Parasites are organisms feeding on other organisms (owners) and harmful to them. Parasites by place of residence of their owners are divided into external (ectoparasites) and internal (endoparasites).
Ectoparasites can be fixed (for example, lice), and time - fleas (see), bugs (see), mosquitoes (see) and other Endoparasites, living in the body of the owner can be located almost in all tissues or organs. Many parasites often make difficult the development cycle, gradually changing owners. The last change is due to the changing conditions of supply in the process of metamorphosis (ticks) or with reproduction and development context larvae (tapeworms).
A number of ectoparasites vector-borne diseases of people and animals - is at the same time the reservoir of these pathogens in nature. The fight against parasites is of great practical significance for the prevention and elimination of the called and they spread diseases.
Cm. also Helminths, Carriers, Natural focality, Protozoa.

Parasites (from the Greek. parasitos - free-parasite) is being fed at the expense of live plants (cenology) or animals (zooparasites) and people temporarily or permanently residing on them or in them. Parasites feeding on the juices of the body tissues or food in the digestive tract of animals and humans. At the place of stay of parasites on the owners distinguish P. external (ectoparasites, Epizoa) and internal (endoparasites, Entozoa).
Some ectoparasites temporarily sit on the surface of the host body for meals (Komar, the gadfly, leech), others live on the body of the owner (lice, Mallophaga, parasitic crustaceans Сорероda on fish). Some parasites - intradermal - live in thicker skin; for example, in the stratum corneum of the epidermis mammals borovic moves and lives itch Sudan; in malpighian layer of the epidermis person can hostparasite larva gastric gadfly (Gastrophilus); wholly immersed in epidermal covers the female sand fleas (Sarcopsylla penetrans). In the corneal layer of the skin and subcutaneous tissue penetrate the larvae of flies, skin gadfly (Hypoderma) and other Abdominal P. live in the cavities of the body (nose, ear, mouth, conjunctiva of the eye), having wide connections with the external environment (larvae Ovodov Oestrus and Rhinoestrus).
Endoparasites can live in almost any organ or tissue of the host. Depending on localization distinguished: a) of endoparasites bodies, communicating with the external environment (lungs, intestines, urinary organs); these include various flagellate, amoeba, worms, etc. b) the blood parasites with the division on parasites that live in the blood plasma, red blood cells and white blood cells (presented, haemosporidia, trypanosomes); C) human endoparasites tissue, living in host tissues, for example in striated muscle (cercopoidea, trechini), in the brain (the trypanosomes, Toxo, Finnish tapeworms), in the cartilage (myxosporidia some fish), in connective tissue (myxosporidia), in the nerve fibers (Myxobolus) and others; d) of endoparasites coelomic cavity (some parasitic worms, gregorini).
Given the division of parasites conditional, as some of P. in different periods of life migrate through the body of the host, entering into various tissues and organs. Seat of the Isthmus is the part of your body where P. substantiated finally; on this basis, the trichina considered P. muscles, although they live larvae of these nematodes, and reproduction occurs in the gut.
Some Polynesians have a wide host range (tick Ixodes ricinus drink the blood of many mammals, birds, lizards); P. called polyphages, masoodi or polyxenida (mnogochislennymi) P. Them antipodes - stenotaenia, monoklinnye or Oligocene P., living at the expense of the owners of one kind or a few species (head lice, causing malaria).
Also there are obligate [from obligatus (lat.)- mandatory] and optional [from facultas (lat.)- the opportunity, ability] hosts of parasites. For example, many mammals are obligate the owners for ticks Ixodes ricinus, while some reptiles (lizards, and other)- optional owners.
Parasites often does a complex life cycle and lives by a few owners (geterogennyi P.). So, Ixodes ricinus in each stage of metamorphosis attacks to power on the host (vertebrates) of the same or different species, using three owners separately for larvae, nymphs and adult mite. Owners can be a few individuals of the same kind or of different kinds, families and even orders. For example, Ixodes persulcatus lives on the chipmunks, the wolves, the hazel grouse, moles, hedgehogs and other masters; he attacks and per person.
Along with trencsenyi mites are duhozanye species. The main (final, definitive) the owner of the animal or person in which sexual reproduction P.; in the intermediate - P. reproduces asexually. For malaria Plasmodium man - intermediate host, because his blood is schizogony (multiple division) Plasmodium; at the same time, specific carrier malaria parasite is the female Anopheles mosquito is the main owner, as it Plasmodium passes the reproductive cycle of development.
There is hardly any kind of animal, specimens of which would be free from P. vegetative and animate nature; however, infection with P. various host species varies considerably: some animals are heavily infested with parasites, others poorly. The total number of P., living at the expense of the person, is several hundreds; often open up new P. person. Among the different types of wildlife P. distributed unevenly. Thus, among the simplest lot P.; some classes of simple, such as sporewiki, composed of P. Coelenterates (Coelenterata) have two parasitic species. Lowest worms (Vermes) - flukes, tapeworms, skrebni - all are Many P. P. among the nematodes. Among annelids on, are leeches. Among echinoderms parasites there. Poor P. shellfish. Arthropods rich P., but by classes and groups unevenly distributed. P. among the lower crustaceans (Entomostraca, Rhizocephala, Copepoda and others). Among arachnids P. presents ticks (see). Many insect parasites, some of endoparasites; units Mallophaga, Anoplura and Aphaniptera consist P. Among the chordates (including vertebrate) almost no P. (excluding mixin and fish Fierasfer found in holoturia).
Lifestyle parasites is diverse. Permanent P. that live on the skin, mucous membranes or in tissues, organs, cavities owner, provided with meals;
they don't need to search for food; therefore, in the process of evolutionary adaptations of their ancestors to a parasitic way of life functions movement weaken and bodies of motion reduced, and sometimes disappear. Since the loss of the master P. doomed to death (except intermittent supply P.), P. developed authorities attach: prehensile feet, powerful mouthparts, special appendages, hooks, strong muscle suckers and others Have long feeding ticks mouthparts are and authorities attach to the protection of the owner. I quickly sucking the blood of female mosquitoes needle-like mouthparts easily caliauta and retrieved from the skin; sucking the blood associated with the injection of salivary glands in the skin that extends the capillaries of the owner and increases blood flow to the oral authorities P. connected With it and toxic skin irritation under the influence of saliva ectoparasite. In saliva and digestive juice bloodsuckers have anticoagulant; other Ii. allocate a proteolytic enzyme that destroys the tissue of the host (amoeba dysentery, causing ulceration of the colon, cercariae schistosomes, penetrating through the veils of human rights in Vienna). Parasites that live in the gut (see Helminths), have antifermental and not digested because of digestive juices owner.
For endoparasites the external environment or habitat, is the host organism, organ or tissue. The impact of environmental factors on human endoparasites, but not direct, but indirect host organism. The degree and nature of these impacts are determined (in addition to the properties of the P.) characteristics of the host organism. The poikilothermic animals fluctuations of body temperature cause the heating or cooling and living in them P. So, malaria parasites living in the body of wintering Komar, subjected to sharp temperature influences that could be disastrous for P. In contrast, in human blood malaria parasites living with insignificant fluctuations of temperature. The age of the owner and various state also influence P.
In some cases, the presence of parasites in the body of the owner is not reflected considerably on his health. However, there is evidence of pathogenicity P., manifested both local and General pathological changes. Sometimes they are only outlined in the host organism; then they can be opened only by special studies; part P. causes the owners of illness called invasive, or parasites. Private pathology parasitosis is not that other, as the study of basic biological question parasites on the hosts. Perubolivia some parasitic diseases cause the immune system (see).
The possibility of invasion master P. determined by the combination of many accidents. Human infection with parasitic worms - a consequence of a number of conditions which, taken together, contribute to the germ., in the egg invazirujutsja state. To such conditions for eggs of worms include: the nature of the environment, in particular of the soil, which were from faeces some eggs of worms, its moisture content, degree of lighting or shading eggs, the duration of their stay in the external environment, and move, which they may be subjected. However, the egg only then will give a new parasite when you get in the alimentary canal of the host where conditions will favour the development of it For p. P., developing with intermediate hosts, this circuit conditions even more difficult. For example, Fasciola hepatica must consistently pass the following habitats: water, freshwater mollusk, again the water, damp meadow, and the body's ultimate master (usually cattle). For P. little chance to pass successfully all the obstacles and to complete the cycle of life. This is an unfortunate side parasitism for P. from the biological point of view.

Natural selection, the parasites have developed properties, compensating influence of randomness; one of these properties is their exceptional fertility. So, Ancylostoma gives a day before 25 000 eggs, and Ascaris human - up to 200 000 eggs. One individual tapeworm naked can give during the day, about 5 million per year - up to 440 million eggs. Hence the very strong development in P. reproduction organs, along with what is happening reduction "unnecessary" for PA agencies.
The value of P. as pests human health, domestic and commercial animals and as determinants of the number of wild animals, very great. In the conditions of nature and agriculture are of special importance P. harmful animals, P. parasites, secondary P. and superpurity (or hyperparasite). In the kernel Jodamoeba butschlii parasitize Nucleophaga intestinalis, and in the cytoplasm of Entamoeba coli-Sphaerita sp. The same superpurity described and some other intestinal protozoa.
The first prerequisite for rational treatment of parasitic diseases should be accurate diagnosis of P. the presence of P. in the body of the owner indicates detection of parasite (balantidia, amoeba, Toxo, Leishmania, trypanosomes, causing malaria, segmented worms, pinworms) or their eggs, cysts and other Survey of the subject of allocation of those bodies inhabited by Polynesians (faeces, urine, sputum or host tissue (blood, lymph nodes, muscles and other).
Recently, to diagnose some parasitic diseases are increasingly used serological and allergic methods of research: the reaction to the dye, the RJC and skin test with toxoplasmosis, the reaction of IgM antibodies and haemagglutination reaction in malaria and toxoplasmosis, the reaction of precipitation in some nematodosis, the reaction of Casoni when echinococcosis and other
To avoid the error in the diagnosis of parasitic diseases, it is necessary to make correct Zoological species identification of the parasite on really its inherent characteristics. In the diagnosis of parasitic diseases should be borne in mind that some other micro - and mikroorganizmy, cells, etc., that are not Ii. may be due to some morphological similarities taken for P. This so-called pseudoparasitism. These can be in the study of blood platelets, pollen caught on the drug during the preparation of smears of blood, algae, who divorced in the pot with water intended for cultivation of paints, etc. Some parasitic organisms may accidentally get into the bowels of one or another of the host in which they can develop. So, for example, oocysts (rabbit in the case, when people ate the liver of an animal infected with these parasites, can be in his gut and in faeces, and they are easy to spot. This so-called transit parasites.
Cm. also Ticks, Mosquitoes, Parasitoses, Vector, Protozoa, Natural focality.