The parasite that hijacks the brain Toxoplasma gondii seems to be almost everywhere. The microscopic invader is thought to infect up to 50 percent of humans, and a number of studies suggest it can alter human behavior in addition to that of many other animals.
The parasite has been linked to a variety of neurological disorders, including schizophrenia and psychotic episodes, and scientists are uncovering more and more mysterious effects that can result from infection.
In one such new study, researchers found that men and women infected with the parasite were ultimately rated as more attractive and healthier looking than uninfected individuals.
At first glance, this may sound strange and improbable. But hypothetically, the phenomenon could make sense from an evolutionary biology perspective, scientists say.
Above: Composite images of 10 females and males infected with Toxoplasma (a), alongside 10 composite images of 10 non-infected females and males (b).
Amid the many neurobiological changes T. gondii that appears to induce infection in their hosts, researchers theorize that some of the effects may benefit occasionally infected animals – which could then benefit the parasite as well, by subsequently helping to boost its own transmission capabilities.
“In a study, toxoplasma-infected male rats were found to be more sexually attractive and preferred as sexual partners by uninfected females,” researchers explain in a new paper led by first author and biologist Javier Borráz-León of the University of Turku in Finland.
Much research has been devoted to examining whether similar effects can be observed in humans T. gondii Infection.
The evidence is far from conclusive, but some evidence suggests that infected men have higher levels of testosterone than uninfected men.
It’s likely that men with higher testosterone levels are more likely to be infected by the parasite because they engage in higher-risk behaviors with the hormone.
However, an alternative view is that the parasite may be able to subtly alter its host phenotype by manipulating chemicals in the animal’s body, such as neurotransmitters and hormones, for its own later purposes.
These changes could be far-reaching, suggest Borráz-León and his team.
“Some sexually transmitted parasites, such as T. gondiican produce changes in the appearance and behavior of the human host, either as a byproduct of infection or as a result of manipulating the parasite to increase its spread to new hosts,” the researchers write.
To test this hypothesis, the researchers compared 35 people (22 men, 13 women) with those infected T. gondii against 178 people (86 men, 92 women) who did not carry the parasite.
However, all participants (including those infected) were healthy students whose blood had previously been tested for another study T. gondii.
After a series of different tests with the participants – including surveys, physical measurements and visual assessments – the researchers found toxoplasma-Infected individuals had significantly less fluctuating facial asymmetry than the uninfected individuals.
Fluctuating asymmetry is a measure of deviation from symmetrical traits, with lower degrees of asymmetry (i.e., higher symmetry) being associated with better physical health, good genes, and attractiveness, among other things.
Additionally, women who carried the parasite were found to have lower body mass and BMI than uninfected women, and they reported both higher self-perceived attractiveness and a higher number of sexual partners.
In a separate experiment, a group of 205 independent volunteers reviewed photos of the participants’ faces, and the reviewers found that the infected participants looked both significantly more attractive and healthier than the uninfected participants.
The researchers interpret the results and say that this is possible T. gondii Infection can be characterized by changes in endocrine variables such as B. the testosterone level, lead to changes in the facial symmetry of their hosts.
In addition, the parasite could also affect metabolic rates in hosts and nudge infected humans in ways that could affect their perceptions of health and attractiveness.
However, all of this is speculation at this point, and the team concedes that other interpretations are also possible, including the idea that highly symmetrical, attractive people somehow better bear the physiological costs associated with parasitism, which are otherwise considered burdensome could contribute to health.
It is not possible to say with certainty which interpretation is correct from this one study alone, and the researchers acknowledge that the small sample size of their experiment is a limiting factor in its statistical analysis.
For this reason, future studies with larger numbers of participants are 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 seemingly 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 the parasite and the host,” the researchers write.
The results are reported in peerJ.