Near the end of the Straight Dope article, Cecil says:
The 1973 article is almost certainly referring to people who submerse themselves in the Amazon river, and then just let go and pee while their urethra is under water – not people standing next to the river and peeing into it.
You can go on an eco-tour to swim with pink dolphins in the Amazon. Conspicuous by its absence in the tour’s website is any mention of the fact that an “oh no!” might swim up your “unh-unh”. Is that because they don’t want to frighten off potential customers or because these critters don’t live upstream in Peru?
With all the people visiting the Amazon lately, I would think that if this really were an established urological problem, that somehow we’d have more data. But all that came up on Google under “candiru urology” were the two websites below. Why aren’t there more?
There are lots and lots of websites concerning “Our Trip To the Amazon”, but nobody mentions how funny it was that Bob went in without a swimsuit and had to be airlifted to a hospital in Rio. And there are lots and lots of websites that will tell you coyly, “If you go, don’t pee in the pool!” But they don’t have any personal experience to relate. Just “they say…”
But who wants to be the one to actually test it? Not me.
This one has the John Herman article.
This one gives three references, one of which is the Gudger book, and one of which is the Herman article again.