Toxoplasma gondii is a single-celled parasite that causes toxoplasmosis, which is manifested clinically in severe cases by jaundice, vision impairment, and liver and spleen enlargement. The microscopic protozoan is often found encysted in nerve tissue, and proliferative forms and cysts can infect numerous species of birds and many species of warm-blooded animals, which act as intermediate hosts. Feline species are the only definitive hosts for the organism. Cats harbor sexual stages of the parasite in their gastrointestinal tract and shed infectious oocysts in their stool. Intermediate hosts become infected by ingesting the sporulated oocysts from cats. Humans can contract the disease by ingesting inadequately cooked meat (especially lamb, pork, and mutton) of the intermediate hosts that have been infected, or by ingesting oocysts from soil or water contaminated with cat feces. In fact, toxoplasmosis is one of the leading causes of death attributed to food-borne illness, and precautions to ensure food safety must be followed. See also: Parasitology; Protozoa; Saliva-based immunoassay of waterborne pathogen exposure; Sporozoa; Toxoplasmea; Toxoplasmida; Zoonoses

Of the many millions of people who carry the T. gondii parasite, very few display symptoms because a healthy person’s immune system typically prevents the organism from causing illness. In some cases, mild flu-like symptoms may occur in otherwise healthy individuals before the parasite enters a chronic and dormant phase, which has previously been regarded as symptom-free. However, pregnant women and individuals who have compromised immune systems must be vigilant because serious health problems could arise from a Toxoplasma infection. For example, it is known that toxoplasmosis in the brain can be fatal in people with severely weakened immune defenses and in fetuses (who can be infected through the mother). See also: Immunological deficiency; Medical parasitology; Opportunistic infections; Public health

Intriguing research indicates that T. gondii has an effect on risk-taking and other behaviors. For example, infected rats are unafraid of cats and are attracted by the scent of cats, endangering the survival of these rats. In humans, the presence of the parasite has been demonstrated to influence behavior, affecting the degree to which an individual might be aggressive, extroverted, or willing to take a risk. Other studies in humans confirm that mental diseases, including schizophrenia, depression, and anxiety disorders, are more prevalent in people with toxoplasmosis. Notably, investigations have shown that the parasite affects its host even during the dormant phase. See also: Anxiety disorders; Depression; Phobia; Schizophrenia

With regard to its mechanism of action, the parasite enters the brain and likely manipulates the host by regulating neurotransmitter levels. In one experiment, human dendritic cells (key immune-defense components) were infected with T. gondii. After infection, the cells secreted the chemical messenger GABA (γ-aminobutyric acid), which is a known inhibitor of fear and anxiety. In another investigation, researchers followed the movement of infected dendritic cells in the bodies of live mice after introduction of the parasite into the brain, from where it spread and persisted to influence the GABA system. These types of disturbances of the GABA system are also seen in individuals with mental disorders, indicating a strong connection between T. gondii and behavior. See also: Brain; Neurobiology