An effect of parasites on host behaviour was tested on the toxoplasma-human model. Three hundred and thirty-eight (338) people were assessed with Cattell's personality questionnaire and then tested for Toxoplasma gondii infection with a delayed type hypersensitivity test for Toxoplasma. A highly significant correlation between chronic toxoplasmosis and two personality factors (G-Low Superego Strength and L-Protension) was found (p= 0.0032 and 0.0020, respectively). A correlation of the intensity of the personality factor-shifts with the duration of the infection (estimated from antibody titer) suggests that toxoplasmosis induces the shift in human personality, rather than the personality factors G and L influence an acquisition rate of Toxoplasma gondii infection.

The effect of parasitism on host behaviour is being demonstrated in a growing number of host-parasite systems. The induced behavioral patterns often promote transmission of the parasite. One hypothesis suggests that such modification of host behaviour is a sophisticated product of parasite evolution aimed at host manipulation rather than an accidental byproduct of other physiological activities of the parasite (B a rna rd and B ehnke 1990, D aw kin s 1982). Among parasitic protozoa from the suborder Eimeriina, the phenomenon has been observed in Sarcocystis (H 0 0 genboom and Dijkstra 1987), Eimeria (K a val ier sand Co 11 well 1982) and Toxoplasma gondii Nicolle et Manceaux, 1906 (H ay et al. 1985). The last organism is an intestinal coccidian of felids with an unusually wide range of intermediate hosts, including humans. After an acute phase of infection (promoted by tachyzoites), in Toxoplasma-infected subjects cysts are formed mainly in neural and muscular tissues. The parasites (bradyzoites) inside the cysts usually cause little or no harm in immunocompetent individuals and probably persist as viable parasites for the life of the host (R em ing ton and K ra hen buh 1 1982). The transmission of the parasite from the intermediate to the definitive host is mediated by carnivorism. Since sexual reproduction of Toxoplasma can be accomplished only in the feline, there might be a strong selection pressure on the parasite to evolve a mechanism to manipulate intermediate host-behaviour so as to favour predation by felines. A high prevalence of lifelong toxoplasmosis (about 32% of pregnant women in New York City, 22% in London and 84% in Paris (Desm 0 nt sand C 0 uvreur 1974)) offers an opportunity to study the possible influence of the parasitism on human behaviour by random screening of normal population. Here we studied an effect of chronic toxoplasmosis on human personality in an experimental set of 338 humans.