Zombie roaches and other parasite tales | Ed Yong

Ed Yong’s central point is that parasites can alter their hosts’ behavior in ways that serve the parasite, and that this is a widespread, consequential feature of nature rather than an oddity. The emerald cockroach wasp illustrates the precision involved: sensory bumps on its stinger help it locate a cockroach’s brain, where it injects venom into two clusters of neurons. Research by Frederick Libersat and Ram Gal found that the venom does not kill or sedate the cockroach; it removes its motivation to walk. The wasp can then lead it by the antennae, lay an egg on it, and leave the immobilized host as food for its offspring.

Other parasites change both the appearance and behavior of their hosts to make transmission more likely. Tapeworm-infected brine shrimp can turn red, live longer, become castrated, and gather in conspicuous swarms. The tapeworm needs a flamingo to reproduce, and these changes make the shrimp more likely to be seen and eaten by one. Gordian worms take a different route: they release proteins that alter a cricket’s behavior, driving it to jump into water and drown, where the worm can mate.

Such manipulation can reshape food webs, not just the fate of an individual host. In a stream studied by Takuya Sato, crickets and grasshoppers driven into the water by Gordian worms accounted for about 60% of local trout diets. In three California estuaries, trematodes surveyed by Kevin Lafferty and colleagues collectively weighed as much as all the fish and three to nine times as much as all the birds. These findings show how parasites can redirect energy through ecosystems and affect animals that are not themselves the parasite’s immediate host.

Some parasitic wasps manipulate hosts that are still alive after the parasites have fed. Wasp larvae eat a caterpillar from within, but some larvae remain behind and control it; the caterpillar then defends the wasp larvae’s siblings while they develop in cocoons. Yong used examples such as these to argue that behavioral manipulation by fungi, viruses, worms, and insects is common and important in nature, even though the particular mechanisms and outcomes differ.

Toxoplasma gondii offers a case where the behavioral effect is better understood in rodents than in people. The parasite can infect many mammals but reproduces sexually only in cats. Research by Joanne Webster and others found that infected rats and mice may move toward the smell of cats, potentially making them more likely to be eaten and allowing the parasite to reach its reproductive host. The precise mechanism is unresolved: Toxoplasma releases an enzyme that produces dopamine and targets parts of rodents’ brains, but researchers do not yet know how those effects combine to produce the behavior.

Some estimates suggest that one in three people worldwide carry Toxoplasma in their brains, where it usually remains dormant without overt illness. Studies have associated infection with differences in personality-questionnaire scores, a slightly higher risk of car accidents, and a greater likelihood of schizophrenia. Those associations do not establish that the parasite changes human behavior, however; the evidence remains inconclusive, and researchers disagree about whether such effects occur.