Toxoplasmosis is a parasitic infection, present all over the world, caused by the protozoan (unicellular organism) Toxoplasma gondii – from the Greek toxon, “arc”, and plasma, “form” because of its shape –, whose definitive host is a felid. However, during its reproductive cycle, this parasite also infects other warm-blooded animals, including humans. While the prevalence of toxoplasmosis is low in Asia or America, it is 20 to 50% in southern Europe and humid regions of Africa and up to 70% in western Europe.
The Toxoplasma gondii parasite was first described in 1908 at the Institut Pasteur in Tunis by two French doctors, Charles Nicolle and Louis Herbert Manceaux. This parasite can be found in three forms: a vegetative form when it is alone and which is particularly vulnerable; a cystic form measuring between 50 and 200µm, more resistant, surrounded by a thick membrane of spherical shape which can contain up to several thousand parasites; and in the form of eggs, very resistant even to bleach, which are found in the external environment and which can resist several months in the ground. Infection by Toxoplasma gondii is in the vast majority of cases asymptomatic but during pregnancy, it represents a serious risk for the fetus. In immunocompromised patients, it can take an acute form accompanied by fever and can cause various lesions (ocular, cardiac, pulmonary, even neurological). In all cases, the cysts that form persist, particularly in the brain tissue.
During pregnancy, the risk of contamination of the fetus only occurs if the pregnant woman contracts the disease for the first time during pregnancy. An infection by Toxoplasma gondii during pregnancy can contaminate the fetus and strongly impact the central nervous system, causing serious psychomotor delays, intracranial calcifications, convulsions, hydrocephalus or on the contrary microcephaly, ventricular dilatations, modification of reflexes, hypotonia or hypertonia and eye damage.
Lifecycle of Toxoplasma gondii. CDC/Alexander J. da Silva, PhD/Melanie Moser, Public domain, via Wikimedia Commons
Can our brain be held hostage by certain parasites?
The question is asked! It is now well known that a very large number of parasites manipulate the behavior of their host and this phenomenon is very frequent. Concerning Toxoplasma gondii, it is also capable of modifying the functioning of neurons in order to influence the behavior of its host. Indeed, the Ajai Vyas team from Stanford University revealed in 2007 that Toxoplasma gondii lodges in the amygdala, an area involved in fear (see the post “Predation: a matter of neural circuits” by Antoine Besnard), and modifies the mouse’s reaction to cat urine, which instead of running away, rushes in and increases the chances of being eaten. This could confer an essential advantage to the parasite in order to increase its chances of infecting its privileged host, the cat, and thus reproducing more efficiently. The cellular and molecular mechanisms underlying this change in behavior have not yet been elucidated, but it seems that the infection increases the level of dopamine in the brain, which can induce greater risk-taking. Similar studies carried out in infected monkeys and humans show similar results, that is to say a less strong repulsion or even an attraction for the urine of felids. These observations have led a number of scientists to wonder what the effects of an infection are on behavior and personality in humans, particularly in certain psychiatric illnesses such as depression, bipolar disorder, obsessive-compulsive disorder, drug addiction or schizophrenia.
Some correlations have been highlighted between the presence of toxoplasmosis and certain behavioral disorders such as increased risk-taking, slowed reactions and a feeling of insecurity and doubt. Nevertheless, it is important to note that there is currently no formal proof of the impact of toxoplasmosis on human behavior. In the case of schizophrenia, the possibility that toxoplasmosis is one of the causes of the disease has been studied since the 1950s. certain mood stabilizer treatments, also known as having an action against toxoplasmosis, lead to better results in schizophrenic patients infected with Toxoplasma gondii. The researchers therefore suggest in the context of certain psychiatric disorders to carry out a screening for toxoplasmosis to better adapt the treatments. In 2006, an article even suggested that the prevalence of toxoplasmosis would have large-scale effects on the culture of a country, enough to raise questions about the manipulation of our brain by certain parasites!
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