Virulence

Virulence (from the Latin virulentus—poisonous), the sum of properties of a microbe that determines its disease-causing action. Only microbes capable of penetrating and multiplying in the depths of living tissues can be disease-causing. To a significant extent, virulence depends on the energy of growth and multiplication of the microbes. The implanted microbes are subjected to the destructive action of the protective forces of the host organism. Therefore, the micro-parasite must be, as Welch expressed it, "immune" with respect to the organism in which it vegetates. The biological basis of virulence consists in the fact that the vital activity of disease-causing microbes disturbs the physical and biochemical equilibrium of the host organism, and in many cases the microbes produce strong poisons (toxins). Infection with very virulent microbes ends in the rapid death of the animal. Many microbes are virulent only for one or a few species of animals or plants and are not virulent for the rest; for example, the cholera vibrio under natural conditions causes disease only in humans. Some microbes are virulent for many species of animals or plants (for instance, plague bacilli are virulent for humans, rodents, camels, and many other domestic animals), but there are no microbes virulent for all species of animals or plants without exception. Therefore, virulence is a relative concept: it defines the properties of a microbe only in relation to a given species of animal or plant. Thus, the virulence of a microbe depends to a significant extent on the resistance of the host organism. A microbe incapable of causing disease in a well-protected organism, and therefore non-virulent for it, may cause disease in other, less well-protected animals of the same species or even in the same animal, but during a temporary weakening of its protective forces. The virulence of some microbes is characterized by great constancy (plague bacilli); the virulence of other microbes (pneumococcus) is subject to great fluctuations. Especially easily does virulence weaken under the influence of the protective forces of the host organism, during the cultivation of microbes on artificial nutrient media, especially when the temperature exceeds the optimal one, during drying, under the action of sunlight, weak concentrations of disinfecting substances, etc. An increase in virulence is often observed under natural conditions; artificially, it is possible to increase the virulence of many microbes by means of successive passage through animal organisms (so-called "passages"). In this process, part of the microbes adapts to the conditions of life in the host organism and acquires resistance with respect to its protective forces. The unadapted individuals die. However, passages through animals can sometimes cause a decrease in the virulence of the microbe for humans. For example, the virulence for humans of the smallpox virus decreases after passage through a calf. One can achieve an increase in virulence by cultivating microbes in a collodium sac containing a suitable nutrient medium for the given microbe and sutured into the abdominal cavity of the corresponding animal. In some cases, the same goal can be achieved by cultivating microbes on the blood serum of the corresponding animal (best of all an immune animal). However, in general, on nutrient media outside the organism it is difficult to achieve an increase in virulence. The fluctuation of virulence undoubtedly has great significance for the origin and subsidence of epidemics. The degree of virulence is determined either by the smallest amount of microbes which always causes infection of animals of a given species and a given weight ("minimum infective dose"), or by the smallest amount of them which causes the death of the animal ("minimum lethal dose", "dosa letalis minima"—d. l. m.). Some micro-parasites are virulent only in known stages of development; for example, the gametes of the malaria plasmodium are not virulent for humans (inoculation of humans with blood containing only gametes gives a negative result), but they are virulent for mosquitoes. Some authors (Nicolle, Friedberger) believe that many pathogenic bacteria in a known stage of development form very small forms invisible under a microscope and passing through bacterial filters ("filtering forms"), and that it is precisely these small forms that are characterized by the greatest virulence.