Cestodoza: general characteristics of cestodes

Cestodoza - diseases, the causative agents of which belong to the class Cestoidea.

Of medical importance are mainly representatives of two orders: lentets - Pseudophyllidea and Chain - Cyclophyllidea, belonging to the subclass of these tapeworms (Eucestoda).

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Structure of cestodes

The body of cestodes (from the Greek cestos - belt, ribbon) is usually ribbon-like, oblate in the dorsoventral direction, consists of a head (scolex), neck and strobila, divided into segments (proglottids). The length of the entire cestode, depending on the species, can vary from a few millimeters to 10 meters or more, and the number of proglottids varies from one to several thousand. At the scallops the scolex is more or less round in shape, has four suckers with muscular walls. At the top of the scolex is a muscular outgrowth - a proboscis carrying armament in the form of one or more rows of hooks. The number, size, shape and location of the hooks are important for determining the types of chains. In the Lentets the scolex is elongated, provided with two suction pits (botryas). Behind the scolex is a narrow, short, non-segmented area of the body - the neck, which serves as a growth zone. From it young segments are budded, as a result of which older ones gradually move to the back of the strobila.

The body of the cestodes is covered with a dermal-muscular layer (a skin-muscular sac), consisting of a cuticle and a subcuticle. Cuticle is a dense non-cellular formation on the surface of cells of epithelial tissue. It consists of three layers: external, containing keratin, medium - cytoplasmic, rich in proteins and lipids, and internal - fibrous or basal. Keratin together with mineral substances and proteins imparts mechanical strength to the cuticle; lipids contribute to its water resistance. Due to the stability of the cuticle to the action of host enzymes and the release through it of substances neutralizing the effect of enzymes, cestodes can exist in an aggressive environment of the intestine of humans and vertebrates. The cuticle is covered with villi-like outgrowths - microtrichia, which come into close contact with the microvilli of the intestinal mucosa, which increases the efficiency of absorption of nutrients. In the subcuticle there is a layer of cells of the immersed epithelium, as well as the outer annular and inner longitudinal layers of smooth muscle fibers.

Inside the body of the cestode is filled with parenchyma, consisting of large irregularly shaped cells, the processes of which are intertwined. In the surface layers of the parenchyma are unicellular skin glands, as well as stocks of nutrients - proteins, lipids and glycogen. The latter is of great importance in the processes of anaerobic respiration. Here lie "calcareous bodies", containing phosphates and calcium and magnesium carbonates, with the participation of which the buffer properties of the medium are regulated.

The deeper layers of the parenchyma are located excretory, nervous and reproductive systems. There are no digestive, respiratory and circulatory systems. Food is provided through the covers of the body.

The excretory system of cestodes is constructed according to the protonephridial type. It consists of numerous cells with a "flickering flame" and thin tubules, which, joining together, flow into large lateral longitudinal outflow channels. These channels in each segment are connected with each other by a posterior transverse canal. When the segment is torn off, the lateral excretory canals open outward on the separation surface.

The nervous system consists of longitudinal nerve trunks, the largest of which are lateral. In Skolex, they are connected by transverse commissures, which are connected with a rather complex head unit. The sense organs are not developed.

The reproductive system in almost all cestodes is hermaphroditic. In most of them, the genitalia have a very complex structure. The hermaphroditic reproductive system is repeated in each proglottid. The first segments, budding from the cervix, do not yet have a reproductive apparatus. As the strobila grows and the segments are removed from the cervix, the organs of the male reproductive system are formed in them, consisting of the majority of species from numerous testicles, which have the appearance of vesicles scattered in the parenchyma of the segment. From them go the vas deferens, flowing into the seminal duct, ending with the copulatory organ (cirrus), which is in the bursa cirri. The sex bag opens, as a rule, on the lateral (sometimes ventral) side of the segment on the sexual tubercle in a special depression called the genital cloaca.

Later, a more complex female reproductive system appears. The female genital pore is located in the genital cloaca next to the male. It leads to a narrow canal of the vagina, which at the inner end forms an extension - the spermatheca - and opens into a special chamber - an ootype. The ducts of the ovaries (oviducts), vitellaria and Melis's corpus enter the ootype. Through the oviduct, oocytes enter the ootype from the ovaries, and spermatozoa accumulate through the vagina, accumulated in the spermatheca after copulation. Oocyte fertilization of eggs and the formation of eggs. They are formed from the nutrient material coming from the vitellaria, and their membranes are created from the secrets of the Melis gland. The formed eggs move into the developing uterus. As the eggs enter it, the uterus grows in size and occupies an increasing part of the joint volume, and the hermaphrodite reproductive system is reduced in a posterior fashion. The terminal segments of the strobila are entirely occupied by the uterus filled with a huge number of eggs.

Segments containing developed genitalia are called hermaphroditic, and those filled with only one uterus are mature. In mature women, the mature womb is closed. She has no communication with the sexual pathways and the external environment. Eggs come out of it only when separating the final proglottids, which is accompanied by destruction of the joint and uterine wall tissues.

In the Lentets the uterus is open, through its outer opening, the eggs enter the host's intestines, and then with the feces are removed to the external environment. Lentets eggs have a lid similar to the trematode eggs.

Eggs of chain are rather monotonous in their structure, so it is often not possible to determine their species belonging to microscopy. Mature eggs of oval or spherical shape are covered with an extremely delicate transparent outer shell, through which the inside larva - oncosphere is clearly visible. It is surrounded by a thick, radially striated inner shell - an embryophor, which performs the basic protective function. Oncosphere has six embryonic hooks, driven by muscle cells. With the help of hooks and the secretion of glandular cells, the larva penetrates into the host tissues during migration. Onkosfery often colorless, less often painted in yellow or yellowish-brown color. In the study of faeces, oncospheres, covered only with embryophore, are found, since the outer shell quickly disintegrates.

Cestode development cycle

All cestodes are biohelminths; Postembryonic development of most of their species occurs with a double (in chain) or triple (in Lentets) change of hosts.

In the intestine of the final host, in the presence of two or more worms, mutual fertilization takes place between different individuals. If only one cestode parasitizes, fertilization can occur between its different proglottids; perhaps self-fertilization of the same proglottid. In chains, oncosphere formation ends in the uterus; in Lentets it occurs in the external environment (usually in water). The cap comes into the water of the mature egg, and the lid opens, and from it comes coracidium - a globular, free-swimming larva, covered with a layer of ciliated cells and armed with six hooks.

Further development of the larvae continues in the intermediate hosts.

Oncospheres, caught with food or water in the gastrointestinal tract of the intermediate host, are released from the embryo, enter the intestinal wall and migrate, getting with the blood into various internal organs, where depending on the species the cestodes develop into the corresponding type of larvae - larvocysts . The larva of the - the larva and Greek. kystis - bubble). Some of these larvocysts (prices, echinococci, alveococci) in the body of the intermediate host can reproduce asexually.

The main types of larvocysts are:

1. Cysticercus (Cysticercus) - a small bubble formation, filled with liquid and containing a submerged scolex with organs of fixation. Upon ingestion of the final host, the scolex extends from the larval bladder, just as the gloved finger is turned out. Cysticercus is the most common of the larvocysts found in the tissues of vertebrates.
2. The cysticercoid consists of an inflated vesicle with a scolex embedded in it and a neck and a caudal appendage (cercomera), on which there are three pairs of embryonic hooks. Cysticercoid develops usually in the body of invertebrate intermediate hosts: crustaceans, ticks, insects.
3. Coenums - a bubbly larvocyst with multiple scolexes embedded in it, each of which subsequently gives rise to an individual strobile. Thus, a large number of parasites develops from one oncosphere (asexual reproduction by budding). The zenur is characteristic of the genus Malticeps, found in sheep and some rodents.
4. Larvocyst of cystic echinococcus (Echinococcus granulosus) is the most complex larva of cestodes. It is a single-chamber bubble filled with a liquid. Its internal germinative shell can produce brood capsules with simultaneous formation of germ scolexes (proto-scolexes) and secondary and then tertiary bubbles in them, due to which the process of asexual reproduction acquires a special intensity. In the body of the intermediate host, echinococcus takes on various modifications. Parasitizes in mammals.
5. Larvocyst alveococcus (Echinococcus multilocularis) is a conglomerate of a large number of small, irregularly shaped vesicles, from the outer surface of which the daughter vesicles bud. Protocolexes develop in the bubbles. The larvocyst tends to germinate into neighboring tissues.

In the lower cestodes (lentets), larvae parasitizing in intermediate hosts are elongated, in shape resembling worms. Their basic forms.

1. Protserkoid (Procercoid) - larval stage of Lentets formed in the first intermediate host (crustacean) from the coracidium. Its length is about 0.5 mm. At the front end is a depression (primary botry). The posterior end of the body (cercomerium) is separated by a constriction and is equipped with chitinous hooks.
2. Plerocercoid (Plerocercoid) - larval stage of lentets, developing from procercoid in the second intermediate host (fish). Some species of lentils can reach several tens of centimeters in length. At the anterior end of the body there are botries.

Definitive hosts become infected when fed by intermediate hosts, invaded by plerocercoids.

Thus, the development of the Lenters consists of five phases:

1. an egg, embryogenesis in which occurs in water;
2. coracidium, hatching from the egg and leading a free way of life;
3. procercoid, developing from coracid in the body of copepods;
4. Plerocercoid, developing from procercoid in fish;
5. adult cestoda (marita), formed from plerocercoid in the intestines of warm-blooded animals.

[2], [3], [4]