It’s one of the most striking pieces of lore about the rainforests of South America: don’t stop to relieve yourself in a river, the legend says, because there’s a fish that can swim up your urine stream and launch itself into your urethra. Once there, it’s impossible to remove without full amputation of the penis, which you’d better do quickly, before the little invader eats through your internal mucus membranes or lays eggs in your bladder.

It’s terrifying – but should it be? Is the “vampire fish,” or “toothpick fish,” or (to use its less sensational name) the candiru really something we need to be worried about?

What is the "penis fish"

The first clue that everything may not be quite kosher with the stories of the Amazonian penis-invading fish – apart from the description “Amazonian penis-invading fish” – is probably the fact that the first Europeans to describe the animal barely seem to agree on what it looks like.

“The taxonomy of South American catfishes is complex, much revised, and appears, at times, controversial,” explains Irmgard Bauer, Adjunct Senior Lecturer at James Cook University’s College of Healthcare Sciences, in a 2013 review paper on the semi-mythical ichthyoid.

“Adding to the problem, explorers individually named the specimen they came across for lack of reference works. It is often not even clear if they talk about the same fish, especially when descriptions and sizes of the fish vary tremendously,” she continues. "Given the similarity of many species, and the early explorers' lack of suitable instrumentation to distinguish between them, the lack of agreement is not surprising … Usually, fish [specimens] were kept in any grog at hand and deteriorated to the point where they could not be typified at all.”

Now, the candiru is definitely a real fish – it’s a type of catfish. They’re mostly pretty small, which does make sense if you’re going to claim it can fit up a penis, generally measuring 17 centimeters (7 inches) not including the tail. The specific type of candiru usually pinpointed as the penis-eating fish is Vandellia cirrhosa, which is even smaller – usually no more than 3 to 5 centimeters (1.2 to 2 inches) in length.

They also do deserve some of their rather icky reputation. They’re a parasitic fish, who survive by swimming into the gills of their hosts – mostly other catfish – and feasting on their blood. To lock itself into position, it uses spines around its own gills to latch on to its host – which, if you believe the rumors, may be a human urethra.

Can a candiru swim up my urethra?

Short answer: almost certainly not.

Sorry to all the gore fans out there, but there are a few problems with this idea, and they range in severity from “mildly suspicious” to “patently ridiculous.” Let’s start with the least damning and work up.

“Considering the alleged voracious habit of the little fish, the geographical size of its habitat, and the considerable number of people living along the river system, should one not expect by now a few confirmed cases in the medical literature?” asked Bauer.

It’s a good point: almost all the “evidence” for supposed urethral candiru attacks come from 19th and early 20th century explorers, and most of that comes from questionably-translated reports from Indigenous locals who, let’s face it, had pretty good reasons to want all these European fellas to leave (or at the very least, to stop peeing in their river.)

On top of that, Bauer points out, there are quite a few examples where people who you’d definitely expect to at least mention the existence of a mental penis-invading fish have apparently zero to say about it. Victorian explorers like Alexander von Humboldt and Alfred Russel Wallace, both of whom spent years and years in the rainforests of South America, never mentioned the candiru, and Henry Walter Bates, who accompanied Wallace in his exploration of the area, wrote that the locals were “almost amphibious” – a surprising lifestyle to adopt for a people apparently concerned with a fish swimming up their urethra.

Next, there are the biological implications. Imagine you’re a tiny fish, used to the vast expanse of the Amazon or the Rio Negro, and suddenly you find yourself inside some human’s urethra: a tube less than 8 millimeters (0.3 inches) in diameter and notably lacking in fresh water. Forget wiggling your way into a bladder and reproducing – you’re going to be dead in a few minutes.

Sure, you may say, but the website I read says it’s attracted to the smell of urine! It simply can’t help itself! Well, that brings us to another reason to doubt the claims: candiru don’t seem to be all that attracted to the smell of human urine.

“In 1829 German biologist and explorer Carl Friedrick Philipp von Martius … hypothesized that the odor of urine attracted candirus, as primary prey of the fish emit urea from their gills,” notes a 2015 report in the Journal of Urology. “However recent research showed that the fish hunt by sight and are not attracted to human urine.”

And now for the real kicker: that thing about the fish swimming up your urine stream? It’s impossible.

And we don’t mean “unheard of” – we mean literally impossible. “During an expedition in … 1855, French naturalist Francis de Laporte de Castelnau was told by local fishermen that candirus ‘spring out of the water and penetrate into the urethra by ascending the length of the liquid column’,” notes the Journal of Urology.

“These claims … defy the laws of physics,” the authors immediately note.

Will a candiru attack me?

Given the huge amount of mythos around the candiru, you’d think there must be quite a few cases of the fish getting into a human body some way or another. In fact, though, there are hardly any – and only one recorded instance from the modern era.

One notable account of a candiru getting lodged in human genitalia comes from 1891, from the naturalist Paul Le Cointe – but it didn’t involve the little fish swimming up a urine stream to enter a penis. In fact, like a full 100 percent of early recorded cases – actually witnessed by the recorder, not “a friend of a friend heard” stories – the candiru was found stuck in the vaginal canal of a woman, and removed without any drastic intervention.

In fact, there’s only been one case ever recorded of a candiru getting stuck in a male urethra, and it happened in 1997. The victim, a 23-year-old from Itacoatiara, Brazil, claimed that the fish had indeed jumped from the river into his penis while he urinated, something that, we’ll remind you, is impossible.

“He reported trying to grab hold of the fish, but it was very slippery, and it forced its way inside with alarming speed,” a case report explains. “The candirú’s forward progress was blocked by the sphincter separating the penile urethra from the bulbar urethra. With the passage blocked, the fish had made a lateral turn and bitten through the tissue into the corpus spongiosum, creating an opening into the scrotum.”

Apparently, by the time the poor guy got to the hospital, the fish had died. This meant both good news and bad news for the patient: the fish’s spines had relaxed post-mortem, meaning it could be removed the same way it went in rather than requiring surgery through the perineum; on the other hand, the case report noted that the fish was too decayed to precisely determine its species, which sounds even worse than it being there in the first place.

But is this proof positive that candiru can and do attack urethra? Some people – and by that, we mean the experts and scientists whose job it is to know these things – aren’t so sure.

“There is a tendency to cling to the one much publicized case from Brazil,” wrote Bauer. “Unfortunately, there are too many inconsistencies and irregularities attached to this case to rely on it with confidence, such as the victim's insistence that the fish jumped out of the water and ascended the urine column.”

Not only that, but when marine biologist Stephen Spotte investigated the case in 1999, he noted several inconsistencies that threw the case into even more doubt. For one thing, the fish was described in the case report and photos as having “measured 134 millimeters (5½ inches) [long], with a head width of 11.5 millimeters (7/16 inches)”, which you may recognize as way bigger than a standard urethra.

And remember how the case report said that the fish “made a lateral turn and bitten through the tissue into the … scrotum”? According to Spotte, that just doesn’t add up: candiru teeth aren’t strong enough to chew through the wall of a urethra.

Basically, there’s reason to remain skeptical, is what we’re saying.

Conclusion

So is there really a fish out there that wants to swim up your penis and chomp on your urethra? Well, maybe – it’s a big old world out there, after all, and we’ve barely discovered even a fifth of the species that live in it.

But that report you heard in the pub about a friend of a friend’s cousin’s fiancé who got a fish in his dick? Definitely didn’t happen. And if it did, there are a few top-rated journals that will want to interview him.

“Travelers to the Amazon who are precious about their urethras can be told that there is no evidence of candirus waiting in the rivers ready to attack humans,” wrote Bauer, “though tight‐fitting bathing suits will alleviate any anxiety and do no harm.”

“This verdict may disappoint a great many people,” she adds, “but until very welcome confirmed evidence exists of this fish's interaction with humans, travelers to the Amazon who feel tempted to urinate in the river, perhaps with spine‐tingling trepidation, will most likely not return home with heroic survival stories to tell.”

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.