Alberta researchers discover how zombie parasite controls ant brains with on-off switch

A parasite that turns ants into zombies has a trick no other parasite in nature appears to use: it can switch its control on and off.

The discovery comes from researchers at the University of Calgary and University of Lethbridge studying Dicrocoelium dendriticum in Cypress Hills Provincial Park, in Alberta's southeast corner. The parasite infects ants and compels them to climb plants and cling to shrubs, waiting to be accidentally swallowed by a grazing mammal like elk, deer, or cattle. Once inside the final host's liver, the parasite completes its life cycle.

The problem: deer and elk actively avoid ants. So the parasite's solution is to send a single larva — the "brainworm" — next to the ant's brain to compel the behaviour. But clinging to plants exposes the ant to predators, prevents it from eating, and leaves it susceptible to dehydration. The ant would die before the grazing mammal ever ate it.

"The solution to that problem is to detach from a plant and go back to the nest. That's where this on-off switch comes from," says Cameron Goater, parasitologist and professor emeritus at the University of Lethbridge. The brainworm senses temperature: when it drops below approximately 19 C, the ant climbs and clings. When it warms, the ant returns to normal behaviour and survives until the next cold snap.

Most parasite-induced host death results in the host dying — think of the Cordyceps fungus from "The Last of Us", where infected ants climb plants and die so the fungus can burst out and spread. Or the horsehair worm that drives crickets to jump into water so the worm can emerge and mate. Dicrocoelium is different: its complex life cycle requires keeping the ant alive as long as possible. That biological incentive created a mechanism researchers say keeps them awake at night — because the on-off switch itself is something they do not yet fully understand.