Rude Houseguests Have Nothing on These Parasitic Insects

Nature abounds with inventive (some might say gross) ways species barge in where they’re not welcome.

(Photo: Wim van Egmond/Getty Images)

May 23, 2014· 3 MIN READ
Richard Conniff is the author of House of Lost Worlds: Dinosaurs, Dynasties, and the Story of Life on Earth and other books.

They aren’t as sexy as vampires, but roughly 40 percent of species on Earth live on the flesh, blood, and brains of other species. They’re parasites, and in contrast to vampires’ monotonous penchant for giving their victims hollow eyes and prominent canines, parasites are highly imaginative in the ways they turn their hosts into instruments of their will.

The classic example is a parasitic fluke that infects a land snail, causing the snail to cough up slime balls, which deliver the fluke to an ant. But the ant is a way station en route to the fluke’s ultimate destination inside a sheep. So the fluke induces the ant to climb up on the tip of a blade of grass—the opposite of its normal instinct for self-preservation—and wait till a grazing sheep comes along to gobble it up along with the grass. The parasite then reaches its adult life as a fluke in the sheep’s liver. After it mates there, its eggs find their way back to other snails by way of the sheep’s droppings.

This is the strange circle of life, and since the discovery of that bizarre strategy in the 1950s, researchers have documented hundreds of other such parasite-host associations, in every animal phylum. For instance, a South American ant, normally black, sometimes develops a bulbous, bright red abdomen and then climbs up to assume the position, abdomen skyward, among the similarly colored fruit on a berry bush. The ant is mimicking a berry under orders from a nematode parasite, which is intent on achieving its destiny in the gut of a berry-eating bird.

Biologists are of course fascinated and regularly reveal intricate new parasite career plans. Thus a new study in Animal Behaviour describes a solitary fly in Virginia that ambushes a worker bumblebee as it forages among the flowers. Both fall to the ground. The fly (a conopid fly also known as “the thick-headed fly”) then uses can opener–like extrusions on its abdomen to shove apart the segments of the bee’s carapace and fire an egg into its abdomen. The bee soon recovers and goes on about its work gathering nectar for another week or two—while a tiny, wriggling maggot develops in its gut. Then, just before the maggot becomes a pupa, the thick-headed-fly-to-be somehow causes the bumblebee to dig its own grave and bury itself alive.

As is common with parasites, the fly larva may be triggering a mechanism that already exists in the host species, according to the study, by the University of Virginia’s Rosemary L. Malfi and her coauthors. Young bumblebee queens, having mated, normally dig an underground hibernaculum to wait out the winter, and they emerge again in the spring to start a nest. But worker bees aren’t supposed to dig, and they never get to emerge.

Instead, the fly pupa hibernates through the winter inside the shelter of its dead host’s corpse and emerges fatter and healthier than it might have if it had endured the aboveground hazards of extreme weather, dehydration, and predators—not to mention parasites that parasitize other parasites. The strategy is so effective that in one of the three bumblebee species Malfi looked at, 45 percent of individuals carried the parasite.

We’re unlikely to experience a vampire attack, but we humans are the playground of numerous parasites, including hookworms, roundworms, pinworms, and tapeworms. We have blood flukes, which, like sheep flukes, spend part of their lives in snails and—in the disease called schistosomiasis—cause severe damage in humans. By way of mosquito bites, we also sicken and sometimes die at the hands of the parasites malaria and filariasis (which can lead to elephantiasis). But most of these old friends can seem thankfully remote in the developed world.

The parasite Toxoplasma gondii, on the other hand, infects 1.1 million people in the United States every year, and they generally stay infected for life. According to the Centers for Disease Control and Prevention, about 60 million Americans now carry it; it disproportionately afflicts African Americans, Latinos, and the poor. The parasite develops in cats, and because there are millions of cats in this country, it’s all around us. It spreads to humans through cat feces, in litter boxes, sandboxes, and gardens, or from improperly cooked meat of infected livestock. Because it often occurs without symptoms, there’s a tendency to treat it as if it were trivial. But toxoplasmosis ranks as the second-leading cause of death and the fourth-leading cause of hospitalizations among food-borne illnesses. It can cause blindness or severe developmental defects in children.

Recent research also connects infection with a variety of psychiatric disorders, including schizophrenia, bipolar illness, and self-directed violence and suicide, as well as memory loss in the elderly. In one study, women infected with toxoplasmosis were 54 percent more likely to attempt suicide. All of this makes preventing the disease, by keeping cats indoors and other measures, an increasingly urgent issue in public health.

Beyond that, evidence suggests that the parasite manipulates the neurology of rodents to make them less fearful and more likely to be caught and eaten by cats. The chilling implication is that it has similar effects in the human brain. That is, parasites are manipulating the minds not just of ants or bumblebees or mice—but of people too.