Parasitism (encompasses all forms of asymptomatic)

PARASITISM encompasses all forms of asymptomatic colonization of the organism by pathogenic phyto- and zooparasites. In practice, this term is often specifically applied to animal parasites (protozoa, helminths), while latent bacterial parasitism is designated by the term "bacillary carriage." Such a distinction of concepts is however incorrect, since bacillary carriage is in reality merely a special case of P. in general. From a general biological point of view, P. can be characterized as a special state of symbiosis that establishes under certain conditions between a pathogenic parasite and the host as a result of their mutual equilibrium. As a state of balanced symbiosis, P. can be relatively stable (Maltese fever in goats) or, conversely, labile (latent states in malaria); it can be short-term (latency in relapsing fever) or indefinitely prolonged (sequential latency in some piroplasmoses). The distinction between P. and infection in the proper sense is conditional, since both states are connected by a whole series of intermediate forms of parasitism and can directly transition into one another (e.g., labile recurrent infections of protozoal etiology). On the other hand, in terms of comparative frequency and prevalence, P. can compete with clinically expressed forms of infection (e.g., amebic dysentery).-From the standpoint of etiology, P. can be diverse. In particular, it occurs in infections of bacterial origin ("bacillary carriage"), in protozoal infections ("protozoal carriage"), in infections caused by filterable viruses, in helminth invasions ("helminth carriage"), etc. The forms of P. are different; their classification is conditional. First of all, a category of primary P. in the proper sense can be distinguished. This includes all forms of asymptomatic colonization of the organism by parasites without subsequent clinical manifestations. Belonging to this category, so-called "healthy parasite carriers," although infected, do not become ill. However, even in this category, P. can leave a deep trace in the host organism, e.g., in the form of acquired non-susceptibility. In this case, "active parasitism of healthy individuals" is identified with so-called "silent" or "occult" infections (see Silent infection). This category in turn is adjacent to "ambulatory" forms of infection.-The second group of P. consists of cases of latent parasitism preceding clinically expressed infection or observed during its course. This type is very characteristic of recurrent labile infections. Its varieties (besides the incubation period) include: 1) phenomena of primary prolonged latency, representing an elongated incubation period (e.g., in malaria, brucellosis); 2) phenomena of latent parasitism during typically recurrent infection (e.g., in relapsing fever, in malaria); 3) phenomena of secondary prolonged latency in recurrent infections (e.g., in malaria, in brucellosis, in amebiasis, in syphilis).-The third group of P. consists of cases of latent parasitism developing as a result of a successfully overcome infection. This includes prolonged bacillary excretion and bacillary carriage observed in convalescents; to this can also be attributed phenomena of sequential latency that occur in some infections after an acute form of the disease has been overcome (example-piroplasmoses of animals, which give rise to an asymptomatic latency of indefinite duration after an acute attack). Finally, phenomena of P. related to the developmental cycle of parasites in an intermediate host can be specially noted (e.g., finna stages of some tapeworms). The localization of pathogens in P. can be just as diverse as in infections in the proper sense. Despite the asymptomatic nature of P., it can be accompanied by complete preservation of the virulence of the pathogen (e.g., malaria, piroplasmoses, syphilis, typhus, brucellosis, diphtheria, etc.). At the same time, latent colonization of the organism by parasites can cause a well-expressed state of resistance to superinfection (premunizing properties of piroplasmoses, syphilis, brucelloses, tuberculosis, etc.). On the other hand, a consequence of latent parasitism is non-susceptibility (e.g., development of antitoxic immunity in diphtheria). Recognition of P. is based on the application of various methods. Microscopic detection of pathogens in P. is a valuable and in some cases almost sole (though giving relative) method, as e.g., in amebiasis, malaria, helminth invasions, etc. In P. of bacterial origin, the method of cultures is applied. Immunobiological methods in a number of cases can also give valuable results (e.g., allergic reactions in tuberculosis, agglutination reactions in brucellosis, alexin fixation reaction in canine leishmaniasis, etc.). The experimental method (inoculation to susceptible animals) in a whole series of infections is the basic method for detecting P. (e.g., inoculation of large quantities of blood in latent piroplasmosis, transplantation of lymph nodes in latent forms of experimental syphilis in rabbits, inoculation of guinea pigs with blood in mild forms of typhus, etc.). Recently Olitsky and Long (1929) discovered latent virus of vaccinia in a rabbit by applying the method of cataphoresis (114-133 days after infection). In some infections, retrospective diagnosis of P. is possible (Schick reaction in diphtheria, Dick reaction in scarlet fever, agglutination reaction and intracutaneous melitin test in Malta fever, determination of immunity in typhus). Hidden forms of some infections can be actively detected by various "provocation" methods causing a decrease in the body's resistance (e.g., injection of foreign protein in malaria, vaccination in piroplasmosis of cattle, splenectomy in rat bartonellosis, overheating in latent tetanus in guinea pigs, etc.). To this can also be attributed activation under the influence of various influences of latent foci formed by "exit microbes." Thus, the presence of P. can be detected by various methods and under various conditions, but ultimately its recognition represents a very difficult task, solvable only to a very imperfect degree at the current level of methodology. Therefore, statistical data on the frequency of P. only to a very relative degree illustrate the actual prevalence of this phenomenon. Despite all the insufficiency of information about P. and its prevalence, the very fact of its detection in almost all infections and invasions, along with direct findings of a very high percentage of latent parasitism in some infections, speaks with sufficient objectivity for the fact that P. represents a very common and obviously very widespread phenomenon. An illustration of this can serve the wide prevalence of immunity to diphtheria toxin, the commonness of asymptomatic forms of brucellosis among animals and humans, the extremely wide prevalence of healthy carriers of dysentery ameba (up to 30% and higher) in endemic foci of amebiasis, and the wide prevalence of latent forms in piroplasmoses, in malaria, in tuberculosis, etc.-Analysis of the phenomena of P. establishes a continuous connection between clinically expressed forms of infection and asymptomatic colonization of the organism by pathogenic agents. A striking example of this kind is rat bartonellosis: this infection, widespread among the mentioned rodents, remains asymptomatic, but transforms into a very severe disease after splenectomy (Mayer). No less indicative is the carriage of dysentery ameba, widely spread not only in hot countries with epidemic amebiasis, but also in northern latitudes, where amebiasis is actually absent. The significance of P. for epidemiology is great. On the one hand, P. creates a reservoir of the virus and thus plays a major role in the spread of infections and invasions; on the other hand, being a cause of premunization or immunization of people and animals, P. in certain cases becomes a factor contributing to the formation of population resistance to a given infection. Finally, P. as such can serve as a source of clinical forms of infection for the carriers themselves (autoinfection and recurrences).-Measures against P. are very difficult in view of the difficulty of detecting carriers themselves and the frequent impossibility of freeing them from the infectious agent (bacillary carriage). The latter is possible only where there is specifically acting therapy (this concerns e.g., malaria, ankylostomiasis, partly amebiasis, etc.). Basically, measures against parasitism coincide with measures against bacillary carriage (see).