INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE METAZOANS (EUMETAZOA)

PHYLUM NEMATHELMINTHES (ROUNDWORMS)

CLASS NEMATODA (NEMATODES)

Subclass Rhabditia

This is a large subclass comprising free-living saprobiotic and specialized parasitic forms. The vast majority of them are small animals, with only a few parasitic species exhibiting gigantism, reaching 40–50 cm (Ascaris) and even 1 m (Dracunculus medinensis).

Somatic bristles are absent; the main receptors are papillae; amphids are small, located on the Lips; onchia are small.

The most important orders of the subclass are Rhabditida, Tylenchida, Strongylida, Ascaridida, and Spirurida.

Order Rhabditida. Representatives of this order include free-living forms (aquatic, soil-dwelling) as well as parasites of invertebrate and vertebrate animals.

Many free-living rhabditids are saprophages that contribute to the Mineralization of organic matter and thus participate in nutrient cycling. This order also includes insect parasites, such as parasites of bark beetles, which help control the populations of these dangerous forest pests, as well as parasites of vertebrates, for example, Rhabdias bufonis, an inhabitant of amphibian Lungs.

Order Tylenchida

Tylenchids inhabit soil and freshwater bodies, with some having adapted to parasitism in plants and insects. Among them are stem nematodes (genus Ditylenchus), which infest potatoes, onions, buckwheat, beets, and other cultivated crops. Their characteristic feature is the presence in the Oral Cavity of a sharp stylet that can be protruded to pierce the thin walls of PLANT Cells AND inject digestive Enzymes into them. They exhibit extraintestinal Digestion, which liquefies the contents of plant cells, facilitating food Absorption in the gut.

Among plant parasites, mention should also be made of a group of nematodes (genus Meloidogyne) characterized by a swollen, nearly spherical body that tapers only at the anterior end (see Fig. 189). They parasitize plant roots, inducing Hypertrophy of the cells surrounding the parasite and The formation of a ROOT Swelling, or gall. A 5 mm piece of such a gall may contain up to 500 sexually mature nematodes. Galls stunt Plant Growth and frequently lead to their death. Root-knot nematodes parasitize many plant species, including greenhouse crops, where they are extremely difficult to control. They cause the greatest damage to cucumbers, tomatoes, peppers, as well as watermelons, melons, potatoes, carrots, sugar beets, etc. Some species parasitize insects.

Order Strongylida

Nematodes of this order parasitize the digestive tract of many mammals, including domestic animals (horses, cattle, small ruminants, pigs), as well as humans. The oral opening of these nematodes is surrounded by six lips, though lips are sometimes absent. The human parasite is the duodenal hookworm (Ancylostoma duodenale) (Fig. 204). These are relatively small worms: females grow up to 10–14 mm in length, and males reach 8–11 mm. The anterior end of their body is hooked in shape, which explains the common name of this parasite—hookworm. Sexually mature worms inhabit the duodenum and upper Small Intestine of humans. They embed their anterior end deeply into the mucous membrane. In their oral cavity (stoma), strongylids possess sharp Teeth with which they damage the integrity of the mucosa and feed on Blood.

Class="center">

Fig. 204. Order Strongylida: male Ancylostoma duodenale:

1 — Mouth; 2 — copulatory bursa; 3 — spicules

Fertilized females lay a tremendous number of eggs—up to 10,000 per day—and live for about five years. The eggs are expelled to the exterior with feces. Under favorable conditions (constant warmth and humidity), larvae hatch from the eggs within one to two days, undergo two molts, and become infective to humans. They can survive in the soil for a long time, even during winter, migrating to deeper soil layers (down to 1 m).

Human infection occurs via two routes: through the Skin and through the mouth. In the first case, larvae actively penetrate human skin during contact with soil (earthwork, working in mines, etc.). Subsequently, the larvae enter the bloodstream, are carried to the lungs, migrate into the respiratory tract and Pharynx, are swallowed, and reach the duodenum. The second route of infection is the ingestion of infective larvae with contaminated food or Water. The larvae enter the intestine and reach sexual maturity there without migration.

Ancylostoma duodenale causes a disease in humans known as miner's anemia or chlorosis. It is characterized by anemia, inflammation, and ulceration of the duodenum and lungs. In cases of high infection intensity, the disease can be fatal. This parasite is distributed mainly in subtropical and tropical countries and southern Europe, frequently affecting miners and earthwork laborers.

Order Oxyurida

Oxyurids differ from nematodes of other orders by a specific Structure OF THE Esophagus, the posterior end of which has a distinct spherical expansion called a bulb. These are obligate parasites of the digestive tract of vertebrates and Arthropods. The pinworm (Enterobius vermicularis) is a widely known human parasite that inhabits primarily the Large Intestine of children, causing an unpleasant disease known as enterobiasis. The length of adult females is 9–12 mm, and that of males is 3–5 mm (Fig. 205). The anterior end of the pinworm is surrounded by a transparent bubble-like cuticular expansion (vesicle). The posterior end of the male is spirally curved, while in the female it is awl-shaped, elongated, and pointed. Following copulation, the female lays eggs that mature within 4–6 hours. In the external environment, the eggs remain viable for up to three weeks.

A person becomes infected by ingesting infective eggs. In the small intestine, larvae emerge from the eggs and migrate to the large intestine. After 12–14 days, the worms reach sexual maturity. Following copulation, males usually die, while females gradually descend into the rectum, crawl out through the anus, and deposit eggs around the perianal region. The total lifespan of the parasite does not exceed one month. Throughout its life, a single female lays 5,000 to 17,000 eggs. By crawling in the perianal area, pinworms provoke severe itching, which most commonly occurs at night. Patients with enterobiasis experience disrupted Sleep, weakness, and decreased working capacity. Infection occurs via hands and food contaminated with pinworm eggs. The parasite's eggs accumulate in large numbers on undergarments and clothing. Enterobiasis is most common in children, who continuously re-infect themselves (autoinfection) with new batches of eggs. Therefore, the Prevention of enterobiasis, based on strict adherence to Sanitary and hygienic rules, is of utmost importance.

Fig. 205. Enterobius vermicularis (order Oxyurida) — female (a), male (b):

1 — tail; 2 — anus; 3 — genital pore; 4 — bulb; 5 — esophagus; 6 — mouth

Order Ascaridida

This order comprises exclusively parasites of vertebrates. Among them, the human roundworm (Ascaris lumbricoides), which causes the dangerous disease ascariasis, as well as the pig roundworm (A. suum) and others, are widely known. Ascarids reach considerable sizes, which distinguishes them from other nematodes. Ascaris lumbricoides occurs in almost all Regions of the globe (see Fig. 192, 193).

The spindle-shaped body of female ascarids reaches 40–44 cm in length. Males are somewhat smaller, measuring 12–25 cm, with their posterior end curved ventrally, and the oral opening surrounded by three lips. In the anterior half of the female's body, There is a small constriction on the ventral side of which the genital pore is located.

Sexually mature males and females live in the human intestine. Fertilized females lay eggs that pass outside with feces. Larvae develop within the eggs in moist soil. Because the eggs have a thick shell, they easily withstand drying and other adverse conditions. They are carried by dust, water, or flies and land on food products. In the human intestine, where the eggs arrive with food or water, larvae hatch from them, burrow through the intestinal wall, and enter the bloodstream. Carried by the blood, they migrate to the Liver, then to The Heart, and from there via the pulmonary artery to the lungs. Destroying the walls of the pulmonary capillaries, the larvae enter the alveoli, and from there migrate via the respiratory tract through the Larynx into the Digestive System, where they develop into adult males and females. During this migration through The Human Body, which lasts about two and a half months, the larvae molt and grow. Ascarids live for about 11–12 months.

Ascariasis proceeds in two stages: the first is associated with larval migration, and the second with the parasitism of adults in the intestine. During migration, the larvae damage the intestinal wall, liver, and especially the lungs. Patients exhibit allergic reactions to the parasite, such as urticaria, itching, and coughing. Sometimes Pneumonia, Pleurisy, or Bronchial Asthma even develop. The presence of adult worms in the host's intestine is accompanied by general intoxication of the Organism by the parasite's metabolic products, as well as intestinal dysfunction. Symptoms include decreased appetite, acute abdominal pain, sleep disorders, anemia, and intestinal volvulus and obstruction. Ascarids may migrate into the liver, causing blockage of the Bile ducts, or into the esophagus, pharynx, and through the oral cavity into the respiratory tract, which can lead to asphyxia. Ascarids cause particularly severe damage to children.

The source of ascariasis spread is infected individuals who release a huge number of eggs. Ascariasis is most frequently contracted by children and agricultural workers. Preventive sanitary and hygienic measures of great importance in combating ascariasis include: washing hands before eating, washing and heat Treatment (scalding with boiling water) of fruits and berries (especially strawberries and wild strawberries) before consumption, and disinfection of human feces.

The life cycles of other ascarid species are similar to that of the human roundworm, for instance in A. suum, which parasitizes exclusively in pigs.

Order Spirurida

Spirurids are the most species-rich order of nematode parasites of vertebrates. Representatives of this order parasitize all Tissues and Organs and cause dangerous diseases in humans and domestic animals. Among them, the human guinea worm (Dracunculus medinensis), known since ancient times, as well as the CAUSATIVE AGENT OF "elephantiasis" — Bancroft's filaria (Wuchereria bancrofti) — stand out.

The guinea worm, or human filarial worm (D. medinensis), is distributed mainly in countries with warm climates — in Africa, South Asia, Latin America, and certain regions of Central Asia. Females reach over 120 cm in length and 0.5–1.7 mm in width; males are considerably smaller, measuring 12–13 cm in length and 0.2–0.4 mm in width. The definitive host of the guinea worm is human, dog, or cat. Sexually mature females parasitize the subcutaneous tissue, localizing predominantly on the legs, and occasionally on the arms, neck, genitalia, etc. A blister forms around the parasite, which eventually ruptures, and the end of the coiled female protrudes outward. Upon contact with water (during bathing or washing a wound), the guinea worm's body wall bursts, and the Uterus protrudes, releasing a multitude of tiny larvae (microfilariae) into the water. For further development, the larvae must enter an intermediate host — the copepod crustacean Cyclops (Fig. 206).

Fig. 206. Life Cycle of Dracunculus medinensis (order Spirurida):

a — general appearance of the guinea worm; b, d — definitive hosts; c — intermediate host (Cyclops); e — infective larva (for the intermediate host)

The crustaceans swallow the microfilariae; from the Cyclops's intestine, the larvae penetrate the body cavity, grow, and become infective by the 12th day. Humans become infected by consuming raw water containing cyclops with infective larvae. Freed from the crustacean's tissues, the larvae penetrate the definitive host's intestinal wall and travel via the blood beneath the skin. After a few months, the parasites reach sexual maturity. Males die shortly after fertilizing the females.

Parasitizing beneath the skin, the guinea worm causes abscesses that cause great suffering to humans — when the parasite localizes near a joint, the patient becomes unable to move.

Dracunculiasis is a vector-borne zoonotic (naturally focal) disease. For example, in the territory of Uzbekistan, a focus of dracunculiasis persisted for a long time (until 1930) in the area of Old Bukhara, where the sole Sources of drinking water were ponds (hauzes) where people and dogs bathed and where cyclops multiplied in great numbers. In the fight against dracunculiasis, besides chemical preparations, preventive measures are of great importance: cleaning water bodies of organic matter that promotes cyclops reproduction, banning the consumption of raw water in areas where the disease is prevalent, and improving water supply systems. The TRADITIONAL METHOD OF treating guinea worm infection is winding the protruding end of the parasite's body onto a stick or gauze and slowly extracting it from the abscess over the course of two weeks.

Bancroft's filaria (Wuchereria bancrofti) also occurs in tropical and subtropical countries. These are thin, thread-like nematodes; females reach 10 cm in length (0.24–0.30 mm thick), and males reach 4 cm. Adult worms

inhabit Lymphatic vessels and nodes. Sexually mature females produce a huge number of larvae (microfilariae) that migrate into the bloodstream. The presence of microfilariae in the human body is characterized by a certain periodicity — during the day they localize in the Blood Vessels of The Heart and lungs, and at night they appear in peripheral vessels.

The intermediate hosts of Bancroft's filaria are blood-sucking mosquitoes, which are most active precisely at night. When feeding on the blood of infected humans carrying filariae, the larvae enter the mosquito's gut along with the blood, from where they penetrate the body cavity and musculature to continue their development. Eventually, they reach the mosquito's mouthparts. When feeding on the blood of a healthy person, the larvae actively emerge from the proboscis onto the skin surface and then penetrate the blood through cracks in the skin or the wound formed by the mosquito bite. After migrating through the human body, the parasites end up in The Lymphatic system, where they reach sexual maturity (Fig. 207). The worms live in the human body for up to 17 years, and microfilariae in the bloodstream remain viable for up to 70 days. They cause blockage of lymphatic vessels, leading to inflammation of the Lymph Nodes AND lymph stasis. This results in the pathological enlargement of various body parts, most commonly the limbs, and the affected areas can reach enormous sizes (elephantiasis).

Fig. 207. Life cycle of Wuchereria bancrofti (order Spirurida):

a — adult filarial worm from a human lymph node; b — definitive host; c — intermediate host (mosquito); d — microfilariae in the mosquito;

d — microfilariae in subcutaneous capillaries

Humans are the only definitive hosts of Bancroft's filaria. Therefore, the primary control measure against this parasite is, above all, the medical treatment of Selection/30.html">The population as the main source of infection. In addition, combating their vectors—blood-sucking mosquitoes—is of great importance.

Nematodes are one of the most numerous and widespread groups of animals. Almost all Multicellular animals and plants from various taxonomic groups, as well as human organs and tissues, have become habitats for nematodes.

Free-living nematodes play a tremendous role in the overall global biogeochemical cycle of matter and energy. The population density of nematodes in natural biocoenoses is enormous. The total number of nematodes inhabiting 1 m² of soil or the bottom of water bodies reaches millions of specimens. Nematodes are a critical group of organic matter decomposers, owing to their High Metabolic Rate and short life cycles combined with vast population sizes. No decomposition process occurs without the participation of nematodes. Along with Bacteria and lower Fungi, nematodes ensure the ultimate mineralization of organic material, returning it to the soil and creating conditions for The Development of countless new generations of living organisms. Furthermore, nematodes are integrated into ecosystem food webs. Feeding on bacteria and organic detritus, they in turn serve as food for numerous crustaceans and fish in aquatic biocoenoses, as well as for mites and insect larvae in the soil. A significant number of nematode species are predators that feed on other nematodes, rotifers, and the like. According to modern scientific data, a substantial portion of the Energy Flow in benthic systems passes through nematodes. Consequently, nematodes take an active part in energy transformation within ecosystems.

They also play an indirect role in marine ecosystems. Nematodes can stimulate the growth of bacterial flora. By secreting vast amounts of mucus, they alter the Physical Properties of soils.