The brain-hijacking parasite Toxoplasma gondii seems to be almost everywhere. The microscopic invader is thought to infect up to 50% of people, and a range of studies suggest it can alter human behavior, in addition to that of many other animals.
The parasite has been linked to a wide range of neurological disorders, including schizophrenia and psychotic episodes, and scientists continue to uncover more mysterious effects that can result from infection.
In one such new study, researchers found that men and women infected with the parasite ended up being viewed as more attractive and healthier than uninfected individuals.
At first glance, this may seem strange and unlikely. But hypothetically speaking, the phenomenon could make sense from an evolutionary biology perspective, the scientists say.
Above: Composite images of 10 women and men infected with Toxoplasma (a), next to 10 composite images of 10 uninfected women and men (b).
Amid the many neurobiological changes T. gondii infection appears to cause in its hosts, the researchers hypothesize that some of the effects may sometimes benefit infected animals – which could then also benefit the parasite, subsequently helping to boost its own prospects for transmission.
“In one study, Toxoplasma-infected male rats were perceived as more sexually attractive and were preferred as sexual partners by uninfected females,” the researchers explain in a new paper led by first author and biologist Javier Borráz-León from the University of Turku in Finland .
Much research has been devoted to determining whether similar effects can be observed in human cases of T. gondii infection.
The evidence is far from clear, but some evidence suggests that infected men have higher testosterone levels than uninfected men.
Arguably, men with higher levels of testosterone might be more likely to become infected with the parasite in the first place, due to higher levels of risky behaviors associated with the hormone.
Another view, however, is that the parasite might be able to subtly alter its host’s phenotype, by manipulating chemicals in the animal’s body, such as neurotransmitters and hormones, for its own further purposes. .
These changes could be far-reaching, suggest Borráz-León and his team.
“Certain sexually transmitted parasites, such as T. gondiimay produce changes in the appearance and behavior of the human host, either as a by-product of infection or as a result of manipulation of the parasite to increase its spread to new hosts,” the authors write. researchers.
To test this hypothesis, the researchers compared 35 people (22 men, 13 women) infected with T. gondii against 178 people (86 men, 92 women) who did not carry the parasite.
All of the participants (including those infected) were nevertheless healthy students, who had already undergone a blood test for another study looking at T. gondii.
After a number of different tests involving the participants – including surveys, physical measurements and visual assessments, the researchers found Toxoplasma-infected subjects had significantly less fluctuating facial asymmetry than uninfected subjects.
Fluctuating asymmetry is a measure of deviation from symmetrical characteristics, with lower levels of asymmetry (i.e. higher symmetry) being linked to better physical health, good genes, and attractiveness, among others.
Additionally, female carriers of the parasite were found to have lower body mass and BMI than uninfected females, and they reported both higher self-perceived attraction and a higher number of sexual partners. .
In a separate experiment, a group of 205 independent volunteers rated photographs of participants’ faces, and raters found that infected participants appeared both significantly more attractive and healthier than uninfected participants.
Interpreting the results, the researchers say it is possible that T. gondii infection can produce changes in the facial symmetry of its hosts through changes in endocrinological variables, such as testosterone levels.
Additionally, the parasite could also influence metabolic rate in hosts, pushing infected people in ways that could influence their health and perceptions of attractiveness.
That said, this is all speculation at this point, and the team acknowledges that other interpretations are also viable, including the idea that highly symmetrical and attractive people might somehow better withstand the physiological costs associated with it. to parasitism, which in other respects is seen as a burden. for health.
As to which interpretation is correct, it is impossible to say with certainty based on this study alone, and the researchers acknowledge that the small sample size of their experiment is a limiting factor for its statistical analysis.
For this reason, future studies with larger numbers of participants will be needed to confirm or refute their overall hypothesis.
But maybe – just maybe, they say – this confusing parasite isn’t necessarily our enemy after all.
“It is possible that the apparently non-pathological and potentially beneficial interactions between T. gondii and some of its intermediate hosts, such as rats and humans, are the result of co-evolutionary strategies that benefit, or at least do not harm, the fitness of both parasite and host,” the researchers write.
The findings are reported in PeerJ.