Parasitism

PARASITISM. - It denotes that form of association between organisms of different species in which one of the two associates (the parasite) lives for a more or less long period of time on the surface or inside the body of the other (the host), taking from it the substances with which to supply its own vital needs, but without destroying it. This association, very widely diffused in nature both in the vegetable and in the animal kingdom, occurs between two plants, between two animals, between a plant and an animal.

Parasitology, the science to which the study of this phenomenon is assigned, comprises a phytoparasitology, for the study of plant parasites, and comprises bacteriology (viruses, rickettsiae, schizomycetes) and mycology (hyphomycetes, ascomycetes, phycomycetes), and a zooparasitology, for the study of animal parasites, which comprises protozoology (protozoa), helminthology (worms: platyhelminths and nemathelminths) and entomology (mites and insects). Since in current usage, when one speaks of parasitology, one refers to zooparasitology, it is to this that particular reference will be made, it being understood that the general principles that will be set forth are applicable to all parasitic organisms.

P. as defined above implies the existence of a series of interrelations, of reciprocal influences between host and parasite that can even lead to the possibility of the coexistence of both: a state of equilibrium is thus established, albeit an unstable one, making possible the life of both associates; only when this state of equilibrium is disturbed does the association come to an end, concluding either with the death of the parasite, if it is the organic defenses of the host that prevail, or, if instead these are overwhelmed, with the death of the host, which however can obviously also be followed by that of the parasite.

P. can be facultative or obligatory. In the first, the parasitic condition does not represent a necessity of life for the species but only a possible modus vivendi, the species itself normally leading a free life; facultative p. is, for example, that of the larvae of certain diptera that usually live as saprozoites, on corpses or organic substances in decomposition, but are also sometimes capable of becoming parasites of man and of animals, causing in them the so-called myiases. P. is instead obligatory when the parasitic condition, more or less long, constitutes an absolute necessity for the species, outside of which there is no possibility of life. Parasites can be: a) temporary, if they limit the period of their stay on the body of the host to the strict time necessary to feed on it, escaping immediately afterward and leading a free life for the rest; to this group belong ticks, fleas, mosquitoes and, practically, almost all hematophagous insects and mites; b) permanent, if they inhabit the host for a more or less long period, corresponding to one or more stages of the life cycle, or for their entire existence. Parasites that live on the surface of the host's body or in natural and artificial cavities of easy access are called ectoparasites; those instead that live inside the body, in deep cavities or in the thickness of the tissues or inside the cells, endoparasites. Quite often a constant preference is found in parasites toward a determined organ or tissue, so much so that it is exceptional to find them in a site different from their usual one (in such a case one speaks of erratic p.); there exist nevertheless numerous examples, especially in parasites in the larval state, in which this organ specificity is not felt, so that one and the same parasite can be found in very different sites.

Many parasites, whether or not they present phases of free life, complete their biological cycle with the intervention of a single host and are therefore called monoxenous; many others instead possess a more complicated biological cycle, for the completion of which they pass through two or more different hosts, in each of these only one part, and a well-defined one, of the cycle taking place; in this second case the parasites are called heteroxenous, and it is customary to distinguish the different hosts into intermediate, which harbor the larval forms (metazoa) or the schizogonic phase of the cycle (protozoa), and definitive, which harbor the adult form (metazoa) or the anfigonic phase (protozoa) of the cycle itself.

The variety of the biological cycles of parasites is practically innumerable, each parasite presenting in its own cycle some specific particularity of its own. The most precise possible knowledge of the biological cycle of parasites, besides being of great scientific interest, is of great practical utility for the species of interest to the human economy, that is, those pathogenic for man, for domestic animals and for cultivated plants, in order to derive from it the indication of the stage in which the parasite proves most vulnerable, for the purposes of prophylaxis (v., e.g., MALARIA).

Very numerous cases are noted of parasites that are exclusive to a single host species or to a small number of species of related hosts (stenoxenous parasites), others instead in which the parasites seem to find good conditions of existence in numerous hosts of distant species (euryxenous parasites), and still others that present intermediate conditions. This parasitic specificity is of great interest, providing perhaps the only indications for establishing the antiquity or otherwise of a parasitic association, a strict specificity presumably implying a complete evolution of the relations between parasite and host that cannot be accomplished except through a very long period of time; and from the practical point of view important elements concerning the real possibilities of success in the struggle against certain parasites, since, for example, it is evident that that against euryxenous parasites has much less probability of success than that against stenoxenous ones, as well as sometimes indications usable in the struggle itself. It suffices, for example, to recall, in this regard, how one of the forms of antimalarial prophylaxis, zooprophylaxis, consisting in interposing between human dwellings and anopheline breeding foci the stabled animals, is based precisely on the observation of the low parasitic specificity of the majority of the anophelines that are vectors of the disease. In some cases one can observe the finding of parasites with a marked parasitic specificity 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; one therefore finds in them particularities of a morphological and physiological order in close connection with their parasitic habitat, and on this depends, as the comparison between the free forms and the parasitic forms belonging to one and the same zoological group demonstrates whenever possible. The extent of the modifications derived from parasitic adaptation is related to the degree of p.: as a general rule, the modifications are in fact lesser in ectoparasites temporary ectoparasites, become more pronounced in permanent ectoparasites, and reach their maximum in endoparasites. A phenomenon common to all permanent parasites, whether ectoparasites or endoparasites, is the reduction or total loss of the organs of locomotion, correlated with a more or less pronounced development of organs of adhesion. In lice in general, and in those of the pubis (Phthirius pubis) in particular, one notes, for example, besides the loss of wings, the reduction of the legs and the transformation of their distal extremities into organs of adhesion; and in helminths, the absolute absence, in the parasitic phases, of organs of locomotion is matched by a great development of special organs of adhesion, such as the suckers of trematodes and cestodes and the proboscis of acanthocephalans. Modifications affecting the organs of nutrition are also almost constant: the buccal apparatus of hematophagous arthropods becomes, for example, wounding and capable of suction; the intestine may undergo transformation of some of its parts so as to become a true reservoir of food, which thus allows the nutrition of the parasite even at long intervals (ticks, bedbugs), or it may be reduced in its terminal part, if the food is such as to leave little or no residue, or even disappear altogether, as in cestodes and acanthocephalans that live immersed in substances already digested by the host and feed by osmosis. Finally, a strengthening, often of a very high degree, of reproductive capacity appears generalized in parasites. This strengthening is achieved through various mechanisms, namely: either directly through the very considerable increase in the production of eggs (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, far more rapid and effective producers of individuals, of asexual reproduction (as, for example, in malarial plasmodia); or with the intervention of reproductions in the larval state (paedogenesis), whereby from each egg laid there may potentially arise even very numerous individuals (as in trematodes and certain cestodes); or with the establishment of a hermaphroditic condition which, if autogamous (cestodes), allows the possibility of reproducing even in the isolated individual, and if dicogamous permits the bisexual utilization of all individuals.

Parasites exert various actions in the host: spoliative, mechanical and toxic. The first, absolutely constant, since the very concept of parasitism is centered on their presence, consist in the subtraction from the host of the substances necessary for the vital requirements of the parasite, and are of very different importance, depending on whether they are exercised on substances merely ingested and elaborated by the host or on substances that form part of its living structures. Mechanical actions, on the other hand, are due to the presence of the parasite with a physical entity acting as a foreign body, and therefore capable of provoking phenomena of occlusion or compression, or to morphological peculiarities of the parasite itself that determine lesions of continuity of the tissues. As for toxic actions, today recognized as certainly prevalent in the determinism of many pathological pictures of parasitic diseases, they are due to the absorption by the host of substances produced by the parasite. To these actions, the host opposes defense reactions that tend, when possible (ectoparasites), to the elimination of the parasitic agent, or to its isolation (for example, in connective-tissue cysts), or to neutralizing its toxic action (production of specific substances, eosinophilia). The importance of the parasitic phenomenon in the field of the economy of nature is truly immense: many examples have by now demonstrated that parasites enter as most principal components into the maintenance of the biological equilibria established in a given environment. Nor is the importance of parasitism in the field of human economy any less, and this both directly through the action of many parasites on man himself, and indirectly through that exerted on domestic animals and cultivated plants; it suffices to recall in this regard the parasitoses in the field of medicine, veterinary medicine and plant pathology.

Marcello Ricci