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 [1]. 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 [3]. They found that 47.7% of patients
with schizophrenia tested positive for toxoplasmosis, whereas control groups
were found to be 20.4% positive [3]. 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
[1]. The same study also showed lower intelligence, particularly for men, lower
levels of academic achievement, and slowed psychomotor performance [1]. 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 [2].
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.
Resources
[1]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.
[2]
Lindova, Priplatova, Flegr. “Higher Extraversion and Lower Conscientiousness in
Humans Infected with Toxoplasma”. Wileyonlinelibrary.com
July 15, (2011).
[3]
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.
Photos:
http://en.wikipedia.org/wiki/Toxoplasmosis#Treatment
