Parasite Controls Ants' Minds to Turn Them into 'Zombies'

In something straight out of HBO's hit zombie show The Last of Us, a parasite has been found to be able to control the minds of ants, forcing them to do its bidding.

The parasites, called lancet liver flukes, are microscopic organisms that infect ants in the first stage of their complicated life cycle, taking over the ant's brain in order to infect their next hosts.

They do this by forcing the ants to climb up blades of grass in order to get the ants eaten by grazing animals like cows and deer. New research in the journal Behavioral Ecology reveals that the climbing is induced by the parasite specifically during dawn and dusk to avoid the heat of the midday sun.

"We found a clear correlation between temperature and ant behavior. We joked about having found the ants' zombie switch," Brian Lund Fredensborg, co-author of the paper and associate professor of organismal biology at the University of Copenhagen, said in a statement.

ant on grass
A parasite has been found to be able to control the minds of ants, forcing them to do its bidding. The ants climbs up and clamp their powerful jaws onto the top of a blade of grass, making it more likely to be eaten by grazers such as cattle and deer, which then become infected. University of Copenhagen

"The successful completion of the life cycle requires that the grazing mammal eats infected ants," he told Newsweek. "Forcing the ants to the tips of grass blades increases the [otherwise very small] chance that the ant is eaten by the deer.

"The point of making the ant climb the grass at dawn and dusk is to synchronize zombie ant behavior with the grazing activity of the deer who primarily eat at dawn and dusk. It makes really good sense that the ant is 'released' during the midday hours where the chance of getting eaten is less and where the damaging effects of sunrays may kill the infected ant sitting on the grass and thereby also killing the parasite itself."

The lancet liver fluke, or Dicrocoelium dendriticum, is a tiny parasitic worm that has a complex life cycle, passing between several hosts to reproduce. The flukes first infect ants, invading their bodies, with a single fluke making its way into the ant's brain.

The fluke then makes the ant crawl up high in grassy areas, causing it to be eaten by grazers. Next, the flukes that were inside the ant find their way to the liver of the grazer, developing into their full adult forms and laying eggs. These eggs are then excreted in the feces of the grazer.

CDC graphic of the life cycle of Dicrocoelium dendriticum. CDC

Occasionally, humans can be the host of the flukes at this stage of the cycle, and the parasites may infect our liver, leading in some cases to cholecystitis or liver abscesses, according to the U.S. Centers for Disease Control and Prevention.

After the eggs are excreted, they are consumed by snails, within which the eggs hatch and develop into larvae, which reproduce asexually. Then, the snails excrete the larvae in a ball of slime, which is fed upon by ants, starting the cycle again.

The new research indicates that the fluke's mind control is much more sophisticated than first assumed. The researchers made the discovery after observing infected ants in Denmark, watching how they behaved and recording the light, humidity, time of day and temperature. They found that when it was cooler, the ants were more likely to climb up the grass, crawling back down when it got hotter.

"One of the aims of the study was to test the effect of various factors on when the ants changed behavior including time of day. While temperature demonstrated a very clear effect on ant behavior there was no evidence that the time of day itself had an effect," Fredensborg said.

"Since temperature usually is low from evening until the following morning, we believe that temperature is a predictable indicator for when grazing mammals are active, and temperature is likely a simpler indicator to these ectotherms than the time of day."

This indicates that the flukes specifically make the ants avoid the hot midday sun, possibly to avoid the ant or the parasite dying before it can be transferred to the next host. The findings reveal a new and more sophisticated facet of mind-controlling parasitism, the authors say.

tagged ant
Researchers tagged several hundred infected ants in the Bidstrup Forests near Roskilde, Denmark. University of Copenhagen

The researchers hope to further study this strange parasite, and find out exactly how it controls the minds of the ants.

"[It] has long been a mystery just how the fluke controls the ant host behavior," Fredensborg said. "In our study we found that temperature is the trigger that switches the altered behavior on or off [crawling up vegetation]. However, we still need to pin down what the exact mechanism is. It could be that the one parasite that migrates to the brain of the ant stimulates increased levels of neuromodulators [serotonin and dopamine] in a temperature-dependent manner, but that needs to be verified."

Luckily for us, while these flukes can infect humans, they cannot take over our brains like they do the ants.

"Humans are not part of this parasite's usual life cycle, and when it [rarely] happens, it would be in the place of the mammalian host where no behavioral changes occur," Fredensborg said.

"Behavioral changes of intermediate hosts to make them more likely to get eaten by the final host are relatively common in parasites with complex life cycles. However, humans almost always take the role as final host where no parasite control of host behavior takes place."

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