The brain-hijacking parasites Toxoplasma gondii seem to be almost everywhere. A range of studies show that the invader may alter human behavior, as well as that of many other animals.
The parasites have been linked to a wide range of neurological disorders, including schizophrenia and psychotic episodes, and scientists keep uncovering more mysterious effects that may result from infections.
The study found that men and women who were exposed to the parasites were rated as more attractive and healthy looking than non-exposed individuals.
It might sound strange and unlikely. Scientists say that the phenomenon could make sense from an evolutionary biology standpoint.
There are 10 Toxoplasma-infected women and men and 10 non-infecting women and men.
Some of the effects of the T. gondii infection may occasionally benefit animals that have been exposed to it, researchers theorize.
In one study, Toxoplasma-infected male rats were perceived as more sexually attractive and were preferred as sexual partners by non-infecting females.
There is a lot of research going on to see if the effects of T. gondii can be seen in humans.
Some evidence suggests that men who have been exposed to HIV have higher levels of testosterone.
Men with higher levels of testosterone may be more likely to become sick from the parasites if they take more risks with the hormone.
An alternative view is that the parasites might be able to alter its host's body chemistry for its own purposes.
Borr and his team suggest that the alterations could be far-reaching.
Some sexually transmitted parasites, such as T. gondii, can cause changes in the appearance and behavior of the human host, either as a result of the infection or as a result of the manipulation of the parasites to increase its spread to new hosts.
The researchers compared the 35 people with T. gondii with the 178 people who did not have the parasites.
The healthy college students who had previously had their blood tested for T. gondii were still in the study.
The researchers found that the Toxoplasma-infected people had lower facial asymmetry than the non-infecting people.
Fluctuating asymmetry is a measure of deviation from symmetrical features and is linked with better physical health, good genes, and attractiveness.
Women carrying the parasites were found to have lower body mass and a higher number of sexual partners.
In a separate experiment, a group of 205 independent volunteers rated photographs of the participants' faces, and the raters found that the participants were more attractive and healthier than the non-infecting participants.
The researchers say that it is possible that T. gondii could cause changes in the facial symmetry of its hosts through changes in endocrinological variables.
The parasites could be influencing the metabolism in hosts, nudging them in ways that might affect their health and attractiveness perception.
The team acknowledges that there are other interpretations that are viable, including the idea that attractive people might be able to afford the costs of parasites, which are considered a burden to health.
The researchers acknowledge that the small sample size of their experiment is a limiting factor for their statistical analysis, so it is not possible to say for sure which interpretation is correct.
Future studies with larger numbers of participants will be needed to confirm or deny the hypothesis.
Maybe, just maybe, they say, this puzzling parasites isn't our enemy after all.
It is possible that the 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 findings are reported in a journal.
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