Infrapopulation of parasites
Frequently, only a small number of infectious stages of the parasite, and sometimes only one individual, enter the host organism. This individual can reproduce here and must form the infectious stage. A subpopulation of the parasite bound to a single host organism, called an infrapopulation, survives there for a certain time; this period of time is limited at one end by the moment of infection and, at the other end, by the moment when the host manages, for example through immune mechanisms, to destroy the parasite or when the host organism dies a natural death or as a result of pathogenic manifestations of parasitosis. From the standpoint of the growth rate of the overall parasite population, and thus from the standpoint of the evolution of the relevant parasite species, two parameters are most important – the number of infected individuals and the number of propagules, i.e. infectious stages that the individual parasite infrapopulation produces. In general, we can state that, under certain conditions, the rate of production of infectious stages by the particular subpopulation per time unit is more important while, under different conditions, the number of infectious stages produced over the entire time of existence of the parasite subpopulation, i.e. for the duration of infection of a single host, is more important. This is thus a system that, in a certain sense, is analogous to the model of turbidostatic and chemostatic selection. Parasite subpopulations bound to individual hosts can be exposed either to selection for greater rate of reproduction, for example, when there is a danger of frequent superinfection (i.e. the infection of an already infected individual), or selection for better (more economical) use of resources, in this case infected hosts. In the latter case, the critical parameter is not the actual growth rate of the infrapopulation, but the effectiveness of use of the infected host individual. Because the growth rate is not of fundamental importance, it can be quite low in some parasites and, in some cases, even zero. The parasite does not reproduce in the infected individual, only survives in a constant number of individuals and produces infectious stages.
A low or zero growth rate of the parasite infrapopulation is, in addition, also advantageous from the standpoint of protection against identification by the immune system of the host. Some components of this system react, not to the presence or absence of a foreign antigen, but rather to its potential dangerousness and thus, for example, to damage to the tissues of the host organism or to the very dynamics of increase in the amount of antigen in the organism. If a parasite reproduces exponentially in an organism, it becomes an easy target for the sensory components of the immune system and thus a probable object of attack by its effector components. If, on the other hand, the concentration of antigens does not change in the organism, or if it increases only slowly, the immune system does not act against the particular instigation. This “sneaking through” of an antigenic agent plays a negative role, not only in defense against a slowly reproducing parasite, but also in antitumour immunity, specifically in immunity failure against slowly growing tumours.
Reproduction or, to the contrary, absence of reproduction of a parasite infrapopulation in an organism currently forms a basis for separation of parasitic organisms into macroparasites and microparasites. While this division is correlated to a certain degree with parasite size or, to be more exact, with the ratio of the size of the bodies of the parasite and the host, the decisive criterion is, however, whether the size of the infrapopulation and thus most pathological manifestations of the infection are proportional to the number of parasite individuals infecting the particular host. Such a direct dependence exists for macroparasites, including, for example, tapeworms, nematodes and ticks; however, such a dependence is more or less lacking for microparasites, including, for example, Plasmodium, Toxoplasma, bacteria and viruses. Thus, from an ecological standpoint, some microscopic parasites belong amongst macroparasites, e.g. the cilita protozoan Ichthyophthirius, and, on the other hand, some macroscopic parasites would tend to be classified amongst microparasites, e.g. the larvae of flukes in snails, barnacles of the genus Sacculina (Kuris & Lafferty 2000).