The Parasites


Parasites are organisms that feed on and damage other organisms (hosts). Parasites at the place of their residence on the hosts are subdivided into external (ectoparasites) and internal (endoparasites).

Ectoparasites can be either permanent (for example, lice) or temporary fleas (see), bugs (see), mosquitoes (see), etc. Endoparasites, which are constantly in the host's body, can be found in virtually all of its tissues or organs. Many parasites often make a complex development cycle, successively changing their hosts. The change of the latter is associated with changing feeding conditions during metamorphosis (mites) or with reproduction and conditions of development of larvae (tapeworms).

A number of ectoparasites - carriers of pathogens of vector-borne diseases of humans and animals - is at the same time a reservoir of these pathogens in nature. The fight against parasites is of great practical importance for the prevention and elimination of diseases caused and spread by them.

See also Helminths, Carriers , Natural foci , Protozoa .


Parasites (from the Greek parasitos - parasite, parasite) are creatures that feed on living plants (phytoparasites) or animals (zooparasites) and people temporarily or permanently residing on them or in them. Parasites feed on juices, body tissues or food in the digestive canal of animals and humans. At the site of parasites on the hosts, parasites of external (ectoparasites, Epizoa) and internal (endoparasites, Entozoa) are distinguished.

Some ectoparasites temporarily sit on the surface of the host's body for food intake (mosquito, horsefly, leech), others live on the host's body (lice, Mallophaga, Parasitic crustaceans of Soreroda on fish). Some parasites - intradermal - live in the thickness of the skin; for example, in the stratum corneum of the mammalian epidermis, it moves and moves the itchy itch; in the malpighian layer of the human epidermis, the larva of the gastric gadfly (Gastrophilus) can parasitize; The female of the sand flea (Sarcopsylla penetrans) is completely immersed in the epidermal covering. Larvae of flies, skin gadfly (Hypoderma), etc. penetrate the horny layer of the skin and into the subcutaneous tissue. Cavity parasites live in the body cavities (nose, ear, mouth, eye conjunctiva) having a wide communication with the external environment (Oestrus and Rhinoestrus larvae) .

Endoparasites can live in almost any organ or tissue of the host. Depending on the localization, they are distinguished: a) endoparasites of organs communicating with the external environment (lungs, intestines, urogenital organs); These include various flagellates, amoebas, worms, and others; b) blood-parasites with a division into parasites living in blood plasma, in red blood cells and white blood cells (microfilariae, hemosporidia, trypanosome); c) tissue endoparasites living in various host tissues, for example, in striated muscles (sarcosporidia, trichina), in the brain (trypanosomes, toxoplasma, tapeworm tapeworms), in cartilage (myxosporidia in some fish), in connective tissue (myxosporidia), in nerve fibers (Myxobolus), etc .; d) endoparasites of coelomic cavities (some parasitic worms, gregarines).

The above subdivision of parasites is conditional, as some parasites migrate through the body of the host during various periods of life, entering various tissues and organs. The place of parasites' stay is that part of the body where the parasites are grounded definitively; on this basis Trichina is considered a parasite of muscles, although in them live the larvae of these nematodes, and reproduction occurs in the intestine.

Some parasites have a wide range of hosts (the mite Ixodes ricinus drinks the blood of many mammals, birds, lizards); such parasites are called polyphages, euryzoid or polyxene (multi-host) parasites. Their antipodes are stenozoid, monoxenous or oligoxic parasites living at the expense of hosts of one species or a few species (head louse, malignant pathogens).

Also distinguish obligate [from obligatus (lat.) - obligatory] and optional [from facultas (lat.) - the ability, ability] of the hosts of parasites. For example, many mammals are obligate hosts for ticks Ixodes ricinus, while some reptiles (lizards, etc.) are facultative hosts.


Parasites often make a complex development cycle and live at the expense of several hosts (heterosexual parasites). Thus, Ixodes ricinus attacks in each stage of metamorphosis to feed on the host (vertebrate) of the same or different species, using three hosts separately - for the larva, the nymph and the adult mite. The hosts may be several individuals of the same species or different species, families and even orders. For example, Ixodes persulcatus lives on chipmunks, wolves, hazel grouses, moles, hedgehogs and other hosts; it also attacks a person.

Along with three-host mites, there are two-host species. The main (final, definitive) host is an animal or a person in which the parasite reproduces sexually; in the intermediate - the parasite reproduces asexually. For a malarial plasmodia, man is an intermediate host, since plasmodia occurs in his blood (plural fission); At the same time, a specific carrier of the malarial plasmodium - the female Anopheles mosquito - is the main host, since in it the plasmodium passes the sexual cycle of development.

There is hardly a kind of animal whose individuals would be free from parasites of plant or animal nature; However, the parasite infestation of different host species varies considerably: some animals are heavily infected with parasites, others - poorly. The total number of parasites living at the expense of man is estimated at several hundred; often open up new human parasites. Among different types of wildlife parasites are distributed unevenly. So, among the protists there are a lot of parasites; some classes of protozoa, such as sporoviks, consist exclusively of parasites. The coelenterata have two parasitic species. Lower worms (Vermes) - flukes, tapeworms, Acanthocephala - are all parasites. There are many parasites among nematodes. Among the ringworms parasites are leeches. There are no parasites among echinoderms. The clams are poor in parasites. Arthropods are rich in parasites, but by classes and orders they are distributed unevenly. Parasites are among the lower crustaceans (Entomostraca, Rhizocephala, Copepoda, etc.). Among spiders, parasites are represented by ticks (see). Many insects are ectoparasites, some are endoparasites; Detachments Mallophaga, Anoplura and Aphaniptera consist of only parasites. Among the chordates (including vertebrates), there are almost no parasites (excluding myxine and Fierasfer fish found in holothuria).

The way of life of parasites differs in originality. Constant parasites that live on the skin, mucous membranes or in tissues, organs, host cavities are provided with nutrition; they do not need to look for food; Therefore, in the process of evolutionary adaptation of their ancestors to the parasitic way of life, the functions of motion are weakened and the organs of motion are reduced and sometimes disappear. Since the parasite is doomed to death when the host is lost (except for cases of intermittent feeding of the parasite), the attachment organs are developed in the parasites: tenacious legs, powerful mouth organs, special appendages, hooks, strong muscular suckers, etc. The mouth organs, which feed for a long time, organs of attachment to the veil of the host. In rapidly sucking blood of female mosquitoes, the needle-shaped oral organs are easily punctured and extracted from the skin; sucking of blood is associated with the injection of the secretion of salivary glands into the skin, expanding the capillaries of the host and enhancing the blood flow to the oral organs of the parasite. This is associated with toxic irritation of the skin under the influence of saliva ectoparasite. In the saliva and in the digestive juice of bloodsuckers there are anticoagulins; other parasites secrete a proteolytic enzyme that destroys the tissues of the host's organs (dysentery amoeba causing ulceration of the colon, cercariae with schistosomes, penetrating through human veins into veins). Parasites that live in the intestine (see Helminthes), have antiferment and therefore are not digested by the digestive juices of the host.

For endoparasites, the external environment, or biotope, is the host's body, organ or tissue. Effects of environmental factors on endoparasites occur, but not direct, but are mediated by the host organism. The degree and nature of these influences are determined (in addition to the properties of the parasite itself) by the characteristics of the host organism. In poikilothermic animals, fluctuations in body temperature cause heating or cooling and parasites living in them. So, the malarial plasmodium, living in the body of the wintering mosquito, undergoes severe temperature effects, which can be fatal to the parasite. In contrast, in human blood, the malarial plasmodium lives with insignificant temperature fluctuations. The age of the host and its various states also affect the parasite.

In some cases, the presence of parasites in the host does not significantly affect his health. Nevertheless, there is evidence of the pathogenicity of parasites, manifested as local, and general pathological changes. Sometimes they are only indicated in the host's body; then they can be discovered only by special research; part of the parasite causes in the hosts of the disease, called invasive, or parasitosis. The particular pathology of parasitosis is nothing more than the study of the general biological question of the effect of parasites on hosts. The disease with some parasitoses causes a state of immunity (see).

The possibility of parasite invasion by the host is due to a combination of many accidents. The infection of a human with parasitic worms is a consequence of a number of conditions, which, together, contribute to the embryo's attainment of a parasite in the egg of the invading state. Such conditions for eggs of worms include: the nature of the environment, in particular the soil on which eggs of some worms, its moisture, the degree of illumination or shading of eggs, the duration of their stay in the external environment and the displacement that they may be subjected to here have fallen with excrement. With all this, the egg will only give a new parasite when it enters the host's gastrointestinal canal, the conditions in which will favor the development of parasites in it. For parasites developing with intermediate hosts, this chain of conditions is even more complicated. For example, Fasciola hepatica must consistently pass through the following habitats: water, freshwater molluscs, again water, wet meadow and the organism of the final host (usually cattle). For parasites, there is little chance of successfully passing through all obstacles and making a full cycle of life. This is the unfavorable side of parasitism for parasites from a biological point of view.

Natural selection of parasites developed properties that compensate for the effect of randomness; one of such properties is their exceptional fertility. So, hookworm gives for a day up to 25 000 eggs, and ascarid human - up to 200 000 eggs. One individual of the unarmed man can give about 5 million a day, and a year - up to 440 million eggs. Hence a very strong development in the parasites of the reproductive organs, along with what happens the reduction of "unnecessary" for the parasite organs.

The importance of parasites as pests of human health, domestic and commercial animals and as factors regulating the number of wild animals is very high. In conditions of nature and in agriculture, parasites of harmful animals, parasite parasites, secondary parasites and superparasites (or hyperparasites) are of particular importance. In the nucleus Jodamoeba butschlii parasitize Nucleophaga intestinalis, and in the protoplasm Entamoeba coli-Sphaerita sp. The same superparasites are also described in some other intestinal protozoa.

The first prerequisite for the rational treatment of parasitosis should be an accurate diagnosis of parasites. The presence of parasites in the body of the host is indicated by the detection of the parasite itself (balantidium, amoeba, toxoplasm, leishmania, trypanosome, malaria pathogens, worm segments, pinworms) or its eggs, cysts, etc. The organs of parasites (feces, urine, sputum) or host tissue (blood, lymph nodes, muscles, etc.).

Recently, serological and allergic methods of investigation have been increasingly used to diagnose some parasitic diseases: reaction with dye, RSK and cutaneous test for toxoplasmosis, fluorescent antibody response and haemagglutination reaction for malaria and toxoplasmosis, precipitation reaction for some nematodes, Casoni response in echinococcosis and other

To avoid errors in the diagnosis of parasitosis, it is necessary to make a correct zoological determination of the type of parasite according to the characteristics inherent in it. In the diagnosis of parasitic diseases, it should be borne in mind that some other micro- and macroorganisms, cells, etc., which are not parasites, can be, because of some morphological similarity, taken as parasites. These are the so-called pseudoparasites. Such can be in the study of blood platelets, pollen of plants, caught on the preparation during the preparation of a blood smear, algae, which divorced in a jar with water intended for the dilution of paint, etc. Some parasitic organisms may accidentally enter the intestine of one or another a host in which they can not develop. So, for example, oocysts of rabbit coccidia, in that case, when a person ate a liver of an animal infected with these parasites, can appear in his intestines and in feces, and they are easy to detect. These are the so-called transit parasites.

See also Ticks, Mosquitoes, Parasitocenosis, Carriers, Protozoa, Natural foci.