In this blog post, we take an interesting look at how parasites can change the behavior and even the personality of their hosts through various examples and scientific research.
Every day, we make many choices and take many actions. But are these choices entirely our own? Most people would probably answer “yes.” However, in reality, our thoughts and actions may be influenced by something else. That something else may even be alive, and it may be something you find very disgusting. It is an unwelcome guest hiding inside your body: a parasite.
First of all, what are parasites? According to the dictionary, parasites are “invertebrates that live in the body of other organisms, feeding on them and relying on them for their environment.” Tapeworms, roundworms, and pinworms are common examples of parasites, and familiar diseases such as sleeping sickness and malaria are also caused by parasites. Looking at the body structure of parasites, most parts are related to reproduction. This is advantageous for them to leave offspring. In addition, they are believed to have evolved into a very simple structure in order to live in the host’s body, and have degenerated into a very primitive form.
But how can such simple creatures control our minds? As mentioned earlier, parasites are organisms that live by parasitizing their hosts. This means that they cannot survive without other organisms. In fact, parasites spend their entire lives inside their hosts. This life cycle does not occur in a single host species, but rather in multiple hosts. Take the malaria-causing Plasmodium parasite as an example. When a mosquito infected with malaria bites a person, the Plasmodium parasite enters the person’s body. There, it divides and creates reproductive cells. During this process, the cells dissolve and cause malaria. When mosquitoes suck them up, they combine in the digestive tract and create new malaria parasites. In other words, the life cycle of malaria parasites is completed by moving between humans and mosquitoes.
In order to complete this life cycle, parasites learn how to control their hosts. Let’s look at an example of how parasites control their hosts. In the case of the malaria parasite mentioned earlier, it must move between humans and mosquitoes to complete its life cycle. How can the parasite increase its chances of being transferred to mosquitoes or humans?
The method by which malaria parasites are easily transferred to mosquitoes is simple. It makes people infected with malaria attractive to mosquitoes. When a mosquito bites a person, its blood clots immediately due to platelets, so it must secrete various chemicals to prevent this. However, people infected with malaria have their blood clotting prevented by the malaria parasite, allowing mosquitoes to suck more blood when they bite. Therefore, mosquitoes prefer people with malaria, and the malaria parasite is transmitted to mosquitoes. The method of transmission to humans is also not complicated. It is to increase the number of times mosquitoes bite people. Mosquitoes infected with the malaria parasite are prevented from producing substances that prevent blood from coagulating. This means that the amount of blood they can suck at one time is reduced. Eventually, mosquitoes want blood more often and bite people more.
Do you remember the question I asked at the beginning? As you have seen, hosts are manipulated by parasites in various ways. So, what about higher animals such as humans?
Toxoplasma gondii originally moves between cats and their prey to complete its life cycle. Humans are not involved in this process, but they can become infected with toxoplasma by accidentally eating contaminated meat or through soil dust. Approximately one-third of the world’s population is infected, and in Korea, about 5% of the population is estimated to be infected. These parasites do not pose a significant threat to humans. Once inside the body, toxoplasma begins to divide. The divided toxoplasma enter a shell that is harmless to the human body and cease all activity. In rare cases, when this shell breaks, they secrete a substance that actually strengthens the human immune system. When this happens, the toxoplasma outside the shell are eliminated by the immune system, and only those inside the shell survive.
At this point, it seems that the toxoplasma surrounded by the shell has no effect on us. Of course, they can be dangerous, but what we want to focus on here is something else. It is the fact that they can change our personality. According to research by Czech scholar Plegr, men infected with toxoplasma tend to have a weaker sense of law-abidingness and fear of punishment, while women tend to become more assertive. This happens because it weakens the host’s fear and awareness of danger. Weakening fear and awareness is a natural behavior in animals, which increases their chances of surviving in the wild. In humans, the tiny Toxoplasma gondii can change our behavior and thoughts.
Parasites are often considered inferior organisms that cannot survive without other organisms. However, contrary to our beliefs, they have evolved in clever ways to manipulate their hosts, even us. Sometimes, this process is so sophisticated that we have not yet fully understood it. Research is actively underway to uncover these secrets, and the process by which parasites manipulate their hosts will gradually be revealed.
