Incubation Period

INCUBATION PERIOD, incubation (from Latin incubare - to lie down), denotes the period separating the moment of infection from the phase of the macroorganism's apparent reaction. Since the discovery of carrier status, indicating that the penetration of a microbe alone is insufficient for infection, the concept of the incubation period as a definite interval between the introduction of the microbe and the body's reaction to this introduction can be maintained only in relation to established infectious diseases and in relation to experimental infections when the exact time of contact or experimental introduction of the infectious agent into the body is known. Historically, the term 'incubation' originates from the sacred mystical healing rites of ancient Greece and Rome. The seriously ill were brought to the temples of Aesculapius and Apollo, and after performing various sacred rites, the priests would leave the patients overnight in the temples, either by putting them to sleep with hypnosis or by having them inhale intoxicating vapors obtained from burning narcotic herbs and CO₂ from pouring vinegar on the so-called 'Memphis stone'. The incubation period is not a period of mechanical residence of the microbe in the macroorganism. During this period, a series of processes takes place, the nature of which has become clearer in recent years, after the introduction of methods of immunization with live microbial cultures and experimental infection with certain infectious diseases for therapeutic purposes (treatment of progressive paralysis with vaccinations of malaria and relapsing fever). The incubation period is determined by changes occurring both in the microbe and in the macroorganism. According to Koch's theory, which is based exclusively on the properties of the microbe, the latter, finding appropriate conditions in the macroorganism, multiplies in it as in a test tube with a nutrient medium. Koch's views were modified by Pfeiffer, who took into account the significance of factors weakening the macroorganism and the quantity of microorganisms introduced. According to Metchnikoff's teaching, the duration of the incubation period depends both on the degree of reaction of phagocytes at the site of infection and on the general phagocytic reaction throughout the body. According to the opinion of Toussaint and Buchner, the duration of the incubation period is related to the time required for the microbe to penetrate into the organ of its predominant fixation (pneumococcus - into the lungs, rabies virus - into the central nervous system, etc.). With the discovery of anaphylaxis, the dynamics of the incubation period were reduced to the process of sensitization (Pirquet, Vaughan). If these theories are reduced to a common scheme, the process of incubation breaks down into two phases: 1) the introduction of the microbe into the tissues of the macroorganism, its multiplication and partial disintegration with the formation of protein toxic bodies; 2) the completion of the local reaction, when the sensitized organism begins to break down bacteria that have penetrated into the general circulation. The beginning of this phase coincides with the development of the clinical symptoms of the given disease. Withoutredka believes that during the incubation period, the mobilization of cells of the reticuloendothelial apparatus takes place, both stationary and those acquiring mobility. The role of the reticuloendothelial apparatus in the dynamics of incubation is undoubtedly great, but has been clarified only in part with respect to the phase of absorption of microbes. According to the nervous theory, during the incubation period, fixation of bacterial toxins in the central and autonomic nervous systems occurs, and when irritation of the centers of metabolism and temperature begins, the general clinical reaction appears. The school that places the basis of the processes occurring in infectious disease on colloidal changes believes that during the incubation period there is a gradual disturbance of the colloidal state due to the adsorption of colloids by bacterial bodies (Freundlich, Muller, Duclaux). If to the old theory of side chains we add the latest discoveries in cellular and intercellular pathology, then the clinical concept of the dynamics of incubation is reduced to the following. The macroorganism possesses general antibacterial properties, partly received by it through hereditary transmission, partly acquired due to immunization with small doses. During the incubation period (Salé), these antibacterial properties act on the penetrated microbe, and for it to maintain its viability, it must change both functionally and morphologically. In those cases where such a change does not occur, the microbe must die, but if it does occur, the macroorganism begins to produce specific anti-infectious properties. The beginning of the production of these anti-infectious properties corresponds to the end of the incubation period and the beginning of the disease. Clinical studies in the incubation period have shown that with artificial infection of humans (malaria), as early as the first days of the incubation period, oxidation increases by 25%, while the temperature remains normal. According to observations by Birke in the incubation period of chickenpox and measles, nitrogenous decay increases, and before the loss of N, its deposition occurs in some depots (subcutaneous tissue, muscles). The decay is the result not of intoxication, but of irritation of the metabolic centers. The same was found regarding the mobilization of sugar and glycogen in the incubation period. According to Bogendorfer, already in the incubation period, irritation of the central nervous apparatus stimulates the formation of agglutinins, and the same phenomenon is observed with respect to some other antibodies. The incubation period is accompanied by changes in the nature of circulation under the influence of the autonomic nervous system (e.g., in the incubation period of measles). During this period, changes in the leukocyte content in the blood are also observed. A number of immunity reactions can be altered or even disappear during the incubation period. The duration of the incubation period (see table) can vary depending on various conditions: on the number of microbes that have penetrated the macroorganism, on their virulence, on local and general trauma to the body (physical and mental), on weakening of the body by hunger, excesses, preceding diseases.

In determining the duration of incubation, the distance from the site of entry of microbes to the selectively affected organ or tissue, and the nature of the portals of entry of infection, are also significant. With combined infection by two infectious diseases, even when the incubation period ends at the same time, they usually do not manifest simultaneously; the incubation period for one of them is abnormally prolonged (measles and scarlet fever); in others, on the contrary, the incubation period is shortened (measles and diphtheria). A number of infectious forms, especially those associated with droplet infection, may be contagious during the incubation period, in the absence of external clinical symptoms; such diseases include, for example: measles, in which droplet infection is possible from the 4th-5th day of incubation, infantile poliomyelitis, whooping cough, scarlet fever, rubella, smallpox and chickenpox, mumps, anginas (especially strepto- and diplococcal), diphtheria of the throat, influenza, cerebrospinal meningitis. Diseases in which the pathogen is excreted with feces, sometimes long before the manifestation of the disease (typhoid fever, dysentery, cholera), are also of highly current interest. Some diseases transmitted by blood-sucking parasites can be transmitted during the I. p. - typhus (from the 6th day), relapsing fever (2-3 days before the first attack). In some infectious diseases, usually at the end of the I. p., it is possible to detect either a reaction of increased sensitivity or the presence of antibodies by using antigens, introducing them into the skin or onto mucous membranes. Such reactions are usually obtained in diseases with a prolonged I. p. For example, the introduction of an autolyzate of typhoid bacilli gives a skin reaction in the I. p. of typhoid fever starting from the 8th-10th day of incubation; the disappearance of the Shick reaction is observed at the end of the I. p. of diphtheria. However, the mechanism of these reactions, their appearance or, conversely, their disappearance during the I. p. has not yet been finally established, and they have no practical significance at present. (See also Infection, Infectious diseases.)

N. Rosenberg.