Zombie roaches and other parasite tales | Ed Yong (re-release)

Parasites can do more than feed on their hosts: fungi, viruses, worms, and insects can alter or override host behavior. Ed Yong says accounts of why animals group together often assume the animals themselves control their actions, even though parasites may be driving some of those behaviors. Scientists have identified hundreds of examples of manipulation, making parasites a significant force in how animals interact with their environments and with one another.

The Emerald Cockroach Wasp demonstrates how precisely a parasite can act. Sensory bumps on its stinger help it locate a cockroach’s brain, and research by Frederick Libersat and Ram Gal indicates that its venom targets two clusters of neurons. The venom removes the cockroach’s motivation to walk without killing or sedating it. The wasp can then lead the cockroach by an antenna to its lair and lay an egg on it; the developing larva feeds on the host. In another example, parasitic wasp larvae eat caterpillars from within, while some wasps remain in cocoons and control the caterpillar to make it defend them.

Other parasites redirect hosts into situations that complete the parasite’s life cycle. Gordian worms can cause crickets and grasshoppers to jump into streams and drown, allowing adult worms to emerge and mate. In one stream, drowned insects accounted for about 60% of the local trout’s diet. Tapeworm-infected brine shrimp, meanwhile, turn bright red, live longer, become castrated, and swim in conspicuous groups. These changes make the shrimp easier for flamingos to eat, where the tapeworm reproduces. In three California estuaries, trematodes collectively weighed as much as all the fish and three to nine times as much as all the birds, illustrating the scale of parasites’ presence in ecosystems.

Toxoplasma gondii raises the question of whether parasite manipulation extends to people. The parasite reproduces sexually only in cats, and infection can make rats and mice run toward the smell of cat urine, increasing the likelihood of reaching a host in which it can reproduce. Researchers know that T. gondii releases an enzyme that makes dopamine and targets parts of the rodent brain, but they do not know exactly how those effects produce the behavior. Ed Yong also describes a wasp leading a cockroach around like a dog on a leash, another vivid instance of a host’s actions being redirected.

Some estimates suggest that T. gondii infects the brains of one in three people worldwide. Infection typically causes no overt illness and can remain dormant for a long time, but proposed links to human personality differences, car accidents, and schizophrenia remain inconclusive. Researchers disagree over whether the parasite affects human behavior at all. Yong argues that it is implausible that humans alone are unaffected by manipulation so widespread in other species, but that argument is not proof of a human behavioral effect. The episode leaves the human question open while showing how clearly parasites can alter behavior in other animals.