This parasite that turns ants into zombies

Dissected ant and where you can see the encapsulated parasites (white oval structures) spilling out of the hind body. Photo: Brian Lund Fredensborg CREDIT Photo: Brian Lund Fredensborg
Dissected ant and where you can see the encapsulated parasites (white oval structures) spilling out of the hind body. Photo: Brian Lund Fredensborg CREDIT Photo: Brian Lund Fredensborg
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Meet the parasite that turns ants into zombies. Lancet liver flukes are parasitic flatworms that have a gruesome strategy for survival. After infecting ants, they manipulate their nervous systems, turning the unsuspecting insects into literal zombies. These zombified ants are coerced into climbing plants and clamping onto leaves, where they eventually become easy prey for the flukes’ next host, typically a grazing mammal. It’s a fascinating, albeit chilling example of nature’s intricate web of survival tactics.

Lancet liver flukes infect a variety of animals, including humans. They are found all over the world, but they are most common in tropical and subtropical regions. They have a complex life cycle that involves three different hosts: snails, ants, and mammals. The flukes start out as eggs in the feces of infected mammals (cows, sheep, deer). The eggs are eaten by snails, where they hatch into larvae. The larvae then develop into a stage called sporocysts, which produce another type of larva called cercariae.

Here is when the zombification starts. The cercariae leave the snail and attach themselves to ants. The ants then climb up to the tips of blades of grass and wait to be eaten by a mammal. When a mammal eats an infected ant, the cercariae travel to the liver and develop into mature flukes.

The mature flukes can live in the liver for many years, causing damage and reducing liver function. In severe cases, lancet liver fluke infection can lead to death.

A new study published in the journal Behavioral Ecology has found that lancet liver flukes may be using air temperature to their advantage. The study found that the flukes were more likely to infect ants on warm days.

The researchers believe that the flukes are using air temperature to cue their behavior. On warm days, the flukes are more likely to leave the snails and attach themselves to ants. This is because the flukes know that the ants are more likely to be active and foraging for food on warm days.

Not only this, the parasite can get the ant to crawl back down the blade of grass when it gets too hot. The scientists observed that when it was cooler the ants were more likely to be attached to the top of a blade of grass. During hotter temperature they let go of the grass and crawled back down.

“We found a clear correlation between temperature and ant behavior,” said Fredensborg. “We joked about having found the ants’ zombie switch,’”

“Getting the ants high up in the grass for when cattle or deer graze during the cool morning and evening hours, and then down again to avoid the sun’s deadly rays, is quite smart. Our discovery reveals a parasite that is more sophisticated than we originally believed it to be,” University of Copenhagen biologist and study co-author Brian Lund Fredensborg said in a statement.

The team of scientists tagged hundreds of infected ants in the Bidstrup Forests near Roskilde, Denmark in the study. “It took some dexterity to glue colors and numbers onto the rear segments of the ants, but it allowed us to keep track of them for longer periods of time,” said Fredensborg.

The findings of this study suggest that lancet liver flukes are adapting to their environment in order to increase their chances of infection. This information could be used to develop new strategies for controlling lancet liver fluke infection.

After the liver fluke has infected the ant hundreds of parasite will invade the ant’s body but only one of these will make it to the brain. Once there, it then influences the ant’s behaviour, while the remaining liver flukes conceal themselves in the ant’s intestine. From the intestine they make their way to the bile ducts and liver, where they feed on blood and grow up into adult flukes that can lay eggs. The eggs are excreted out via faeces.

“Here, there can be hundreds of liver flukes waiting for the ant to get them into their next host. They are wrapped in a capsule which protects them from the consequent host’s stomach acid, while the liver fluke that took control of the ant, dies. You could say that it sacrifices itself for the others,” said Fredensborg.

After the lancet liver fluke eggs are excreted, they can remain on the ground until a snail eats them. Inside the snail, the eggs hatch into larvae, which multiply asexually. Thousands of larvae can develop in a single snail, which are then expelled via mucus that comes out as the snail is made to cough. Ants are attracted to this ball of mucous and eat it. And so begins the cycle once again.

“Historically, parasites have never really been focused on that much, despite there being scientific sources which say that parasitism is the most widespread life form,” said Fredensborg. “This is in part due to the fact that parasites are quite difficult to study.”

“We now know that temperature determines when the parasite will take over an ant’s brain. But we still need to figure out which cocktail of chemical substances the parasite uses to turn ants into zombies,” Fredensborg said. “Nevertheless, the hidden world of parasites forms a significant part of biodiversity, and by changing the host’s behavior, they can help determine who eats what in nature. That’s why they’re important for us to understand.”

Halloween fun fact

Did you know that lancet liver flukes are sometimes called “Halloween parasites”? This is because the flukes are most active during the fall months, when the weather is cooler.

Read the full study here.

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I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.

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