The toxoplasmosis parasite reduces anxiety in infected mice—not just fear of predators

For the past twenty years or so, the Toxoplasma gondii parasite has been suspected of manipulating the brains of rodents to make them less afraid of predators, particularly cats. A study published in *Cell* shows that, in addition to reducing fear of cats, the toxoplasmosis parasite decreases general anxiety in infected rodents, as well as their aversion to a wide range of threats.

The goal of the T. gondii parasite is to continue its life cycle, doing everything it can to

its ability to transition from a rodent to a feline predator. Thus, it can reproduce only in the cat’s intestine, the sole host capable of shedding the toxoplasma in its feces and thereby infecting herbivores, such as sheep. It’s worth noting that Toxoplasma gondii can infect any warm-blooded vertebrate, including humans, who become infected by eating undercooked meat contaminated with the parasite or poorly washed vegetables. However, it is indeed in the cat’s intestine that it reproduces sexually and takes the form of oocysts, which spread and infect other animals. Toxoplasma can persist for long periods, even for life, within the host’s body.

Behavioral observations of infected mice reveal that they do not seek to avoid contact with cats or rats and are more willing to explore their surroundings. Scientists have sought to understand why the parasitic manipulation is limited to reducing fear of predators. There is no evolutionary reason for this brain manipulation to focus on cats in particular. The parasite’s primary goal is for the infected rodent to be killed and eaten by another animal—any animal at all!

Recent studies have therefore focused on the immune response of the infected rodent, particularly in the cerebral cortex, where the parasitic cysts are concentrated, even though the observed process of infection and spread appears to be random. Analysis of brain tissue revealed the presence of markers of inflammation. The degree of inflammation is proportional to the prevalence of cysts and, most importantly, to the extent of changes in the host’s behavior.

Toxoplasmosis is a significant parasitic disease in animal health. But it should not be forgotten that one-third of humans are believed to be carriers of this infection, which has worldwide prevalence. In the United States, more than 40 million people are thought to harbor the parasite. Furthermore, in recent years, a link has been established between the presence of the parasite in humans and certain mental illnesses such as schizophrenia, Parkinson’s disease, and bipolar disorder, as well as traffic accidents and suicide attempts. Toxoplasmosis is considered one of the leading causes of death attributable to foodborne illnesses in the United States. Infection during pregnancy can lead to fetal death and also poses a threat to immunocompromised patients, such as those infected with HIV. Its impact on human health is therefore considerable.

Future studies should therefore analyze the actual impact of parasitic cysts on the human brain and their influence on behavioral changes in humans. It should be noted that inflammation in the human brain is much more limited than that induced in mice under specific laboratory conditions. However, a better understanding of this neuroinflammation should eventually make it possible to modify certain behavioral traits such as anxiety, sociability, or curiosity. Nevertheless, the potential effect of a T. gondii infection on neuronal function in humans should not be underestimated.

https://vetitude.fr/conservation-chat-ou-biodiversite-faudra-t-il-choisir/