Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

A great many parasites act through hormones excreted into the blood or, through local mediators produced directly in the central nervous system of the host.The fact that a number of endoparasites are located in the nerve tissue of the host’s body is connected with the fact that the CNS is a privileged organ from the viewpoint of the immune system of the organism.Immune reactions against foreign agents do not occur there or occur with only low effectiveness, so that the parasite is relatively well protected against the defense mechanisms of the host.Another reason for location of parasites in the CNS organs could also be that the parasite is capable of very effectively influencing the host’s behavior here through natural mediators, their analogues or factors stimulating synthesis of these substances directly in the nerve cells of the host.Once again, the parasitic protozoa Toxoplasmosa gondii can serve as an example; its cysts are tolerated for a long time in various tissues of the intermediate host but are nonetheless mostly located in the brains of infected individuals (Stibbs 1985).

The second way in which a parasite can intervene in the nervous system of its host is through signals transferred to the CNS by the relevant sensory organs of the host.This includes, for example, excretion of various pheromones that, under normal circumstances, are used for communication between members of the host species and through which the parasite is capable of initiating even very complicated patterns of behavior in its host.Insect parasites of ants are a typical example; some of them are capable of employing pheromones to force their host to take them into the nest, feed them and, where this is useful for the parasite, even kill their own queen (Fiedler, Hölldobler, & Seufert 1996).

Pheromones are not the only means through which a parasite can act on the sensory organs of its host.In certain cases, it can employ relatively simple unspecific stimuli.Elevated sexual appetite of the host, increasing the frequency of sexual intercourse and thus the frequency of transmission of a parasite spread by this pathway, can be very effectively influenced simply by the parasite (for example Neisseria, causing gonorrhea) producing irritation in the sexual organs of its host.The mechanism of the action of some aphrodisiacs, for example cantharidine, is based on a similar principle in humans (Dawkins 1976).

Pinworms (Enterobius vermicular) employ a similar mechanism, not to increase sexual appetite, but for a much more prosaic purpose.At night, females climb out of the rectum of infected children and lay their eggs.They apparently simultaneously cause itchiness through mechanical irritation, so that the child scratches itself around the rectum and the eggs, which stick to its hand, then infect it again and also infect other hosts (Herrstrom et al. 1997).