PARASITISM. — This term denotes that form of association between organisms of different species in which one of the two associates (the parasite) lives for a longer or shorter period on the surface or inside the body of the other (the host), drawing from it the substances needed to meet its vital requirements without, however, destroying II. Such an association, which is extremely widespread in nature both in the plant and animal kingdoms, may occur between two plants, between two animals, or between a plant and an animal.
Parasitology, the science devoted to the study of this phenomenon, includes phytoparasitology, which studies plant parasites, and comprises bacteriology (viruses, rickettsiae, schizomycetes) and mycology (phomycetes, ascomycetes, phomycetes), as well as zooparasitology, which studies animal parasites and includes protozoology (protozoa), helminthology (worms: platyhelminths and nemathelminths) and entomology (mites and insects). Since, in current usage, when parasitology is mentioned reference is made to zooparasitology, it is this branch that will be given particular consideration, without prejudice to the applicability of the general principles to be expounded to all parasitic organisms.
Parasitism, as defined above, implies a series of interrelationships and reciprocal influences between host and parasite that may even lead to the possibility of the coexistence of both: a state of equilibrium, albeit unstable, is thus established, making it possible for both associates to live; only when this state of equilibrium is disturbed does the association come to an end, resulting either in the death of the parasite if the host’s organic defences prevail, or, if these are overcome, in the death of the host, which may of course be followed by that of the parasite.
Parasitism may be facultative or obligatory. In the former case, the parasitic condition is not a necessity for the species’ life but merely a possible way of life, the species normally leading a free existence; facultative parasitism is exemplified, for instance, by the larvae of certain diptera that usually live as saprozoites on cadavers or decomposing organic matter but are sometimes capable of becoming parasites of man and animals, causing the so-called myiases. Parasitism is obligatory, on the other hand, when the parasitic condition, whether long or short, constitutes an absolute necessity for the species, within which there is no possibility of life. Parasites may be: a) temporary, if they limit the period of their stay on the host’s body to the time strictly necessary for feeding, then quickly escaping and leading a free life for the rest of the time; to this category belong ticks, fleas, mosquitoes and, practically speaking, almost all blood-sucking insects and mites; b) permanent, if they remain on the host for a longer or shorter period corresponding to one or more stages of their life cycle, or for their entire existence. Parasites that live on the surface of the host’s body or in natural or artificial cavities of easy access are called ectoparasites; those that live inside the body, in deep cavities or in the thickness of the tissues or within the cells, are called endoparasites. Very often parasites show a constant preference for a particular organ or tissue, so much so that it is exceptional to find them in a different site from their usual one (in which case one speaks of erratic parasitism); there are, however, numerous examples, especially among parasites in the larval stage, in which such organ specificity is not observed, so that the same parasite may be found in very different sites.
Many parasites, whether or not they have free-living phases, complete their life cycle with the intervention of a single host and are therefore said to be monoxenous; many others, however, have a more complex life cycle, passing through two or more different hosts, in each of which only a well-defined part of the cycle takes place; in this second case the parasites are said to be heteroxenous, and it is customary to distinguish the various hosts as intermediate, which harbour the larval forms (metazoa) or the schizont stage of the cycle (protozoa), and definitive, which harbour the adult form (metazoa) or the amphogonic stage (protozoa) of the cycle itself.
The variety of the life cycles of parasites is practically innumerable, each parasite presenting some specific peculiarity in its own cycle. Knowledge, as precise as possible, of the life cycle of parasites, apart from being of great scientific interest, is of great practical utility for species of interest to human economy, namely those pathogenic to man, domestic animals and cultivated plants, except for indicating the stage in which the parasite is most vulnerable, for the purposes of prophylaxis (cf., e.g., MALARIA).
There are numerous cases of parasites that are exclusive to a single host species or to a small number of closely related host species (stenoxenous parasites), others in which parasites seem to find good conditions of existence in numerous hosts of distant species (eurixenous parasites), and still others that present intermediate conditions. Such parasitic specificity is of great interest, as it may provide the only indications, perhaps, for establishing whether a parasitic association is ancient or not, strict specificity presumably implying a complete evolution of the relationship between parasite and host that can only be achieved over a very long period of time; and from a practical point of view, important elements regarding the real possibilities of success in the fight against certain parasites, since it is evident, for example, that the fight against eurixenous parasites is much less likely to succeed than that against stenoxenous ones, as well as sometimes providing indications useful in the struggle itself. Suffice it to recall, in this connection, how one of the forms of antimalarial prophylaxis, zoo-prophylaxis, which consists in placing domesticated animals between human dwellings and anopheline breeding grounds, is based precisely on the observation of the low parasitic specificity of most anopheline vectors of the disease. In some cases, parasites with a marked parasitic specificity may be found in an unusual host; this is referred to as accidental parasitism.
Like every living being, parasites are perfectly adapted to the environment in which they live; consequently, they exhibit morphological and physiological peculiarities closely connected with their parasitic habitat, as is demonstrated, whenever possible, by comparing free-living forms with parasitic forms belonging to the same zoological group. The extent of the modifications deriving from parasitic adaptation is related to the degree of parasitism; in general, the modifications are less pronounced in ectoparasites.
Temporary adaptations are accentuated in permanent ectoparasites and reach their peak in endoparasites. A phenomenon common to all permanent parasites, whether ectoparasites or endoparasites, is the reduction or total loss of locomotory organs, accompanied by a more or less pronounced development of attachment organs. In lice in general, and particularly in pubic lice (Phthirius pubis), for example, one observes, in addition to the loss of wings, the reduction of legs and the transformation of their distal extremities into attachment organs; and in helminths, the absolute absence of locomotory organs in the parasitic phases is matched by a great development of special attachment organs, such as the suckers of trematodes and cestodes and the proboscis of acanthocephalans. Almost constant are also the modifications affecting the organs of nutrition: the mouthparts of blood-sucking arthropods, for example, become piercing and capable of suction; the intestine may undergo transformation of some of its parts so as to become a true food reservoir, which thus allows the parasite to feed even at long intervals (ticks, bedbugs), or its terminal part may be reduced if the food leaves little or no residue, or even disappear altogether, as in cestodes and acanthocephalans, which live immersed in substances already digested by the host and feed by osmosis. Finally, in parasites there is a general enhancement, often of a very high degree, of reproductive capacity. This enhancement is achieved through various mechanisms, namely: either directly through a considerable increase in egg production (it is estimated, for example, that the egg-laying capacity of a Taenia solium is 50,000,000 per year, and that of a female Ascaris lumbricoides is 64,000,000); or by alternating sexual reproduction with phases of asexual reproduction, which are much faster and more effective in producing individuals (as, for example, in malarial plasmodia); or by the intervention of larval reproduction (paedogenesis), whereby each egg laid can potentially give rise to a very large number of individuals (as in trematodes and some cestodes); or by the establishment of a hermaphroditic condition, which, if self-fertilizing (as in cestodes), allows reproduction even in an isolated individual, and if dichogamous, permits the bisexual use of all individuals.
Parasites exert various actions on the host: deprivational, mechanical, and toxic. The first, which are absolutely constant since the very concept of parasitism hinges on their presence, consist in the withdrawal from the host of substances necessary for the parasite’s vital needs, and vary greatly in importance depending on whether they affect substances merely ingested and processed by the host or substances that are part of the host’s 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 occlusion or compression phenomena, or to morphological peculiarities of the parasite itself that result in tissue discontinuity lesions. As for toxic actions, now recognized as decisively predominant in determining many pathogenic pictures of parasitic diseases, these are due to the host’s absorption of substances produced by the parasite. Against these actions, the host mounts defensive reactions that tend, when possible (ectoparasites), to eliminate the parasitic agent, or to isolate it (e.g., in connective cysts), or to neutralize its toxic action (production of specific substances, eosinophilia). The importance of the parasitic phenomenon in the economy of nature is truly immense: many examples have now shown that parasites act as principal components in maintaining the biological equilibria established in a given environment. Nor is the importance of parasitism in the realm of human economy any less, whether directly through the action of many parasites on humans themselves or indirectly through their action on domesticated animals and cultivated plants: suffice it to recall in this regard parasitosis in the fields of medicine, veterinary science, and phytopathology.