Silent Infection

SILENT INFECTION (stumme Infektion), a term proposed by Reiter (H. Reiter; 1925) to denote those cases of infection which are not accompanied by clinically expressed signs of disease, but in the majority of cases lead to the immunization of the organism. Such infections are also called asymptomatic, hidden, "unmanifested," latent, or dormant infections (see Infections). However, the term "silent infection" is the most accepted among both clinicians and epidemiologists. Silent infection, by Reiter's definition, means an externally unmanifested act of infection, while a latent infection also means a specific stage of disease, which can be the result of both a hidden and an expressed (or so-called manifesting) form of infection. The doctrine of silent infection, developed by Friedemann, Reiter, Degkwitz, and others, is based on the assumption of the wide spread of pathogenic microbes among the population and the conditional pathogenicity of the majority of microbe-causative agents, which cause expressed disease only in a certain percentage of all cases of infection. According to these views, the penetration of a microbe into a human or animal organism can produce different results depending on the type of microbe and macroorganism, the quantity of microbes and their virulence, the constitutional features of the affected organism, and external conditions that exert an influence on the micro- and macroorganism. Therefore, the constitution of the macro- and microorganism must be considered as a changing, mobile state. Furthermore, with absolute equality of the infective agent, we will have a wide gamut of different responses of the organism. One should speak of the "kinetics of infection" (Reiter), which gives an idea of the varying intensity of reactions to the infective agent. Reiter proposed a scheme allowing for four possible outcomes of the meeting of a microbe-causative agent with a macroorganism: 1) silent infection without immunity, 2) silent infection with subsequent immunity, 3) expressed atypical infection, 4) expressed typical infection. Depending on the species sensitivity of a human to one or another type of microbe-causative agent, we will have different quantitative ratios between these four groups in mass infections. Thus, for smallpox among an unvaccinated population, 95% of all infections will be in the group of expressed reactions to infection (typical and atypical diseases); this percentage will be somewhat lower for measles, and even less for diphtheria and scarlet fever. For acute infectious diseases of the central nervous system (meningitis, poliomyelitis, and encephalitis), the number of expressed diseases compared to the total number of infections is so small that Lentz proposed calling them diseases that strike in the order of selection (Auslesekrankheiten). Thus, the type of microbe-causative agent already determines to a significant degree in which group the majority of infected subjects will end up—in the group of expressed or silent infections. Furthermore, the distribution of forms of infection is influenced by the constitution of the micro- and macroorganism and the conditions of the external environment. Silent infections reach their greatest spread with the droplet method of transmission of infection, especially in urban conditions with high density and crowding of the population. The possibility of silent infection was experimentally proven in laboratory animals for relapsing fever, Weil's disease, and syphilis; however, on the basis of epidemiological observations, the great importance of silent infection is also admitted for diphtheria, scarlet fever, measles, meningitis, influenza, tuberculosis, and a number of other infectious diseases, including typhus, as has been established by the works of recent years. The epidemiological significance of silent infection is determined, on the one hand, by their role in the spread of the infectious agent, and on the other, by the immunity caused by undergoing a silent infection. Accordingly, the epidemiological study of silent infection is based 1) on the study of the prevalence of bacillus carriage among healthy people and the ratio of the number of bacillus carriers to the number of diseases in a given locality; 2) on the determination by immuno-biological methods of the number of non-susceptible people in various age or social groups of the population; and 3) on the statistical study of changes in age-related morbidity and the territorial distribution of diseases depending on previously occurring epidemics. The significance of bacillus carriage, which is also one of the forms of silent infection, and the role of hidden immunization (latente Durchseuchungimmunität), arising as a result of such carriage, were most thoroughly clarified for diphtheria in the works of Degkwitz, Friedemann, and de Rudder. These works establish the unusually wide spread of diphtheria bacillus carriage, which increases with the increase in density and crowding of the population. Therefore, healthy bacillus carriers in diphtheria generally play a greater role in the spread of diphtheria than patients with diphtheria. According to Friedemann, 39% of the residents of Berlin were carriers of diphtheria bacilli during 1926, and 97.6% of all cases of diphtheria in this city had healthy bacillus carriers as the source of infection. On the other hand, at the basis of non-susceptibility to diphtheria, which increases with age, lies specific immunity to diphtheria, acquired by a significant part of the population as a result of silent infection, i.e., bacillus carriage. This immunity is registered by a negative Schick reaction (see Schick reaction) and explains the features of morbidity and mortality from diphtheria in the city and village, among poor and wealthy strata of the population. Glover's observations on meningococcus carriage in English barracks during the World War established the importance of silent infection for the emergence of an epidemic independently of expressed cases of meningitis disease. Glover found that under the best conditions of housing in barracks, soldiers had 2-5% meningococcus carriers among them, but there were no cases of meningitis. With the deterioration of these conditions (crowding), the number of bacillus carriers increased; with an increase in bacillus carriage to 20%, cases of meningitis appeared. Contrary to common notions, bacillus carriage preceded the epidemic in this case and conditioned it. The significance of silent infection for the immunization of the population was shown by Wernstedt using the example of poliomyelitis in Sweden, where in the 1911 epidemic, those regions that were affected in the 1905 epidemic were completely untouched, despite the fact that in 1905 such an insignificant part of the population had been ill that it was impossible to attribute the revealed immunity to having undergone expressed forms of poliomyelitis disease. At the same time, it was shown that the share of the childhood age (up to 6 years) in the total mortality from poliomyelitis is higher the greater the population density; with a decrease in population density, mortality among the more adult population relatively increased. These facts have been repeatedly confirmed and explained by the fact that in large cities the possibility of infection and hidden immunization is highest, so that older children and adults turn out to be immune. On the basis of statistical comparisons, Reiter also established the great importance of silent infection for the movement of scarlet fever epidemics in Mecklenburg in 1903-27. The fact, long known to epidemiologists, of the different susceptibility to typhoid fever of residents of endemic regions and visitors from places free of typhoid fever is also an expression of the influence of silent infection on the first group. Likewise, silent infections lie at the basis of differences in the severity of disease observed in visitors and inhabitants of endemic foci of typhoid and typhus. In all these cases, permanent residents are imperceptibly immunized thanks to silent infection, acquiring a greater or lesser degree of non-susceptibility. However, the greatest volume of silent infection and the immunity created by them exists in droplet infections, provided there is a low susceptibility of humans to them.

A. Zakharov. NEMENOV, Mikhail Isayevich (born in 1880), director of the State Roentgenological, Radiological, and Cancer Institute in Leningrad, professor at the Military Medical Academy. He graduated from the medical faculty of the University of Berlin in 1905. In 1916, he defended his dissertation on the effect of roentgenization of the testicles on the prostate gland. In 1930, Nemenov took the chair of roentgenology at the Military Medical Academy. While a student, he joined the RSDLP. In 1905, he worked in military social-democratic organizations. He took part in the February Revolution. In 1918, the State Roentgenological and Radiological Institute was founded based on Nemenov's project, which is still under Nemenov's leadership to this day. The Russian Society of Roentgenologists and Radiologists, founded by Nemenov in 1913, was reorganized in 1924 into the All-Union Association, of which Nemenov is the chairman. Nemenov is the author of about 60 scientific works published in the Soviet and foreign press. Nemenov has been awarded the Order of the Red Banner of Labor. Separate editions published include: "Roentgenotherapy" (Petrograd, 1920); "Roentgenology" (parts 1-2, Moscow-Leningrad, 1926-30) and a number of popular science brochures. Under Nemenov's editorship, the "Bulletin of Roentgenology and Radiology" (Leningrad-Moscow) has been published since 1920.