ocation has been proposed as an important factor in the behavioural changes observed in rodents infected with the protozoan Toxoplasma gondii. During the chronic stages of infection, encysted parasites are found in the brain but it remains unclear whether the parasite has tropism for specific brain regions. Parasite tissue cysts are found in all brain areas with some, but not all, prior studies reporting higher numbers located in the amygdala and frontal cortex. A stochastic process of parasite location does not, however, seem to explain the distinct and often subtle changes observed in rodent behaviour. One factor that could contribute to the specific changes is increased dopamine production by T. gondii. Recently, it was found that cells encysted with parasites in the brains of experimentally infected rodents have high levels of dopamine and that the parasite encodes a tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of this neurotransmitter. A mechanism is proposed that could explain the behaviour changes due to parasite regulation of dopamine. This could have important implications for T. gondii infections in humans.
Parasite-augmented mate choice and reduction in innate fear in rats infected by Toxoplasma gondii
Typically, female rats demonstrate clear mate choice. Mate preference is driven by the evolutionary need to choose males with heritable parasite resistance and to prevent the transmission of contagious diseases during mating. Thus, females detect and avoid parasitized males. Over evolutionary time scales, parasite-free males plausibly evolve to advertise their status. This arrangement between males and females is obviously detrimental to parasites, especially for sexually transmitted parasites. Yet Toxoplasma gondii, a sexually transmitted parasite, gets around this obstacle by manipulating mate choice of uninfected females. Males infected with this parasite become more attractive to uninfected females. The ability of T. gondii to not only advantageously alter the behavior and physiology of its host but also secondarily alter the behavior of uninfected females presents a striking example of the ‘extended phenotype’ of parasites. Toxoplasma gondii also abolishes the innate fear response of rats to cat odor; this likely increases parasite transmission through the trophic route. It is plausible that these two manipulations are not two distinct phenotypes, but are rather part of a single pattern built around testosterone-mediated interplay between mate choice, parasitism and predation.
Influence of latent Toxoplasma infection on human personality, physiology and morphology: pros and cons of the Toxoplasma–human model in studying the manipulation hypothesis
The parasitic protozoan Toxoplasma gondii infects about one-third of the population of developed countries. The life-long presence of dormant stages of this parasite in the brain and muscular tissues of infected humans is usually considered asymptomatic from the clinical point of view. In the past 20 years, research performed mostly on military personnel, university students, pregnant women and blood donors has shown that this ‘asymptomatic’ disease has a large influence on various aspects of human life. Toxoplasma-infected subjects differ from uninfected controls in the personality profile estimated with two versions of Cattell’s 16PF, Cloninger’s TCI and Big Five questionnaires. Most of these differences increase with the length of time since the onset of infection, suggesting that Toxoplasma influences human personality rather than human personality influencing the probability of infection. Toxoplasmosis increases the reaction time of infected subjects, which can explain the increased probability of traffic accidents in infected subjects reported in three retrospective and one very large prospective case-control study. Latent toxoplasmosis is associated with immunosuppression, which might explain the increased probability of giving birth to a boy in Toxoplasma-infected women and also the extremely high prevalence of toxoplasmosis in mothers o