BASICS OF MEDICAL BIOLOGY - 2012

Medical Helminthology

Medical helminthology is a branch of medical parasitology that studies parasitic worms (helminths) in humans, the diseases they cause, and control measures against them. Diseases caused by helminths are called helminthiases.

The Classification of helminths into bio- and geohelminths was proposed by K.I. Skryabin and R.S. Shults (1931). Helminths whose development involves the mandatory change of hosts (intermediate and definitive) are called biohelminths (e.g., cat Liver fluke, beef tapeworm). Geohelminths develop without intermediate hosts (e.g., Ascaris, whipworm). Soil is the most favorable environment for The Development of their eggs, which is where the term "geohelminths" comes from (Greek geo - earth). Accordingly, helminthiases are divided into biohelminthiases and geohelminthiases. A person infested, i.e., infected with bio- or geohelminths, is not directly contagious to others.

In 1952, E.S. Shulman proposed identifying an additional group—contact helminthiases—which included two infections: enterobiasis and hymenolepiasis. The Specific features of their Epidemiology do not allow them to be classified as either biohelminthiases or geohelminthiases. One can become infected with the eggs of these helminths directly from an infected person, primarily via hands contaminated with the eggs.

Human parasitic worms belong to two phyla: 1) Phylum Flatworms (Plathelminthes) and 2) Phylum Roundworms (Nemathelminthes).

Class="center">Phylum Flatworms (Plathelminthes)

The phylum Flatworms comprises about 7,300 species. In the course of evolution, flatworms originated from Coelenterates and, in turn, gave rise to roundworms and segmented worms. Animals of this phylum are characterized by: 1) triploblastic development (ectoderm, endoderm, and mesoderm develop in embryos); 2) presence of a integumentary-muscular sac (body wall fused with Muscles); 3) absence of a body cavity (the space between Organs is filled with parenchyma); 4) bilateral body Symmetry; 5) dorsoventrally flattened body shape; 6) presence of well-developed Organ Systems: muscular, digestive, excretory, nervous, and reproductive; 7) absence of circulatory and respiratory systems.

Of medical importance are parasitic flatworms of humans that belong to two classes: 1) Class Trematodes (Trematoda) and 2) Class Tapeworms (Cestoidea).

Class Trematodes (Trematoda)

This class comprises about 3,000 species. All trematodes are parasitic organisms. The body is unsegmented, mostly leaf-shaped, and dorsoventrally flattened. A characteristic feature is the presence of suckers, usually two: oral and ventral (hence the class name). Suckers function as organs of attachment of the parasite to the host's body. The body is covered with an integumentary-muscular sac consisting of the tegument (outer covering) and muscles fused with it. The musculature consists of three layers: circular, diagonal, and longitudinal. The cuticle is covered with epithelium and often bears tiny spines.

The Digestive System consists of an ectodermal foregut (Mouth, muscular Pharynx, Esophagus) and two branches (branched or unbranched) of the endodermal midgut that end blindly. There is no anus. Undigested food residues are expelled through the mouth.

The excretory system is of the protonephridial type. It is represented by numerous stellate (star-shaped) terminal Cells containing canaliculi with a tuft of cilia (flame cells). Metabolic products from the parenchyma enter the lumen of the terminal Cell canaliculus, and from there, driven by the movement of cilia, they advance into one or two secretory canals that open to the outside via an excretory pore.

The Nervous system has a Structure typical of flatworms (orthogon). It consists of a circumpharyngeal nerve ring from which three pairs of nerve trunks extend, with the lateral ones being the most developed. The nerve trunks are interconnected by commissures. Overall, the nervous system resembles a lattice (gangliate-ladder type of nervous system).

Almost all trematodes are hermaphrodites. The Male Reproductive System consists of two Testes, two vas deferens that merge into an ejaculatory duct, and a copulatory organ (cirrus). The cirrus is enclosed in a cirrus sac located above the ventral sucker. The FEMALE REPRODUCTIVE SYSTEM is complex. The Ovary, vitelline glands (vitellaria), and seminal receptacle open into the ootype, where Fertilization and the final formation of eggs take place. Nutrient material for the eggs from the vitellaria and secretions from special glands (Mehlis' glands) also flow here. From the ootype, the eggs move into the Uterus and are excreted to the outside through the genital opening located next to the male opening. In some trematodes, fertilization occurs in the seminal receptacle. Fertilization is usually cross-fertilization. Self-fertilization is observed less frequently.

Life cycle involves alternation of hosts and multiple larval generations. Definitive hosts are vertebrates and humans, while the intermediate host is invariably a mollusk. In addition, some trematodes have a second intermediate host. A characteristic feature of The life cycle is the reproduction of larval stages by parthenogenesis (without fertilization). The sexually mature form of trematodes is called a marita.

Medical significance. Several species of trematodes parasitize humans. Diseases caused by trematodes are called trematodiases.

Liver fluke (Fasciola hepatica) is the CAUSATIVE AGENT OF fascioliasis.

Geographical distribution: ubiquitous.

Localization: intrahepatic Bile ducts, Gallbladder, Pancreas.

Morphology. Marita: size 3-5 cm, anterior end elongated into a cone shape (Fig.). The ventral sucker is located close to the oral sucker. Immediately behind the ventral sucker lies the multilobed uterus, behind which is the ovary; the testes occupy the entire middle part of the body. The two intestinal branches have numerous ramifications. The ovary and testes are also branched. Numerous vitellaria are located along the sides of the body. Eggs are yellow-brown, oval, with a lid (operculum) at one of the poles; dimensions are 135x80 µm.

Life cycle. The liver fluke is a biohelminth. Development occurs with a change of hosts (Fig.). Definitive hosts include herbivorous mammals, cattle, small ruminants, horses, pigs, rabbits, and occasionally humans. There is a single intermediate host—the pond snail Limnea (Galba) truncatula. Eggs are released into the environment with the feces of the definitive host and must reach Water. Here, a ciliated larva—the miracidium—develops inside the egg. The miracidium possesses protonephridia, a nerve ganglion, an eyespot, and specialized Germ Cells. The anterior end of the body features an apical gland, which produces Enzymes capable of dissolving living tissue upon penetration into the intermediate host. In the light, the egg operculum opens, and the miracidium leaves the egg membranes. It swims actively, nourishes itself on the egg's nutrients, and then actively penetrates the body of the mollusk, enters the liver, and transforms into a sporocyst. Sporocysts are sac-like, immobile, and lack excretory and nervous systems. Inside the sporocyst, new larvae—rediae—develop parthenogenetically from germ cells (i.e., without fertilization). Rediae possess a mouth, pharynx, blind gut, and germ cells, from which the next larval stage—cercariae—develops within the body of the redia. A cercaria has a long muscular tail that provides propulsion. The cercaria leaves the mollusk, swims actively in water, then attaches to aquatic plants or other objects, casts off its tail, encysts, and transforms into an adolescaria. Cattle feed on plants and thereby swallow the adolescents (adolescariae). Humans become infected by drinking raw water from open, especially stagnant, bodies of water or by consuming unwashed vegetables and greens (most often lettuce) irrigated with water from a river or lake containing adolescent stages. In the human intestine, the cysts dissolve, releasing young flukes that migrate into the host's liver. Thus, in the Life Cycle of Fasciola, the infective stage is the miracidium for the intermediate host (mollusk) and the adolescaria for the definitive hosts.

Pathogenic action. Fascioliasis is a biohelminthosis, a natural focal disease, and a zoonosis. Fasciola flukes cause toxic-allergic reactions, mechanical damage to the bile ducts and liver Tissues, and lead to obstructive jaundice due to the blockage of biliary tracts. Prolonged infection results in inflammation of the biliary tracts with fibrous tissue proliferation, and liver cirrhosis.

Laboratory Diagnostics. Detection of eggs in the feces of an infected person. Eggs may also be detected in a healthy person if they have eaten the liver of animals infected with Fasciola (transit eggs). To avoid diagnostic errors, liver should be eliminated from the patient's diet and the analysis repeated.

Prophylaxis. Personal: do not drink raw water from open, especially stagnant, water bodies; thoroughly wash vegetables eaten raw. Public: veterinary measures, Treatment of infected animals (deworming), pasture rotation, and health education.

Lancet liver fluke (Dicrocoelium lanceatum) is the causative agent of dicrocoeliosis.

Geographical distribution: ubiquitous.

Localization: bile ducts of the liver, gallbladder, pancreas.

Morphology. The body is lanceolate in shape, with a size of 8-10 mm. There are two suckers: oral and ventral. Two unbranched intestinal branches extend along the sides of the body toward the posterior end, where they end blindly. Two rounded testes are located behind the ventral sucker. The female reproductive system consists of a small, rounded ovary situated behind the testes, paired vitelline glands lying along the sides of the body, and a highly developed uterus occupying the posterior part of the body (see figure). The eggs are elongated and asymmetrical, with an operculum at the narrowed end; their color ranges from yellowish to dark brown, and they measure 38-45 µm.

Life cycle. The lancet liver fluke is a biohelminth. Its life cycle involves a change of hosts. The definitive hosts include small and large ruminants, other herbivorous mammals, and occasionally humans. There are two intermediate hosts: the first are land snails of the genera Helicella and Zebrina, and the second are ants. The eggs are released into the external environment with the feces of the definitive host and must be swallowed by a snail. The egg already contains a miracidium. Inside the snail, the miracidium hatches from the egg membranes, penetrates the liver, and transforms into a first-order sporocyst, which develops second-order sporocysts, which in turn produce cercariae. The redia stage is absent. The cercariae emerge from the sporocysts and migrate to the Lungs of the snail, where they encyst, grouping together in clusters to form collective cysts. The latter are expelled outward with mucus and deposited onto vegetation. If eaten by the second intermediate host (ants of the genus Formica), each cercaria sheds its coat inside the ant's body and develops into the next larval stage—the metacercaria. Definitive hosts become infected by ingesting ants containing metacercariae. Humans are infected accidentally, also by swallowing ants infected with metacercariae. Infective stages: for the first intermediate host (snail) — an egg containing a miracidium; for the second intermediate host (ant) — a cercaria; for the definitive hosts — a metacercaria.

Pathogenic effect. Dicrocoeliosis is a biohelminthiasis, a natural focal disease, and a zoonosis. It is accompanied by impaired liver function and inflammatory processes, which may lead to cirrhosis. The liver is tender and enlarged. Obstruction of the bile ducts can cause jaundice.

Laboratory diagnostics. Detection of eggs in the feces of an infected person. Transit eggs may also be present.

Prevention. The main measures are veterinary: treatment of infected animals (deworming), pasture rotation, and health education. Personal prevention includes washing vegetables and fruits before consumption.

Cat fluke or feline liver fluke (Opisthorchis felineus) is the causative agent of opisthorchiasis. The disease was first described in Siberia in 1891 by K.N. Vinogradov.

Geographical distribution: along the banks of rivers in Western Siberia, particularly the Ob-Irtysh basin. Isolated foci have been recorded in the Baltic states and the basins of the Kama, Volga, Dnieper, and Southern Bug rivers.

Localization: bile ducts of the liver, gallbladder, pancreas, and its ducts.

Morphology. The anterior end of the body is narrower than the posterior. Size: 8-13 mm. The intestinal branches are unbranched and end short of the posterior end of the body. The uterus occupies the middle part of the body. Behind it are the rounded ovary and bean-shaped seminal receptacle. In the posterior part of the body lie two rosette-shaped testes, between which the S-shaped canal of the excretory system is visible. The vitellaria are located between the intestinal canals and the body margin. The eggs are very small (23-34 × 10-19 µm), oval, slightly asymmetrical, narrowed at the anterior pole, and yellow in color. The narrowed pole bears an operculum, while the opposite pole features a distinct knob; the shell is thick and double-contoured, and the content is finely granular and includes a miracidium.

Fig. 52. Life cycle of the liver fluke (Fasciola hepatica).

Life cycle. The cat fluke is a biohelminth with a developmental cycle involving host alternation. Definitive hosts include humans, cats, dogs, foxes, otters, muskrats, and other fish-eating mammals. There are two intermediate hosts: the first is the freshwater snail Bithynia leachi, and the second comprises cyprinid fish (ide, roach, common carp, asp, etc.). The parasite's eggs are eliminated into the environment with the feces of the definitive host, must reach water, and be swallowed by a snail. Inside the snail, the miracidium emerges from the egg, penetrates the liver, and transforms into a sporocyst. Rediae develop parthenogenetically within the sporocyst, followed by cercariae. The latter leave the snail into the water, swim freely, and are subsequently swallowed by fish or attach to their bodies, actively penetrating the subcutaneous tissue and muscles. Here they encyst and transform into metacercariae. Two membranes surround the parasite: a hyaline membrane produced by the parasite and a Connective Tissue capsule formed by the host. Metacercariae become infective within 6 weeks.

Humans become infected by consuming insufficiently cooked, raw, or dried fish containing metacercariae. In the body of the definitive host, the larvae enter the digestive tract, shed their membranes under the action of digestive juices, and migrate to the gallbladder and liver, where they reach sexual maturity. Infective stages: for the first intermediate host (snail) — an egg with a miracidium; for the second intermediate host (cyprinid fish) — a cercaria; for the definitive host (human, cat, etc.) — a metacercaria.

Pathogenic effect. Opisthorchiasis is a biohelminthiasis, a natural focal disease, and a zoonosis. The causative agent is especially prevalent in regions where eating raw, lightly frozen fish is customary (Siberia). In the early phase of opisthorchiasis, symptoms include allergies, fever, liver pain, and eosinophilia. Parasite accumulation leads to bile stasis and liver cirrhosis. Complications of opisthorchiasis include biliary Peritonitis, primary liver Cancer, or Pancreatic Cancer. In severe cases, death may occur.

Laboratory diagnostics. Detection of eggs in the feces or duodenal contents of an infected person.

Prevention. Personal: do not consume raw, undercooked, or underfried river fish. Public: quality control of fish culinary preparation processes, identification and treatment (deworming) of infected individuals, protection of water bodies from fecal contamination, refraining from feeding raw fish to domestic dogs and cats, and health education.

Fig. 53. Life cycle of the cat liver fluke.

Clonorchis sinensis is the causative agent of clonorchiasis.

Geographical distribution: Far East, China, Japan, Korea.

Localization: bile ducts of the liver, gallbladder, pancreatic ducts.

Morphology. Dimensions: 10-25 mm. The anterior end is narrowed, and the posterior end is rounded. The oral sucker is larger than the ventral sucker. The uterus and vitellaria occupy the middle part of the body. Two branched testes are located in the posterior region of the body. The eggs are yellow-brown, small, measuring 26-30 × 15 µm.

Pathogenic action. Clonorchiasis is a biohelminthiasis, a natural focal disease, and a zoonosis. The accumulation of parasites causes bile stasis and liver cirrhosis. Their lifespan in The Human Body is up to 25–40 years.

Laboratory diagnostics. Detection of eggs in the patient's feces. The eggs are practically indistinguishable from those of the cat liver fluke.

Prophylaxis. Personal — avoid eating insufficiently heat-treated fish and freshwater crayfish. Public — detection and treatment of infected individuals, protection of water bodies from fecal contamination, control over the preparation technology of fish culinary products, avoiding feeding raw fish to domestic dogs and cats, and health education.

Fig. 54. A — liver fluke:

1 — oral sucker; 2 — pharynx; 3 — anterior part of the intestine; 4 — cirrus pouch (copulatory organ pouch); 5 — uterus; 6 — ovary; 7 — Laurer's canal; 8 — transverse vitelline duct; 9 — longitudinal vitelline duct; 10 — vitellaria; 11 — terminal part of the uterus; 12 — vas deferens; 13 — ventral sucker; 14 — Mehlis' gland; 15 — yolk reservoir; 16 — vas deferens; 17 — Testis.

B — cat liver fluke; C — lancet liver fluke.

Lung fluke (Paragonimus ringeri, syn. P. westermani) — the causative agent of paragonimiasis.

Geographical distribution — East Asia, the Far East, South America, Africa.

Localization — small Bronchi, lungs; extrapulmonary localization — liver, Spleen, Brain, muscles.

Morphology. The body is egg-shaped, reddish-brown, covered with spines, and up to 10 mm in size (see fig.). The testes are located in the posterior third of the body, while the ovary and uterus are situated above the testes. The eggs are broad and oval, operculated, golden-brown, and up to 100 µm in size.

Life cycle. The lung fluke is a biohelminth. Its development involves alternating hosts. Definitive hosts include canids, felids, procyonids (raccoons), pigs, and less frequently humans. There are two intermediate hosts: the first is freshwater Mollusks of the genus Melania, and the second is freshwater crayfish and crabs. The parasite's eggs are released into the external environment with the definitive host's sputum and feces. For further development, they must enter water. A miracidium hatches from the egg and actively penetrates the mollusk, where the larval stages—sporocyst, redia, and cercaria—develop sequentially. The cercaria leaves the mollusk and actively penetrates the second intermediate host (freshwater crayfish, crabs), where it encysts into a metacercaria. Humans become infected by eating insufficiently cooked freshwater crayfish and crabs infected with metacercariae, or via water (metacercariae can survive in water for up to 25 days after the death of the crustacean). Infective stages: for the first intermediate host (mollusk) — miracidium; for the second intermediate host (freshwater crayfish and crabs) — cercaria; for definitive hosts — metacercaria.

Fig. 55. Lung fluke (Paragonimus ringeri):

1 — vitellaria; 2 — ovary; 3 — uterus; 4 — testes.

Pathogenic action. Paragonimiasis is a biohelminthiasis, a natural focal disease, and a zoonosis. In the lungs, paragonimids cause inflammation, hemorrhages, and cyst-like cavity formation. A cough with Blood-tinged sputum develops, which may resemble tuberculosis. Cases of extrapulmonary localization of the parasite also occur. Cerebral paragonimiasis often leads to blindness, paralysis, convulsive seizures, and mental disorders.

Laboratory diagnostics. Detection of eggs in sputum or feces.

Prophylaxis. Personal — do not eat raw freshwater crayfish and crabs. Public — protection of water bodies from fecal contamination, and health education.

Nanophyetus salmincola schikhobalowi — the causative agent of nanophyetiasis.

Geographical distribution — the Far East (Amur River basin), the northern part of Sakhalin Island, North America.

Localization — Small Intestine.

Morphology. The smallest human trematode, measuring 0.6 × 0.4 mm, nearly spherical in shape, yellowish-gray, and armed with spines. Elongated-oval, large testes are situated symmetrically in the posterior half of the body. The ovary is located in the right body cavity anterior to the testes. Small vitelline follicles are scattered throughout the body. The eggs are oval, gray or yellowish, operculated, very similar to the eggs of the broad tapeworm, and measure 0.07 × 0.04 µm (see fig.).

Life cycle. Definitive hosts include humans, cats, dogs, and various wild mammals. The eggs are shed with the definitive host's feces. There are two intermediate hosts: the first is gastropod mollusks, and the second is freshwater fish. Humans become infected by consuming insufficiently cooked fish infected with nanophyetid larvae.

Pathogenic action. Nanophyetiasis is one of the most common helminthiasis cases in the Far East. The disease manifests as digestive disorders (damage to the mucous membrane of the small intestine).

Laboratory diagnostics. Detection of eggs in the patient's feces.

Prophylaxis. As in opisthorchiasis.

Metagonimus yokogawai is the causative agent of metagonimiasis.

Geographical distribution: Japan, China, South Korea, Indonesia, and the Amur River region. In Ukraine, it is found in the basins of the Dnieper, Dniester, and Danube rivers.

Localization: Small intestine.

Morphology. Length: 1–2.5 mm; width: 0.4–0.7 mm. The body is covered with tiny spines. The oral sucker is significantly smaller than the ventral one. The intestinal branches extend to the posterior end of the body, where two round testes are located. The vitellaria are situated laterally in the posterior third of the fluke (Fig.). The eggs are light brownish, lemon-shaped, with an operculum at one pole and a small knob at the opposite pole; dimensions are 26–28 × 15–17 µm.

Fig. 56. Nanophyetus salmincola schikhobalowi, Metagonimus yokogawai:

a) Nanophyetus salmincola schikhobalowi; b) Metagonimus yokogawai: 1 - oral sucker; 2 - ventral sucker; 3 - genital sucker; 4 - pharynx; 5 - esophagus; 6 - intestine; 7 - testes; 8 - ovary; 9 - vitellaria; 10 - cirrus pouch; 11 - uterus with eggs; 12 - seminal receptacle; 13 - Seminal Vesicle.

Life cycle. Metagonimus is a biohelminth. Definitive hosts include humans, dogs, cats, foxes, pelicans, cormorants, and others. There are two intermediate hosts: the first is a snail of the genus Melania, and the second comprises various species of cyprinid and salmonid fish. With the feces of the definitive host, the egg is released into the environment and must reach water. A miracidium hatches from the egg and passively penetrates the body of the first intermediate host (the snail), where it develops into the cercaria stage. Cercariae emerge into the water and actively penetrate the body of the second intermediate host (fish), encysting as metacercariae in its muscles, scales, and fins. Humans become infected by consuming undercooked, poorly fried, raw, or dried fish infected with Metagonimus metacercariae. Invasive stages: miracidium for the snail, cercaria for fish, and metacercaria for humans and other definitive hosts.

Pathogenic effect. Metagonimiasis is a biohelminthiasis, a naturally focal disease, and a zoonosis. It is accompanied by intestinal disorders. The larvae burrow through the intestinal mucosa, causing inflammatory processes.

Laboratory diagnostics. Detection of eggs in the feces of an infected person.

Prophylaxis. Individual: avoid consuming raw, dried, or insufficiently thermally processed fish. Public: protection of water bodies from fecal contamination, and sanitary and educational outreach.

Blood flukes, or schistosomes, are the causative agents of Schistosomiasis, belonging to the family Schistosomatidae—the only family of dioecious trematodes.

Geographical distribution: Countries with tropical and subtropical climates.

Localization: Lumen of Blood Vessels, typically Veins.

Morphophysiological characteristics. Unlike other trematodes, schistosomes are dioecious. The body is narrow and cylindrical. The male is significantly shorter and broader than the female, measuring 10–15 mm in length. A groove, the gynaecophoric canal, runs along his ventral side, in which the long, slender female (about 20 mm in size) resides during fertilization. At the middle of the body, the two intestinal branches unite into a single unpaired trunk that ends blindly near the posterior end of the parasite.

Life cycle. The definitive host is humans and various mammals; the intermediate host is freshwater snails. The life cycle includes two generations of sporocysts (First and Second order), while rediae are absent. The invasive stage for definitive hosts is the cercaria. Infection occurs via the active penetration of cercariae through the Skin or mucous membranes upon contact with water. The eggs possess a spine. Species parasitizing humans include Schistosoma haematobium, Schistosoma mansoni, and Schistosoma japonicum (Fig.).

Fig. 56. Schistosomes.

Schistosoma haematobium is the causative agent of urogenital schistosomiasis (bilharziasis).

Geographical distribution: Africa, the Middle East, Central and South America. Localization: VEINS OF THE lesser pelvis and Urogenital System organs.

Morphology. Male size: 10–15 mm in length; female: up to 20 mm. The body is covered with spines (Fig.). The eggs lack an operculum and feature a terminal spine; dimensions are 150 × 62 µm.

Life cycle. Urogenital schistosomiasis is an anthroponosis. Definitive hosts are humans and monkeys. Intermediate hosts are snails of the genera Bulinus and Planorbis. The fertilized female leaves the male's gynaecophoric canal and lays eggs in small venules close to the lumen of the Urinary Bladder. The egg features a sharp spine used to pierce the vessel wall, while the miracidium inside the egg secretes enzymes. With the aid of these adaptations, the egg penetrates the cavity of the urinary bladder and is excreted externally with urine. For further development, the egg must reach water, where a miracidium hatches and penetrates the body of the intermediate host (the snail). Two generations of sporocysts (mother and daughter) and cercariae develop within the snail's body. Humans become infected while spending time in water. Cercariae actively penetrate the skin during swimming, washing clothes, or working in rice paddies. Via the lymphatic and blood vessels, cercariae pass through the right ventricle into the lungs and subsequently into the liver veins. Adult forms migrate to the veins of the lesser pelvis and urogenital system. Infection is also possible by drinking water containing cercariae. They can live in the host Organism for up to 40 years.

Pathogenic effect. At the site of cercarial penetration, punctate hemorrhages and papules appear, accompanied by skin itching and edema of the affected tissues. Migration of schistosomes may be accompanied by fever, cough, and chest pain. The eggs, with their sharp spines, destroy the tissues of infected organs, leading to inflammation and ulceration.

Laboratory diagnostics. Detection of eggs in urine. The parasite's eggs enter the urine only during the hottest time of day, when the probability of them reaching water is higher. This must be considered during Diagnosis: schistosome eggs are not detected in the patient's morning urine.

Prevention. Personal: in schistosome-endemic areas, avoid swimming in contaminated water bodies, drinking raw water, or washing with unfiltered water. Community: protecting water bodies from urine contamination, snail eradication, and health education.

Schistosoma mansoni is the causative agent of intestinal schistosomiasis.

Geographic distribution: Africa and South America (especially Brazil).

Localization: mesenteric veins, intestinal veins, and hemorrhoidal plexus.

Morphophysiological characteristics. Structurally similar to Sch. haematobium, though slightly smaller: males up to 10 mm in length, females up to 15 mm. The body is covered with somewhat larger spines (see fig.). The egg features a large lateral spine rather than a terminal one, measuring 140×61 µm.

Life cycle. Definitive hosts include humans and monkeys; intermediate hosts are snails of the genera Planorbis and Bulinus. Fertilized females deposit eggs in small venules close to the intestinal lumen. The eggs pass into the intestinal lumen and are expelled with feces.

Pathogenic action. The spines of the eggs damage the intestinal wall, leading to ulceration and polypoid growths. A severe complication in the late stage of the disease is liver Damage caused by trapped schistosome eggs.

Laboratory diagnostics. Detection of eggs in feces.

Prevention. Similar to that of urogenital schistosomiasis.

Schistosoma japonicum is the causative agent of Japanese schistosomiasis.

Geographic distribution: China, Southern Japan, Indonesia, and the Philippine Islands.

Localization: intestinal veins, mesenteric veins, and the portal Venous system.

Morphology. It differs from other schistosomes by its smooth body surface, devoid of spines. Male size reaches up to 20 mm, female size up to 26 mm. The eggs are rounded, with a rudimentary knob-like spine located laterally, measuring 85×60 µm.

Life cycle. Japanese schistosomiasis is a zoonotic, naturally focal disease with a wide range of hosts. Definitive hosts include humans as well as domestic and wild animals (cattle, horses, pigs, dogs, rats, etc.); however, humans remain the primary source of infection. The intermediate host is the snail genus Oncomelania. Development does not differ significantly from other schistosomes.

Pathogenic action. Compared to other schistosomiases, Japanese schistosomiasis is characterized by a malignant course and high mortality (Katayama fever).

Laboratory diagnostics. Detection of eggs in feces.

Prevention. Similar to that of other schistosomiases.

Class Tapeworms (Cestoidea)

Tapeworms are obligate endoparasites. In their adult stage, they parasitize the intestine of humans and vertebrates, causing diseases known as cestodiases. The body is dorsoventrally flattened and ribbon-like. The anterior end bears a holdfast organ or scolex, followed by a neck and a strobila consisting of segments called proglottids. The scolex is equipped with fixation organs: suckers (usually 4) or bothria (slit-like attachment grooves). In many species, In addition to suckers, the apex of the scolex bears a rostellum with cuticular hooks. The neck serves as the growth zone of the helminth, where new segments are continuously formed. The body is externally covered by a tegument-muscular sac, the surface layer of which is the tegument. The tegument contains a series of digestive enzymes and an anti-proteolytic enzyme that prevents the parasite from being digested in the host's intestine. Beneath the tegument lie three layers of Muscle: an outer circular layer, an inner longitudinal layer, and a diagonal layer situated between them.

The digestive system is completely absent. Nutrients present in the host's intestine are absorbed osmotically across the entire body surface. The tegumental cells secrete digestive enzymes that facilitate food Digestion and assimilation.

Excretory organs are protonephridia. There are two main excretory canals running along the sides of the strobila over its entire length, opening via excretory Pores in the terminal segment. In each proglottid, the longitudinal canals are connected by a transverse canal.

The nervous system is of the ganglionated ladder-like (orthogon) type. It consists of an anterior nerve mass or ganglion located in the scolex, and two main lateral nerve trunks extending along the entire body. Sense Organs, apart from tactile receptors, are absent.

The circulatory and respiratory systems are absent.

The reproductive system is extraordinarily developed and highly complex compared to other organ systems. Cestodes are hermaphroditic. Each proglottid contains a repeating set of both male and FEMALE REPRODUCTIVE ORGANS, endowing tapeworms with immense fecundity. In the youngest segments directly behind the neck, the reproductive system is not yet differentiated; subsequently, Male reproductive organs appear, and as these proglottids are pushed further back, female organs also develop, rendering the segment hermaphroditic (immature). Later on, in many species, part of the reproductive organs within the segment regresses, leaving only the uterus filled with eggs containing oncospheres; such a segment is termed gravid. It can detach from the strobila and be expelled into the environment. The male reproductive system is represented by numerous testes and vas efferentia that unite into a common vas deferens, opening through the male genital pore into the genital atrium. The distal end of the vas deferens Functions as a copulatory organ, the cirrus. The female reproductive system comprises the ovary, vitelline glands, Vagina, ootype, and uterus. The vagina is a specialized tube for receiving male Gametes; one end connects to the ootype, while the other opens into the genital atrium adjacent to the male genital pore. In many cestode species, the uterus lacks an external opening (blind uterus). In such cases, as it accumulates eggs, it enlarges and gradually fills the entire proglottid, while other PARTS OF THE hermaphroditic reproductive system partially atrophy. In certain cestodes, the uterus possesses a uterine pore (open uterus), through which eggs containing oncospheres are discharged externally. The structure of the uterus serves as a key taxonomic feature for identifying cestode species. Fertilization in cestodes typically occurs between different proglottids of the same individual (self-fertilization) or between different individuals (cross-fertilization).

The life cycle is characterized by alternation of hosts and the presence of several larval stages. The definitive host is a vertebrate animal or human, while the intermediate host is predominantly vertebrate, though occasionally invertebrate. The developmental cycle of all cestodes invariably includes two larval stages: the oncosphere and the metacestode (bladder worm/plerocercoid/etc.). The oncosphere, or first larval stage, develops within the egg while it is still in the proglottid; it is spherical and bears six hooks. In the intestine of the intermediate host, the oncosphere hatches from the egg, uses its hooks to penetrate the blood vessels, and is passively carried by the blood to various organs, where it transforms into the second larval stage, the metacestode. The following types of metacestodes are distinguished:

Cysticercus is a small, fluid-filled bladder about the size of a pea, with a single inverted scolex inside;

Coenurus is a bladder containing multiple inverted scoleces;

Cysticercoid is a double-walled bladder with an inverted scolex and a caudal appendage;

Echinococcal cyst is a large cyst containing toxic fluid, numerous scoleces, daughter cysts, and granddaughter cysts;

Alveolar cyst is a conglomerate of small vesicles with a colloid content, containing a small number of scoleces inside;

Plerocercoid is a worm-like larva with sucking grooves (bothria) at the anterior end.

The development of bladder worms into the sexually mature stage occurs in the intestine of the definitive host, where the HEAD everts under the action of digestive juices and attaches to the intestinal wall, while the bladder disintegrates.

Medical significance. Several species of tapeworms are causative agents of human cestodiases.

Pork tapeworm, or armed tapeworm (Taenia solium) is the causative agent of taeniasis in its sexually mature stage, and of cysticercosis in its larval stage (bladder worm - cysticercus).

Geographical distribution is ubiquitous wherever pig farming is developed.

Localization: the sexually mature form parasitizes the small intestine, while the bladder worms inhabit various organs, most frequently the muscles and the Central Nervous System.

Morphology. The body is ribbon-like, white, and 2-3 m long. The scolex is microscopic (2-3 mm) and bears 4 suckers and a double crown of hooks (hence the name - armed tapeworm). An unsegmented neck is located right behind the scolex (Fig.). The strobila contains up to 1,000 or more proglottids. Hermaphroditic proglottids are square-shaped. The ovary has three lobes - two large ones and a third small (accessory) one, which is a species-specific diagnostic feature. The vitelline gland is located beneath the ovary. Testes are found in the lateral parts of the proglottid. The genital pore alternates irregularly from the left side in one proglottid to the right side in the next. The uterus is a blind-ending sac. In mature proglottids, the uterus forms 7-12 branches on each side (a diagnostic feature).

Fig. 57. Pork tapeworm (Taenia solium):

a - strobila; b - scolex; c - hermaphroditic proglottid; d - mature proglottid; 1 - hooks on the scolex; 2 - suckers; 3 - ovary; 4 - third (accessory) ovarian lobe; 5 - testes; 6 - vas deferens; 7 - vagina; 8 - cirrus pouch; 9 - main uterine stem; 10 - lateral uterine branches; 11 - Mehlis' gland; 12 - vitelline gland.

Life cycle. The pork tapeworm is a biohelminth. Development involves a change of hosts (Fig.). The definitive host is exclusively human, while the intermediate host is a pig, and rarely humans. A person infected with taeniasis excretes mature proglottids with feces, which contain eggs with oncospheres. Pigs rooting in sewage may swallow proglottids or individual eggs. In the pig's Stomach, a six-hooked larva—the oncosphere—hatches from the egg. Using its hooks, the oncosphere penetrates blood vessels and is carried by the blood to various Internal Organs (liver, muscles, brain), where it transforms into a cysticercus within 2 months.

The cysticercus can be seen with the naked eye and reaches up to 10 mm in diameter. Humans become infected by consuming raw or insufficiently heat-treated pork infested with cysticerci. Under the action of digestive juices, the scolex everts from the cysticercus and attaches to the wall of the small intestine; then, proglottids begin to bud from the neck, and within 2-3 months the helminth reaches sexual maturity. Its lifespan in the human body is 10 years or more. Thus, the infective stage for the intermediate host is the egg with the oncosphere, and for the definitive host, it is the cysticercus.

Pathogenic effect. Taeniasis is a biohelminthiasis and zoonosis. Pathogenic effects are caused by mechanical damage, depletion of the host's digested nutrients, and the toxic impact of Metabolic waste products from the parasite. Digestive disorders, anemia, and general weakness are observed. The pork tapeworm is also dangerous because humans can act as an intermediate host upon accidental ingestion of eggs or As a result of autoinfection (when taeniasis patients vomit, proglottids may enter The Stomach). In the stomach, oncospheres hatch from the eggs and are carried by the bloodstream to various organs, where they develop into cysticerci. Localization of cysticerci in the brain is dangerous and can sometimes cause death, as well as in the visual organ, which may lead to blindness. Treatment for cysticercosis is exclusively surgical.

Laboratory Diagnosis of taeniasis. Detection of mature proglottids in the feces of an infected person. The presence of 7-12 lateral uterine branches indicates taeniasis. The uterus has no opening, so eggs are rarely found in feces.

Laboratory diagnosis of cysticercosis. X-ray examinations, immunological tests.

Prevention of taeniasis. Personal: avoid consuming pork that has not undergone veterinary and sanitary inspection. Public: detection and treatment of infected individuals, especially those working in animal husbandry; veterinary inspection of pork; protection of soil from human fecal contamination; indoor housing of pigs.

Prevention of cysticercosis. Personal: strictly follow personal hygiene rules. Public: health education campaigns.

Fig. 58. Life cycle of the pork tapeworm.

Beef tapeworm, or unarmed tapeworm (Taeniarhynchus saginatus) is the causative agent of taeniorhynchiasis.

Geographical distribution - worldwide.

Localization - small intestine.

Morphology. One of the largest human helminths, measuring 4–7 m in length (with some specimens reaching up to 10 or even 18 m). Its structure is similar to that of the pork tapeworm. It differs by the absence of hooks on the scolex (hence the name unarmed) and the lack of a third (accessory) ovary lobe in the hermaphroditic proglottid (the ovary has only two lobes). In addition, the uterus in the mature proglottid features 17–35 lateral branches. Detaching from the strobila, mature proglottids can independently crawl out of the anus and move across the patient's body and underwear. The mature proglottids of the armed tapeworm lack this ability.

Fig. 59. Unarmed tapeworm (Taeniarhynchus saginatus):

a - general appearance; b - scolex; c - hermaphroditic proglottid; d - mature proglottid; 1 - ovary; 2 - vitelline gland; 3 - Mehlis' gland; 4 - uterus; 5 - testis; 6 - vas deferens; 7 - vagina; 8 - cirrus pouch; 9 - genital atrium; 10 - excretory canal; 11 - nerve cord.

Life cycle. The unarmed tapeworm is a biohelminth. Its development involves a change of hosts. The definitive host is exclusively human, while the intermediate host is cattle. A patient infected with taeniorhynchiasis passes mature proglottids containing eggs with oncospheres in their feces. If the proglottids or eggs enter the digestive tract of cattle, the eggshell is digested, and the hexacanth oncosphere penetrates the intestinal wall into blood vessels, being carried by the blood to various Organs of the host. Here, the oncosphere loses its hooks and transforms into a bladder worm, known as a cysticercus. Cysticerci can live in muscles for years without developing further. Subsequent development is possible only in the intestine of the definitive host, which is humans. Humans become infected by consuming insufficiently cooked beef infected with cysticerci. In the human intestine, the cysticercus scolex everts, attaches to the wall of the small intestine, and the growth of the strobila begins from the neck. Thus, the infective stage for the intermediate host (cattle) is the egg containing the oncosphere, whereas for the definitive host (human) it is the cysticercus (fin). The lifespan is approximately 10 years.

Pathogenic effect. In its clinical course, taeniorhynchiasis is similar to taeniasis.

Laboratory diagnostics. Detection of mature proglottids in the patient's feces. The uterus in a mature proglottid has 17–35 branches on each side. Since the uterus in the unarmed tapeworm is also closed, eggs are frequently not found in the patient's feces. If they are found (in scrapings from perianal folds), they are morphologically indistinguishable from the eggs of the pork tapeworm.

Prophylaxis. Individual prevention involves avoiding meat that has not undergone veterinary and sanitary inspection. Public measures include the detection and treatment of infected individuals; veterinary and sanitary inspection of beef; protection of soil from contamination with human feces; and health education.

Dwarf tapeworm (Hymenolepis nana) - the causative agent of hymenolepiasis.

Geographical distribution - worldwide.

Localization - human small intestine.

Morphology. Its size is small compared to other cestodes: 1.5–2 cm in length, rarely up to 5 cm (hence the name dwarf). The scolex is pear-shaped, bearing four suckers and a retractable rostellum with hooks. The neck is long and thin (fig.). The strobila consists of 100–200 proglottids. The genital pores of all proglottids are located on the same side. The posterior proglottids, containing a sac-like uterus, are very fragile. Upon detaching from the strobila, they disintegrate rapidly, releasing eggs with oncospheres into the lumen of the small intestine.

Life cycle. In the life cycle of the dwarf tapeworm, humans serve as both the definitive and intermediate host, representing the sole source of infection. The parasite's eggs are discharged into the external environment with human feces and are immediately infective. Infection occurs via the oral route. Unwashed hands serve as the primary vehicle for transmitting the causative agent of hymenolepiasis. In the intestine, oncospheres hatch from the eggs, penetrate the villi of the small intestine, and develop into a cysticercus. The cysticercoid grows, destroys the villus, and drops into the intestinal lumen. Under METABOLISM/18.html">The Influence of digestive juices, the scolex of the cysticercoid everts, attaches to the small intestinal wall, and proglottid growth begins. A sexually mature individual forms within 14–15 days. It is believed that autoinfection (the development of parasite eggs within the intestine without passing into the external environment) can occur in hymenolepiasis. Under such conditions, the infection persists for a long time. A patient with hymenolepiasis who fails to observe personal hygiene may introduce eggs of their own parasites back into the mouth (autoreinfection).

Rats and mice can also act as definitive hosts and sources of infection for humans, with the confused flour beetle and its larvae serving as intermediate hosts.

Pathogenic effect. Hymenolepiasis is a contact biohelminthiasis and an anthroponosis. The source of infection is an infected person. The transmission mechanism is fecal-oral, occurring through the ingestion of eggs. Preschool children are most commonly infected. The parasite occurs in large numbers (up to 1,500 specimens), which accounts for its significant pathogenic impact on the host organism, consisting of the destruction of intestinal villi and the Toxic effects of the helminths' metabolic products. Hymenolepiasis is characterized by headaches, abdominal pain, digestive and nervous system disorders, general weakness, and rapid fatigue. Children become fussy and irritable.

Laboratory diagnostics. Detection of eggs in the patient's feces.

Prophylaxis. Individual measures include the strictest adherence to personal hygiene rules, instilling hygienic habits in children, and washing hands before meals and after using the restroom. Public measures include the detection and dehelminthization of infected individuals; isolating infected children from healthy ones in childcare groups; health education among parents and staff of children's institutions; and rodent control.

Fig. 60. Dwarf tapeworm (Hymenolepis nana):

a - general appearance; 1 - scolex; 2 - neck; 3 - body; 4 - immature proglottid; 5 - hermaphroditic proglottid; 6 - mature proglottid; b - developmental cycle in the human intestine: 1 - body of the adult worm; 2 - eggs; 3 - oncospheres hatched from eggs within the mucosal villi; 4 - cysticercoid; 5 - exit of the cysticercoid with an everted scolex from the villus into the intestinal lumen; 7 - growth of the worm's body.

Echinococcus (Echinococcus granulosus) - the causative agent of echinococcosis.

Geographical distribution - worldwide, particularly in sheep-farming regions. In Ukraine, it is found predominantly in the southern regions.

Localization - unlike other cestodes, it parasitizes humans in the larval (cystic) stage, affecting the liver, lungs, brain, and tubular bones, though it may occur in any organ.

Morphology. The body of a mature parasite is 3-6 mm long. The scolex bears 4 suckers and a rostellum with a double crown of hooks. The neck is short. The strobila consists of 3-4 proglottids. The penultimate proglottid is hermaphroditic, while the ultimate one is mature and contains a uterus with lateral outgrowths filled with eggs containing oncospheres (500-800 eggs).

Life cycle. Echinococcus is a biohelminth. Its development involves a change of hosts. Mature adults parasitize the small intestine of dogs, wolves, and jackals, which serve as definitive hosts. Intermediate hosts include humans, cattle, small ruminants, pigs, camels, rabbits, and many other herbivorous mammals. The terminal proglottid detaches from the strobila and is either passed with feces or actively crawls out of the anus of the definitive host (predominantly dogs), moving actively across the fur while releasing eggs. Proglottids shed with feces crawl onto the grass. Domestic herbivores consume the grass and inadvertently ingest the Echinococcus eggs.

In the intestine of intermediate hosts, the oncosphere hatches from the egg and penetrates the blood vessels. Via the portal vein, it reaches the liver (the most frequent site of infection). Oncospheres that bypass the liver are carried through the right side of The Heart into the lungs. Frequently, oncospheres are disseminated by the bloodstream to the brain and other organs. Here, the oncosphere develops into a metacestode (finna)—the hydatid cyst. The cyst grows slowly and can reach large sizes: hydatids weighing up to 64 kg have been described in animals, and in humans, they can grow as large as a newborn's head. Cysts persist in the intermediate host's body for years. For further development, they must reach the intestine of a definitive host. Infection of dogs and other definitive hosts occurs upon ingestion of organs from intermediate hosts infected with hydatid cysts. In the definitive host's intestine, a single cyst gives rise to A large number of tapeworm forms. Invasive stages: for definitive hosts — the cyst; for intermediate hosts — eggs containing oncospheres.

Fig. 60. Echinococcus and Alveococcus:

a — Echinococcus; b — Alveococcus; 1 — scolex; 2 — neck; 3 — hermaphroditic proglottids; 4 — mature proglottid; 5 — uterus filled with eggs.

Humans most commonly contract echinococcosis through poor personal hygiene around infected dogs whose fur carries eggs. Unwashed hands transfer the eggs to the mouth. Infection is also possible from sheep whose wool is contaminated with Echinococcus eggs shed by infected guard dogs. The causative agent of echinococcosis is likewise transmitted via unwashed vegetables and garden produce contaminated with the feces of infected dogs. Humans represent a biological dead-end (blind branch) in the life cycle of Echinococcus, because the cyst typically is not transmitted to animals after human death, but simply perishes.

Pathogenic effect. Echinococcosis is a biohelminthiasis, a naturally focal infection. The first signs of the disease appear a considerable time after infection, once the hydatid cysts have grown to a certain size. Disease manifestations depend on the cyst's localization and the parasite's developmental phase. The pathogenic effect is driven by the toxicity of the cyst fluid and its mechanical impact on tissues (tissue compression), which disrupts organ function. Treatment is exclusively surgical. Rupture of a hydatid cyst is extremely dangerous: the toxic fluid it contains can trigger anaphylactic Shock and instant death. Furthermore, a rupture leads to the dissemination of brood capsules and daughter scolexes into the Abdominal cavity, resulting in secondary multi-organ echinococcosis. Sometimes dissemination occurs as a result of fluid containing scolexes escaping from the cyst during the Surgical treatment of echinococcosis.

Laboratory diagnostics. The diagnosis is established on The basis of immunological tests. Radiological examinations are also employed. To detect infected dogs, their feces are examined for the presence of Echinococcus eggs.

Prophylaxis. Individual prophylaxis involves observing personal hygiene rules, washing hands after contact with dogs, washing vegetables, boiling water, and the preventative deworming of domestic dogs twice a year. Public health measures include controlling stray dogs; identifying and deworming infected working dogs; destroying internal organs affected by echinococcosis; and conducting public health education.

Fig. 62. The developmental cycle of Echinococcus.

Alveococcus (Alveococcus multilocularis) is the causative agent of alveolar echinococcosis (alveococcosis).

Geographical distribution: encountered less frequently than echinococcosis — Ukraine, Siberia, Alaska, Northern Canada, Southern France, Germany, Northern Kazakhstan, Northern Vietnam.

Localization — fundamentally similar to that of echinococcosis, though the liver is primarily and predominantly affected, while other organs are rarely involved.

Morphology. The tapeworm form of Alveococcus resembles the Echinococcus tapeworm. It differs in its smaller size (1.2–3.7 mm), the number of hooks on the scolex, and the shape of the uterus (in Alveococcus it is spherical, lacking lateral outgrowths). The genital pore in Alveococcus is located in the anterior part of the lateral margin of the proglottid, whereas in Echinococcus it is situated posteriorly. The metacestode of Alveococcus consists of a cluster of small vesicles. The vesicles contain no fluid, bud externally, and progressively infiltrate tissues much like malignant tumors, forming metastases.

Life cycle. Alveococcus is a biohelminth. Its development involves a change of hosts. Definitive hosts include the red fox, arctic fox, wolf, dog, and occasionally the cat; intermediate hosts are murid rodents, and rarely humans. The life cycle is similar to that of Echinococcus. Eggs containing oncospheres are expelled into the environment with the feces of definitive hosts. Humans become infected by consuming forest berries contaminated with Alveococcus eggs; by handling the pelts of infected foxes, arctic foxes, or dogs; or through contact with infected dogs or objects soiled with dog feces containing Alveococcus eggs. Humans serve as a biological terminal link in the life cycle of Alveococcus.

Pathogenic effect. Alveococcosis is a biohelminthiasis, a naturally focal disease. In humans, the primary localization of the parasite is predominantly in the liver. Hepatic alveococcosis proceeds asymptomatically for a long time (sometimes years). Pain emerges at later stages. The disease is characterized by a malignant clinical course. Metastatic nodes may form in the lungs and brain. Treatment is exclusively surgical. The cyst must be excised along with a portion of the affected organ because it lacks a well-defined capsule.

Laboratory diagnostics. Immunological tests are used to establish the diagnosis. Diagnosis is usually made at advanced stages when surgical intervention is complicated or impossible.

Prophylaxis. Individual prophylaxis — observing personal hygiene rules when handling dogs, Processing the pelts of foxes, arctic foxes, and dogs, washing berries, and boiling water. Public health measures — organizing specialized facilities for skinning pelts; sanitary and educational outreach.

Broad tapeworm (Diphyllobothrium latum) is the causative agent of diphyllobothriasis.

Geographical distribution. Diphyllobothriasis is found among populations engaged in fishing, particularly in Karelia, the Baltic region, and along the banks of the Volga and Dniester rivers.

Localization — the small intestine.

Morphology. The largest human helminth (8-10 m in length, with individual specimens reaching up to 20 m). The scolex is elongated, bearing two longitudinal sucking grooves — bothria — instead of suckers. The proglottids have a characteristic shape: their width is several times greater than their length. The structure and arrangement of the reproductive organs differ from those of tapeworms of the family Taeniidae, which has diagnostic significance. The genital atrium is located not on the lateral margin, but on the ventral surface of the proglottid near its anterior edge. Vitelline glands are situated in the lateral parts of the proglottid ventral to the testes. The uterus is coiled into a rosette-like loop, possesses its own opening through which eggs are expelled as they are produced, and does not form lateral branches.

Definitive hosts are humans and carnivorous mammals (cats, foxes, arctic foxes, dogs, bears). There are two intermediate hosts: the first is the copepod crustacean Cyclops, and the second is freshwater fish (pike, burbot, zander, etc.). Definitive hosts excrete eggs with their feces, which must reach water. A larva — the coracidium (a spherical, six-hooked oncosphere covered with cilia) — hatches from the egg and swims freely in the water for some time (3-4 days) until ingested by a Cyclops. Within the body of the Cyclops, the coracidium uses its hooks to penetrate the intestinal wall into the body cavity, where it develops into a procercoid. The Cyclops is eaten by a fish. In the stomach of the second intermediate host, the Cyclops is digested, while the procercoid penetrates the musculature and transforms into a plerocercoid. The plerocercoid is a white, worm-like larva about 6 mm long, possessing two sucking grooves at its anterior end. If an infected fish is eaten by another predatory fish, the plerocercoid retains its viability. Consequently, predatory fish serve as paratenic (reservoir) hosts, within whose bodies a large number of plerocercoids can accumulate. Humans become infected by consuming raw, sun-dried, or insufficiently thermally processed fish containing plerocercoids. In the definitive host's body, the plerocercoid attaches to the intestinal walls via its bothria and develops into a sexually mature adult. The broad tapeworm lives in the human body for up to 28 years. Invasive stages: for the Cyclops — the coracidium; for the fish — the procercoid; for the definitive host (humans, etc.) — the plerocercoid.

Life cycle. The broad tapeworm is a biohelminth. Its development proceeds with a change of hosts.

Fig. 63. Broad tapeworm (Diphyllobothrium latum):

a - body; b - scolex (bothria visible - 1); c - hermaphroditic proglottid; d - mature proglottids; 2 - uterus; 3 - vitelline gland; 4 - uterine loops; 5 - cirrus pouch; 6 - ovary; 7 - Mehlis' gland; 8 - testes.

Pathogenic effect. Diphyllobothriasis is a biohelminthiasis and a natural-focal disease. Using its bothria (sucking grooves), the parasite damages the intestinal mucosa, leading to tissue necrosis. Large accumulations of parasites can cause intestinal obstruction. Patients frequently develop severe anemia associated with a deficiency of cyanocobalamin (vitamin B₁₂), which the parasite selectively absorbs onto its surface from the intestinal contents.

Laboratory diagnosis. Detection of eggs in the feces of an infected person. The eggs are broad, oval, with a smooth, transparent, double-contoured shell, yellowish or gray in color, featuring an operculum and measuring up to 70 µm. The diagnosis is often confirmed by finding strobila fragments of varying lengths in the feces. A wide strobila with short proglottids and a centrally located rosette-shaped uterus indicates diphyllobothriasis.

Prevention. Individual - avoiding the consumption of raw or undercooked fish and caviar. Public - detection and dehelminthization of infected individuals; sanitary maintenance of vessels and settlements; protection of water bodies from contamination with human feces; sanitary control over compliance with fish processing regulations; and health education.

Fig. 64. Life cycle of the broad tapeworm.