Parasitism (one of the forms of)

PARASITISM, one of the forms of cohabitation (symbiosis), in which one organism lives at the expense of an organism of another species. A human, animal, or plant that feeds a parasite is its host. True parasitism refers to all those cases where a parasitic lifestyle is a species trait for the corresponding parasite (see Pseudoparasites). Parasitism can be temporary or permanent. The gradations of temporary parasitism are diverse: 1) the parasite lives freely and attacks the host only for feeding and for the duration of feeding, e.g., females of blood-sucking mosquitoes; however, the calendar duration of feeding time can be significantly prolonged (e.g., in Ixodidae ticks up to 7-8 days of continuous food intake); in such cases, the amount of blood-sucking can be unlimited or, conversely, strictly distributed according to the stages of the parasite's transformation; 2) the parasite lives without a host in its dwelling, nest, burrow, or den, whereby a) only the adult form moves onto the host to suck blood, b) all stages of the parasite's development feed on the host (e.g., bedbug, tick Argas persicus, Ornithodorus moubata, etc.); 3) the parasite spends all or most of its time on the host during one or another stage of its life (e.g., larvae of the warble fly Hypoderma, whereas the pupa and imago of this species reside in the external environment); such parasitism is called periodic, as it covers a specific period of the parasite's life. Permanent parasitism is characterized by the fact that the parasite is never found outside the body of its host or generally cannot exist off its host. Trichinae and the malaria plasmodium are examples of the first variant, and chewing lice and lice are examples of the second. The latter reproduce on the host itself and attach their eggs there. Parasitism phylogenetically developed from various forms of organism relationships. If a commensal (see Commensalism), while consuming the host's food scraps, begins to encroach upon them to a greater extent, then the indifferent attitude toward the host fundamentally shifts to a harmful one, and upon crossing a certain quantitative threshold, the commensal qualitatively becomes a parasite. Many parasites are similar to predatory animals in that both feed on living organisms, but with a significant difference: predators as a rule destroy their prey, whereas parasites feed repeatedly at the expense of their host, who in the process remains alive. Predators are often much larger than their prey and in any case possess great strength or skill in attacking the victim. Parasites, on the other hand, are immeasurably smaller than their hosts (e.g., mosquito and human). However, such gradations are quite relative; the same animal can be interpreted as both a predator and a parasite, e.g., a leech sucks the blood of a newt and the latter dies (predation); the same leech, having attached itself to a human, is a parasite. It can be assumed that some parasites originated from predators upon a change in the object and method of feeding. Finally, true parasitism could have arisen from accidental parasitism, e.g., fly larvae living in excrement, when eaten by another animal, enter the stomach and do not die there, but, on the contrary, live with a certain benefit to themselves. With frequent and long repetition, such relationships, under the influence of natural selection, are fixed in such a definite form that the free-living animal effectively becomes a parasite. In interpreting the definition of parasitism, it is implied that the life of a parasite at the expense of another organism is associated with harm to the latter. Indeed, many pathogenic parasites are known, e.g., phylloxera for grapevines, the warble fly for deer, the malaria plasmodium for humans. At the same time, regarding a large number of parasites, one cannot speak with certainty about their pathogenic properties, e.g., Trypanosoma melophagium for sheep, trypanosomes of sleeping sickness for antelopes, etc. Along with this, cases of parasite carriage (see) are known, e.g., for the dysentery amoeba. The result of the parasite's action on the host is the resultant of many factors, namely: 1) the basic pathogenic properties of the parasite in relation to the given host species, 2) the general susceptibility of the host to the influence of the corresponding parasite, 3) the modifying influence of external factors on the parasite and, independently of it, on the host itself, 4) special conditions of the state of the parasite or host. The action of parasitism on hosts consists of various moments: 1) endoparasites living in the intestine absorb food digested by the host and thereby cause it damage; 2) they secrete toxic excretory or secretory products which are absorbed into the host's organism and cause it chronic intoxication; the saliva of many ectoparasites also possesses toxic properties; 3) the mechanical action of parasitism can be significant in its effect: intestinal obstruction by a ball of ascarids, thrombosis of the ductus thoracici by Bancroft's filaria, wounding of the integument by piercing ectoparasites, atrophy of organ parenchyma from the pressure of echinococcus cysts, etc.; 4) reflex effects: various symptoms of nervous disorders (neuralgia, convulsions, chorea, visual disturbances in parasite carriers, which pass after the expulsion of the parasites); 5) inoculation by parasites of pathogenic agents due to the violation of the integrity of the skin or mucous membranes (pinworms and appendicitis); 6) a special form of the influence of parasitism on hosts is parasitic castration—atrophy, underdevelopment of the gonads, or direct destruction of them by parasites; e.g., bark beetles and parasitic nematodes, crabs and rhizocephalan parasitic crustaceans; 7) usually parasites influence the host through a combination of forms of impact, the total effect of which can be quite effective (infantilism in ancylostomiasis and Chagas disease, sleeping sickness in trypanosomiasis, pernicious anemia, etc.). Along with this, cases are not rare where even heavy infestation with some parasites does not reflect noticeably on the condition of the host (see Parasite Carriage, Parasites).