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 your 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, what is a parasite? According to the dictionary, a parasite is “an invertebrate animal that lives in the body of another organism, feeding on its body fluids and excretions.” Tapeworms, roundworms, and pinworms, which we are familiar with, are 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, it is believed that they 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 malaria parasite enters the person’s body. There, it divides and produces reproductive cells. During this process, the cells dissolve and cause malaria. When mosquitoes suck up these cells, they combine in the digestive tract and produce new malaria parasites. In other words, the life cycle of the malaria parasite is completed by moving between humans and mosquitoes.
In order to complete this life cycle, parasites learn how to manipulate their hosts. Let’s look at an example of how parasites manipulate 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 parasites increase their chances of being transferred to mosquitoes or humans?
The method by which malaria parasites are easily transmitted to mosquitoes is simple. It is to make people infected with malaria attractive to mosquitoes. When a mosquito bites a person, the blood platelets immediately coagulate, so the mosquito must secrete various chemicals to prevent this. However, people infected with malaria have malaria parasites that interfere with coagulation, 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 toxoplasma weakens the host’s fear and awareness of danger. Weakening fear and awareness is a natural behavior in animals, which increases their chances of survival by reducing the risk of being eaten by predators. In humans, the tiny Toxoplasma gondii parasite can change behavior and thinking.
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 this mystery, and the process by which parasites manipulate their hosts will gradually be revealed.
