Nematodes or roundworms themselves (Nematoda) are a kind of protostomes, protocavities, bilaterally symmetrical moulting animals.
Table setting. Nematodes are one of the most widespread animal species that have been able to colonize a wide range of habitats – from the interstitium (the space between sand grains) and bryophytic communities to arctic ice (e.g.Theristis Melnikovi and Cryonema crissum, in the thickness of multi-year ice in the central Arctic Ocean).Parasitic nematodes are of particular interest to researchers, among other things, because of the diversity of their hosts.
Construction plan. Thin fusiform body, tapering towards the end, round in cross-section.The mouth is at the front end and the anus (anus) is 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), which does not have its own epithelial lining, is filled with fluid.
Digestive system. The mouth opening at the front end of the body is surrounded by protrusions - lips (usually three) and leads into a muscular ectodermal pharynx with a triangular lumen.The pharynx leads to the endodermal midgut from a single layer of columnar epithelial cells.This is followed by a short ectodermal hindgut that opens into the anus.
Selection system. The excretory organs are unicellular glands that replaced the protonephridia.There is usually a neck gland at the front of the body, 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.Respiration takes place through the skin.Anaerobic metabolism is also possible (anaerobic breakdown of glycogen into butyric acid and valeric acid in parasites).
Nervous system. The nervous system is scalariform type.It is represented by a nerve ring and six longitudinal trunks.The two nerve trunks running along the abdominal and dorsal lines are stronger and are connected by semicircular nerve bridges (commissure).
Sensory organs. Around the mouth are papillae and stipules - the organs of touch.Some marine representatives have primitive eyes - pigment spots.Chemical sense organs, amphitheatres, are usually pocket, spiral, or slit-shaped.They are located on the sides of the tip of the head and are particularly well developed in males, as they help in finding females.
Reproduction and development. Nematodes are dioecious animals.The internal genitalia are paired and tubular in structure.Reproduction is only sexual.Sexual dimorphism is pronounced: females are larger, males have a curved back end of the body.Fertilization occurs internally, life force.During their development, nematodes go through four larval stages, which are separated by molting and are accompanied by shedding of the cuticle.The third stage of some species (including the famous Caenorhabditis elegant) changes into a so-called permanent state - a resting larva - under unfavorable conditions.
Parasitism. Currently, about half of the more than 24,000 described nematode species are parasites.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. They include ecto- and endoparasites of plants, vertebrates and invertebrates, including other nematodes and even protozoans.
The most important representatives of roundworms from the point of view of medical parasitology are described below.
Human roundworm(Ascaris lumbricoides)
Appearance.The body, pointed at the ends, is pinkish-white.Sizes: males - 15-25 cm, females - 20-40 cm.The body is covered with a ten-layer flexible cuticle that protects against mechanical stress and the host's digestive enzymes.
Table setting. The species is cosmopolitan - it is spread everywhere, but the rate of infected people varies in different countries.In Japan, for example, more than 90% of the population is infected with roundworms due to the use of human excrement as fertilizer.In areas with hot, dry climates, roundworms are less common.
Life cycle.The development takes place without a change of ownership.Adult worms parasitize in the small intestine, causing ascariasis.A person is usually affected by dozens of roundworms (the record is 900 pieces).The lifespan in the intestines is about one year.Roundworms are dioecious like other nematodes.A sexually mature female lays approximately 200,000 oval-shaped eggs per day, which are passed into the external environment with feces.Roundworms belong to geohelminths - they need to develop a larval stage in the soil.As a result of favorable conditions (moist soil at around 25 °C and sufficient oxygen supply), a larva develops in the egg.The development period ranges from 16 days to several months and depends on the air temperature.Eggs containing such larvae are 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 bore through the intestinal wall, enter the blood vessels and the liver, then migrate through the inferior vena cava to the right atrium and right ventricle.From the latter, the larvae reach the lungs via the pulmonary circulation, where they travel from the blood to the alveoli, bronchi, trachea and the 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" nematodes is associated with molting (usually four of them).
Clinical picture of ascariasis. During the migration phase of ascariasis, cough (it helps the larvae to enter the throat), chest pain, allergic reactions and fever are observed.
In the intestinal section, the intestinal mucosa is damaged and the body is poisoned with toxic metabolic products.Symptoms: nausea, vomiting, stool disorders, loss of appetite.
Long-term effects of the infection: general decrease in performance, sleep disorders.When worms enter the bile ducts and airways, the outcome is fatal.In addition, roundworm larvae can enter the brain (for example, from the inferior vena cava to the superior vena cava and then along the brachiocephalic vein), causing meningoencephalitis, which is accompanied by migraine.
Prevention. Washing hands before eating and preparing food.Washing vegetables and fruits.The eggs are also carried by flies, so the fight against such two-winged insects, for example with the help of Velcro, also helps prevent ascariasis.
Interesting fact. There are studies that have shown the positive effects of tapeworm infection in alleviating the symptoms of autoimmune diseases and increasing female fertility.Scientists attribute this to the parasite's effect on 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. Gray-white nematode, males 2-5 mm, females 8-14 mm long.The end of the tail is pointed (hence the name).The characteristic swelling of the esophagus can be seen at the front end of the body.
Life cycle.Pinworms parasitize the lower part of the small intestine and the large intestine, causing enterobiasis.Life span 1-2 months.The anterior end of the pinworm attaches to the intestinal wall.A sexually mature female crawls out of the large intestine through the anus and lays 5-15 thousand eggs on the skin near the anus and then dies.
Climbing out of females is accompanied by itching.When the skin is scratched, the eggs are transferred to the hands and so on.Flies are also involved in the transfer of eggs.Infection occurs through ingestion.The larvae hatch from eggs that enter the intestines.
Epidemiology and clinical picture of enterobiasis. Enterobiasis is widespread, especially common 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.It reduces the effectiveness of vaccinations.
Symptoms: abdominal pain, loss of appetite, headache, allergic manifestations, perianal itching (leads to sleep disorders, increases irritability).
Trichinella(Trichinella spiralis)
Description.A small nematode 2-4 mm long.It parasitizes the mucous membrane of the small intestine.It is widespread in Eurasia and North America.
Life cycle. The development of trichinella requires a change of host.Usually these are wild animals (fox, wolf, bear, boar), as well as people and livestock.Females are attached to the front end of the body in the intestinal epithelium and give birth to 1-2 thousand larvae.Ovoviviparity is characteristic: the larvae hatch from the eggs in the female genital tract.The larvae travel throughout the body through the blood and lymphatic vessels and settle in the striated muscles.At this stage, they have a cuff, which they use to destroy muscle tissue, causing the host to form a capsule in which they curl up and stay in the future.After a few months, the capsule is soaked in lime.Such muscle trichina can exist for years and persist 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 escape from the capsule, penetrate the mucous membrane, and within a few days, after undergoing four molts, transform into adult worms.
Clinical picture of trichinosis. Increased temperature, swelling of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is spread through food and contaminated meat.Therefore, in order to prevent the disease, the meat must be examined by a veterinarian and properly prepared - cook for 2-3 hours.Cooking methods such as smoking and salting do not kill trichinella.
Whipworm(Trichocephalus trichurus)
Appearance.The maggot is whitish in color and about 4 cm long.The front is thin and resembles hair (hence the name).
Table setting.They prefer countries with a humid and warm climate.
Life cycle.The worm is a parasite in the initial part of the large intestine, only in humans.It causes Trichuriasis.A person's lifespan is several years.Its thin end 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 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 for a month at a certain humidity and temperature (25-30 °C).After that, infection occurs when the egg is ingested;they hatch into larvae in the host's intestines, penetrate the intestinal villi and grow within them for about a week.Then, after the villi are destroyed, they exit into the lumen of the intestine, reach the large intestine, settle there, and reach it within a month.
Clinical picture of trichocephalosis. The worm damages the lining of the large intestine and poisons the host with waste products.The whipworm is a hematophagous, so it can lead to anemia.Trichocephalosis is accompanied by abdominal pain, headache and dizziness.Because the whipworm adheres to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
Appearance.A thin, whitish nematode, females are 30-120 cm long, males up to 4 cm.There is a small spine on the tail.
Distribution: Tropical countries of Asia and Africa.
Life cycle.Infection occurs when you drink unboiled water containing cockroaches.The crustaceans in the stomach are killed by the hydrochloric acid, but the larvae of the marine worm survive and spread throughout the body through the lymphatic system.They then penetrate the body cavity, spawn there 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 the best for pain relief.
The development of the eggs causes the female to begin to move her "head" forward towards the surface of the skin, leaving behind an inflammatory process, turning into a purulent abscess, which then bursts.When the female's uterus enters the water, it ruptures and the larvae hatch from the eggs.To ensure that development is not interrupted, the larvae must infect the cyclops crustacean, which is an intermediate host.Larvae that remain in the water die.After the crustaceans are ingested by the definitive host, gastric acid dissolves the crustaceans and the larvae easily enter the intestine, pass through its wall and end up in the lymph nodes, where the development cycle continues.The disease caused by the marine 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 is already sick with dracunculiasis, purulent abscesses begin to form at this time.The only salvation from pain is the pond.The relief is immediate, but upon contact with water the bubbles burst and the sea worm throws the larvae into the water.Crustaceans eat them and the life cycle begins again.
In the treatment of dracunculiasis, an incision is often made at the site of the blister, and the worm is gradually pulled out and wrapped around a stick.This takes days, sometimes weeks (the worm must be pulled out slowly and carefully to avoid breakage).It is assumed that the appearance of a guinea worm wrapped around a stick became a kind of prototype of the symbol of medicine - the staff of Asclepius entwined with a snake.
Bancroft's filament (filaria), or Bancroft's string(Wuchereria bancrofti)
Appearance.White thread nematode, females are 10 cm long, males are 4 cm long.
Distribution. Tropics and subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually occur in the lymph nodes and blood vessels, obstructing the drainage of lymph and causing persistent swelling.Females produce larvae - nocturnal microfilariae - which appear in the peripheral blood at night and penetrate deep into the body (pulmonary vessels and kidneys) during the day.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 up, then accumulate near the snout, from where they are transferred to humans by sucking blood.Bancroft threads cause elephantiasis, or elephantiasis, or elephantiasis.It is worth noting that this disease can also be caused by other nematodes.
Clinical picture and treatment of elephantiasis. Enlargement of any part of the body occurs due to hyperplasia (painful growth) of the skin and subcutaneous tissue caused by inflammatory thickening of the walls of the lymphatic vessels and lymphatic stasis due to blockage of the lymphatic vessels by adult Bancroft filamentous individuals.The skin of the affected part of the body is covered with ulcers.
Treatment of elephantiasis aims to improve fluid outflow.The use of anthelmintic drugs is effective.In the later stages, surgery may be necessary.
