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see parasiteparasite,
plant or animal that at some stage of its existence obtains its nourishment from another living organism called the host. Parasites may or may not harm the host, but they never benefit it.
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an association between two organisms of different species in which one—the parasite—uses the other—the host —as a habitat and source of food; the host either partly or fully regulates the parasite’s interaction with the environment.

Like the concepts of planktonic, benthic, and edaphic modes of life, the concept of parasitism is of most interest in ecology. The outstanding feature of parasitism, as compared with other modes of life, is the parasite’s habitat—another living organism that actively reacts to the parasite’s presence. The parasite’s environment, as shown by E. N. Pavlovskii and V. A. Dogel’, is dual in nature: the distinction is made between the first-order environment, that is, the host’s organs and tissues that surround the parasite, and the second-order environment, which surrounds the host. The parasite’s relations with the second-order environment are regulated chiefly through the host, although environmental factors, for example, temperature, may act directly on the parasite; the extent of this action depends on the species of parasite. The extent of metabolic interaction between a parasite and its host is rather great.

Many parasites are antigens and stimulate the formation of the host’s antibodies, which, in turn, provoke an immune response. Ectoparasitism, in which the parasite lives on the surface of the host’s body, is distinguished from endoparasitism, in which the parasite lives within the host’s body. Parasites are classified according to the duration of infestation as temporary or permanent; the permanent parasites can be intermittent.

Parasitism developed over the course of evolution in several ways, initially occurring as instances of inquilinism, predation, commensalism, or symbiosis. Other mechanisms were the chance ingestion of one organism by another or the accidental settling of one organism on another’s body. Parasites frequently, but not always, harm the host to some extent and give rise to a variety of diseases of man, animals, and plants. Parasitology, microbiology, virology, and plant pathology are concerned with parasitism and the development of methods of controlling parasites.

In animals. Parasitic species are found in most groups of animals, except echinoderms and brachiopods. Among chordates, a semiparasitic mode of life is maintained by lampreys, by hagfishes, and by bats of the family Desmodontidae. The members of certain orders and classes are all parasites, for example, sporozoans among protozoans; trematodes, members of the class Monogenoidea, and tapeworms among flatworms; acanthocephalids among roundworms; and fleas and true lice among insects.

The extent to which a parasite harms its host varies. Parasitism usually causes the host to become sick. Sometimes a pathogenic parasite does not cause disease but is simply spread by the host; such parasitism is called the parasite carrier state. Various animal and plant species can be hosts of parasites. A host is often simultaneously infested with several parasitic species, which interact not only with the host but also with each other. The aggregate of parasites that exist on a single host constitutes the parasite community, or parasite biocenosis.

Ectoparasites settle on the surface of an animal or plant host’s body; these include many sucking insects, for example, true lice, fleas, aphids, and members of the family Trichodectidae and of the superfamily Analgesoidea. Cases where parasites settle in the skin or in body cavities that open to the outside represent a transitional type between ectoparasitism and endoparasitism. In endoparasitism, a distinction is made between celozoic parasitism and tissue parasitism. In the former case, the parasite lives within body cavities, for example, the intestinal lumen. In tissue parasitism, the parasite lives in the body tissues; for example, sarcosporidians and larvae of the genus Trichinella infest skeletal muscles, and nematodes live in the tissues of potatoes, tomatoes, tobacco, and other plants. In tissue parasitism, the parasites may penetrate into individual cells; for example, the causative agents of malaria penetrate into human red blood cells, and coccidians penetrate into intestinal epithelial cells. The duration of temporary parasitism varies from a few seconds, as in the sucking of human blood by female mosquitoes, to several days or months, as with ticks of the family Ixodidae and the larvae of botflies and warble flies.

The development of some endoparasites occurs both outside and within the host’s body. For example, the larvae of hookworms develop in soil but mature in the body of an animal or human being. With other parasites, including whipworms, ascarids, and the tapeworm Echinococcus granulosus, the larvae develop within the eggs, which drop from the host; subsequently, the alimentary canal of the host can be reinvaded by the eggs, which now contain mature larvae.

Parasitism varies in degree of specificity, that is, in the ability of parasites to adapt to a particular species or group of host species. Often, a parasite needs two or three hosts on which to complete its life cycle; sometimes the hosts are taxonomically distant from each other. For example, the hosts to the causative agent of malaria are man and mosquitoes of the genus Anopheles; to the swine tapeworm, swine and man; to the beef tapeworm, cattle and man; to the tapeworm Echinococcus granulosus, man and dogs; to the liver fluke, mollusks and cattle; and to the broad fish tapeworm, copepods (for example, Cyclops), fish, predatory mammals, and man. The succession of hosts is connected with the alternation of parasite generations.

A host in which a parasite matures and multiplies sexually is called definitive, or final; hosts in which larval stages exist are called intermediate. The routes by which a parasite enters the body of its host are quite varied. For example, the parasite may penetrate into the digestive tract with food, actively burrow through the skin, or penetrate by means of such carriers as bloodsucking insects, ticks, and mites.

The geographic distribution of parasites is controlled by the distribution of their hosts and by a given region’s geography. Therefore, parasites are not distributed evenly throughout their range, since the distribution depends on numerous factors, including the feeding habits, migratory patterns, and hibernation habits of the host, as well as the size of the range, domestication patterns, and landscape and climatic features.

The study of parasites’ life cycles, the ways in which hosts can be infested, and the ecology of hosts and carriers is extremely important for preventing parasitic diseases and for limiting the populations of causative agents of diseases of man, domestic animals, and game animals. The size of parasite populations can be reduced by drugs or by resorting to various measures that interrupt the parasites’ life cycle and destroy the intermediate hosts and carriers. Plant pests can be controlled by using parasites of these pests, for example, such hymenopterous insects as ichneumon flies; this is an example of biological control.


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In plants. Many parasitic fungi, bacteria, viruses, mycoplasms, and flowering plants as well as a few parasitic algae are known. Parasitism is not encountered among mosses, ferns, and gymnosperms. Plants, animals, and even man can be hosts of parasitic plants; man is parasitized chiefly by bacteria and certain fungi. Some human diseases that are caused by parasitic bacteria are tuberculosis, diphtheria, tonsillitis, dysentery, plague, cholera, and systemic sepsis. Fungi usually attack the skin and hair and occasionally the lungs, eyes, and other organs. Entomophthoraceous fungi are parasitic on insects, which they often destroy in large numbers; this happens, for example, with houseflies at the end of summer. Fungi of the order Mucorales and the genus Aspergillus cause disease by infesting the respiratory tract of birds.

Plants that are parasitic of other plants are classified according to whether they contain chlorophyll. Green parasites, which contain chlorophyll, synthesize organic matter through photosynthesis while receiving mainly water and minerals from the host. Non-chlorophyll-containing plants that are parasitic of other plants are complete parasites, or holoparasites, and thus must receive both organic and inorganic nutrients from the host. Plant parasites of other plants vary in the degree to which they are facultative or obligate, and these variations are viewed as stages in the evolution of the parasitic mode of life.

In ectoparasites, for example, powdery mildews, dodder, and members of the genus Lathraea, a large part of the body is outside the host, and only the feeding organs—haustoria—come into direct contact with living cells within the host. In endoparasites, including many parasitic bacteria and fungi and flowering plants of the family Rafflesiaceae, all or almost all of the parasite’s body is buried in the living tissue of the host; only the reproductive organs remain on the surface. This morphology is the most efficient arrangement in terms of feeding, which in endoparasites as in ectoparasites probably takes place by osmosis.

Most parasitic fungi live in the interstitial spaces, burying only their haustoria into the host’s cells. The lower fungi, bacteria, viruses, and mycoplasms are cytozoic parasites, living within the host’s cells. The organization of many parasites changed over the course of evolution to conform to the parasite state. The principal change was a simplification, or reduction, of certain functions and corresponding organs. For example, owing to a loss of photosynthesis, dodder and members of the genus Lathraea underwent a reduction of the leaves, which develop only as small, colorless glumes. In plants of the family Rafflesiaceae, and in some broomrapes and mistletoes, both the leaves and stems are reduced. In some parasites only the flowers are visible; these can be very large—in some Rafflesiae they are up to 1 m in diameter. The vegetative structure is completely embedded in the host’s tissues and consists of cellular filaments that resemble fungal hyphae. Parasitic algae also exhibit morphological reduction, although of a less pronounced type than in flowering parasites. In some fungi, for example, certain ascomycetes, morphological reduction sometimes occurs as underdevelopment of the fruiting bodies.

As an organism assumes the parasitic mode of life, its enzyme systems become limited until ultimately only specialized enzymes remain, which permit infestation of only a limited variety of plants. Some plant physiologists consider saprophytic flowering plants and their mycorrhizal fungi to be mutually parasitic (seeMYCORRHIZA).

Effect of parasite on host. Facultative parasites often attack only weakened plants or parts of plants that are inactive, for example, stored fruits and vegetables, and they quickly kill the affected part. Fungal pests penetrate vegetables in storehouses through surface bruises. The fungus’ mycelium settles at first on dead tissues, where it proliferates. Its hyphae then release toxins and enzymes. The toxins kill living cells, while pectinase enzymes macerate the tissues. Thus, fungal hyphae do not come into contact with living cells of the host plant but rather feed and grow only on the constituent substances of dead cells. Strictly speaking, such nutrition can be called saprophytic, although it might also be considered parasitic, since the fungus prepares the dead substrate from living tissues.

Obligate and near-obligate parasites, for example, rust fungi, smut fungi, and the powdery mildews, mainly attack mature plants, but the effects are manifested rather slowly. Sometimes the host’s cells are even stimulated to produce more chlorophyll and grow better. The stimulating effect lasts for a few days, a few weeks, or sometimes even through the entire growth season and beyond, after which the “peaceful” coexistence of the parasite and host ends. The host’s photosynthesis ceases and its respiration is intensified, and the host cells begin to die off. Eventually, the fungal mycelium begins forming spores and soon dies off as well.


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A symbiotic relationship in which the host is harmed, but not killed immediately, and the species feeding on it is benefited.
References in periodicals archive ?
In the models of microparasites and macroparasites Anderson and May (1), (2) have analyzed how the threshold phenomena for the persistence of epidemics are modified when population size is variable.
Despite this, microparasites (generally prions, viruses, bacteria, fungi, and protozoans) often remain the singular focus of research on emerging infectious diseases (35,36).