From Big Medical Encyclopedia

PARASITES (grech, parasitos the parasite) — the organisms eating at the expense of individuals of other look and temporarily or constantly staying on the surface of their body or in an organism. The organism providing a parasite with food and a constant or temporary habital is his owner.

Items are subdivided into zooparasites — parasitizing on animals, napr, many protozoa (see), helminths (see), arachnoid (see), insects (see), etc., and phytoparasites — parasitizing on plants, napr, mushrooms (see), nematodes (see), etc. On a habital in an organism of the owner P. divide on outside, or ectoparasites, and internal, or endoparasites. Nek-ry belong to ectoparasites bloodsuckers (see) and blood-sicking insects. Endoparasites parasitize in fabrics, internals and perigastriums of the owner. Depending on localization among endoparasites distinguish: intradermal, or intradermal (e.g., scabby zuden — Sarcoptes scabiei, etc.); fabric (e.g., larvae of a trichina of Trichinella spiralis, Finns of nek-ry cestodes, etc.); band (e.g., nek-ry helminths, gregarines, etc.); endoparasites of the bodies which are reported with the environment — a nasal cavity, an ear, a mouth (e.g., amoebas, nek-ry flagellates, helminths, etc.); cellular, or intracellular (e.g., koktsidiya, toxoplasma, leushmania, etc.); endoparasites of plasma and uniform elements of blood (e.g., filarias, haemo sporidium, flagellates, etc.); endoglobular — living in red blood cells. Such division of parasites is conditional since during the different periods of the life nek-ry of them change a habital and migrate on an organism of the owner.

Originally P. allocated in absolutely isolated group. With P.'s inclusion in the general system of live organisms exact definition of the concept «parasites» was required. The main sign of P. considered a way of food in the beginning. Therefore any organism eating at the expense of other live organism without killing him as it does a predator, carried to P. Vposledstviya such definition of P. was added with a sign of harm of P. for an animal organism, for the account to-rogo it lives and a cut is his owner (see. Parasitism ).

Among P. are available constant and temporary. Constant P. all the life, at all stages of development, live on a body or in a body of the owner. Out of the owner they cannot exist. Ectoparasites belong to constant P. — louses (see), lice (see), vlasoyeda, etc.; the parasites passing a development cycle with change of owners, napr, plasmodiums also treat them. Temporary P. are connected with the owner only during meal and carry out the most part of the life out of the owner. These are mosquitoes, gadflies (see), bloodsuckers (see), etc.

Duration of food of P. is various — from several minutes to several days. Many types of ixodic mites (see) during the life eat blood of the owner in phases of a larva, the nymph and a female (only three times), every time remaining on the owner till several days. Nek-ry types of P. live near the owner (in a nest, a den), and in some cases at the expense of the owner only adult individuals, napr eat, fleas (see). At the expense of the owner P. can be fed also throughout all phases of development, napr, argasovy mites (see), bugs (see), etc.

It is necessary to distinguish P. from lozhnoparazit, or pseudoparasites.

Lozhnoparazitami is called by such free living forms, to-rye, having got accidentally into a human body or an animal, some time is able to live in it and to eat at his expense (e.g., larvae of a cheese fly). Lozhnoparazita can be true and imaginary. Unlike true, imaginary lozhnoparazita are found in excrements of the owner where get from the outside. If P. develops in an organism of the nonspecific owner where gets accidentally, then it is called gosteparazity or a xenoparasite. E.g., at accidental hit in a human body of larvae of a rat tapeworm (Hymeno-lepis diminuta) in intestines development of a tape (puberal) form of a parasite is possible.

Items are most adapted for the owners and special living conditions. In the course of evolution they had a number of devices to specific living conditions. It is accepted to allocate two basic groups of devices — morphological and physiological adaptations. To morfol, to adaptations belong: difficult arranged bodies of fixing (a sucker, kryuchya, prisasyvatelny cracks, chitinous outgrowths on body surfaces and extremities etc.); the specific cover fabric performing generally protective function (e.g., a cuticle at nematodes); the simplified structure of a nervous system and, as a rule, a reduction of organs of sight (at endoparasites, mites); a reduction of circulatory and respiratory systems (at helminths); a hermaphroditism (at flat worms). To fiziol, to P.'s adaptations refer the maximum development of a reproductive system, significant increase in amount of the eggs allocated to P., complication of life cycles of development of P., development of the alimentary system promoting absorption of large volumes of blood (e.g., at mites), or, on the contrary, its reduction due to development of ability to absorption of food by a body surface (e.g., at flat worms).

Life cycle of P. includes all phases of development, beginning from egg and finishing a puberal individual. An organism, in P.'s Krom reach puberty and breed sexually, is called the final or definitivny owner. An organism, in Krom larval phases P. live, and its reproduction happens in the sexless way, is called the intermediate owner. Nek-ry P. have not one, and several intermediate owners.

A row P., called by geteroksen-ny, can parasitize at representatives of several animal species, in other words, has a wide range of owners. So, e.g., females of mosquitoes suck blood not only at the person and various mammals, but also at birds. The items living at the expense of representatives of one type of owners (a head louse of Pediculus capitis at the person), are called monok-hay or specific. Mono-ksennye P. cannot parasitize on others, even relatives, animal species or plants.

Infection of the owner P., belonging to protozoa, helminths, mites and insects, can occur in various ways: alimentary, napr, swallowing eggs or larvae of helminths with dirty vegetables and fruit, and also with meat of cattle, pigs or other intermediate owners of parasites; at drink of water from reservoirs where there can be larvae of parasites (blood flukes, or shistosoma, a parasitic worm of Dracunculus medinensis); through integuments (scabby zuden — Sarcoptes scabiei, larvae ankilostomid — Ancylostoma duode-nale, etc.); through a placenta (Toxoplasma gondii toxoplasma, a trypanosome of Trypanosoma equiperdum, etc.); through blood at a sting of blood-sicking arthropod carriers (e.g., a malarial mosquito of the sort Anopheles).

Source of an invasion is the infected (sick) animal or the person. The carrier can be a factor of a chain of transfer — the final owner of a parasite (e.g., a mosquito of Anopheles), in Krom occurs a cycle of sexual development of a plasmodium, or intermediate hozyarsh (midges for onkhotserk, mosquitoes for filarias).

Each member of couple a parasite — the owner not only influences the partner, but also itself experiences his influence. Parasitizing can be followed by mechanical action, otnyatiy at the owner of the substances necessary for his normal life activity, release of the toxicants operating on an organism of the owner, opening of ways for penetration in an organism of the owner of pathogenic microbes or viruses (i.e. other parasites), the general weakening of an organism of the owner and sensitization to other diseases. All these various effects of parasitizing are interconnected, one influence can lead to influence of other character and mention all organism in general.

For the owner P. are always the alien organisms operating with the secrets, excretes and fabric substances and causing response. Depending on a stage of development of P. can have various pathogenicity. During the parasitizing during the life they pass various phases of development in one owner, and pathogenetic and immunological processes at the same time can be various intensity. Sometimes, staying in an organism of the owner, P. do not exert visible impact on it. However in most cases parasitizing does not take place for an organism of the owner without effects.

Responses of an organism of the owner have protective value and can be divided on cellular, fabric and humoral. Cellular and fabric reactions are expressed in development of inflammatory processes and a hypertrophy of one or several cells of body tissue struck with P. in formation of tilation, or zootsetsidiyev. At the same time on site P.'s localizations big swelling or growth, hollow is formed inside owing to what P. is placed in a cavity that interferes with its penetration into the next cells. Humoral reactions from the owner are closely connected with the phenomenon immunity (see) are also expressed in education in blood of the owner of antibodies, specific in relation to various P. Formation of antibodies in an organism of the owner is followed by development of resistance in relation to the Item. By means of this defense reaction the organism of the owner or gets rid of P., or interferes with its harmful effects.

Diagnostic methods are subdivided on macroscopic, microscopic and immunological.

Macroscopic methods consist in detection of parasites on outside covers of the owner or in his excrements. By means of microscopic methods reveal P. in blood smears, in an intercellular lymph, pieces of the muscles received by means of a biopsy and also in allocations (a phlegm, excrements). In a basis immunol, methods lie immunol, reactions, to-rye develop after hypodermic introduction to the person of a small amount of antigens P. Immunol, methods find broad application at diagnosis of a trichinosis, echinococcosis, migratory ascaridosis, etc.

See also Infectious diseases , Parasitic diseases .

Bibliography: Bogoyavlensky Yu. K. Structure and functions of cover fabrics of parasitic nematodes, M., 1973; Dogel V. A. Course of the general parasitology, JI., 1947; Pavlovsky E. H. Guide to parasitology of the person, t. 1 — 2, M. — JI., 1946 — 1948; Parasitology of the person, under the editorship of G. S. Pervomaysky and V. Ya. Podolyan, JI., 1974; Scriabin K. I. and Petrov A. M. Bases of a veterinary nematodologiya, M., 1964; Geiman Q. M of Comparative physiology, mutualism, symbiosis and parasitism, Ann. Rev. Physiol., v. 26, p. 75, 1964; O d e n i n g K. Parasitismus, B., 1974.

Yu. K. Bogoyavlensky.