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. The human, animal, or plant that provides nourishment for the parasite is its host. True parasitism refers to all those cases where a parasitic lifestyle is a species characteristic 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 during the time of feeding, e.g., female blood-sucking mosquitoes; however, the calendar duration of the feeding time can be significantly prolonged (e.g., in Ixodidae ticks up to 7–8 days of continuous food intake); in such cases, the number of blood-feedings may be unlimited or, conversely, strictly distributed according to the stages of the parasite's transformation; 2) the parasite lives without the host in its dwelling, nest, burrow, or lair, whereby a) only the adult form moves onto the host for blood-sucking, b) all stages of the parasite's development feed on the host (e.g., the bedbug, the tick Argas persicus, Ornithodoros moubata, etc.); 3) the parasite spends all or most of its time on the host during one stage or another of its life (e.g., the larvae of the skin botfly Hypoderma, while the pupa and imago of this species remain 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 without its host. Trichinella and the malaria plasmodium are examples of the first variant, while biting lice and lice are examples of the second. The latter reproduce on the host itself and attach their eggs there. Parasitism developed phylogenetically from various forms of relationships between organisms. If a commensal (see Commensalism), consuming the host's food scraps, begins to encroach upon the food in a larger amount, then the indifferent relationship toward the host fundamentally transitions into 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 repeatedly feed at the expense of their host, which 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., a mosquito and a human). However, such gradations are quite relative; the same animal can be treated as both a predator and a parasite; for example, a leech sucks the blood of a newt and the latter dies (predation); the same leech attached to a human is a parasite. It can be assumed that some parasites originated from predators through a change in the object and method of feeding. Finally, true parasitism could have arisen from accidental parasitism; for example, fly larvae living in manure, when eaten by another animal, enter the stomach and do not perish 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 a free-living animal effectively becomes a parasite. In interpreting the definition of parasitism, it is implied that the parasite's life at the expense of another organism is associated with harm to the latter. Indeed, many pathogenic parasites are known, e.g., phylloxera for the grapevine, the skin botfly 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, the trypanosomes of sleeping sickness for antelopes, etc. Along with this, cases of parasite carrying (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 relative 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 the host. The action of parasitism on hosts consists of various elements: 1) endoparasites living in the intestine absorb food digested by the host and thereby cause it harm; 2) they secrete toxic excretory or secretory products that are absorbed into the host's body and cause chronic intoxication; the saliva of many ectoparasites also possesses toxic properties; 3) the mechanical action of parasitism can be significant in its effect: blockage of the intestine by a ball of ascarids, thrombosis of the ductus thoracicus by Filaria bancrofti, wounding of the integuments by piercing ectoparasites, atrophy of organ parenchyma from the pressure of echinococcus cysts, etc.; 4) reflex influences: various symptoms of nervous disorders (neuralgia, convulsions, chorea, visual disturbances in parasite carriers, passing after the expulsion of the parasites); 5) inoculation of pathogenic agents by parasites 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 overall effect of which can be very significant (infantilism in ankylostomiasis and Chagas disease, sleeping sickness in trypanosomiasis, malignant anemia, etc.). Along with this, there are frequent cases where even heavy infestation with certain parasites does not reflect noticeably on the host's condition (see Parasite Carrying, Parasites).

E. Pavlovsky. PARASITOLOGY, the science that studies the complex and diverse complex of phenomena of parasitism and, as such, based on its primary objects of study, belongs to the category of biological disciplines. According to the nature of the subject of study, a distinction is made between human, animal, and plant parasitology. The task of parasitology includes the study of the systematics of parasites and the identification of their fauna for a particular locality; the grouping of relevant facts is carried out either by hosts as a whole or by their individual organs. Furthermore, the study of the structure of parasites and their physiology allows for the observation and explanation of the unique forms and functions of parasitic organisms associated with the peculiarities of their habitat. For endoparasites, this environment is the host organism, and various changes in it can significantly affect the parasites living within it. Due to such relationships, the ecology of parasites includes, for example, the study of the therapy of invasive diseases, since the introduction of medicinal substances into the host organism changes the parasite's habitat. The next important task is the study of the biology of parasites, often associated with complex life cycles, and the influence of parasites on their hosts. This latter direction closely links parasitology with general pathology, pathological anatomy, serology, and the pathology and therapy of parasitic diseases. This same direction connects general or theoretical parasitology with applied parasitology of medical or veterinary content. Assessing the harmfulness of parasites provides material for judging the economic significance of parasites and for identifying species of primary practical importance. In connection with the pathogenic significance of parasites, the necessity of studying parasitological diagnostics of parasitic diseases arises, which is based on special methods of parasitological research. The therapy of parasitic diseases also has many unique features depending on the nature of the pathogen in each individual case. The natural conclusion of applied parasitological work is the search for measures to combat parasites and methods for preventing infection by them (respectively, the prophylaxis of parasitic diseases). For its purposes, the named branches of parasitology use a wide variety of research methods, which makes parasitology as a whole a vast and important complex of various sections of theoretical and applied sciences (biology, medicine, veterinary medicine, zootechnics, agronomy, economics, etc.). The official subdivision of parasitology into branches or departments is carried out either on the principle of the parasites belonging to specific hosts or by the systematic position of the parasites. In the first case, a distinction is made between medical (i.e., human) parasitology, veterinary parasitology, and the parasitology of commercial and wild animals. According to the second principle, parasitology is divided into protozoology (see), or the study of parasitic protozoa; helminthology (see), or the study of worms; and arachnology and entomology (see)—the study of parasitic arachnids and insects. In the French understanding, parasitology also encompasses plant parasites. The history of parasitology consists of the development of its aforementioned systematic subdivisions and certain pivotal stages in the evolution of related disciplines. Parasites have been known to humans since antiquity. The Egyptians knew lice, fleas, ascarids, pinworms, and taeniae. Until the 17th century, there was no progress in parasitology, as for all the preceding time (about 16 centuries!) the meager data of Hippocrates and Galen on several parasitic worms dominated. The second half of the 17th century was marked by the use of the microscope for studying small objects; Leeuwenhoek discovered parasitic protozoa—oocysts of coccidia in the rabbit liver and flagellates in human feces (Giardia intestinalis). Redi is the father of parasitology. His manual on parasites provided a wealth of new information, particularly on worms found in the most diverse organs of hosts. Redi proved the absence of spontaneous generation of insects. In 1687, the itch mite was discovered. The works of Swammerdam and other microscopists on the anatomy and biology of parasitic insects also stand out. The study of parasitic worms in the 18th century degraded due to the refusal to recognize the pathogenicity of parasitic worms. However, the 19th century was the beginning of the true flourishing of parasitology, a science in which experimentation became increasingly established (Küchenmeister, Leuckart, and others). The complex development cycles of worms and parasitic protozoa were unraveled; intermediate hosts were established for various parasitic worms, and these hosts turned out to be not only vertebrates but also arthropods in the form of crustaceans and insects. In the field of protozoology, the 19th century was marked by the discovery of parasitic amoebae of humans, the malaria plasmodium (Laveran), trypanosomes, and many others. Especially important was the establishment of the facts of the transmission of certain parasitic protozoa and pathogenic bacteria through vectors (see). The development of parasitology is closely linked to the development of tropical medicine, the need for which was dictated by the imperialist colonial policy of European states. Depending on the parasites belonging to three types of animals, three branches of parasitology became specialized: protozoology, helminthology, and entomology; however, the presence of intermediate hosts and the simultaneous parasitism of various parasites in the host organism smooth out the sharp boundaries between the branches of parasitology; this applies especially to those sections of it that can be characterized as general parasitology, which deals with the study of the patterns of parasitism and the entire complex of questions regarding the relationship between parasite and host. The 20th century is marked by the further brilliant development of parasitology and the recognition of this science's extraordinary practical importance. The further development of parasitology is ensured by its great theoretical and enormous practical significance. There are special parasitological institutes (e.g., The Molteno Institute for Research in Parasitology at Cambridge University); the zoological departments of some universities have chosen various branches of parasitology as their specialty (Königsberg under M. Braun); parasitology has received special development in institutes of applied significance, where it is differentiated into departments and laboratories according to its systematic subdivisions. This is the case in tropical institutes (London, Hamburg, etc.), some Pasteur institutes (Paris, Algiers, Tunis), and special laboratories of a sectoral nature (on plant pests, ichthyological institutes, fur-bearing animal nurseries, etc.). In the USSR, parasitological work is conducted in the Department of Parasitology and in the Malaria Commission of the Zoological Museum of the Academy of Sciences of the USSR, in some departments of the State Institute of Experimental Medicine and the State Institute of Experimental Veterinary Medicine (protozoology and helminthology), in tropical institutes, some sanitary-bacteriological and veterinary-bacteriological institutes, in the sector for livestock pests of the All-Union Institute for Plant Protection of the Lenin Academy of Agricultural Sciences, in the laboratory of the Department of General Biology and Parasitology of the Military Medical Academy, and others. The Russian Parasitological Society also functions in Leningrad. In addition to various laboratory studies, complex parasitological expeditions are finding increasingly wide application as a method (e.g., the Central Asian Parasitological Expedition of 1928 of the Academy of Sciences of the USSR, the Murgab Parasitological Expedition of the Academy of Sciences of the USSR and the People's Commissariat of Health of Turkmenia in 1930, the Kara-Kala Parasitological Expedition of 1931 of the Academy of Sciences of the USSR, etc., expeditions of the Moscow and Bukhara tropical institutes, etc.). The teaching of parasitology is provided along medical lines by the establishment of an independent course in parasitology at the Military Medical Academy under the Department of General Biology, and in the Kazan, Central Asian, and some other medical institutes; in veterinary institutes, departments of parasitology and invasive diseases have been organized; some biological faculties of universities also pay attention to the teaching of parasitology (Leningrad). Finally, in Leningrad, there is a Department for Livestock Pests, which is effectively a higher educational institution training specialists in the fight against ectoparasites of domestic animals and humans (in particular, on the use of the aerial method in combination with ground-based mosquito control—within the system of anti-malaria measures), organized by the Association for Pest Control (OBV). In the Leningrad Institute for Pest Control (LINBOV), a correspondence course has also opened. Within the OBV system, the first technical school for pest control workers specifically for ectoparasites is being organized (in Kustanai). In addition, regular or sporadic courses on various branches of parasitology are held from time to time (Tropical Institute in Moscow, local courses at various republican and regional institutes). Finally, it is necessary to mention specialized parasitological laboratories, e.g., the laboratory for fish diseases of the Leningrad Ichthyological Institute, which covers the study of the entire complex of fish parasites.