Nematodes or roundworms themselves (Nematoda) are a type of protostomes, protocavities, bilaterally symmetrical molting animals.
Scattering. Nematodes are one of the most widespread species of animals that have been able to colonize a variety of habitats - from the interstitium (the space between sand grains) and moss communities to arctic ice (such asTeristis Melnikovi and Cryonema chrysum, found in the thickness of perennial 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.
Building plan. Slender fusiform body, tapering towards the ends, round in cross-section.The mouth is located at the front end and the powder (anus) is at the back.From the outside, 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 fluid.
Digestive system. The oral 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 into the endodermal midgut of a single layer of columnar epithelial cells.This is followed by a short ectodermal hindgut that opens into the anus.
Excretory system. The excretory organs are unicellular glands that replace the protonephridia.In the front 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 to butyric and valeric acid in parasites).
Nervous system. The nervous system is of the scalariform type.Represented by a nerve ring and six longitudinal trunks.The two nerve trunks running along the ventral and dorsal lines are more powerful and are connected by semicircular nerve bridges (commissures).
Sense organs. There are papillae and bristles - 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 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 genitalia are paired and have a tubular structure.Reproduction is only sexual.Sexual dimorphism is pronounced: females are larger, in males the rear end of the body is curved.Fertilization is internal and quickening occurs.In their development, nematodes go through four larval stages, separated by molting, which is accompanied by shedding of the cuticle.The third stage in some species (including the well-known Caenorhabditis elegans) under unfavorable conditions passes into the so-called dauer stage - a resting larva.
Parasitism. Currently, of the more than 24,000 described nematode species, 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 the body cavity, etc.Among them are both ecto- and endoparasites of plants, vertebrates and invertebrates, including other nematodes and even protozoa.
The following are descriptions of the most significant 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.Sizes: male - 15-25 cm, female - 20-40 cm. The body is covered with a ten-layer flexible cuticle that protects against mechanical stress and digestive enzymes of the host.
Scattering. The species is cosmopolitan - distributed everywhere, but different countries have different rates of infection.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 a hot and dry climate, roundworms are less common.
Life cycle.The development continues without a change of owners.Adult worms parasitize the small intestine, causing ascariasis.A person is usually affected by several dozen roundworms (the record is 900).The life span in the intestine is about one year.Roundworms are dioecious, like other nematodes.A sexually mature female lays about 200 thousand 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 exposed to favorable conditions (moist soil at a temperature of about 25 °C and 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 eggs are ingested with food or water;transmission does not occur directly from person to person.In the intestine, the larvae pierce the intestinal wall, enter the blood vessels and liver, and then migrate through the inferior vena cava into 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 into the pulmonary vesicles, the bronchi, the trachea and the oral cavity.Secondary infection occurs in the oral cavity: the larvae are swallowed, enter the intestines and after three months become sexually mature.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, coughing (helps the larvae to enter the throat), chest pain, allergic reactions and fever are observed.
In the intestinal stage, there is damage to the intestinal mucosa and poisoning of the body with toxic metabolic products.Symptoms: nausea, vomiting, stool disorder, loss of appetite.
Long-term consequences of the infection: general decrease in working capacity, sleep disorders.When the worms crawl into the bile ducts and respiratory tract, the outcome is fatal.Also, ascaris 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 effect of roundworm infection in alleviating the 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 the mechanism is currently too poorly understood to draw reliable conclusions.
sedges(Enterobius vermicularis)
Appearance. Grey-white nematode, males 2-5 mm long, females 8-14 mm long.The end of the tail is pointed (hence the name).At the front end of the body, a characteristic swelling of the esophagus is noticeable.
Life cycle.Pinworms parasitize the lower part of the small intestine and large intestine, causing enterobiosis.The life span is 1-2 months.The front end of the scutes attaches to the intestinal wall.The 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, then dies.
The crawling of females is accompanied by itching.When combing the skin, the eggs are transferred to the hands, etc.Flies are also involved in egg transfer.Infection occurs through ingestion.Larvae hatch from eggs that enter the intestine.
Epidemiology and clinical picture of enterobiosis. Enterobiosis is widespread, especially often in children due to non-compliance with the rules of personal hygiene and "accumulation" in kindergartens and schools.It is transmitted from person to person without an intermediate host.Reduces the effect of vaccinations.
Symptoms: abdominal pain, loss of appetite, headache, allergic manifestations, perianal itching (leads to sleep disturbances, increases irritability).
Trichinella(Trichinella spiralis)
Description.A small nematode 2-4 mm long.It parasitizes the mucosa of the small intestine.Widespread in Eurasia and North America.
Life cycle. The development of Trichinella requires a change of hosts.Usually these are wild animals (foxes, wolves, bears, wild boars), as well as people and livestock.Females are anchored with the front end of the body in the intestinal epithelium and give birth to 1-2 thousand larvae.Ovoviparity is characteristic: hatching of larvae from eggs occurs in the female genital tract.The larvae are carried in 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 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 a muscle 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 carcass of the previous one), the larvae are released from the capsule, penetrate the mucous membrane and within a few days, after undergoing four molts, turn into adult worms.
Clinical picture of trichinellosis. Increased temperature, puffiness of the face, muscle pain, allergic reactions.
Prevention. Trichinellosis is transmitted through food through infected meat.Therefore, in order to prevent the disease, the meat must be subjected to a veterinary examination and 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).
Scattering.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 life span of a person is a few 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: for the eggs to become invasive, they must remain in the soil at a certain humidity and temperature (25-30 ° C) for a month.Infection then occurs when the eggs are ingested;the larvae emerge from them into the host's intestines, penetrate the intestinal villi and grow within them for about a week.Then, having destroyed the waves, they go out into the intestinal lumen, reach the large intestine, settle there and reach maturity within a month.
Clinical picture of trichocephalosis. The worm damages the lining of the large intestine and causes poisoning of the host with waste products.Whipworm is hematophagous, so it can cause anemia.Trichocephalosis is accompanied by abdominal pain, headache and dizziness.Because the whipworm attaches itself to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rushing(Dracunculus medinensis)
Appearance.A thin whitish nematode, females 30-120 cm long, males no more than 4 cm.There is a small spike on the tail.
Distribution: the tropical countries of Asia and Africa.
Life cycle.Infection occurs by drinking unboiled water containing copepods.Crustaceans in the stomach die under the influence of hydrochloric acid, but the larvae of the sea worm survive and spread throughout the body through the lymphatic system.They then 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 forms, accompanied by burning and pain.Cold water is best for pain relief.
The development of the eggs forces the female to start moving "head" forward to the surface of the skin, leaving an inflammatory process in its path, turning 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 crustacean, which is an intermediate host.Those larvae that remain in the water die.After the crustaceans are ingested by the definitive host, under the influence of stomach acid, the crustaceans dissolve and the larvae easily enter the intestine, pass through its walls and end up in the lymph nodes, where the cycle of development continues.The disease caused by guinea worm is called dracunculosis.
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 the pain is the lake.Relief is immediate, but upon contact with water the bubbles burst and the sea worm expels the larvae into the water.Crustaceans consume them and the life cycle begins again.
In the treatment of dracunculosis, an incision is often made at the site of the blister and the worm is gradually pulled out by wrapping it around a stick.This takes 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 of the symbol of medicine - the staff of Asclepius entwined with a snake.
Bancroft thread (filariae) or Bancroft thread(Wuchereria bancrofti)
Appearance.White filamentous nematode, female 10 cm long, male 4 cm long.
Distribution. Tropics, subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually appear in the lymph glands 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 enter deep into the body (in the pulmonary vessels and kidneys).This is due to the fact that the intermediate host is mosquitoes, which usually suck blood in the evening and at night.The larvae enter the mosquito's stomach, then the body cavity, where they grow, then 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 picture 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 adult filamentous Bancroft individuals.The skin of the diseased part of the body is covered with ulcers.
Treatment for elephantiasis is aimed at improving fluid drainage.The use of anthelmintic drugs is effective.In later stages, surgery may be required.
