Nematodes, or roundworms themselves (Nematoda), are protostome, 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 interstitium (the space between sand grains) and lichen communities to arctic ice (such asMelnikovi therist and Cryonema crissum, found in multi-year ice thickness in the central Arctic Ocean).Parasitic nematodes are of great interest to researchers, partly due to the wide variety of their hosts.
Building plan. Body thin fusiform, tapering towards tip, round in cross-section.The mouth is at the front end, and the anus is at the back.The outer part of the body is covered with a multi-layered elastic cuticle - a non-cellular formation secreted by the hypodermis.The hypodermis, or epidermis, lies beneath the cuticle.Muscles are represented by a layer of longitudinal oblique striated muscle fibers.The primary body cavity (schizocoel), without its own epithelial lining, is filled with fluid.
Digestive system. The mouth 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 into the endodermal midgut from a single layer of columnar epithelial cells.Next comes the short ectodermal hindgut, which opens into the anus.
Excretory system. The excretory organ is a unicellular gland that replaces the 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 collect insoluble metabolic products that are not removed from the body.
Circulatory and respiratory systems. This system does not exist.Respiration 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.Represented by a nerve ring and six longitudinal rods.The two nerve trunks running along the ventral and dorsal lines are stronger and are connected by semicircular nerve bridges (commissures).
Sense organs. There are papillae and setae - touch organs located around the mouth.Some marine representatives have primitive eyes - pigment spots.Chemical sense organs, amphids, usually have a pocket, spiral or slit shape.It is located on the side of the tip of the head and is well developed in males, as it helps in finding females.
Breeding 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 rear end of the body is curved.Fertilization is internal and viviparity occurs.In 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, it turns into a stage called dauer - a resting larva.
Parasitism. Currently, of the more than 24,000 species of nematodes described, about half are parasites.They can affect almost all tissues and organs: connective tissue, muscles, blood and lymphatic vessels, gonads, sensory organs, as well as body cavities, etc.Among them there are both ecto- and endoparasites of plants, vertebrate and invertebrate animals, including other nematodes, and also protozoa.
The following is a description of the most important representatives of roundworms from the point of view of medical parasitology.
human roundworm(Ascaris lumbricoides)
Appearance.The body, pointed at the end, is pinkish white.Dimensions: male - 15-25 cm, female - 20-40 cm.The body is covered with a ten-layer flexible cuticle that protects against mechanical stress and host digestive enzymes.
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 roundworms due to the use of human excrement as fertilizer.In areas with hot and dry climates, roundworms are less common.
Life cycle.Development continues without changing owners.Adult worms are parasites of the small intestine, causing ascariasis.A person is usually affected by several dozen roundworms (the record is 900 pieces).The lifespan in the intestine is about one year.Ringworms 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 the 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), the larvae develop in the eggs.The development period varies from 16 days to several months and depends on the air temperature.Such eggs containing larvae 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 penetrate the intestinal wall, enter the blood vessels and the 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: larvae are swallowed, enter the intestine 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. In the migration stage of ascariasis, coughing is observed (helping the larvae enter the throat), chest pain, allergic reactions, and fever.
At the intestinal level, 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 decline in performance, sleep disturbances.When the worms crawl into the bile ducts and respiratory tract, the results are 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. Wash hands before eating and preparing food.Wash vegetables and fruits.Eggs are also carried by flies, so combating this dipteran using, for example, Velcro also helps prevent ascariasis.
Interesting facts. There are studies showing the positive effects of ringworm infection on relieving the symptoms of autoimmune diseases and improving female fertility.Scientists attribute this to the parasite's effect 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.
pinworms(Enterobius vermicularis)
Appearance. Gray white nematode, male 2-5 mm long, female 8-14 mm long.The tip of the tail is pointed (hence the name).At the anterior end of the body, the characteristic swelling of the esophagus is noticeable.
Life cycle.Pinworms parasitize the lower part of the small intestine and large intestine, causing enterobiasis.Shelf life 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 5 to 15 thousand eggs on the skin near the anus, after which it dies.
Crawling out of the female 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 intestine.
Epidemiology and clinical picture of enterobiasis. Enterobiasis is widespread, especially common in children due to non-compliance with personal hygiene rules and "overcrowding" in kindergartens and schools.Transmitted from person to person without an intermediate host.Reduces the effect of vaccination.
Symptoms: abdominal pain, loss of appetite, headache, allergic manifestations, perianal itching (leads to sleep disturbances, increases irritability).
Trichinella(Trichinella spiralis)
Description.Small nematodes 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 host is necessary.Usually these are wild animals (foxes, wolves, bears, wild boars), as well as humans and livestock.The female anchors by the anterior end of the body into the intestinal epithelium and gives birth to 1-2 thousand larvae.Ovoviviparity is typical: hatching of larvae from eggs occurs in the female genital tract.Larvae are carried throughout the body through blood and lymph vessels and settle in 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, rolled up, they live in the future.After several months, the capsules are soaked in lime.Such 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 it eats the corpse of the previous one), the larva is released from the capsule, penetrates the mucous membrane and in a few days, after undergoing four molts, turns into an adult worm.
Clinical picture of trichinosis. Increased temperature, swelling of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is spread through food through contaminated meat.Therefore, to avoid the disease, the meat must undergo a veterinary inspection 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.Worms are parasitic in the early part of the large intestine, only in humans.Causes trichuriasis.A person's lifespan is several years.The thin tip 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 roundworms, whipworms are 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 intestine, penetrate the intestinal villi and grow inside for about a week.Then, after destroying the villi, they go out into the lumen of the intestine, reach the large intestine, become established there and reach maturity within a month.
Clinical picture of trichocephalosis. Worms damage the mucous membrane of the colon and cause poisoning of the host with waste products.Whipworms are hematophagous, so they can cause anemia.Trichocephalosis is accompanied by abdominal pain, headache and dizziness.Because whipworms attach to the intestinal wall, they are more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
Appearance.Thin white nematode, female 30-120 cm long, male 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 raw water containing copepods.The crustaceans in the stomach die under the influence of hydrochloric acid, but the guinea worm larvae survive and spread throughout the body through the lymphatic system.Then they penetrate into the body cavity, where they molt 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 best for pain relief.
The development of the egg forces the female to start moving "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 uterus of the female enters the water, it breaks, and the larvae that hatch from the eggs come out.To ensure uninterrupted development, the larvae must infect cyclops crustaceans, which are intermediate hosts.Larvae that remain in the water will die.After the crustacean is swallowed by the definitive host, under the influence of stomach acid, the crustacean dissolves, and the larvae easily enter the intestine, pass through its wall and end up in the lymph nodes, where the cycle of development continues.The disease caused by guinea worms 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 fallen ill with dracunculiasis, at this time a purulent abscess begins to form.The only salvation from pain is the pool.The relief is immediate, but upon contact with water, the bubbles burst and the guinea worms expel the larvae into the water.Crustaceans eat 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 the wood.This takes days, sometimes weeks (you have to remove the worm slowly and carefully so it doesn't break).It has been suggested that the appearance of guinea worms wrapped around a stick became a kind of prototype for a medical symbol - the rod of Asclepius entwined with a snake.
Bancroft's filament (filaria), or Bancroft's string(Wucheria bancrofti)
Appearance.White thread nematode, female 10 cm long, male 4 cm long.
Distribution. Tropical, subtropical Asia, Africa, Central and South America.
Life cycle. Adults usually occur in lymph nodes and vessels, obstructing lymph flow 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 and renal tracts).This is due to the fact that the intermediate hosts are mosquitoes, which usually suck blood in the evening and night.The larvae enter the mosquito's stomach, then into 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.Keep in mind 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 lymphatic stagnation, which occurs due to blockage of lymphatic vessels by adult Bancroft filamentous individuals.The skin on the diseased part of the body becomes covered with ulcers.
Treatment of elephantiasis aims to increase the outflow of fluid.The use of anthelmintic drugs is effective.At a later stage, surgery may be required.
