PARASSITISMO. — This designates that form of association between organisms of different species in which one of the two associates (the parasite) lives for a more or less prolonged period on the surface of, or within the body of, the other (the host), depriving it of substances with which to meet its own vital needs, but without destroying II. This association, extremely widespread in nature both in the plant and the animal kingdoms, occurs between two plants, between two animals, or between a plant and an animal.
Parasitology, the science devoted to the study of this phenomenon, comprises phytoparasitology, concerned with the study of plant parasites—which includes bacteriology (viruses, rickettsiae, schizomycetes) and mycology (hyphomycetes, ascomycetes, phycomycetes)—and zooparasitology, concerned with the study of parasitic animals, which includes protozoology (protozoa), helminthology (worms: flatworms and roundworms), and entomology (mites and insects). Since in current usage, when one speaks of parasitology, one is referring to zooparasitology, particular reference will be made to the latter, while the applicability to all parasitic organisms of the general principles set forth below remains understood.
P. as defined above implies the existence of a series of interrelationships and reciprocal influences between host and parasite that may also lead to the possibility of coexistence between the two: a state of equilibrium is thus established, however unstable, making the life of both associates possible; only when this state of equilibrium is disturbed does the association cease, ending either in the death of the parasite, if the host’s organic defenses prevail, or, if these are instead overcome, in the death of the host, which may of course also be followed by that of the parasite.
P. may be facultative or obligatory. In the former case, the parasitic condition does not represent a vital necessity for the species, but merely a possible modus vivendi, the species itself normally leading a free life; facultative p. is, for example, that of the larvae of certain dipterans, which usually live as saprozoites on corpses or decomposing organic substances, but are also sometimes capable of becoming parasites of humans and animals, producing in them the so-called myiases. P. is instead obligatory when the parasitic condition, for a more or less prolonged period, constitutes an absolute necessity for the species, outside of which there is no possibility of life. Parasites may be: a) temporary, if they limit the period of their presence on the host’s body to the time strictly necessary for feeding upon it, then immediately escaping and leading a free life for the remainder of the time; ticks, fleas, mosquitoes, and practically the great majority of blood-sucking insects and mites belong here; b) permanent, if they remain on the host for a more or less prolonged period, corresponding to one or more stages of the life cycle, or throughout their entire existence. Parasites that live on the surface of the host’s body or in natural and artificial cavities that are readily accessible are called ectoparasites; those, on the other hand, that live inside the body, in deep cavities, within the thickness of the tissues, or inside cells are called endoparasites. Very often parasites display a constant preference for a specific organ or tissue, so much so that it is exceptional to find them in a location other than their customary one (in such a case one speaks of erratic p.); nevertheless, there are numerous examples, especially among parasites in the larval state, in which this organ specificity is absent, so that the same parasite may be found in very different locations.
Many parasites, whether or not they have free-living phases, complete their biological cycle with the intervention of a single host and are therefore called monoxenous; many others, however, possess a more complicated biological cycle, for whose completion they pass through two or more different hosts, only one specific and well-defined part of the cycle taking place in each of them; in this second case the parasites are called heteroxenous, and the different hosts are distinguished as intermediate hosts, which harbor the larval forms (metazoans) or the schizogonic phase of the cycle (protozoa), and definitive hosts, which harbor the adult form (metazoans) or the gamogonic phase (protozoa) of the cycle itself.
The variety of parasites’ biological cycles is practically innumerable, each parasite displaying some particular feature of its own in its cycle. The most precise possible knowledge of the biological cycle of parasites, besides being of great scientific interest, is of great practical usefulness for species of importance to human economy—that is, those pathogenic to humans, domestic animals, and cultivated plants—since it indicates the stage at which the parasite is most readily attacked, for purposes of prophylaxis (v. MALARIA).
Very numerous cases are found of parasites that are exclusive to a single host species or to a small number of closely related host species (stenoxenous parasites); in other cases, parasites seem to find favorable conditions for existence in numerous hosts belonging to distant species (eurixenous parasites); and still others present intermediate conditions. This parasitic specificity is of considerable interest, providing perhaps the only indications for establishing whether or not a parasitic association is ancient, close specificity presumably implying a complete evolution of the relations between parasite and host, which can be achieved only over a very long period of time. From the practical standpoint, it also provides important elements concerning the actual chances of success in combating certain parasites, since, for example, it is evident that the fight against eurixenous parasites is much less likely to succeed than that against stenoxenous parasites, while it sometimes also offers indications that can be employed in the struggle itself. It is enough, for example, to recall in this regard how one form of antimalarial prophylaxis, zooprophylaxis, consisting in placing housed animals between human dwellings and anopheline breeding grounds, is based precisely on the observation of the low parasitic specificity of most of the anopheline vectors of the disease. In some cases, parasites known to possess marked parasitic specificity may be found in an unusual host; in such a case one speaks of accidental p.
Like every living being, parasites too are perfectly adapted to the environment in which they live; consequently, morphological and physiological features are found in them that are closely connected with their parasitic habitat, as is demonstrated whenever possible by comparison between the free-living and parasitic forms belonging to the same zoological group. The extent of the modifications resulting from parasitic adaptation is related to the degree of p.: generally speaking, the modifications are in fact less pronounced in ectoparasites.
temporary ones; they become more pronounced in permanent ectoparasites, and reach their maximum in endoparasites. A phenomenon common to all permanent parasites, both ectoparasites and endoparasites, is the reduction or total loss of locomotory organs, correlated with a more or less marked development of organs of attachment. In lice generally, and particularly in those of the pubis (Phthirius pubis), one notes, for example, in addition to the loss of the wings, the reduction of the legs and the transformation of their distal extremities into organs of attachment; and in helminths, the absolute absence, during the parasitic stages, of locomotory organs is matched by a great development of special organs of attachment, such as the suckers of trematodes and cestodes and the proboscis of acanthocephalans. Modifications affecting the organs of nutrition are also almost constant: the mouthparts of hematophagous zittropods become, for example, piercing and capable of sucking; the intestine may undergo transformation of some part of it so as to become a true food reservoir, thereby permitting the nourishment of the parasite even at long intervals (ticks, bedbugs), or may be reduced in its terminal part if the food is such as to leave little or no residue, or may even disappear altogether, as in cestodes and acanthocephalans that live immersed in substances already digested by the host and feed by osmosis. Finally, an enhancement of reproductive capacity, often to a very high degree, appears to be general among parasites. This enhancement is achieved by various mechanisms, namely: either directly, through a considerable increase in egg production (it is calculated, for example, that the egg-laying capacity of a Taenia solium is 50,000,000 per year, and that of a female Ascaris lumbricoides 64,000,000); or by alternating with sexual reproduction phases of asexual reproduction that are much more rapid and effective in producing individuals (as, for example, in malarial plasmodia); or through the occurrence of reproduction in the larval state (paedogenesis), whereby each egg laid may potentially give rise to very numerous individuals (as in trematodes and certain cestodes); or through the establishment of a hermaphroditic condition which, when autogamous (cestodes), permits reproduction even in the isolated individual, and, when dichogamous, allows the bisexual utilization of all individuals.
Parasites exert various actions on the host: depletive, mechanical, and toxic. The first, which are absolutely constant, since their presence constitutes the very basis of the concept of p., consist in depriving the host of substances necessary for the parasite’s vital requirements, and are of widely differing importance according to whether they act upon substances merely ingested and processed by the host or upon substances forming part of its living structures. Mechanical actions, on the other hand, are due to the presence of the parasite as a physical entity acting as a foreign body, and thus capable of causing obstruction or compression, or to morphological characteristics of the parasite itself that produce breaches in tissue continuity. As for toxic actions, now recognized as unquestionably predominant in the determination of many pathogenic forms of parasitic diseases, they are due to the host’s absorption of substances produced by the parasite. To these actions, the host responds with defensive reactions that tend, when possible (ectoparasites), toward elimination of the parasitic agent, or toward its isolation (for example, in connective-tissue cysts), or toward neutralizing its toxic action (production of specific substances, eosinophilia). The importance of the parasitic phenomenon in the sphere of the economy of nature is truly immense: many examples have by now demonstrated that parasites enter as highly important components into the maintenance of the biological equilibria established in a given environment. Nor is the importance of p. any less in the sphere of human economy, both directly, through the action of many parasites on human beings themselves, and indirectly, through that exerted on domestic animals and cultivated plants; it is enough to recall in this connection parasitic diseases in the fields of medicine, veterinary medicine, and phytopathology.
BULL. P. P. Grassé. Parasites et parasitisme. Parigi 1936; J. G. Baer, Le parasitisme, ivi 1946; E. Brumpt, Précis de parasitologie, 6e ed., ivi 1949; A. C. Chandler, Introduction to parasitology, with special reference to the parasites of man, 8ª ed., Nuova York 1949; M. Caullery, Parasitisme et symbiose, 2ª ed., Parigi 1950; J. G. Baer, Ecology of animal parasites, Urbana 1951. Marcello Ricci