Can Rare Steak Make You Stupid?
An analysis of Toxoplasmosis and its effects on human behavior
A common parasitic infection once considered to be relatively benign has recently been linked to some disturbing effects. Current studies of Toxoplasma Gondii, the protozoa responsible for the condition known as Toxoplasmosis, have shown some very interesting and not so benign links to human behavior. One of the primary vectors for transmission is undercooked meat, which means Americans and Europeans are ripe for infection due to high levels of meat consumption compared with the rest of the world. In fact, thirty to sixty percent of the world population are infected, with the North American and European continents claiming the highest rates . Some of the startling effects being unearthed are: a possible link to schizophrenia, increased rates of depression, decreased ability to achieve academic success, decreased intelligence, decreased reaction time, decreased conscientiousness, and an increase in risk taking to list a few [1,2,3].
Toxoplasma gondii is a species of parasitic protozoa that sexually reproduce mainly in cats, then are passed to other mammals through feline feces where they propagate through asexual reproduction in the new host, called the intermediate host. Once in the general mammalian and bird population they easily become part of the human diet. When meat is fully cooked the T. gondii is killed, but whenever meat is ingested “rare” there is an increased risk of infection. Humans can also get infected from cat feces, which is a good reason to keep the cat litter box away from a food area, and to wash hands after cleaning a litter box.
T. gondii Tachyzoites T. gondii constructing daughter scaffolds within the mother cell.
T. gondii is the cause of toxoplasmosis, which is characterized by the widespread formation of cysts, particularly in the brain and muscle tissue. When T. gondii invades a cell it forms a space in the cell called a parasitophorous vacuole. Inside the vacuole bradyzoites are formed, which are slow replicating versions of T. gondii. Those parasitophorous vacuoles are what become the cysts found in those infected, and because they are inside the cells they avoid detection by the immune system. As the slowly replicating T. gondii increase in number, the cell becomes unable to contain the population and bursts, releasing the bradyzoites. Outside the cell the T. gondii takes the form of fast replicating tachyzoites, which roam freely through the organism but are now fully detectable by the immune system. The antibodies from this immune response are how we test for infection. While most of the tachyzoites are destroyed by the immune system, enough are able to invade other cells to maintain the infection. When the infected cysts are ingested, the parasite is passed to a new host. This is where the undercooked meat comes in. Another common source of infection comes from cleaning cat litter boxes. The T. gondii is inhaled, gets imbedded in mucous, then swallowed.
Toxoplasmosis comes in two forms: acute and latent, with the acute form being more active which calms down into the chronic latent form. Until recently most of the effects and symptoms were considered benign. Most of those infected are unaware they have toxoplasmosis. When women are infected early in pregnancy their fetuses are at risk of disability or death. Other populations at risk are those who are immunocompromised, such as people with HIV, AIDS, and organ transplant recipients or others who are on immunosuppressant therapy. One of the greatest risks of the acute form is encephalitis which can cause permanent disability or death.
Acute toxoplasmosis can be treated with antibiotics and antimalarial drugs, but with limited results. These therapies don’t completely wipe out the T. gondii, they just slow it down and often convert it to latent toxoplasmosis. Once in the latent form, toxoplasmosis is resistant to most drugs because most of the T. gondii resides in the cells in the bradyzoite form, protected from direct contact with the blood stream. Acute toxoplasmosis is still treated with the drug therapies in the high risk groups because the latent form is much more benign.
Researchers have known for decades that mice infected with T. gondii behave differently. Now recent studies have found behavioral differences in humans who are infected. A 2009 study has found a correlation between toxoplasmosis and schizophrenia . They found that 47.7% of patients with schizophrenia tested positive for toxoplasmosis, whereas control groups were found to be 20.4% positive . An earlier 2003 study showed that toxoplasmosis positive people were found to have higher dopamine levels, which has been linked to schizophrenia although the exact mechanism is still unknown . The same study also showed lower intelligence, particularly for men, lower levels of academic achievement, and slowed psychomotor performance . In a more recent 2011 study, researchers looked at a broad range of behavioral factors and T. gondii infection. They found that the toxoplasmosis positive population is more extroverted and less conscientious. They surmised that the latent, long term infection of T. gondii had a gradual effect on the personalities of those infected .
These studies all show compelling evidence that toxoplasmosis affects human behavior
in many ways, and further study is clearly warranted. If over one third of the world population is infected with a disease that affects personality, judgment, intelligence, and behavior, how has that affected human society as we know it? Is it possible that these effects have influenced how people vote? How leaders make decisions? Have the infection rates changed over the past decades or centuries? It is difficult to prevent this line of questioning from spiraling into a paranoid abyss of alien mind-controlling parasites dominating our planet undetected, but...in a creepy way, that’s pretty much what we’re looking at.
It is probable that T. gondii has coevolved with us and other mammals over millennia, and are perhaps as much a part of our lives and history as the microbiota in our gut. The studies considered in this paper provide ample evidence that toxoplasmosis warrants further investigation and should be treated as a disease with potentially harmful effects for a significant portion of the population. One thing we can do right now is educate people on the risks and how to prevent infection.
Flegr, Preiss, Klose, Havlicek, Vitakova, Kodym. “Decreased level of psychobiological factor novelty seeking and lower intelligence in men latently infected with the protozoan parasite Toxoplasma gondii Dopamine, a missing link between schizophrenia and toxoplasmosis?” Biological Psychology 63 (2003) 253-268.
 Lindova, Priplatova, Flegr. “Higher Extraversion and Lower Conscientiousness in Humans Infected with Toxoplasma”. Wileyonlinelibrary.com July 15, (2011).
 Dogruman-al, Aslan, Yalcin, Kustimur, Turk. “A possible relationship between Toxoplasma gondii and schizophrenia: A seroprevalence study”. International Journal of Psychiatry in Clinical Practice. (2009); 13(1): 82-87.