Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavities, animals with bilaterally symmetrical moulting.
Spreading. Nematodes are one of the most widespread types of animals that have been able to colonize a variety of habitats - from the interstice (the space between grains of sand) and moss communities to Arctic ice (such asTheristis Melnikovi and Crionema 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, not least because of the wide variety of their hosts.
Construction plan. Thin fusiform body, tapering at the ends, with a round cross-section.The mouth is located at the front end and the powder (anus) is at the rear.The outer part of the body is covered with a multilayer 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 longitudinal obliquely striated muscle fibers.The primary body cavity (schizocele), devoid of its own epithelial lining, is filled with fluid.
Digestive system. The oral opening at the anterior 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 from 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 replaced the protonephridia.There is usually a cervical gland on the front of the body, from which a short excretory duct arises.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 occurs through the skin.Anaerobic metabolism is also possible (anaerobic degradation of glycogen to butyric and valeric acids 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 that run along the ventral and dorsal lines are more powerful and are connected by semicircular nerve bridges (commissures).
Sense organs. There are papillae and bristles - organs of touch located around the mouth.Some marine representatives have primitive eyes - age spots.Chemical sense organs, amphids, usually have the shape of a bag, spiral or slit.They are located on the sides of the head and are especially well developed in males, as they help in locating 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 posterior end of the body is curved.Fertilization is internal and viviparity occurs.In development, nematodes go through four larval stages, separated by molting, which are accompanied by the detachment of the cuticle.The third stage in some species (including the famous Caenorhabditis elegans) under unfavorable conditions passes into the so-called dauer stage - a resting larva.
Parasitism. Currently, of the more than 24 thousand species of nematodes described, around half 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.Among them are ecto- and endoparasites of plants, vertebrate and invertebrate animals, 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.Dimensions: males - 15-25 cm, females - 20-40 cm.The body is covered by a ten-layer flexible cuticle that protects from mechanical stress and the host's digestive enzymes.
Spreading. The species is cosmopolitan – distributed far and wide, 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 excrement as fertilizer.In areas with hot, dry climates, roundworm is less common.
Useful life.Development continues without a change of ownership.Adult worms parasitize the small intestine, causing ascariasis.A person is usually affected by several dozen roundworms (the record is 900 pieces).The shelf life in the intestines is about a year.Roundworms are dioecious, like other nematodes.A sexually mature female lays around 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 in the egg.The development period varies from 16 days to several months and depends on air temperature.These eggs containing a larva can be considered invasive.
Infection occurs when eggs are ingested in food or water;transmission does not occur directly from person to person.In the intestine, larvae pass through the intestinal wall, enter blood vessels and the liver, and then migrate through the inferior vena cava into the right atrium and right ventricle.From the latter, the larvae pass 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 intestine and become sexually mature after three months.The process of “growth” of nematodes is associated with molting (usually four of them).
Clinical picture of ascariasis. In the migratory phase of ascariasis, coughing (helps the larvae to enter the throat), chest pain, allergic reactions and fever are observed.
In the intestinal phase, damage to the intestinal mucosa and poisoning of the body with toxic metabolic products occur.Symptoms: nausea, vomiting, stool disorders, loss of appetite.
Long-term effects of infection: general decrease in performance, sleep disturbances.When the worms penetrate the bile ducts and respiratory tract, the result is fatal.In addition, 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. Wash your hands before eating and preparing food.Wash vegetables and fruits.Eggs are also transported by flies, so fighting these diptera using, for example, Velcro also helps prevent ascariasis.
Interesting fact. There are studies that show the positive effects of roundworm infection in relieving the symptoms of autoimmune diseases and increasing fertility in women.Scientists attribute this to the effect of parasites on the immune system by influencing the level of T cells in the body, but at present the mechanism is too poorly understood to draw reliable conclusions.
moth(Enterobius vermicularis)
Appearance. Grayish-white nematode, males 2 to 5 mm long and females 8 to 14 mm long.The rear end is pointed (hence the name).At the anterior end of the body, a characteristic swelling of the esophagus is noticeable.
Service life.Pinworms parasitize the lower part of the small and large intestine, causing enterobiasis.Life expectancy is 1 to 2 months.The anterior end of the moth attaches to the intestinal wall.A sexually mature female leaves the large intestine through the anus and lays 5 to 15 thousand eggs in the skin near the anus, after which she dies.
Women's crawling is accompanied by itching.When you scratch your skin, eggs are transferred to your hands and more.Flies are also involved in egg transfer.Infection occurs through ingestion.Larvae hatch from eggs that enter the intestine.
Epidemiology and clinical picture of enterobiasis. Enterobiasis is widespread, especially common in children due to non-compliance with personal hygiene rules and “crowding” in kindergartens and schools.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 disturbances, increases irritability).
Trichinella(Trichinella spiralis)
Description.Small nematode 2-4 mm long.Parasitizes the mucous membrane of the small intestine.Distributed in Eurasia and North America.
Useful life. For Trichinella to develop, a change of host is necessary.These are usually wild animals (foxes, wolves, bears, wild boars), as well as people and livestock.Females are anchored by the anterior end of the body in the intestinal epithelium and give birth to 1 to 2 thousand larvae.Ovoviviparity is typical: hatching of larvae from eggs occurs in the female genital tract.The larvae are transported throughout the body through blood and lymphatic vessels and settle in 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 will reside in the future.After a few months, the capsule is soaked in lime.This 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 having eaten the corpse of the previous one), the larvae free themselves from the capsule, penetrate the mucous membrane and in a few days, after four moults, they transform into adult worms.
Clinical picture of trichinosis. Increased temperature, facial swelling, 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.
Whipworm(Trichocephalus trichurus)
Appearance.The worm is whitish in color, about 4 cm long.The front part is thin, resembling hair (hence the name).
Spreading.They prefer countries with a humid and warm climate.
Service life.The worm parasitizes in the initial part of the large intestine, only in humans.Causes trichuriasis.A person's life expectancy is several years.The thin end penetrates the thickness of the mucous membrane of the intestinal wall.It feeds on tissue fluid and blood.
The female lays 1 to 3 thousand eggs, which are released into the external environment through feces.Like the roundworm, the whipworm is related to geohelminths: for the eggs to become invasive, they need to 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;larvae emerge from them in the host's intestine, penetrate the intestinal villi and grow in them for about a week.Then, having destroyed the villi, 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 mucous membrane of the colon and causes poisoning of the host with waste.Whipworm is a hematophagus, which is why it can cause anemia.Trichocephalosis is accompanied by abdominal pain, headaches and dizziness.Because the whipworm attaches itself 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 30-120 cm long and males no more than 4 cm.There is a small spine on the tail.
Distribution: tropical countries in Asia and Africa.
Service life.Infection occurs when drinking unboiled water containing copepods.Crustaceans in the stomach die under the influence of hydrochloric acid, but Guinea worm larvae survive and spread throughout the body through the lymphatic system.They then penetrate the body cavity, where they change and reach sexual maturity.After mating, the male dies and the female passes into the subcutaneous tissue, where a purulent abscess forms, accompanied by burning and pain.Cold water is best for relieving pain.
The development of eggs forces the female to begin moving her “head” forward towards the surface of the skin, leaving an inflammatory process in its wake, 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.Larvae that remain in the water die.After the crustaceans are swallowed 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 development cycle continues.The disease caused by 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 become ill with dracunculiasis, at this time purulent abscesses begin to form.The only salvation from pain is a lake.The relief is immediate, but upon contact with water the bubbles burst and the Guinea worm throws the larvae into the water.The crustaceans consume 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 removed by wrapping it around a stick.This takes days, sometimes weeks (you need to remove the worm slowly and carefully so it doesn't break).It was suggested 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 filament (filaria) or Bancroft string(Wuchereria bancrofti)
Appearance.White thread nematode, females 10 cm long and males 4 cm long.
Distribution. Tropics, subtropics of Asia, Africa, Central and South America.
Useful life. Adults usually occur in lymph nodes 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 penetrate deep into the body (into the pulmonary vessels and kidneys).This is due to the fact that the intermediate host is the mosquito, which usually sucks blood in the afternoon and at night.The larvae enter the mosquito's stomach and then the body cavity, where they grow, after which they accumulate near the proboscis, from where 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 lymphatic vessels and stagnation of lymph, which occurs due to clogging of lymphatic vessels by adult Bancroft filamentous individuals.The skin on the diseased part of the body becomes covered with ulcers.
Elephantiasis treatment aims to improve fluid flow.The use of anthelmintic medications is effective.In later stages, surgery may be necessary.
