January 21, 2020
Written by: Carolyn Keating
Parasite, a movie from director Bong Joon-ho, is getting a lot of buzz this awards season. The film follows the poor Kim family as they worm their way into the lives of much wealthier Park’s. This parasitic relationship was contrived using cunning and gall to psychologically manipulate the Park’s into supporting more and more members of the Kim family. But can one creature really control the mind of another?
Frighteningly, the answer is yes. Some parasites—organisms that live on or in a different species, feeding on it and causing harm—actually manipulate the brain and nervous system of their host in order to change their behavior to benefit the parasite. Many examples can be found in the insect world (Figure 1). For instance, some parasites use their host to guard their offspring1. A type of parasitic wasp inserts an egg into a ladybug, where the larva begins to develop. This larva contains a virus, which it transmits to the ladybug just before emerging from the ladybug’s body and spinning a cocoon between its legs. The virus replicates in the ladybug’s nervous system, and the resulting damage causes the ladybug to twitch its body. These movements protect the wasp pupa from predators.
Other parasites seem to be able to alter the decision-making ability of their host. The parasitic jewel wasp turns cockroaches into “zombies” to provide food for their offspring1. When the wasp stings the cockroach, it specifically targets regions of the roach’s “brain” involved in regulating walking. The venom from the sting seems to inhibit neuronal activity in these regions, resulting in a long-term inability of the cockroach to initiate walking. The roach is still mechanically able to move, as studies have shown that other behaviors like righting, flying, and grooming aren’t affected, and spontaneous walking can be induced by submerging the cockroach in water. Rather, the “motivation” of cockroaches to walk appears to be specifically inhibited. With the cockroaches thus disabled, the jewel wasp lays an egg on the bug that when hatched, will feed on the docile roach.
Another reason for parasites to manipulate host behavior is to get themselves into a more favorable environment. Some parasites can only reproduce in certain organisms, but are able to survive in other “intermediate hosts.” In order to make their way into their final host, the parasite uses the intermediate host to help it out. Take the lancet liver fluke for example1. This worm-like parasite lives in the liver of grazing animals like cows or sheep. It produces eggs which make their way through the grazer’s digestive system and end up expelled in the animal’s droppings. Snails feed on these egg-infested droppings and become infected themselves, eventually expelling the fluke larvae in slime balls. Ants then eat the slime balls, and here is where the story gets interesting. The flukes make their way into the ant’s hemolymph (the insect equivalent of blood) and drift inside its body, until one parasite makes its way to the head. It settles next to the equivalent of the ant’s brain, and presumably releases yet unknown chemicals that take over the ant’s navigational abilities. This causes the infected ant to leave its colony in the evening and climb to the top of a blade of grass, where it clamps its jaws down and waits to be eaten by a grazing animal, allowing the fluke to continue its life cycle. Amazingly, if the ant is not eaten, it will return to the ground the next day and behave normally until the following evening, when it returns to the top of a blade of grass. (Ants seem to have it rough; watch a parasitic fungus similarly hijack the insect’s navigation control.)
Incredibly, this suicidal behavior is also seen in a parasite that infects mammals: Toxoplasma gondii. Like the lancet liver fluke, this single-celled organism can only reproduce in a particular creature (the gut of a cat in this case), but is able to infect other warm-blooded animals like birds, rodents, and even humans. When these intermediate hosts ingest food or water contaminated with the parasite (either eggs from cat droppings or tissue cysts from another intermediate host), they become infected. At first the parasite replicates very quickly and spreads throughout the host, initiating a rapid immune response. But in certain parts of the body—cardiac/skeletal muscles and especially brain in humans and rodents—these fast-dividing cells convert into a slowly-replicating form that make cysts. These cysts evade detection from the immune system and can persist for the life of the host2.
What happens to infected humans? In people with intact and well-developed immune responses, most T. gondii infections are asymptomatic or produce a mild flu-like illness at the time of infection2. But in individuals with compromised immune systems, chronic infection can result in reduced motor functions, memory, and coordination. There may also be correlations with infection and mental health disorders such as depression and schizophrenia3. However, the jury is still out on whether the parasite causes behavioral changes in healthy people.
But in rodents the story is very different3. Remember, the parasite’s final host is the cat. When rats and mice become infected with T. gondii for a long period of time, they eventually lose their fear of cats. How does the parasite cause this behavior? No one mechanism has been nailed down, but there are a few ideas. T. gondii targets neurons, so some scientists believe that the specific neurons or brain regions infected with cysts cause the behavior changes (however, a study in mice using a form of T. gondii that doesn’t form cysts still displayed altered behavior). There is evidence that the parasite can alter levels of neurotransmitters, the compounds neurons use to communication with each other. More globally, T. gondii can impact systemic hormone levels. It’s also possible that inflammation can alter neural circuits. A new study from Switzerland recently showed that the level of brain inflammation, indirectly reflected by the amount of cysts, is related to the severity of behavioral alterations, which included loss of fear of other predators besides just cats4. All of these mechanisms could disrupt the neurons or circuits responsible for controlling specific behaviors. But whatever the reason, the result is clear: rodents have reduced anxiety and decreased avoidance of urine from their predators.
From causing rats to lose their feat of cats, to co-opting an insect to provide food and shelter for its offspring, it’s clear parasites have developed a wide range of ways to manipulate the brains of their host for their own gain. The more we can learn about how these parasites are able to exert their mind control, the more we’ll learn about how our own brains produce behaviors and make decisions.
- Libersat, F., Kaiser, M. & Emanuel, S. Mind control: How parasites manipulate cognitive functions in their insect hosts. Front. Psychol. 9, 1–6 (2018).
- Kochanowsky, J. A. & Koshy, A. A. Toxoplasma gondii. Curr. Biol. 28, R770–R771 (2018).
- Tedford, E. & McConkey, G. Neurophysiological changes induced by chronic Toxoplasma gondii infection. Pathogens 6, (2017).
- Boillat, M. et al. Neuroinflammation-Associated Aspecific Manipulation of Mouse Predator Fear by Toxoplasma gondii. Cell Rep. 30, 320–334 (2020).
Cover image by Gerd Altmann via Pixabay https://pixabay.com/illustrations/design-face-dialogue-talk-psyche-2808312/
Figure 1 reproduced from Libersat, F., Kaiser, M. & Emanuel, S. Mind control: How parasites manipulate cognitive functions in their insect hosts. Front. Psychol. 9, 1–6 (2018). CC BY 4.0.