Nematodes, or roundworms themselves (Nematoda), are a type of protostomes, protocavities, molting animals with bilateral symmetry.
Broadcast. Nematodes are one of the most widespread types of animals, capable of colonizing a variety of habitats – from interstitium (the space between sand grains) and moss communities to arctic ice (such asTheristis Melnikovi And Cryonema crissum, found in the thickness of multi-year ice in the central Arctic Ocean).Parasitic nematodes are of particular interest to researchers, particularly because of the wide variety of their hosts.
Construction plan. Thin fusiform body, tapering towards the ends, round in section.The mouth is located at the front and the powder (anus) at the back.The exterior of the body is covered with a multi-layer 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 obliquely striated longitudinal muscle fibers.The primary body cavity (schizocele), lacking 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 single-celled glands that have replaced the protonephridia.There is usually a cervical gland at 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 eliminated from the body.
Circulatory and respiratory systems. These systems are lacking.Breathing takes place 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 scalariform.Represented by a nerve ring and six longitudinal trunks.The two nerve trunks which 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 - pigment spots.The chemical sense organs, amphids, are generally shaped like a pocket, spiral, or slit.They are located on the sides of the head and are particularly well developed in males, as they help find 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 posterior end of the body is curved.Fertilization is internal and viviparity occurs.During their development, nematodes go through four larval stages, separated by a molt, which is accompanied by a fall of the cuticle.The third stage in certain species (including the famous Caenorhabditis elegans) under unfavorable conditions, it passes into the so-called dauer stage - a resting larva.
Parasitism. Currently, of more than 24,000 described species of nematodes, approximately 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 are both ecto- and endoparasites of plants, vertebrate and invertebrate animals, including other nematodes, and even protozoa.
Here is a description 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 flexible ten-layer cuticle that protects against mechanical stress and digestive enzymes of the host.
Broadcast. 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 feces as fertilizer.In areas with hot, dry climates, roundworms are less common.
Life cycle.The development is taking place without 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).Lifespan in the intestines is approximately one year.Roundworms are dioecious, like other nematodes.A sexually mature female lays about 200,000 oval-shaped eggs per day, which are released into the external environment along with feces.Roundworms are classified as soil-transmitted helminths – they require the development of a larval stage in the soil.When exposed to favorable conditions (moist soil at a temperature of around 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.Such eggs containing a larva may 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 the liver, then migrate through the inferior vena cava to the right atrium and right ventricle.From these, 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 intestines and become sexually mature after three months.The process of "growth" in nematodes is associated with molting (usually four of them).
Clinical picture of ascariasis. In the migratory stage of ascariasis, cough (helps the larvae enter the throat), chest pain, allergic reactions and fever are observed.
At the intestinal stage, damage to the intestinal mucosa and poisoning of the body with toxic metabolic products occurs.Symptoms: nausea, vomiting, bowel movements, loss of appetite.
Long-term effects of infection: general reduction in performance, sleep disturbances.When worms crawl into the bile ducts and respiratory tract, the outcome 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 hands before eating and preparing food.Wash vegetables and fruits.Eggs are also carried by flies, so combating 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(Eterobius vermicularis)
Appearance. Grayish white nematode, males 2 to 5 mm long, females 8 to 14 mm long.The end of the tail is pointed (hence its 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 and large intestines, causing enterobiasis.Shelf life is 1 to 2 months.The anterior end of the pinworm attaches to the intestinal wall.A sexually mature female exits the large intestine through the anus and lays 5 to 15 thousand eggs on the skin near the anus, after which she dies.
Crawling out of females is accompanied by itching.By scratching the skin, the eggs are transferred to the hands and more.Flies also participate in egg transfer.Infection occurs through ingestion.The larvae hatch from eggs that enter the intestines.
Epidemiology and clinical picture of enterobiosis. Enterobiosis is widespread, especially common among 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 to 4 mm long.Parasite 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.These usually include 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 2,000 larvae.Ovoviviparity is typical: hatching of larvae from eggs occurs in the female genital tract.The larvae are carried throughout the body by blood and lymphatic vessels and settle in striated muscles.At this stage they have a stylus, they use it to destroy the 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 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 eating the corpse of the previous one), the larvae are released from the capsule, penetrate the mucous membrane and after a few days, having undergone four molts, transform into adult worms.
Clinical picture of trichinosis. Increased temperature, swelling of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted through food via 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 and measures approximately 4 cm in length.The front is thin, reminiscent of hair (hence the name).
Spreading.They prefer countries with humid and warm climates.
Life cycle.The worm parasitizes the initial part of the large intestine, only in humans.Causes trichuriasis.The lifespan of a person 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 1-3 thousand eggs, which are released into the external environment along with feces.Like the roundworm, the whipworm is related to soil-transmitted helminths: for the eggs to become invasive, they must 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;the larvae emerge in the host's intestines, penetrate the intestinal villi and develop there for about a week.Then, after destroying the villi, they exit 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.The whipworm is a blood feeder, so it can cause 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.
Richta(Dracunculus medinensis)
Appearance.A slender, whitish nematode, females measuring 30 to 120 cm in length, males not exceeding 4 cm.There is a small thorn on the tail.
Distribution : 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 guinea worm larvae survive and spread throughout the body through the lymphatic system.Then they enter the body cavity, molt there and reach sexual maturity.After mating, the male dies, and the female moves to the subcutaneous tissue, where a purulent abscess is formed, accompanied by burning and pain.Cool water is the best solution to relieve pain.
The development of eggs forces the female to begin to move her “head” towards the surface of the skin, leaving on its way an inflammatory process that turns 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 their development is not interrupted, the larvae must infect the cyclops crustacean, which is an intermediate host.The larvae that remain in the water die.Once the crustaceans are swallowed by the definitive host, under the influence of gastric 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 already has dracunculiasis, purulent abscesses begin to form at this time.The only salvation from pain is a pond.The relief is immediate, but on 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 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 apart).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 stick of Asclepius entwined with a snake.
Bancroft filament (filaria) or Bancroft cord(Wuchereria bancrofti)
Appearance.White thread nematode, females 10 cm long, males 4 cm long.
Distribution. Tropics, subtropics of Asia, Africa, Central America and South America.
Life cycle. Adults are usually found in lymph nodes and lymph vessels, obstructing lymphatic drainage and causing persistent swelling.Females produce larvae - nocturnal microfilariae, which appear at night in the peripheral blood and penetrate deep into the body during the day (into the pulmonary vessels and kidneys).This is because the intermediate host is the mosquito, which usually sucks 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 which they are transmitted to humans by sucking blood.Bancroft filaments cause elephantiasis, or elephantiasis, or elephantiasis.It should be noted that this disease can also be caused by other nematodes.
Clinical picture and treatment of elephantiasis. Hypertrophy of any part of the body is due to hyperplasia (painful growth) of the skin and subcutaneous tissue, caused by inflammatory thickening of the walls of lymphatic vessels and stagnation of lymph, due to clogging of lymphatic vessels by adult Bancroftian filamentous individuals.The skin of the diseased part of the body becomes covered with ulcers.
Treatment of elephantiasis aims to improve fluid flow.The use of anthelmintic drugs is effective.In later stages, surgery may be necessary.
