Some species of viruses, fungi, protozoans, wasps, and tapeworms take over the brains and actions of the animals and insects they infect — making them do whatever their new master wants.
A recent issue of the Journal of Experimental Biology was completely dedicated to these parasites, which turn their hosts into zombies.
This is usually controlled by special biochemistry. The parasites release chemicals or manipulate the genes of their host, changing their behaviors. Most them them do this by changing the brain's chemicals, or by inducing "sickness behaviors" by messing with the host's immune system.
"The fact that parasites can so efficiently alter host behaviour is fascinating," guest editor of the journal, Michael Dickinson, from the University of Washington, said in an article in the issue. "There is something horrifying and wondrous about a tiny 'implant' being able to control such a large animal machine."
Hymenoepimecis argyraphaga – These wasps live in the spider Plesiometa argyra. The wasp injects chemicals into the spider that make it abandon its web and build a completely different one, with a thick sheet to protect the wasp's offspring's cocoons, which they build after chewing threw the spider's body.
Baculovirus – This virus infects Gypsy moth caterpillars and releases an enzyme that stops their natural molting cycle, makes them eat continually, and forces them to climb to the top of trees and clamp onto the leaves until it dies and liquefies, raining down viruses across the forest to infect other caterpillars.
Toxoplasma gondii – Picked up from cat poop, this tiny (one-celled) parasite infects the brains of rats, causing them to make extra dopamine and testosterone, which makes them fearless. When they aren't afraid of cat smells, they venture out into the animals' presence, and get eaten, spreading the parasite to the cat. They also mate more, spreading the parasite to other rats.
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