When the 78 year old woman arrived at the hospital, it was clear something was wrong. She’d been suffering from headaches and been in a drowsy fog for weeks. So doctors checked her cerebral spinal fluid, and found it was cloudy and yellow instead of clear. It was brimming with white blood cells, indicating an infection. This, alongside a positive antibody test, led to a diagnosis of angiostrongyliasis—an infestation of the parasite rat lungworm (Angiostrongylus cantonensis).
As the name implies, rat lungworms usually set up shop in rat pulmonary arteries. But their complex lifecycle involves spending time in an intermediate host like a snail before making a home inside a rodent. If we happen to eat that infected host before a rat does, they can end up inside us instead, where they can get lost and cause serious infections in other places. The parasites are most dangerous when they make their way into the brain, causing swelling and inflammation—like the woman was experiencing. But the woman hadn’t been eating snails.
A few weeks later, her 46 year old son was also admitted, and he, too, was diagnosed with angiostrongyliasis. Both were cured after a few weeks on an antiparasitic and steroid. But the question remained as to how they became infected—he wasn’t eating snails, either. But, it turned out, both he and his mother had been consuming raw centipedes from a local market. And that led the doctors to question: could centipedes also carry rat lungworm?The answer, they discovered, is yes.
There’s no reason to initially suspect centipedes would host rat lungworm. The parasites use mollusks as intermediate hosts, and they develop from their first larval form into third stage larvae (the ones that infect us) in them exclusively. Centipedes aren’t even remotely closely related to the snails and slugs the worms usually target, and most parasites are pretty particular about their hosts. Things that live in mollusks don’t usually survive in arthropods, and vice versa.
But, the worms can at least live for a little while in other species—some infections in the past have occurred from eating prawns and other animals that live around the worms’ usual hosts, which led scientists to conclude lots of species can serve as substitute or paratenic hosts. A worm doesn’t develop further in these less-than-ideal hosts, so it has to be already third stage, but it doesn’t die either. And it can still be passed on to its final host if the substitute host is consumed. So it’s possible that centipedes could also act as substitute hosts. Or they could even be true intermediate hosts—until now, no one thought to look.
That’s in part because human infestations of rat lungworm often occur when people eat unwashed vegetables, thus inadvertently consuming small snails or slugs hosting the worms raw—and accidental centipede ingestion isn’t really a thing. It’s pretty hard to overlook a finger-sized centipede on your lettuce leaf.
You might intentionally eat a centipede, of course. While centipedes aren’t common fare for most in the Western world, they are frequently served as street food or in soups in Chinese and other Asian cultures. They’re also dried and powdered for Chinese medicine. But however they’re prepared, they’re usually cooked—which kills the parasite. “We don’t typically hear of people eating raw centipedes, but apparently these two patients believed that raw centipedes would be good for their health,” explained Lingli Lu, who works in the Department of Neurology in Zhujiang Hospital, in a press release.
But it’s one thing to suggest the centipedes were the source, and another to confirm it. The researchers wanted to detect the parasites inside the centipedes directly. So they bought 20 centipedes from the same market in Guangzhou where the family had purchased theirs (animals that were locally caught in China’s Guangxi Province). They then examined the centipedes carefully before mashing them up for DNA extraction. Since the parasites are tiny and hard to identify to species visually, they used DNA analysis to get a clearer positive or negative result.
And they found them. An average of 56 larvae were found per centipede, and rat lungworm DNA specifically was detected in 7 of the 20 centipedes they’d purchased.
Still, the team wasn’t sure if the larvae had ended up there because the centipedes ate an infected snail (thus serving as substitute hosts), or whether the centipedes themselves were serving as intermediate hosts. They tried to infect centipedes with first stage larvae to find out, but all the centipedes died. While that doesn’t make it seem likely they’re intermediate hosts, it’s also possible the experiment didn’t perfectly mimic a wild infection, so the question is still open.
Unfortunately, the test doesn’t tell us how long they can serve as substitute hosts, even if they aren’t true intermediate ones—which makes it difficult to understand how centipedes might fit into the larger picture of the parasites lifecycle or affect our risk of infection. While rat lungworm is native to Asia, it’s made its way around the world, and has become a big problem in places like Africa, Hawaii, the Caribbean, and most recently, the southern United States (especially Louisiana and Florida). And of course, these areas are also home to centipedes—pretty much everywhere somewhat warm is—so it’d be really great for future research to get a better understanding of how these animals interact with rat lungworms.
But, given the team’s findings, it seems that regardless of how they end up with third stage larvae inside them, centipedes are quite capable of transmitting the worms to us. So, moral of the story: don’t go around eating raw centipedes…. Toast them up nicely first.
Citation: Wang et al. 2018. Eating centipedes can result in Angiostrongylus cantonensis infection: two case reports and pathogen investigation. Am J Trop Med Hyg, online early. DOI:10.4269/ajtmh.18-0151