How parasites manipulate the behavior of their hosts

How parasites manipulate the behavior of their hosts

Laith Al-Shawaf writes:

What if some outside force could control your mind and make you act against your own interests? It’s a terrifying prospect—one that captures our imagination and recurs frequently in our fiction. It’s the goal of one of the three Unforgivable Curses in Harry Potter. It’s the purpose of Newspeak, the fictional language in George Orwell’s 1984. It enthralls in classics such as Brave New World and The Manchurian Candidate. In the 1950s, the CIA was so concerned that the communists had developed mind-control techniques that they launched their own secret program called MK-ULTRA, whose purpose was to use hallucinogenic drugs and biological manipulation to achieve mind control in ways that could be used against the United States’ enemies. It didn’t work—but mind control is real, and it can be observed in nature. Parasites do it all the time.

Parasites are organisms that live in or on other creatures, feeding on their hosts and taking their resources. They infect animals and pull their behavioral strings, manipulating them like marionettes. Such parasites are “essentially neuroengineers,” as one recent paper puts it, “capable of controlling the central nervous systems of the hosts they infect.” Their tactics are astonishing.

Consider the fluke parasite Dicrocoelium dendriticum. It masterminds a cycle that starts in the liver of a hoofed animal—a cow or sheep. First, it lays eggs that end up in the animal’s digestive system. When the eggs are pooped out, a snail feeds on them. The parasite’s eggs hatch inside the snail’s intestines. Once the larvae are able, they drill through the snail’s gut into its digestive tract, where they become juveniles. The parasites exit when the snail excretes a ball of slime.

Ants then swallow the slime balls loaded with flukes. The parasite now needs its ant host to be eaten by a hoofed animal to restart the cycle—but this is a problem because hoofed animals eat grass, not ants. The parasite’s solution? Make the ant climb to the tip of a blade of grass and immobilize it there—in the exact location where it has the highest likelihood of being eaten by a grazing cow or sheep. But being in full view of the midday sun might scorch and kill the ant before it has a chance to get eaten. To solve this problem, the parasite makes the ant retreat down the blade of grass when it gets too hot. When the weather cools, D. dendriticum drives the ant back to the top after the risk of death by scorching has passed. [Continue reading…]

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