Is candiru really the scourge of the Amazon?

Ah the candiru. A little fish of legend. Is it really something to be scared of? I mean, look at the little guy. Well, this review article will probably answer that question for you.

Who hasn’t heard the stories about fish swimming up urine streams to get lodged in the urethrae of unsuspecting tourists? If you ask all of your medical friends, I bet greater than 75% of them have heard of stories, and some may anecdotally report being present for extractions. Even nonmedical acquaintances are likely to have heard these stories. So where did they come from?

First encountered verbally by Amazon explorers, written evidence of this mythical creature was first published in 1829 by Carl Friedrich Philipp von Martius (names were better back then). He of course never actually saw one, and never saw the preventive method of tying a ligature around one’s penis either. But by putting it in writing, it now became fact. So much so, it has been repeated over the last 180+ years by multiple sources, and in modern-day has been reported by such reputable sources as River Monsters, Grey’s Anatomy, Dr. Oz, and Weird Planet.

As reported in the review article, the early explorers likely weren’t even all talking about the same fish. Something that small probably evaded most detailed examinations at the time, and lack of good preservation meant most specimens were pretty chewed up in the lab. Add to the fact that they were effectively playing a giant game of telephone with the natives, since there was very little in the way of common language between all the tribes. Lots of pantomiming and hand gestures that could be misinterpreted, as well as simple syntax problems likely caused mistranslations about most things.

With this much plurality of anecdote, it has to be fact at this point, right? Well, sorry to burst your bubble, but no. Even the best documented case from 1997 has serious problems with evidence. The fish that is reported as the source was much too large (11.5mm in diameter!) to enter the urethra of the patient, and the report has it jumping out of the water, or swimming up the stream, as it were. The report also has the fish being attracted to urine. Also, the specimen fish still had spines in place, which had been snipped off in the report of the surgery. The marine biologist who went down to examine the evidence came away with reservations about the report.

Here is a list of “facts” about candiru that are myths

  • They are attracted to urine. Experiments have shown this isn’t true, they hunt by sight
  • They can swim up a stream out of the water. Physics applies to all fish, even small ones.
  • There is documented evidence of urethral invasion. The sparse case reports available do not seem to be valid.

If you want to be cautious, you can simply wear a bathing suit in the water, but that’s more for modesty than for safety. There simply does not appear to be any evidence that this is a problem. The review article searched for literature in many languages and even did a deep dive into the historical data, and came away wanting.

In the end, it was probably just a way for the natives to tell the explorers, “Hey, you guys, stop peeing in our river, we bathe, drink, and get food from there.” It simply snowballed from there.

But hey, who lets facts get in the way of a good story.

Bauer IL. Candiru–a little fish with bad habits: need travel health professionals worry? A review. J Travel Med. 2013 Mar-Apr;20(2):119-24 [PMID 23464720]


Emergency physician with interests in wilderness and prehospital medicine. Medical Director of the Texas State Aquarium, Padre Island National Seashore, Robstown EMS, and Code 3 ER | EBM gone Wild | @EBMGoneWild |

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.