In the slimiest — and perhaps costliest — case of mistaken identity in modern biology, hundreds of scientific papers and years of research could be thrown into doubt, for they may have been based on experiments carried out on the wrong leech.
Three species of bloodthirsty invertebrate have been passing themselves off as the right leech, a discovery that adds powerfully to the shock and confusion.
The evidence of this innocent error, published in a British scientific review, Proceedings of the Royal Society B, could provoke a cascade of consequences in hospitals and pharmaceutical companies around the world, the authors say.
For decades scientists have used what they thought was the Hirudo medicinalis, or "medicinal leech," to develop dozens of new compounds and drugs. Some help thin blood and others are harnessed in the search to fight viral infections, from HIV to hepatitis C.
The leech can, of course, lay claim to a far more venerable place in medicine, as a blood-letter.
It fell out of fashion in this role at the end of the 19th century, but has recently wormed its way back in — this time as the means to ease blood pressure in body parts that have been reattached or traumatized by restorative plastic surgery.
The leeches and the compounds they yield work wonders, but there's still a problem, according to Mark Siddall, a zoologist at the American Museum of Natural History who has crawled over the evidence.
"What has been sold and used as Hirudo medicinalis is usually another species, Hirudo verbana," explained Siddall, who led an international team of researchers in examining dozens of specimens procured from leech farms in Europe and the US.
"Indeed, we have never received a true medicinalis from a commercial supplier," he said in an interview, adding that a few leeches from a third species, H. orientalis, from Turkey or Azerbaijan, may also have crept into the mix.
To the naked eye, even a highly trained one, H. medicinalis and H. verbana seem identical in size and coloration.
Fully grown adults of both species weigh in at about 80g and measure up to 15cm when fully extended.
"The ones we ship to hospitals are a lot smaller," explained Carl Peters, assistant manager of Britain-based Biopharm Leeches, one of the world's two or three largest suppliers of medicinal bloodsuckers.
That's probably a good thing, because leeches — equipped with three mouths and three hundred razor-sharp teeth — can easily siphon off in a single sitting six times their body weight in blood, enough to keep them chubby and happy for half a year.
Leeches deliver built-in anaesthesia, so the incision is painless.
And because of anti-coagulants in their saliva, bleeding continues for six to 10 hours, draining off enough blood to fill half a wine glass.
"In micro-surgery you can suture the two ends of an artery together because they have thick walls," explained Siddall. "But veins are thin-walled and very difficult to reattach, which creates blood congestion and the risk of tissue dying. That's where leeches work best."
No one has accused leech farms of fobbing off fakes on to the world market, and Peters is not terribly concerned about the brouhaha over authenticity, though he does acknowledge that there could be a problem with the US.
The US Food and Drug Administration (FDA) approved the medical use of leeches in 2004, specifying though that H. medicinalis is the only authorized species.
If the US government cracks down, different species from Biopharm and other suppliers could suddenly become contraband.
That still leaves healthy markets in Europe and Asia, Peters said. "Koreans more readily accept the use of leeches, whereas in Britain they are still seen as a last resort."
The real worry stemming from the mislabelling of leeches lies in a large body of scientific research. "The implications of our findings are far-reaching," said Siddall.
Development biologists and neurobiologists "are grappling" with the fact that certain results in their experiments are possibly due to the fact that they were working — without knowing it — with two, or possibly even three, species rather than one.
There are at least 115 bioactive ingredients that have been isolated from what was sold as H. medicinalis, mainly anticoagulants, antistasins and protease inhibitors used to block the reproduction of viruses.
Finding out which of these wonder compounds is attributable to which species may well be a nightmare.
But there is a silver lining, insisted Siddall. "With three species rather than one, there may three times as many interesting compounds to be discovered and harnessed."
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