Toxoplasma gondii is a single-celled protozoan parasite with astounding abilities to manipulate human personalities and behavior. This real-life body snatcher appropriates people’s bodies for its own reproductive benefit, taking control of its host in subtle, but significant, ways.
In nature, T. gondii only develops into its adult stage in a final host, any member of the cat family, Felidae. However, while developing into a larva, the parasite can use a wide range of different intermediate hosts, which consists of most species of warm-blooded animal. Intermediate hosts include many species of mammal and bird. Intermediate hosts even include marine mammals, such as dolphins and seals. The human is an intermediate host.
The parasite’s lifecycle can start in cat feces as an oocyst, which is a dormant, thick-walled zygote. The lifecycle can also start as a tissue cyst in either an intermediate host or a final host. An oocyst or tissue cyst is ingested by another host. Then the cyst converts to a trophozoite, its active, feeding form. Generally, transmission in humans is foodborne and occurs by accidental ingestion of cysts, especially oocysts from cat feces or tissue cysts from undercooked meat. Transmission can also occur congenitally or by organ transplant or blood transfusion. The Centers for Disease Control and Prevention list some ways people can prevent infection by T. gondii.
According to the Agricultural Research Service, this widespread microorganism infects about one third of humanity, including an estimated 16-40% of people in the United States. Toxoplasmosis, which is infection by T. gondii, generally causes no symptoms in an adult with a healthy immune system. Rarely, flu-like symptoms occur and then disappear. Yet the behavioral changes that T. gondii brings about in a person with a healthy immune system often go unregistered as signs of infection, because these behaviors frequently are attributed to ordinary human conduct and activity. Still, this asymptomatic toxoplasmosis is worth taking seriously.
Research published in 2007 and 2002 by parasitologist Jaroslav Flegr et al. and research published in 2006 by parasitologist Kor Yereli et al. shows that asymptomatic toxoplasmosis increases the likelihood of a person being in a traffic accident. Flegr et al. found that the probability of a person with asymptomatic toxoplasmosis being in a traffic accident is 2.65 times that of an uninfected person. Flegr, Yereli, and, in a separate study, parasitologist J. Havlícek et al. found the reason for this: The infection slows the host’s reaction time, decreases their psychomotor performance, and shortens their attention span.
Furthermore, several researchers have demonstrated that asymptomatic toxoplasmosis induces psychological changes in both women and men. T. gondii decreases an infected person’s desire to try new things. Flegr et al. and, separately, Kevin D. Lafferty found that the parasite also makes people apprehensive, insecure, self-doubting, worrisome, and self-blaming. In addition, toxoplasmosis in a fetus, infant, or immunosuppressed adult can engender various mental disabilities.
Additionally, T. gondii causes sex-influenced psychological changes. In research published in 1999 and in the 2007 research, respectively, Flegr et al. found that, in women with healthy immune systems, the parasite makes the women smarter and more attractive to men. The 2007 study also found that the parasite has an opposite effect on men with healthy immune systems, making the men dumber and more repugnant to women. In an article in the International Journal for Parasitology, published in 2006, scientist Jitka Lindová et al. propose a theory for these changes: In early human societies, the parasite used the changes to increase the probability of a man being eaten by a large cat and to increase the probability of a woman becoming pregnant and transmitting the parasite congenitally to her children.
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