Monday, 1 July 2013

The Candiru vs Peer Review

This is reportedly a candiru.
       It's a big one.
Image by em_j_bishop: CC BY 2.0

I think sometimes our faith in a system, and our love of a colourful story, deadens our common sense. Take the candiru, for example – Vandellia spp., the vampire catfish.

“This little charmer normally lives inside the gills of other fish to suck their blood, but is attracted to urine and reputedly able to wriggle up human's urinary tracts, where it lodges itself with sharp spines and can only be removed by surgery.”


The quote above came from the 2002 Lonely Planet guide to Brazil (pg. 75). First of all, these little catfish do not live in the gills of their fish hosts. They visit there to feed for all of about two minutes – more like a mosquito, really, than a parasite.

There is no scientific proof that they are attracted to urine. If one ever wriggled up a human urethra it wouldn't lodge itself so much as become stuck as a result of those spines, designed for a different situation entirely. Finally, there is only one published case – semi-documented and unconfirmed – and it was reportedly resolved by cystoscopy rather than surgery. (Samad, Anoar. "Candiru Inside Urethral." Urology Clinic.)

The Candiru in Academic Literature

While we'd like to expect better of Lonely Planet, it's only fair to excuse this case of bad information – the candiru has been reported widely and legitimized in publications that normally hold themselves to an even higher standard. For example:

  • From The New Encyclopedia Britannica 15th ed. Vol 2. (1990): “... parasitizes man and has been known to enter the urethras of bathers and swimming animals. Once inside the passage, it erects the short spines on its gill covers and may thereby cause inflammation, hemorrhage and even death...” (“Candiru”).
  • From a 1991 article by J. L. Breault in the Journal of Wilderness Medicine (2): “Forced extraction may cause lacerations of the urethral mucus membranes, which has caused death by exsanguination. Remedies have ranged from penile amputation and suprapubic cystostomy to application of a native herb that softens the spines” (“Candiru: Amazonian Parasitic Catfish”).

Judging by these sources and others published by academic presses and peer reviewed journals, subject as they are to editorial scrutiny, peer review, and fact checking, one would think that the consequences of a personal encounter with the candiru were well documented medical knowledge.

The library website at the University of Victoria says “Peer review ensures that an article-and therefore the journal and the scholarship of the discipline as a whole-maintains a high standard of quality, accuracy, and academic integrity. When you consult peer-reviewed sources, you are tapping into a wealth of established, verified knowledge” (, accessed July 1, 2013).

A 2013 Review of the Evidence for Candiru Attacks

Nonetheless, Dr. Irmgard Bauer, in her 2013 investigation of two centuries of literature describing the candiru, found little in the way of verification. “...most reports are...repeated again and again,” she writes, “based on the same stories already described elsewhere... After careful distillation, very little [evidence] remains and of that little, even accounts sounding like first-hand descriptions become suspect.” (“Candiru – A Little Fish With Bad Habits: Need Travel Health Professionals Worry? A Review.” Journal of Travel Medicine; 20:2.)

What happened here? In 200 years, did no one go to the trouble to verify candiru reports until now? The candiru is like the Sasquatch's rich city cousin: in contrast to the Sasquatch, consistently disinherited by science because there is no evidence for it, the candiru has been repeatedly legitimized by science and reputable publishers despite the lack of evidence. All we really have for sure is a translucent little fish that takes a blood meal from a fish larger than itself, and a bunch of anecdotes. It really isn't even a parasite.

And we wonder why it's important to consult original sources?

I wrote about what we know about the Candiru for Decoded Science: "Candiru- A "Don't Pee in the Water" Horror Story Debunked."