Did you know that parasites can influence our thoughts and behaviors? Parasites live out their lives inside their hosts, and they can even manipulate human behavior. We’ve explored how parasites manipulate their hosts through the use of thermophilic worms and toxospores.
Every day, we make choices and take actions, but are these choices entirely yours? Most people would probably answer “yes,” but the reality is that your thoughts and actions are being influenced by something else – something that’s alive and well, and something you might find downright gross. It’s an unwanted guest, a parasite, hiding inside your body.
First of all, what is a parasite? According to the dictionary, a parasite is “an invertebrate that lives as a parasite inside the body of another organism, relying on food and the environment for sustenance.” We’re all familiar with tapeworms, roundworms, pinworms, and other familiar diseases like sleeping sickness and malaria, which are caused by parasites. When you look at the anatomy of a parasite’s body, you’ll see that most of it is related to reproduction. This gives them an advantage in leaving behind offspring. They are also thought to have evolved into very simple structures to live in the host’s body, becoming very primitive.
But how can such a simple creature be able to manipulate our minds? As we said earlier, parasites are creatures that live off of their hosts, which means they can’t live without another organism. In fact, parasites spend their entire life inside their host’s body. This life cycle doesn’t just take place in one species of host, but through multiple hosts. Take for example the falciparas that cause malaria. When a malaria-infected mosquito bites a human, the process transfers the parasite into the human body. Inside the human body, it divides to create reproductive cells. During this process, the cells dissolve, causing malaria. When the mosquito sucks them up, they combine in the digestive tract to create new P. falciparum parasites, completing a life cycle that moves back and forth between humans and mosquitoes.
During this life cycle, the parasite learns to manipulate its host. Let’s look at an example of how a parasite manipulates its host. In the case of the aforementioned trophozoites, they need to move between humans and mosquitoes to complete their life cycle. How can we increase the likelihood that the parasite will transfer to a mosquito or to a human?
The way in which a trophozoite transfers well to a mosquito is simple. When a mosquito sticks its proboscis into a person’s skin, the blood is immediately clotted by platelets, which must be prevented by spitting out a number of chemicals. However, a person with malaria is able to draw more blood when a mosquito bites them because the trophozoites interfere with the clotting process. Therefore, mosquitoes prefer people with malaria, and the trophozoites are transferred to them. The way it transfers to humans isn’t complicated either: the mosquito increases the number of times it bites a person. A mosquito infected with a trophozoite is prevented from producing a substance that prevents blood from clotting, which means it can suck less blood at a time. As a result, the mosquito wants blood more often and bites people more.
Remember the question I asked at the beginning? As you can see, hosts are manipulated by parasites in a variety of ways, but what about the higher animals, humans?
Toxoplasmosis is a life cycle that is originally completed by moving between cats and their prey. Humans are not included in this process, but they can become infected with toxoplasmosis by accidentally eating contaminated meat or through dirt. The number of people so infected is roughly one-third of the world’s population, and in Korea, about 5% of people are infected. They do not pose a significant threat to humans. Once inside the body, toxoplasmosomes begin to divide. Once inside the body, the dividing toxoplasma enter a harmless shell and stop all activity. Sometimes, when this shell is broken, they secrete a substance that actually boosts the body’s immune system. The toxoplasma outside the shell is then eliminated by the immune system, and only those inside the shell survive.
At this point, it would seem that the toxoplasma enclosed in their shells have no effect on us, and while they can be dangerous, there’s something else we’re interested in here. They can change our personalities. According to research by Czech scholar Plegr, men infected with toxoplasmosis become less law-abiding and fearful of punishment, while women become more assertive. This happens because the parasite weakens the host’s fear or alertness to danger, which is what it does to animals that are normally part of the same species to increase their risk of becoming prey to cats. In humans, the tiny toxoplasma results in changes in behavior and thought.
It’s easy to think of parasites as very inferior creatures that other organisms can’t live without, but contrary to what we think, they have evolved to manipulate their hosts, and even us, in insidious ways. Sometimes these processes are very sophisticated and happen in ways we haven’t yet fully understood. Research is actively underway to unravel this, and the mystery of how parasites manipulate their hosts will continue to be lifted.
