Toxoplasma: a mind-controlling parasite
By Maria-Cecília Costa
Imagine a parasite that makes an animal change its habits, care for the parasite’s offspring, or even attempt suicide. Is this the plot of some science fiction movie? Not really! This is a very real phenomenon and affects especially insects. Unfortunately, parasites like these are much closer to us than we think. For example, one in three people has a brain parasite called Toxoplasma gondii, which causes toxoplasmosis. Although most people do not show any symptoms, there is a relationship between infection with this parasite and some psychiatric disorders such as bipolar disorder, schizophrenia, and epilepsy. Luckily, most healthy people recover from toxoplasmosis without even a treatment.
Organisms interact in different ways
We are surrounded by a broad variety of organisms, from the plants that we eat to our human friends and family. With each of these organisms, we interact in very different ways. These organisms also interact with each other in different ways. In some interactions, such as between bees and crocuses or hyacinths, both parties gain benefit. The bees benefit by getting pollen, the primary food source for the hive. The plants benefit by getting their pollen carried to other flowers, which is necessary for their reproduction.
In other interactions, one party benefits while the other neither benefits nor gets harmed. Usually, the benefiting party gets locomotion, shelter, food, or support from the host, which does not get anything in exchange and is not harmed. For example, tree frogs use plants as shelter, which is good for the frogs and makes no difference to the plants.
There are interactions when one party benefits and the other party gets harmed, like the ones between parasites and hosts. Parasites are found in every organism of all existing species on Earth and their presence does not necessarily mean serious injury or even death of the host. Very often, it is in the parasite’s best interest not to kill the host, because it relies on the host to live. For example, fleas are parasites that harm their hosts, such as dogs and cats, when they bite them to get food (the host’s blood), causing them to itch. Although one flea will not kill its host (keep in mind that they can transmit other parasites, which might kill the host), a serious flea infestation might.
Parasite’s life cycle
Someone who has a parasite is called a host. Some parasites require a single host to complete their life cycles, while others require multiple hosts. Usually, parasites that require multiple hosts get more attention because they cause severe diseases to humans and livestock, such as malaria, leishmania, schistosomiasis, sleeping sickness, and toxoplasmosis.
For example, toxoplasma (Toxoplasma gondii), the single cell parasite that causes toxoplasmosis, requires two hosts — a main and an intermediary — to complete its life cycle. Cats (domestic and wild-living) are the main hosts and get infected when they prey on infected intermediary hosts, usually mice and rats. Inside the cats, toxoplasma reproduces sexually, producing millions of egg-like forms of the parasite that will be released in the environment together with the cat’s feces. These egg-like forms can survive for 18 months and possibly years in the water and the soil. For a mouse, eating a single of these egg-like forms may be enough to get infected. In the body of an infected mouse, toxoplasma reproduces asexually, infects virtually any organ, and can be transmitted via the placenta, milk, and sperm.
When toxoplasma reaches a mouse’s brain, it takes control of the mouse’s mind. Toxoplasma manipulates how the mouse thinks (a.k.a. cognitive functions) and switch its behavior from avoiding cat urine to being attracted to it. Toxoplasma does it by affecting the production of dopamine, which plays a key role in the control of locomotion, learning, working memory, cognition, and emotion. This is a very clever way used by the parasite to facilitate its own transmission. You can imagine that when a mouse loses its instinctive fear for the smell of cats, it becomes an easy target for the cat and an efficient transmitter of the parasite. Once a cat eats an infected mouse, the life cycle is completed.
Humans as hosts
Humans can be infected by toxoplasma too! Luckily, we are rarely eaten by felines, so we are dead-end hosts. In humans, toxoplasma forms cysts in muscles, heart, brain, and eyes. These cysts can remain in our bodies for the rest of our lives.
It is estimated that worldwide, over 6 billion people have been infected with toxoplasma. The percentage of people infected varied from less than 10% in some countries to as high as 98% in others. In Germany, almost 50% of adults (18 to 79 years of age) have antibodies against toxoplasma, meaning that they have been contaminated at some point in their lives. This percentage rises around 1% per year of life, from about 20% in young adults (18 to 29 years) to 77% in 70- to 79-year-olds. Interestingly, in Germany, men, cat owners, and the obese (BMI ≥ 30) were more often positive than other groups. In contrast, people with plant-based diets and high socioeconomic status were found to be contaminated less often than other groups.
In case you are wondering how you can find out if you have toxoplasmosis, the diagnosis is quite simple. It is typically made by measuring antibodies in the blood. These antibodies are produced in response to the infection or the presence of the parasite’s DNA in the blood.
Humans can get infected mainly by ingesting the egg-like forms. This can happen by eating undercooked, contaminated meat (especially pork, lamb, and venison) or shellfish (like oysters, clams, and mussels) and unwashed fruits or vegetables grown in contaminated soil. The egg-like forms can also be present in water and unpasteurized goat’s milk. If you have a cat, you need to be especially careful when cleaning the cat’s litter box, as the egg-like forms might be accidentally ingested.
Unborn children can also get contaminated when their mother gets infected during or just before pregnancy. The woman may not have symptoms, but there can be severe consequences for the unborn child, such as diseases of the nervous system and eyes.
Symptoms
Healthy people that become infected with Toxoplasma often do not show symptoms because their immune system usually protects them from the parasite. However, some people get “flu-like” symptoms, such as fever, aching muscles, tiredness, sore throat, and swollen glands. Although these symptoms usually disappear within about 6 weeks, the parasite remains in the person’s body in an inactive state and can become reactivated if their immune system becomes compromised.
It seems that toxoplasma is also able to affect human behavior. Maybe this is why some people get to love cats so much. Joking apart, some studies have suggested that, as the parasite infects the brain, it might make people more reckless. Other studies found a positive correlation between toxoplasmosis and psychiatric and behavioral disorders. These claims need further investigation because two facts being correlated does not necessarily mean that one causes the other.
Prevention
In case you want to avoid being contaminated, the best ways to prevent and reduce the transmission of toxoplasma are washing fruits and vegetables, avoiding consumption of raw and undercooked meat, and washing hands after gardening or handling cats. Most importantly, the water must be clean and free from toxoplasma.
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To learn more
Centers for Disease Control and Prevention
