The Scientific Realities of The Last of Us Series: Can a Parasite Take Over the Human Brain?

There are many diseases that change the behavior of human beings. As reminded Jennifer Ronholm : “There are also very well-known pathogens that alter human behavior – rabies and toxoplasmosis are the main examples that come to mind.“

In both of these cases, the brain is affected by the pathogen and the behavior of the infected human is altered from its normal state.

As Giuseppe Lippi and Gianfranco Cervellin write in a study published in Acta Biomedica, “conventional zombies, as depicted in comic books and movies (23), share similar behaviors with patients infected with the rabies virus“.

Rabies is a virus that attacks the nervous system and like the CDC (US Centers for Disease Control and Prevention) explains it, “at as the disease progresses, the person may experience delirium, abnormal behavior, hallucinations, hydrophobia (fear of water), and insomnia“.

We list in the symptoms the “aggressive behavior” of the infected and knowing that the disease is transmitted via saliva during a bite or by scratching, we quickly put the label “zombie” on the patients of the rage. An idea studied by Lippi et Cervellin : “The two [les zombies et les malades de la rage] experience a varying degree of impairment of consciousness, which tends to be almost identical in the later stages of rabies disease. Both individuals also display fearful facial expressions, increased hyperirritability, and aggressiveness, which can both be greatly heightened by external stimuli (thirst, fear, light, and noise) in enraged patients, and can eventually progress to behaviors violent and ferocious.“

• Toxoplasmosis turns mice into zombies

The parasite toxoplasma gondii is a single-celled microbe that only survives and multiplies when it infects a living cell. The parasite is transferred through food and contact with cat feces, the only animal in which the parasite can reproduce other than by cell division. Once in another host’s system, the parasite tries to grow, infects immune cells and hijacks their identity until it takes control of the brain (amygdala). The parasite can go so far as to prevent its survival instinct. As for example in the mouse which loses its fear of cats and can remain carefree in front of them, with the aim of being eaten and therefore of infecting a new host.

Humans can be infected with toxoplasma gondii, lhe WHO estimates that at least 30% of the world’s human population carries the parasite. But it is often not serious, the immune system allowing to limit the symptoms to a flu while remaining in a “dormant” state in the body. However, there are cases where the immune system is too weak and the parasite wakes up to infect other cells and multiply.

In a study published in 2009, scientists found that the parasite could take over the brains of humans in some cases. “In people without a history of mental illness, T. gondii can cause abnormal behavior“, iThey found a correlation between infection and self-inflicted violence, going as far as suicide. Although other studies have challenged these findings, more studies are needed to attest to the impact of toxoplasmosis on human behavior.

Tomorrow to complete the scientific analysis of the scenario of the series The Last of US, we will see if it is possible that one day a mushroom will take control of our brain and transform us into a zombie.