This parasite uses a clone army to suck out the guts of its enemies

Haplorchis pumilio is far from a household name. But scientists have just discovered a diabolical strategy these microscopic parasites use against their enemies that’s never been seen before.

When these trematode flatworms invade a host—typically the Malaysian trumpet snail—they immediately set to work building a colony of clones, each genetically identical to the others. Over time, the flatworm floods its host’s body cavity with its copies, all the while eating the gastropod’s gonads, castrating it in the process.  

Competition from other parasites inside the host can be fierce. As many as 38 different kinds of trematodes have been known to inhabit these freshwater mollusks.

However, H. pumilio has a trick up its squiggly sleeve.

According to a new study published in the journal PNAS, the flatworms can create an army of highly aggressive warriors with exceptionally tiny bodies, massive mouths, and just one purpose—to locate other species of flatworms, latch onto their sides, and vacuum their guts out.

Because this special form of warrior worm has no reproductive material and cannot create copies of itself, scientists believe it’s the first known example of a dedicated soldier caste in flatworms—something more commonly seen in ants and termites.

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“Their mouthparts are five times larger than what is seen in any of the other worms in the colony, but their body size is only five percent [compared to the others],” says Dan Metz, a parasitologist at the University of Nebraska-Lincoln, and lead author of the study.

“So it’s really surprising. They’re basically just like mobile jaws,” he says.

Why should you care about a war between parasites inside of a snail? Well, human lives might depend on who wins that battle.

Can flatworms infect humans?

Trematodes cannot complete their life cycle without shuffling through multiple, larger organisms, or what scientists call “intermediate hosts.” For H. pumilio, the journey begins with a snail, but progresses to fishes before finally infecting birds and mammals.

While humans are not the preferred host of this flatworm, we can suffer consequences from it all the same. H. pumilio infection can cause abdominal pain, diarrhea, weight loss, and other health concerns.

People become infected with flatworms after eating aquatic plants, such as watercress, or ingesting the meat of animals that are themselves infected, such as mollusks, fish, or even mammals that have eaten those items, according to the World Health Organization.

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The Malaysian trumpet snail host is native to Asia, but the species was introduced to the United States in the 1960s, says Metz, likely through the aquarium trade. And H. pumilio came with it.

While infections are most common in East Asia and South America, foodborne trematode infections lead to around 200,000 illnesses and 7,000 deaths each year—though these are likely underestimates, due to limited reporting in the communities the trematodes hit hardest, says the W.H.O.

“Globally, they are a major concern,” Metz says.

How can we fight back?

While Metz’s hope is that learning more about these snails and trematodes could one day help us better neutralize them, the study’s findings are somewhat bad news on that front. This is because one method for combating trematodes is through bio-control, or using flatworms that can’t infect humans against the ones that can.

“If you can infect that snail that already has a human-infectious trematode with another trematode, then those two worm species will fight inside of that snail,” says Metz.

The enemy of my enemy is my friend, in other words.

The bad news? Armed with its caste of mini-super soldiers, it now appears that H. pumilio is the most formidable trematode on the block. And this limits our options for fighting back against it.

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Bio-control doesn’t work, says Metz, if you can’t find a species dominant to the one you want to suppress. 

Worse still, when Metz and his coauthor, Ryan Hechinger, submitted the H. pumilio soldiers to “attack trials” against other colonies of the same species, the soldiers seemed to stand down altogether. The scientists aren’t sure why this should be the case, since the same soldiers are hyper-aggressive when placed in proximity to other trematode species, but it suggests that separate colonies of H. pumilio won’t compete against each other inside the same host.

Even scarier, it could be that they actually work together.

This phenomenon is strikingly similar to how invasive Argentine ants can create super colonies composed of millions of queens and billions of workers—and it could hint at how H. pumilio has been able to become so successful.

While scientists have been trying to prove the existence of the trematode soldier caste for about 15 years, among “controversy and disagreement,” parasitologist Robert Poulin of the University of Otago in New Zealand says this new study offers “the clearest evidence to date” that a soldier caste exists, at least in H. pumilio.

“Natural selection has favored division of labor as the best way to maximize fitness in many colonial animals. To date, our best examples came from social insects like ants, bees and termites,” says Poulin in an email.

“This latest study makes it clear that division of labor has also independently evolved in trematodes,” he adds.