Mind-Control Parasites
Pathogens that can control the behavior of their hosts
∘ What Kind of Parasites Manipulate Their Host?
∘ Parasite-Induced Behavioral Changes
∘ Are There Parasites That Can Manipulate Humans?
One of my favorite video games when I was younger, was Resident Evil 4. It presents a story about a special agent named Leon Kennedy, who looked to rescue the US president’s daughter from a sect that wanted to infect her with mind-control parasites called “las plagas”.
One thing that I most enjoyed about this video game was that players could find research notes from a biologist (Luis Sera) who compared “las plagas” with real-life parasites.
At that time, I realized that Capcom took care to include some aspects of real pathogens to create their fictional parasites.
While there are no known parasites that can cause zombie mutations in the human body, the idea that a parasite can modify its host’s behavior is now a well-established concept in the study of animal behavior science.
What Kind of Parasites Manipulate Their Host?
Host manipulation by parasites is the ability of certain parasites to modify host behavior, appearance, or physiology to their own advantage [1].
Parasites often exhibit complex life cycles that imply infecting different species at different times.
In the context of altered host behavior, infected hosts are manipulated in ways that facilitate the transmission of their parasites and allow them to complete their life cycles [2].
The two classical textbook examples of host manipulation by parasites are the two trematode species, Dicrocoelium dendriticum, and Leucochloridium spp.
Dicrocoelium dendriticum must be transmitted by accidental ingestion from an ant to a ruminant; it causes infected ants to climb to the tip of grass blades and stay there patiently waiting where they are most vulnerable to being eaten.
Leucochloridium spp. on the other hand, needs to go from a snail to a bird to complete its life cycle. It alters the size, shape, and coloration of the tentacles of its snail and causes them to pulsate violently in response to light (check out this video).
The infected snail makes its way to the top of a plant where it is more visible to the eyes of birds. The coloration and the pulsation of tentacles presumably simulate a potential caterpillar prey that attracts birds.
Host manipulation by parasites has been documented in a few hundred distinct host-parasite associations spanning all major phyla of living organisms [3].
At least some species are capable of host manipulation in the animal phyla Platyhelminthes (classes Trematoda and Cestoda), Acanthocephala, Nematoda, Nematomorpha, and Arthropoda, as well as in the viruses, bacteria, fungi, and protozoans [3].
Parasite-Induced Behavioral Changes
Parasite transmission routes and associated induced changes in hosts’ behavior can be classified in one of the following categories [2].
- Trophic transmission
Some species of parasites must be transmitted from one host to another by predation.
Manipulation consists of altering the appearance or behavior of its host to make it more susceptible to its predators.
For example, worms of the genus Galactosomum (platyhelminth) mainly infect aquatic birds, but some species need to infect a fish as an intermediate host. These parasites settle inside the fish’s brain and cause it to swim closer to the surface to make it more visible to predators.
2. Direct Physical Transport
Some parasites need to release themselves into a habitat other than the one in which the host lives.
Host manipulation involves inducing the host to move to a different habitat, often one that is completely unsuitable for it.
For example, several nematomorpha worm species that infect crickets, require an aquatic habitat to complete their life cycle. They manipulate the brains of crickets to induce their host to seek water and jump into it.
This category also includes several parasitic fungi species that infect insects. Parasitic fungi force their host to settle on the underside of leaves where conditions are best for wind-assisted dispersal of fungal spores.
3. Vector-borne transmission
Some parasites need a vector to be transmitted.
The best-known examples in this category are blood-sucked insect-borne pathogens. They are picked up by the vector (like mosquitoes) during a blood meal and then injected into a new host during a subsequent blood meal.
The parasites manipulate their vectors to shorten the duration of individual blood meals in order to facilitate transmission.
Known parasites that can induce vector behavioral changes include trypanosomes (protozoans mainly known to cause diseases such as African sleeping sickness and Chagas disease), the rabies virus, and Plasmodium spp. (protozoans that cause malaria).
A fun fact is that the common influenza virus could fall into this category because it has been shown to increase in-person sociability during the 48-hour incubation period [4].
4. Host’s Nursery Service
Some parasitic insects (called parasitoids) lay their eggs on a host that will serve as food and shelter until the larvae grow and can fend for themselves.
In this case, manipulation alters the behavior of the host in ways that will provide protection to the parasite from predators or other dangers during its growth.
For example, some wasp larvae species that infect spiders cause their host to start building a strange new pouch-like structure attached to its web. This pouch serves to protect the wasp larva as it pupates after the spider’s death.
Are There Parasites That Can Manipulate Humans?
The protozoan parasite Toxoplasma gondii must be transmitted from a rat to a cat. It reverses the innate aversion of its host to cat odor into an attraction.
Humans can also become infected instead of rats. Although this is a dead-end for parasite transmission, Toxoplasma gondii can induce neurochemical changes in infected humans.
People with latent Toxoplasma gondii infections show personality traits and reaction times that differ from those of uninfected controls.
Infected humans with Toxoplasma gondii have a 2.65 times higher risk of getting into a traffic accident [5].
Infection with the parasite also has been associated with a number of neurological disorders such as schizophrenia and bipolar disorder [6].
Finally, Toxoplasma gondii can also affect the development of human babies during pregnancy [7].
So, if you are a cat lover, don’t forget to take your cats to the vet and deworm them regularly [8].
As you can see, the term “mind control” does not apply to all of these parasites at all.
There is currently a branch of science called Neuroparasitology, which is specifically focused on studying more about how pathogens can control the nervous system of their hosts.
If you can learn more about host-manipulation by parasites, check out these articles [1,2,9,10,11].
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