Last Chance to See - Return (extract from Chapter One)

The worst thing about swimming in the Amazon isn't what you might expect. It's certainly not crocodile-like caiman, which are large enough to eat the smaller members of a film crew, but rarely do. And it's not bull sharks, which rather alarmingly leave the open ocean and make their way as much as 3,700 kilometres upriver, past Manaus and far beyond.

Stingrays, admittedly, can be slightly worrying. They lie flat on the river bed, waiting for you to step on them, and then give you an almighty sting that is so excruciatingly painful you have to run, not walk, to the nearest doctor. But if you shuffle your feet in the muddy or sandy shallows, rather than marching like a trooper, they are more likely to get out of your way than get out their secret weapon.

It's certainly not piranhas, and it's not even two-metre-long electric eels or anacondas the length of mini-buses.

No, the worst thing about swimming in the Amazon is the tiny candiru fish (pronounced can-dee-roo). Otherwise known as the toothpick fish, vampire fish or, more disturbingly, the willy fish, this is a parasitic freshwater catfish just a few centimetres long.

Eel-shaped and translucent (so it's virtually impossible to see underwater) the candiru has a voracious appetite for blood. On a normal day it seeks out unsuspecting larger fish by following the flow of water from their gills. It dives underneath the gill flaps, opens its umbrella-like spines to lock itself in position and draw blood, and then drinks and drinks and drinks. It may consume so much blood that its body visibly expands, like that of a leech. Eventually, the unwelcome guest unhooks its spines and sinks to the bottom of the river to digest its meal.

If you happen to be swimming in the Amazon, and peeing, the candiru fish will happily follow the flow of your urine back to its source. Before you can whip up your trunks it will swim straight into your penis and inconveniently lodge itself right inside your urethra.

The pain, apparently, is spectacular.

If you are unfortunate enough to be candirued, the best option is to get to a hospital before infection causes shock and death, or your bladder bursts. Failing that (a likely scenario if you happen to be in a remote corner of the Amazon) the next option is to chop off your penis.

Alternatively, there is a traditional cure that requires the use of two local plants: the juice of the jagua tree or the pulp of the buitach apple. These are supposed to be brewed into a hot tea that apparently dissolves the skeleton of the fish within a couple of hours (a synthetic version of the brew has been used in the past to dissolve kidney stones). Be wary of some survival books that rather unconvincingly suggest you insert the buitach apple into the affected area.

Photo by Mark Carwardine

The main problem with such traditional cures is that, if you're anything like me, you won't have a clue a) where to find a jagua tree or a buitach apple, or b) how to know if you actually do. But if you think you're capable of calmly flicking through a field guide, inevitably written in Portuguese, with a candiru fish and its open umbrella firmly lodged inside your penis, and then organising a nice little camp fire to brew a piping hot cup of tea with your correctly identified traditional plants, then you might just be okay.

The only good news is that, despite rumours to the contrary, it's perfectly safe to stand on the riverbank and pee into the Amazon below. A candiru fish cannot, no matter how hard it tries, leap in mid-air and work its way upstream like a salmon.

Copyright: Last Chance to See - Return by Mark Carwardine and Stephen Fry, to be published in October 2009 by HarperCollins.