The lancet liver fluke employs a remarkable strategy, seizing control of the minds of unsuspecting ants, compelling them to cling to blades of grass against their will. This intricate life cycle orchestrated by the liver fluke involves snails, ants, and unwitting grazing animals, making it a subject of intense research interest as scientists strive to unveil its precise mechanisms.
Imagine the surreal experience of an ant, its jaws clamped onto the swaying tip of a grass blade, with no recollection of how it arrived in this bizarre predicament. This is the reality for ants infected by the lancet liver fluke, a minuscule parasitic flatworm. The liver fluke’s life cycle is an intricately woven web that commences with the subjugation of the ant’s brain. The manipulated ant dutifully ascends the grass blade, locking its formidable mandibles onto the apex, increasing its chances of being consumed by herbivores like cattle and deer.
Researchers from the Department of Plant and Environmental Sciences at the University of Copenhagen have uncovered the parasite’s capacity for control to be even more ingenious than previously presumed. Astonishingly, the parasite can compel the ant to descend the grass blade when exposed to excessive heat. This manipulation serves the purpose of keeping the ants elevated in the grass during the cooler morning and evening hours, when grazing by cattle and deer is more likely, and then guiding them back down to avoid the scorching sun’s rays. As Associate Professor Brian Lund Fredensborg, who led the study alongside former graduate student Simone Nordstrand Gasque, now a PhD student at Wageningen University in the Netherlands, elucidates, “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.”
This intriguing research on the lancet liver fluke’s manipulation of ants has recently been published in the scientific journal Behavioral Ecology.
The researchers meticulously tagged hundreds of infected ants in the Bidstrup Forests near Roskilde, Denmark, using a technique that involved affixing colored markers and numbers to the ants’ rear segments. This method allowed them to monitor the ants over extended periods. Their observations revealed that temperature significantly influenced ant behavior. When temperatures were low, the ants were more likely to remain affixed to the grass blades’ tips. In contrast, as temperatures rose, the ants relinquished their grassy perches and descended. Brian Lund Fredensborg humorously remarks, “We found a clear correlation between temperature and ant behavior. We joked about having found the ants’ zombie switch.”
Once the liver fluke infects the ant, hundreds of parasites infiltrate the ant’s body, with only one making its way to the brain to exert control over its host’s behavior. The remaining liver flukes take refuge in the ant’s abdomen. Brian Lund Fredensborg explains, “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.” Animals infected with numerous liver flukes can suffer liver damage as these parasites navigate the host’s liver and bile ducts.
Brian Lund Fredensborg emphasizes that parasites that alter animal behavior are more influential in the food chain than commonly acknowledged. He suggests that parasites, which hijack their host’s behavior, play a pivotal role in shaping ecological dynamics. Shedding light on this often underestimated group of organisms, Fredensborg asserts, “Historically, parasites have never really been focused on that much, despite there being scientific sources that say that parasitism is the most widespread life form. This is in part due to the fact that parasites are quite difficult to study. 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.”
The lancet liver fluke is prevalent in Denmark and various temperate regions worldwide. Brian Lund Fredensborg and his colleagues remain dedicated to investigating this enigmatic parasite further, delving into the specifics of how it commandeers an ant’s brain. Fredensborg concludes, “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.”
[Box: Liver Fluke Life Cycle]
– Zombie ant: The liver fluke infects an ant’s brain, compelling it to cling to a blade of grass, where it becomes a target for the next host, such as cows, sheep, deer, or other grazers. Simultaneously, a large group of flukes lies dormant in the ant’s abdomen.
– The grazer: When a grazer consumes an infected ant, it also becomes infected with the liver fluke. The fluke controlling the ant’s brain dies in the grazer’s stomach acid, while the remaining flukes within the ant’s abdomen remain protected by a capsule that dissolves only in the host’s intestine. In this stage, liver flukes infiltrate the host’s liver, feed on its blood, and mature into adult flukes, laying eggs that are excreted in the host’s feces.
– The snail: Fluke eggs excreted in the feces wait for a passing snail to consume them. Inside the snail, the eggs develop into larval flukes, reproducing asexually and multiplying into thousands.
– The slime ball: To exit the snail and move on to its next host, larval flukes trigger the snail to cough, expelling them within a mucus mass. Attracted to this mucus, ants consume it and unwittingly ingest the fluke larvae. This step marks the beginning of a new cycle.]
Reference: “Expression of trematode-induced zombie-ant behavior is strongly associated with temperature” by Simone Nordstrand Gasque and Brian Lund Fredensborg, 24 August 2023, Behavioral Ecology. DOI: 10.1093/beheco/arad064