Roundworms, or roundworms themselves (Nematoda), are a type of protostome, protocavity, bilaterally symmetrical moulting animals.
Spread. Nematodes are one of the most widespread types of animals that have been able to colonize a variety of habitats: from the crevice (the space between grains of sand) and moss communities to Arctic ice (such asTheristis Melnikovi And Cryonema crisum, found in the thickness of multi-year ice in the central part of the Arctic Ocean).Parasitic nematodes are of particular interest to researchers, partly due to the wide variety of their hosts.
Construction plan. Thin, fusiform body, tapering towards the ends, with a round section.The mouth is located at the front and the dust (anus) at the back.The outside of the body is covered with a multilayered elastic cuticle, a non-cellular formation secreted by the hypodermis.The hypodermis, or epidermis, is located beneath the cuticle.The muscles are represented by a layer of obliquely striated longitudinal muscle fibers.The primary body cavity (schizocele), without its 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 the muscular ectodermal pharynx with a triangular lumen.The pharynx leads to the endodermal midgut by a single layer of columnar epithelial cells.Then comes a short ectodermal intestine, which opens into the anus.
Excretory system. The excretory organs are unicellular glands that replaced the protonephridia.There is usually a cervical gland in the anterior part 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 (anaerobic decomposition of glycogen to butyric and valeric acid in parasites) is also possible.
Nervous system. The nervous system is scalariform.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 - pigment spots.The chemical sense organs, amphids, usually have the shape of a pocket, a spiral or a slit.They are located on the sides of the head and are especially well developed in males, as they help to find females.
Reproduction and development. Nematodes are dioecious animals.The internal genital organs are even and have a tubular structure.Reproduction is exclusively 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 moulting, accompanied by the loss of the cuticle.The third stage in some species (including the famous Caenorhabditis elegans) under unfavorable conditions turns into the so-called dauer stage - a resting larva.
Parasitism. Currently, of the more than 24,000 described nematode species, approximately half are parasitic.They can affect almost all tissues and organs: connective tissues, muscles, blood and lymph vessels, gonads, sense organs, as well as body cavities, etc.Among these 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 nematodes from the point of view of medical parasitology.
Human roundworm(Ascaris lumbricoides)
I wait.The body, pointed at the ends, is pinkish-white.Size: males - 15-25 cm, females - 20-40 cm.The body is covered with a ten-layered flexible cuticle that protects against mechanical stress and host digestive enzymes.
Spread. 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, dry climates, roundworms are less common.
Life cycle.The 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 intestine is approximately one year.Roundworms are dioecious, like other roundworms.A sexually mature female lays around 200 thousand oval-shaped eggs per day, which are released into the external environment with faeces.Nematodes 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 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 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 penetrate through the intestinal wall, enter the blood vessels and liver, 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, trachea and oral cavity.The 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 moulting (usually four).
Clinical picture of ascariasis. In the migratory phase of ascariasis, cough (helps the larvae 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 occurs.Symptoms: nausea, vomiting, stool disorders, loss of appetite.
Long-term effects of the infection: general decrease in performance, sleep disturbances.When worms enter the bile ducts and airways, 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 migraine.
Prevention. Wash your hands before eating and preparing food.Wash fruit and vegetables.Eggs are also carried by flies, so the fight against these dipterans using, for example, Velcro also helps to prevent ascariasis.
Interesting fact. There are studies showing the positive effects of roundworm infection on alleviating the 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 the mechanism is currently too poorly understood to draw reliable conclusions.
pinworms(Enterobius vermicularis)
I wait. Greyish-white nematode, males 2-5 mm long, females 8-14 mm long.The end of the tail is pointed (hence the name).At the anterior end of the body, a characteristic swelling of the esophagus is noted.
Life cycle.Pinworms parasitize the lower part of the small intestine and large intestine, causing enterobiasis.Lifespan 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 she dies.
The exit of females 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 very widespread, especially widespread in children due to failure to comply 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, headache, allergic manifestations, perianal itching (leads to sleep disturbances, increases irritability).
Trichinella(Trichinella spiralis)
Description.Small nematode 2-4 mm long.Parasitizes the mucosa of the small intestine.Distributed in Eurasia and North America.
Life cycle. A change of host is necessary for the development of Trichinella.Usually these are wild animals (foxes, wolves, bears, wild boars), as well as people and livestock.Females are anchored at the anterior end of the body in the intestinal epithelium and give birth to 1-2 thousand larvae.Ovoviviparity is typical: the hatching of the larvae from the eggs occurs in the female genital tract.The larvae are transported throughout the body via blood and lymph vessels and lodge 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, curled up, they reside in the future.After a few months, the capsule is immersed 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.
Having reached the stomach of the new host (after it has eaten the corpse of the previous one), the larvae free themselves from the capsule, penetrate the mucosa and within a couple of days, after having undergone four moults, they transform into adult worms.
Clinical picture of trichinosis. Increased temperature, swelling of the face, muscle pain, allergic reactions.
Prevention. Trichinosis is transmitted by food through contaminated meat.Therefore, to prevent the disease, meat must be subjected to veterinary examination and properly prepared: boiled for 2-3 hours.Cooking methods such as smoking and salting do not destroy Trichinella.
Whipworms(Trichocephalus trichurus)
I wait.The worm is whitish in color, about 4 cm long.The front part is thin, reminiscent of hair (hence the name).
Spread.They prefer countries with a humid and warm climate.
Life cycle.The worm parasites in the initial part of the large intestine, only in humans.Causes trichuriasis.The life span of a person is several years.The thin end penetrates the thickness of the mucosa of the intestinal wall.It feeds on tissue fluids 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 need to remain in the soil at a certain humidity and temperature (25-30°C) for a month.Subsequently, infection occurs when the eggs are ingested;the larvae emerge from them into the host's intestine, penetrate the intestinal villi and grow 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 mucosa of the colon and causes poisoning of the host with waste products.Whipworm is a bloodsucker, so it can lead to anemia.Whipworm is accompanied by abdominal pain, headache and dizziness.Because whipworm attaches to the intestinal wall, it is more difficult to remove from the host than other parasites.
Rishta(Dracunculus medinensis)
I wait.A thin whitish nematode, females are 30-120 cm long, males do not exceed 4 cm.There is a small thorn on the tail.
Distribution: tropical countries of Asia and Africa.
Life cycle.Infection occurs when drinking unboiled water containing copepods.Crustaceans in the stomach die under the influence of hydrochloric acid, but the larvae of the Guinea worm survive and spread throughout the body through the lymphatic system.They then penetrate the body cavity, moult there 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.Cool water is best for pain relief.
The development of the eggs forces the female to begin moving her "head" forward towards the skin surface, leaving an inflammatory process along its path, transforming 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, purulent abscesses begin to form at this time.The only salvation from pain is a pond.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 bladder and the worm is gradually pulled out, wrapping it around a stick.It takes days, sometimes weeks (you have to take the worm out slowly and carefully so it doesn't break).It has been 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's filament (filaria), or Bancroft's thread(Wuchereria bancrofti)
I wait.White thread nematode, females 10 cm long, males 4 cm long.
Distribution. Tropics, subtropics of Asia, Africa, Central and South America.
Life cycle. Adults usually find themselves in the lymph glands and vessels, blocking 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 mosquitoes, which usually suck blood in the evening and at 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's filaments cause elephantiasis, or elephantiasis, or elephantiasis.It is worth noting that this disease can also be caused by other roundworms.
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, caused by inflammatory thickening of the walls of the lymphatic vessels and stagnation of lymph, which occurs due to obstruction of the lymphatic vessels by adult Bancroft's filamentous individuals.The skin of the diseased part of the body becomes covered with ulcers.
Treatment of elephantiasis is aimed at improving the outflow of fluids.The use of anthelmintic drugs is effective.In the later stages, surgery may be necessary.
