The candiru, so we are told, is a small, thin, slippery fish that will enter a human urethra if that human dares to pee in the surface waters of the Amazon region of South America (the fish is reportedly attracted to urea in urine).
Once inside, the story goes, the fish anchors itself with spines, sucks blood and even gnaws tissue, resulting in excruciating pain. Some sources claim that the fish will also enter the vagina or rectum, given the opportunity.
The candiru, apparently cannot get back out of a human host, and cannot be removed because of the spines – so this little fish does horrendous damage, leading to laceration, hemorrhage, bladder destruction, penis amputation, even death!
Search for the candiru on the Internet and you will see just how big this fish story can get.
This exquisitely horrifying tale blends our fear of unknown waters with our utter loathing of parasites to produce a nightmare arguably worse than piranha attack, snake bite, or malaria.
The good news is that, even with millions of people living in Amazonia, and millions of tourists visiting each year, well-documented cases of candiru infestation simply do not exist – contrary to the horror stories you may see and hear on television and over the Internet.
Anecdotal Reports of Candiru Attacks
A 2013 paper titled “Candiru – A Little Fish With Bad habits: Need Travel Health Professionals Worry? A review” in the Journal of Travel Medicine explores the history of and scientific evidence for candiru attacks on humans. In an email to Decoded Science, author Irmgard L. Bauer wrote:
“I had heard every now and then of this thing – not through those awful websites but rather in the literature; also medical literature mentioned it, but for my taste, there was nothing that substantiated the rumours.”
Dr Bauer found that tales of the candiru date back to sometime in the 19th century, with the first attributed to Carl Friedrich Philipp von Martius. From there the trail leads through explorers, naturalists (including Alfred Russel Wallace), physicians, missionaries, scientists, and skeptics. Ultimately however, the body of evidence amounts to little more than anecdote and hearsay.
“Most reports,” writes Dr, Bauer in her paper, “are, in fact, repeated again and again based on the same stories already described elsewhere… it is almost impossible to identify genuine eye witnesses of candiru ‘attacks…’”
The Truth About the Candiru
Though the fish most frequently blamed for human attacks is the catfish species Vandellia cirrhosa, the term candiru (alternatively pencil catfish, vampire catfish, or carnero) actually refers to a group of parasitic catfishes that have not been well studied.
Claims that they are attracted by urine, suck blood, and survive inside a human host are based on supposition and embellishment of anecdote over the centuries. This is what we do know:
- Vandellia spp. normally ingest the blood of fish hosts. The catfish attack by entering the gill opening and anchoring near a major blood vessel. Jansen Zuanon and Ivan Sazima studied the feeding habits of several species of vampire catfish including V. cirrhosa, and report their findings in a 2004 paper in the Journal of Ichthyology and Aquatic Biology (“Vampire Catfishes Seek the Aorta, not the Jugular: Candirus of the Genus Vandellia Feed on Major Gill Arteries of Host Fishes”). The researchers observed that the fish do appear to use spines on their gill openings to anchor themselves in the gills of the host fish, but rather than actually sucking blood, they lacerate the host’s artery and feed on blood that pumps out through the wound. Feeding takes less than two and half minutes, and then the candiru leaves the host.
- Entering a human urethra would be as devastating for the fish as it would for the human. This is a vastly different environment from a fish’s gill and, as Dr. Bauer states in her paper, “with no oxygen available and no room to ‘swim’ up the urethra it is unlikely that the fish survives even a few minutes.”
Evidence That the Candiru Attacks Humans
Two hundred years after the first terrifying report of the candiru appeared, we still have no solid proof that it deserves its reputation. There is one case reported by urologist Anoar Samad of Manaus Adventist Hospital in Brazil; however, the case is not documented well enough to constitute proof.
Of this case, Dr. Bauer told Decoded Science:
“I think any interpretation is very shaky… Dr Samad unfortunately did not really do anything that would allow proof to his claim… The fish has disappeared; the photos could be really anything… At this stage, the claims cannot be confirmed.”
Researchers have not proven how the candiru locates the gill opening of its fish host, but it is true that urea is excreted by fish through their gills, and by humans in the urine. It is also possible that the movement of liquid out through the orifice attracts the fish.
Zuanon and Sazima observed and documented a candiru successfully penetrating a fish host’s posterior nostril (where water is also expelled) and taking a blood meal.
These authors write:
“entry into the nostril is… probably due to the catfish having been starved for… five days. This may shed some light on the occasional reports of candirus entering body openings… other than the gill chamber…, and may perhaps also relate to the highly unusual penetration of the human urethra…”
Swimming Fears: Candiru Infestation Risks
From the evidence, we know that the candiru does sometimes swim up the wrong stream, so to speak, but it appears that attacks on humans, if they occur at all, are vanishingly rare – maybe once every hundred years or so.
Bauer, Irmgard L. Candiru – A Little Fish With Bad habits: Need Travel Health Professionals Worry? A Review. (2013). Journal of Travel Medicine. 20:2.
Hara, Toshiaki J., and Zielinski, Barbara. Fish Physiology: Sensory Systems Neuroscience. (2007). Elsevier Academic Press.
Zuanon, Jansen and Sazima, Ivan. Vampire Catfishes Seek the Aorta, Not the Jugular: Candirus of the Genus Vandellia (Trichomycteridae) Feed on Major Gill Arteries of Host Fishes. (2004). Journal of Ichthyology and Aquatic Biology. 8:1. Accessed June 26, 2013.
Samad, Anoar. Candiru Inside Urethral. Urology Clinic. Accessed June 25, 2013.
Decoding Science. One article at a time.