling.Females produce larvae - nocturnal microfilariae, which appear in the peripheral blood at night, and during the day go deep into the body (in 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. Enlargement of any part of the body occurs due to hyperplasia (painful growth) of the skin and subcutaneous tissue, which is caused by inflammatory thickening of the walls of the lymphatic vessels, and stagnation of lymph, which occurs due to blockage of the lymphatic vessels by the individual filamentous vessels of the adult Bancroft.The skin on the diseased part of the body is covered with ulcers.
Treatment of elephantiasis is aimed at improving fluid flow.The use of anthelmintic drugs is effective.In later stages, surgery may be required.
