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the parasite Toxoplasma gondii, which causes toxoplasmosis, is relatively common: its definitive host is the cat (or another cat), but it can infect any mammal or bird, including humans. It is estimated that 30 to 50% of the world’s population carries this parasite. It’s known to change its host’s behavior, but a new study gives it another amazing effect: Humans wear T. gondii appear more attractive and healthier than others.
To date, there is no treatment to eliminate T. gondii of the organism. So once a person is infected, she carries the parasite with her for life. Several studies have linked this microorganism to various neurological disorders; one found, for example, that levels of parasite-specific IgG antibodies were higher than normal in patients with schizophrenia — suggesting that exposure to the parasite may be a risk factor for the mental illness.
Research has shown that T. gondii produces proteins that alter and manipulate the brain chemistry of infected intermediate hosts. The target ? Promote his reproduction to the definitive host in which he can reproduce. For example, one study found that this parasite changes the behavior of infected rats by converting their innate aversion to cat scent into a form of attraction — increasing their chances of being devoured by their predator. An international team of researchers has proven that it can work in a similar way in humans.
A parasite that changes the properties of its host
The way in which T. gondii phenotypic traits in humans has been little studied to date. The results of this new study show that infected men and women are ultimately considered more attractive and healthier than uninfected individuals. A very surprising effect, but all in all logical: as the researchers indicate in the review pearJIf the parasite diminishes its host’s health and attractiveness to such an extent that the parasite’s chances of finding a mate are reduced, it reduces its own chances of reproducing.
Natural selection tends to favor mechanisms that allow organisms to identify and favor phenotypic traits associated with “good genes”, physical strength, and good health. The presence of parasites, which generally cause physiological and/or physical deficiencies in their hosts, is considered a negative factor in mate selection… unless they manage to alter the behaviour, appearance, morphology and/or physiology of the host thus remains “competitive” against the rest of the population.
The neurobiological mechanisms by which T. gondii changes in the behavior of its intermediate hosts are not fully known. However, previous research in rodents suggests that this parasite may suppress neural activity in limbic regions responsible for innate defensive behavior, and increase activity in limbic regions that modulate sexual attraction in response to feline odors. Similarly, another study showed that male rats infected with T. gondii were perceived as more sexually attractive and preferred as mates by uninfected females, the researchers explain.
Some studies of infected men show the same trend: the data shows that these individuals are on average 3 cm taller and that their faces are considered more masculine and dominant by women; other studies have shown that infected men have higher testosterone levels than uninfected men. results suggesting that T. gondii is able to modify the physical and physiological properties of its host to promote the mating of the latter.
A less asymmetrical face that is considered more attractive
To verify this hypothesis, the researchers first compared the self-perceived attractiveness of 35 subjects who carried the parasite and 178 uninfected subjects. They also compared other variables, such as self-rated desirability, number of sex partners, number of minor ailments, body mass index (BMI), grip strength, facial asymmetry or even face width-to-height ratio.
In a second phase, they asked more than 200 people to rate the attractiveness and health of infected and uninfected individuals based on photographs. To do this, they created composite images of several female and male portraits, to represent the ‘average’ face of an infected and uninfected individual.
Researchers report that infected men had lower fluctuating facial asymmetry; no other significant differences were found for the other variables. Infected women had lower body mass and BMI, a propensity for lower fluctuating facial asymmetry, greater self-perception of attractiveness, and a greater number of sexual partners than uninfected women. The team then found that infected subjects were rated significantly more attractive and healthier than uninfected people; no significant sex differences were found for these assessments.
† One possible explanation for our results is that highly symmetrical subjects can afford to bear the physiological costs associated with parasitism. the researchers note. This supports the hypothesis that highly symmetrical traits are honest signals of good health, they add. Another possibility is that the infection may cause changes in host facial symmetry through changes in endocrinological variables (particularly testosterone levels).
The results also support the idea that: T. gondii is able to increase the metabolism of its hosts to influence their perception of health and attractiveness. † In any case, the phenotypic changes seen in Toxoplasma-infected individuals may represent transmission-related benefits to the parasite, as T. gondii can be sexually transmitted ‘, the researchers conclude.