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The scientific study of parasites and of parasitism. Parasitism is a subdivision of symbiosis and is defined as an intimate association between an organism (parasite) and another, larger species of organism (host) upon which the parasite is metabolically dependent. Implicit in this definition is the concept that the host is harmed, while the parasite benefits from the association. Although technically parasites, pathogenic bacteria and viruses and nematode, fungal, and insect parasites of plants are traditionally outside the field of parasitology.
Parasites often cause important diseases of humans and animals. For this reason, parasitology is an active field of study; advances in biotechnology have raised expectations for the development of new drugs, vaccines, and other control measures. However, these expectations are dampened by the inherent complexity of parasites and host-parasite relationships, the entrenchment of parasites and vectors in their environments, and the vast socioeconomic problems in the geographical areas where parasites are most prevalent.
The ecological and physiological relationships between parasites and their hosts constitute some of the most impressive examples of biological adaptation known. Much of classical parasitology has been devoted to the elucidation of one of the most important aspects of host-parasite ecological relationships, namely, the dispersion and the transmission of parasites to new hosts.
Parasite life cycles range from simple to highly complex. Simple life cycles (transmission from animal to animal) are direct and horizontal with adaptations that include high reproduction rates, and the production of relatively inactive stages (cysts or eggs) that are resistant to environmental factors such as desiccation, ultraviolet radiation, and extreme temperatures. The infective stages are passively consumed when food or water is contaminated with feces that contain cysts. The cysts are then activated in the gut by cues such as acidity to continue their development. Other direct-transmission parasites, such as hookworms, actively invade new hosts by penetrating the skin. Physiologically more complicated are those life cycles that are direct and vertical, with transmission being from mother to offspring. The main adaptation of the parasite for this type of life cycle is the ability to gain access to the fetus or young animal through the ovaries, placenta, or mammary glands of the mother.
Many parasites have taken advantage of the food chain of free-living animals for transmission to new hosts. During their life cycle, these parasites have intermediate hosts that are the normal prey of their final hosts. Parasites may ascend the food chain by utilizing a succession of progressively larger hosts, a process called paratenesis. See Food web
Vectors are intermediate hosts that are not eaten by the final host, but rather serve as factories for the production of more parasites and may even carry them to new hosts or to new environments frequented by potential hosts. Blood-sucking athropods such as mosquitoes and tsetse flies are well-known examples. After acquiring the parasite from an infected host, they move to another host, which they bite and infect. Snails are important vectors for two-host trematodes (flukes), which increase their numbers greatly in the snail by asexual reproduction. The stages that leave the snail may either infect second intermediate hosts that are eaten by carnivorous final hosts, may encyst on vegetation that is eaten by herbivorous hosts, or in the case of the blood flukes (schistosomes) may swim to and directly penetrate the final host.
Metabolic dependency is the key to parasitism, and parasites employ many ways to feed off their hosts. The simplest is exhibited by the common intestinal roundworm, Ascaris, which consumes the host's intestinal contents. Parasites require from their hosts not only energy-yielding molecules but also basic monomers for macromolecular synthesis and essential cofactors for these synthetic processes. Many examples of the specific absence of key parts of energy-yielding or biosynthetic pathways in parasites are known, and these missing enzymes, cofactors, or intermediates are supplied by the host. Tapeworms are more complex than Ascaris in nutritional requirements from the host. They lack a gut, but their surface actively takes up, by facilitated diffusion or active transport, small molecules such as amino acids and simple sugars.
Parasites, by coevolving with their hosts, have the ability to evade the immune response. The best-known evasive tactic is antigenic variation, as found in African trypanosomes, which have a complicated genetic mechanism for producing alternative forms of a glycoprotein that virtually cover the entire parasite. By going through a genetically programmed sequence of variant surface glycoproteins, the trypanosome population in a host stays one step ahead of immunity and is not eliminated. Other possible immune escape mechanisms in parasites have been discovered and probably cooperate to prolong parasite survival.
Parasites are not altogether exempt from the effects of immunity. Rather than completely eliminating parasites, the immune system more often functions to control their populations in the host. Thus a balance is achieved between hosts and parasites that have lived in long evolutionary association, with both surviving through compromise. Enhancing these particular antiparasite mechanisms and neutralizing the parasite's evasion mechanisms would tip the balance in favor of the host. See Medical parasitology, Population ecology
a multidisciplinary branch of biology that is concerned with all aspects of parasitism.
Parasitology is largely an ecological discipline, since its main objective is to study the interrelationship and environmental dependence of parasite and host. However, the study of parasitology differs from that of the ecology of free-living animals primarily in that a parasite’s environment is another living organism. The host’s own environment is the outside world, and this mostly affects the parasite indirectly, that is, through the host. Although the main object of study in parasitology is the intricate set of relations that exist between members of the parasite-host-environment complex, parasitology concentrates on various aspects of the parasite itself. Among these aspects are structure, all phases of parasite activity, adaptation to life in the host, life cycle, and geographic distribution. Another primary objective is the elucidation of the parasite’s influence on the host and of the conditions under which this influence is manifested. The practical applications of parasitology to the fields of human health, the health of both farm and game animals, and plant health entail providing a scientific basis for controlling parasites and parasitic diseases as well as researching preventive measures against these diseases. In so doing, parasitology is closely associated with zoology, botany, and other biological, medical, veterinary, agricultural, and chemical disciplines. Specifically, it relies extensively on biochemical, immunological, and electron-microscopical research.
The study of metabolism reveals many of the features that distinguish parasites from free-living organisms. Of note is the predominance of anaerobic over aerobic processes in parasites. Immunological methods are useful in elucidating much of the subtle behavior of the parasite-host complex on the molecular level. Ultrastructural studies have been used to reexamine and modify many ideas regarding the organization and physiology of parasites. Most unicellular parasites are ultrastructurally complex. The study of the ultrastructure of intracellular parasites has revealed previously unknown modes of feeding through pinocytosis and through minute orifices, or microstomes, that are not visible under the light microscope. Electron-microscopical studies on parasitic worms, namely, trematodes, tapeworms, and members of the class Monogenoidea, have radically altered earlier ideas on the structure and function of these creatures’ external coverings. These examples show how closely the development of parasitology is linked to advances in other sciences.
The field of parasitology is divided into botanical, medical, veterinary, and general branches. Further specialized branches of parasitology are named according to the taxonomic group to which the parasites of interest belong. The major groups studied are pathogenic microbes, viruses, and fungi, as well as parasite protozoans, worms, crustaceans, arachnids, and insects.
Parasites cannot be controlled without an accurate knowledge of all aspects of their activity, especially their taxonomic position, anatomy, histology, embryology, physiology, and ecology (including the life cycles). It is very important to know the variety of a given parasite’s hosts, to understand the parasite’s life cycle, and to know the geographic distribution and mutual influence of parasite and host. Also of interest is the parasite’s specificity, that is, the suitability of a parasite for a particular host species or group of species. As confirmed by V. A. Dogel’, a parasite that is specific to a certain host will be found exclusively in that host. The degree of specificity varies. Parasites with narrow specificity are confined to a single species or a few related species. Specificity is broader when the parasites of a single species can exist in or on hosts that belong to different genera, families, and sometimes even orders.
The study of the life cycle of parasites is of primary importance both for understanding the evolution of a given group of parasites and for controlling parasitic diseases by taking action during various specific phases of the causative agent’s development. Hence it is particularly worthwhile to study the changing conditions of the host’s environment and the life cycles of parasites with respect to time and in relation to the life cycle of the hosts.
The parasite habitat consists of a first-order environment—the host organism—and a second-order one—the environment that is external to the host. As discovered by E. N. Pavlovskii and Dogel’, environmental factors affect the parasite indirectly, through the first-order environment. They can also affect the parasite directly, according to the extensive research of the followers of Dogel’, who considered the relationship between the parasite fauna as a whole, on the one hand, and changes in the host’s physiology and surrounding conditions, on the other, to be one of the primary focuses of parasitological study. This very progressive field of research is called ecological parasitology.
If the host organism is the habitat of many parasitic species, then the influence of the first-order environment is not limited to the effects exerted by the host; it also includes the influence of other parasites that live in the same host, as shown by Pavlovskii. The aggregate of parasites on a single host or in a single infested organ is defined as the parasite biocenosis. The composition of a parasite biocenosis changes with changes in the environment that surrounds the host and with changes in the host’s physiology. Parasitology now faces the difficult task of elucidating all the interrelationships of the parasites within an organ or host; in these studies, the second-order environment is also taken into account. The task also involves studying the population dynamics of parasites in terms of the dynamics of the host population. Many modern parasitologists, for example, A. P. Markevich, view their comprehensive investigation of parasite biocenoses as a special branch of parasitology called parasite biocenology. These scientists feel that this is the most promising approach to further progress in parasitology.
The study of parasites, many of which evolved in close contact with certain groups of hosts, provides important additional criteria for establishing the phylogeny and investigating the evolutionary morphology of host fauna. A knowledge of the relationship between parasites and environmental conditions is helpful in elucidating the hosts’ biocenotic relationships and migratory patterns. Thus, parasitology provides valuable data for ecology, paleogeography, the study of speciation, and the investigation of evolutionary theory in general.
The study of the complex relations between parasites and their environment is not only of theoretical interest but is also needed to design methods of parasite control. Thus, parasitology is closely allied with epidemiology and epizootiology, which investigate patterns of infestation and infection, routes by which parasites penetrate into the host, the conditions under which diseases develop, and the etiology of the asymptomatic parasite-carrier state. Of particular importance in establishing the routes of penetration of parasites into the host organism is the study of carriers of the causative agents of diseases—chiefly insects, mites, and ticks. The many diseases that are transmitted by carriers, for example, malaria, seasonal encephalitides, and plague, are called transmissible diseases; some of these are zoonoses.
The concept of the natural geographic focus of a transmissible disease, as elucidated by such researchers as Pavlovskii, P. A. Petrishcheva, N. G. Olsuf’ev, G. S. Pervomaiskii, and A. N. Skrynnik, is of great importance for parasitology as well as for medicine and veterinary science. It has been shown that geographic focuses of disease exist that are independent of human influence. The causative agents of these diseases—viruses, bacteria, and rickettsiae—are circulated among vertebrates including rodents, insectivorous species, and birds, by blood-sucking arthropods—ticks, mites, and insects; the arthropods are called vectors. Humans or domestic animals upon entering such a natural geographic focus become infected when bitten by the vectors, which themselves are ectoparasites. Transmissible diseases that have a natural geographic focus include taiga encephalitis, scrub typhus, Rocky Mountain spotted fever, and cutaneous leishmaniasis.
The study of the pathological effect of parasite on host requires the participation of such sciences as pathological anatomy and physiology, which reveal the structural and functional changes that occur in the host and in the individual host organs. Parasitology is also concerned with the medicinal treatment of parasitic diseases. The findings of parasitology have been applied to the prevention of parasite-induced infections on both the individual and group levels. Parasites can be destroyed in the environment during all phases of their life cycle; food products are monitored by regular inspections; the vectors and causative agents of various diseases are exterminated; repellents and other preparations are used to prevent the vectors from attacking; and finally, the geographic focus itself is rendered unfit for vectors, intermediate hosts, and the parasites themselves by such measures as draining swamps and clearing underbrush. An indispensable aspect of the prevention and control of parasites and transmissible diseases is health education aimed at eradicating age-old customs and habits that promote infestation of humans.
History. In the early days of medicine, physicians dealt only with the very common parasitic worms and external parasites that are readily detected by the naked eye. For centuries it was firmly believed that parasites arise spontaneously in the human organism. Experimental methods that were developed by (among others) the Russian scientists A. P. Fedchenko and N. M. Mel’nikov, the German scientists F. Küchenmeister, K. Vogt, and R. Leuckart, and the Italian scientist G. B. Grassi opened a new era in the history of parasitology, especially in connection with the life cycles of parasites. The invention of the microscope and the development of special microscopical techniques exposed the world of microorganisms, many of which are injurious to humans and domestic animals. All these discoveries provided impetus for the development of parasitology.
Parasitic protozoans were discovered in the second half of the 19th century. These are causative agents of many widespread diseases of man, including malaria, leishmaniases, and amebiases, as well as diseases of domestic animals, for example, babesiasis, theileriasis, and coccidiosis. An important breakthrough was the identification of pathogenic protozoan vectors. For instance, mosquitoes of the genus Anopheles transmit malaria, ticks of the genus Ornithodoros transmit relapsing fevers, and tsetse flies transmit pathogenic trypanosomes.
Several Russian parasitologists made important discoveries. G. Gross was the first to describe amoebas that are parasites of man. D. F. Lambl’ discovered the parasitic protozoan genus Lamblia. A. P. Fedchenko described several parasitic worms and demonstrated experimentally the role of the members of the crustacean genus Cyclops as the intermediate hosts of the guinea worm. These findings were arrived at independently and were of great scientific interest, although parasitology did not yet exist as a science.
Parasitological reports first appeared in Russia at the end of the 19th and the beginning of the 20th century. The first textbook of veterinary parasitology was written by E. K. Brandt, who also translated into Russian and appended Leuckart’s book General Natural History of Parasites in 1881. N. A. Kholodkovskii compiled an atlas of worms that are parasites of man, while A. L. Lovetskii prepared a compendium of medical helminthology. The first major research in the field of parasitology dates to the same period, when K. I. Skriabin began his work in helminthology and E. I. Martsinovskii published several important works on leishmaniases and malaria. Also from this period are V. Ia. Danilevskii’s early studies on parasites of avian blood and V. L. Iakimov’s works in the field of veterinary microbiology.
The number of organizations that sponsor parasitological research as well as the number of parasitologists increased sharply after the October 1917 Revolution. The topics of parasitological research increased in number and complexity. Major studies appeared on the taxonomy and zoogeography of specific groups of parasites. Malarial and other blood-sucking mosquitoes, flies of the genus Phlebotomus, and many other flies that are commensal with man were thoroughly investigated by A. A. Shtakel’berg, A. S. Monchadskii, and A. V. Gutsevich. Also of great value were Iakimov’s researches in veterinary microbiology, G. V. Epshtein’s studies on parasitic intestinal protozoans, V. B. Dubinin’s and A. A. Zakhvatkin’s investigations of lower ticks and mites, and I. G. Ioffe’s work on the taxonomy and biology of fleas and the role of fleas in the transmission of infections.
Much useful data was obtained by parasitological expeditions that investigated regional pathology. The special helminthological expeditions under the direction of Skriabin and the general parasitological expeditions of the co-workers of Dogel’ studied the ecology of specific parasites or groups of parasites. The expeditions of Pavloskii and his students and co-workers investigated the epidemiologic and epizootiological significance of parasitic and transmissible diseases, including malaria. Permanent stations were set up by the expeditions to continue these researches locally. Forest reserves also served as research centers.
Extensive experimentation with parasites has been carried out both in the USSR and abroad, resulting in elucidation of the life cycle of several parasitic worms that were not previously known to have intermediate hosts, for example, oribatid mites, earthworms, and ceratopogonid flies. Other subjects that were investigated included the factors that cause an organism to become a host, as well as the intraspecific and interspecific relationships of intestinal parasites. Pavlovskii developed the theoretical aspects of parasitology in terms of landscape and region; this eventually became the scientific basis for controlling parasites and transmissible diseases. In the field of veterinary helminthology, the students and co-workers of Skriabin worked out the theory behind exterminating sources of infection; this marked a radical turning point in the control of worms that are parasites of farm animals. In the 1950’s, the constant interplay of parasitological theory and practice led to the elimination of malaria in the USSR. Another human disease, dracunculiasis, or guinea worm disease, was completely eradicated in Middle Asia.
Major advances were made in the control of highly contagious infections, including tularemia. Methods have been developed for ridding an area of the geographic focuses of several transmissible diseases, including the desert form of cutaneous leishmaniasis, and tickborne spirochetosis transmitted by the soft ticks of the genus Ornithodoros. Of great importance are the theoretical studies in parasitology and the practical measures that have been taken by socialist countries to curb losses in livestock raising and to boost the productivity of all aspects of animal husbandry. These include efforts to control parasitic diseases that affect the blood of livestock, for example, babesiases; efforts to control helminthiases of domestic and game animals; efforts to control warble flies, which infest cattle and deer; and efforts to control parasitic diseases of pond fish.
Scientific institutions and societies. Parasitological topics are being studied throughout the world, especially in such specialized institutes as the Pasteur institutes and the institutes that are attached to universities, medical schools, and veterinary schools. Parasitological laboratories are often found as subdivisions of marine and freshwater biological institutes and stations. Extensive research is conducted in zoology departments of universities and biology departments of medical institutes, veterinary schools, and schools of tropical medicine. Scientific societies outside the USSR play a major role in the development of parasitology, for example, the Royal Society of Tropical Medicine and Hygiene in London, the Society for Exotic Pathology in Paris, the American Society of Parasitologists, and the Polish Parasitological Society. The first International Congress of Parasitologists was held in Rome in 1964, the second in Washington, D. C, in 1970, and the third in Munich in 1974.
The most widely known periodicals published outside the USSR are Parasitology (Cambridge-London-New York, since 1908), Journal of Parasitology (Lawrence, Kan., since 1914), Experimental Parasitology (New York, since 1951), Annales de parasitologie humaine et comparée (Paris, since 1923), Zeitschrift für Parasitenkunde (Berlin-Heidelberg-New York, since 1928), Acta parasitologica polonica (Warsaw, since 1953), and Folia Parasitologica (Prague, since 1954).
Several major schools of parasitology exist in the USSR (V. N. Beklemishev, medical parasitology; V. A. Dogel’, ecological parasitology, general problems and parasitology of fish; E. N. Pavlovskii, ecological and regional parasitology, geographical focuses of transmissible diseases, parasite biocenoses; K. I. Skriabin, helminthology). A few special parasitological organizations are affiliated with the Academy of Sciences of the USSR, whose Institute of Zoology sponsors a parasitological laboratory. Such laboratories are also found in the zoological institutes of the various Union republics. The health system includes major parasitological organizations, such as are found in the N. F. Gamaleia Institute of Epidemiology and Microbiology of the Academy of Medical Sciences; in the E. I. Martsinovskii Institute of Medical Parasitology and Tropical Medicine; and in the “Mikrob” Institute in Saratov.
Under the jurisdiction of the Ministry of Agriculture are found the All-Union Scientific Research Institute of Plant Protection and the K. I. Skriabin All-Union Institute of Helminthology. Successful parasitological societies exist in the Ukraine (with some oblast branches), in Georgia, in Kazakhstan, and in Leningrad. Very active are the All-Union Society of Helminthologists, which was founded in 1940 and includes a large network of republic and oblast branches, and the All-Union Society of Protozoologists, which also has numerous branches.
The scientific literature that is published in the USSR on parasitology is vast and includes collections of studies, reports of expeditions, proceedings of special-problems conferences, monographs, textbooks, reference books, and handbooks. The journal Parazitologiia has been published since 1967. Specialized medical and agricultural publications also contain material on parasitology.
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B. E. BYKHOVSKII and E. N. PAVLOVSKII