Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavity, bilaterally symmetrical moulting animals.
Spreading. Nematodes are one of the most widespread types of animals that have been able to colonize a variety of habitats - from the interstitium (the space between grains of sand) 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 due to the wide variety of their hosts.
Building plan. Thin 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 rear.The outside 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.The 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 anterior 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 from a single layer of columnar epithelial cells.Next comes a short ectodermal hindgut, which opens into the anus.
Excretory system. Excretory organs are unicellular glands that replaced protonephridia.There is usually one cervical gland in 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 breakdown 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 running along the ventral and dorsal lines are more powerful and are connected by semicircular nerve bridges (commissures).
Sense organs. There are papillae and setae - organs of touch located around the mouth.Some marine representatives have primitive eyes - pigment spots.Chemical sense organs, amphids, usually have the shape of a pocket, spiral or slit.They are located on the sides of the head end 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 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 shedding of the cuticle.The third stage in some species (including the famous Caenorhabditis elegans) under unfavorable conditions, it changes into the so-called dauer stage - a resting 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 tissues, muscles, blood and lymphatic vessels, gonads, sensory organs, as well as the body cavity, etc. Among them there are both 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 with a ten-layer flexible cuticle that protects from mechanical stress and digestive enzymes of the host.
Spreading. 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 roundworm due to the use of human excrement as fertilizer.In areas with hot, dry climates, roundworm is less common.
Life cycle.Development proceeds 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 lifespan in the intestines 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 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 the air temperature.Such 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, the larvae burrow through 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 move from the blood into the pulmonary vesicles, bronchi, windpipe 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. At the migratory stage of ascariasis, a cough is observed (helps the larvae get into the throat), chest pain, allergic reactions, and fever.
At the intestinal stage, damage to the intestinal mucosa and poisoning of the body with toxic metabolic products occurs.Symptoms: nausea, vomiting, stool disorders, loss of appetite.
Long-term effects of infection: general decrease in performance, sleep disturbances.When worms crawl into 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 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 on relieving symptoms of autoimmune diseases and increasing fertility in women.Scientists attribute this to the parasites' effect on the immune system by influencing the level of T cells in the body, but at the moment the mechanism is too poorly understood to draw reliable conclusions.
Pinworm(Enterobius vermicularis)
Appearance. Grayish-white nematode, males 2-5 mm long, females 8-14 mm long.The tail end is pointed (hence the name).At the anterior 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 enterobiasis.Life span is 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 from 5 to 15 thousand eggs on the skin near the anus, after which it dies.
The crawling out of females is accompanied by itching.When scratching the skin, eggs are transferred to the hands and more.Flies are also involved in the transfer of eggs.Infection occurs through ingestion.Larvae hatch from eggs that enter the intestines.
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 vaccinations.
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.
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 are anchored by the anterior end of the body into the intestinal epithelium and give birth to 1-2 thousand larvae.Ovoviviparity is typical: the 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 muscle tissue, causing the host to form a capsule in which, curled up, they reside in the future.After a few 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.
Once in the stomach of the new host (after it has eaten the corpse of the previous one), the larvae are freed from the capsule, penetrate the mucous membrane and within a couple of days, having undergone four molts, turn into adult worms.
Clinical picture of trichinosis. Increased temperature, puffiness of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted by food through contaminated meat.Therefore, to prevent the disease, 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 end is thin, reminiscent of hair (hence the name).
Spreading.They prefer countries with a humid and warm climate.
Life cycle.The worm parasitizes in the initial part of the large intestine, only on humans.Causes trichuriasis.The lifespan of a person is several years.The thin end penetrates into 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 need to remain in the soil at a certain humidity and temperature (25-30 ° C) for a month.After this, infection occurs when the eggs are swallowed; larvae emerge from them in the host’s intestines, penetrate the intestinal villi and grow in them for about a week.Then, having destroyed the villi, they exit into the intestinal lumen, reach the large intestine, become established 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 products.Whipworm is a hematophage, so it can lead to anemia.Trichocephalosis is accompanied by abdominal pain, headaches and dizziness.Because the whipworm attaches to the intestinal wall, it is more difficult to remove from the host than other parasites.
Guinea worm(Dracunculus medinensis)
Appearance.A thin whitish nematode, females 30-120 cm in length, males no more than 4 cm. There is a small spine on the tail.
Spreading: tropical countries of Asia and Africa.
Life cycle.Infection occurs when drinking unboiled water containing copepods.The crustaceans in the stomach die under the influence of hydrochloric acid, but the guinea worm larvae survive and are spread throughout the body through the lymphatic system.Then they penetrate into the body cavity, there they molt and reach sexual maturity.After mating, the male dies, and the female moves into the subcutaneous tissue, where a purulent abscess is formed, accompanied by burning and pain.Cool water is best for pain relief.
The development of the eggs forces the female to begin to move “head” forward towards the skin surface, 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 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 swallowed by the definitive host, under the influence of stomach acid, the crustaceans dissolve, and the larvae easily enter the intestine, make their way 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 fallen ill with dracunculiasis, at this time purulent abscesses begin to form.The only salvation 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.The 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 pulled out, wrapping it around a stick.This takes days, sometimes weeks (you have to pull out 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 staff of Asclepius entwined with a snake.
Bancroft's filament (filaria), or Bancroft's string(Wuchereria bancrofti)
Appearance.White thread nematode, females 10 cm long, males 4 cm long.
Spreading. Tropics, subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually occur 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 go deep into the body (into 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 stomach of the mosquito, 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 picture and treatment of elephantiasis. An 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 clogging of the lymphatic vessels by adult Bancroft's filamentous individuals.The skin on the diseased part of the body becomes covered with ulcers.
Treatment of elephantiasis is aimed at improving fluid outflow.The use of anthelmintic drugs is effective.In later stages, surgery may be required.
