This fish in the Amazon River won't enter your body through urine, but it will suck your blood?

You must have heard the legend that if you pee in the Amazon River, a little guy called Vandellia cirrhosa will crawl into your urethra along with the urine, making you miserable...

Compared to the name of the tendril parasitic catfish, I prefer its other name, the blue vampire. As the name suggests, this is a blood-sucking name. It has a slender body and drills into the gill cover of large fish, piercing the fish flesh with its sharp needle-like teeth to suck blood.

A parasitic catfish burrows under the gills of a fish and sucks blood. Source: Reference 2

Generally speaking, in South American adventure movies, there are more scenes of such strange creatures drilling into the human body. When moving along the river, the protagonist in the movie warned his friends in a popular science tone not to urinate in the river casually, and warned everyone to be careful, after all, the Amazon jungle is mysterious and dangerous. Indeed, this forest is dangerous enough, otherwise Percival Harrison Fawcett would not have disappeared during his exploration of the Amazon River Basin. Back to the topic, from the perspective of the laws of physics alone, it is impossible to swim upstream by relying solely on descending liquid. You may come up with a counterexample. Can't salmon swim upstream? That's right, but the premise is that it comes up along the river, there are obstacles to attach to, and it jumps up layer by layer.

A parasitic catfish Source: gizmdo You can't just let it go against the current. After all, it hasn't evolved a rocket-like device that can spray any substance to get thrust to counter gravity and the resistance of urine moving downward. So, this fish can't crawl into the urethra along the urine. Then you must be curious, why is this seemingly unreasonable rumor so popular? The earliest version of the "pee fish" came from the 1930s, when an ichthyologist named Eugene Willis Gudger wrote a book "The candiru". Its subtitle says it all: The candiru - the only vertebrate that parasitizes humans.

In the book, Gu Jie did not personally see a case of a parasitic catfish entering a human urethra. He also recorded his own case in the book. In the same year, he also wrote a paper about the parasitic catfish, discussing the possibility of this fish entering the human body. There were still no specific cases in the article, but more from the side. So its credibility is still very low.

Document screenshots

The article introduces a wooden device that locals use to protect their urethra from being attacked. Source: Reference 1 Another study also shows that it is unlikely that this fish will follow the urine. Stephen Spotte did a study in 2001. He kept a group of Hematophagous candirus in an aquarium and then threw several attractants, fresh fish paste and human urine. Needless to say, scientists must have been influenced by the literature that parasitic catfish would attack humans through urine, so they thought of such an experiment. The results showed that there was almost no difference in the attraction of the two to parasitic catfish.

Parasitic catfish is attacking a goldfish. Source: Document 4. After that, another experiment was conducted, using two different prey, a slightly larger goldfish and a slightly smaller cichlid. The result was that the goldfish was more likely to be attacked by the parasitic catfish. Scientists speculate that this is mainly because the goldfish is relatively large and can be easily observed by the parasitic catfish with its eyes. In other words, the parasitic catfish is more likely to use vision to find prey rather than smell urine through its sense of smell. I guess the parasitic catfish will not jump out of the water, look around for a man's genitals, and then take a closer look before deciding whether to bite or not. You know, the size of the cichlid is actually not small... But, I just said that the possibility of this fish drilling into the urethra is relatively low, and I didn't say that it will not attack humans.
In 2021, the Journal of the Brazilian Society of Tropical Medicine published an article recording a case of a parasitic catfish attacking a human. (The picture is bloody, so I won't post it) A parasitic catfish bit an adult male's back tightly, looking like a leech. The victim pulled the fish off with force, and his skin was torn open, bleeding. Note that the abdomen of the parasitic catfish is full of blood at this time. Under normal circumstances, parasitic catfish rarely bite people actively, and more often eat dead fish or other dead animals underwater. But you can see that it is not impossible to actively bite people and suck blood, although the probability is relatively low. Since it can suck blood on the back of a person, why bother to find the human genitals first, then drill into the crotch, and use the body with a diameter of about 1 cm to drill into the urethra with a diameter of 0.6 cm to suck blood? Isn't this very strange?

By the way, the introduction to parasitic catfish you see in a certain encyclopedia will "popularize science" that it is normal for it to drill into the human urethra, and the document cited is called "The Legend of the Amazon Parasitic Catfish"... Wikipedia is slightly better, adding a sentence at the end of the article: "But it has never been clinically proven."

Source: Bringing Science Home