October 20, 2009 -- An armed robber has been caught after his DNA was matched with blood in a leech found at the scene of the crime. The extraordinary case of the leech that led to a conviction of a weapon-wielding crook is believed to be a world first - and now the robber is awaiting sentencing on Friday. Crown prosecutor John Ransom told a judge in Tasmania that the chances of the DNA found in the leech matching someone were one in a hundred million. 'It is the oddest way of convicting anyone I have ever been involved in,' Detective Inspector Mick Johnston said outside the court after 54- year-old Peter Cannon pleaded guilty to aggravated armed robbery committed in 2001. 'I have not been able to find any similar cases anywhere in the world - nothing like this at all.'

The leech, the court was told, was found beside a ransacked safe in a farmhouse in northern Tasmania, leading to suggestions that it had dropped off the body of one of the two hooded men who had broken into the property. 'It was the only evidence we found, and as there was no evidence of any leech bites from the victim or the police present, we thought it was a good chance to have come from one of the offenders,' Mr Johnston told The Mercury newspaper. 'We took it from the scene because it didn't belong there.'

Leeches live in humid conditions where water is found, suggesting that Cannon and his unknown accomplice had made their way through bushland before they made their move on the isolated home of 71-year-old Miss Fay Olson. Their faces hidden under black hoods and clutching pieces of wood, they forced their way into Miss Olson's home and, she told police in a statement, began ransacking the house. When she tried to make a phone call, the handset was ripped from the cord. They got the safe opened, took $500 (equivalent to £200 at the time) and found another $50 in Miss Olson's handbag. She was then tied up and a belt was tightened around her ankles.

When police arrived after she was finally able to raise the alarm they began a thorough search for clues. They could find nothing obvious - until Senior Constable Nathan Slater saw the leech, fat with blood, next to the bed. He handed it to Inspector Johnston and later it was forensically examined and DNA was extracted from the blood. All police needed now was a person to match it with. It was seven years before Peter Cannon was arrested and charged with drug offences - and when his DNA was compared with that taken from the leech a match was found.

The Tasmanian court hearing the armed robbery charge was told by defence counsel John Oxley that Cannon's guilty plea had come in spite of the 'relatively weak case' by the Crown. But prosecutor Mr Ransom denied that the case was weak. 'It was a reasonable hypothesis consistent with the facts,' he said. Police have been able to match DNA profiles across Australia following the setting up in 2007 of a national DNA database known as CrimTrac - enabling profiles dating from before the 2001 Forensic Procedures Act to be used. Police are still searching for Cannon's alleged accomplice.

There are three main types of leech - freshwater, land and marine. Not all attach themselves to humans, but those that do remain there until they are full of blood. Once they have 'feasted' they drop off the skin in order to digest the blood. While it is not known how the leech that attached itself to Cannon became detached, ways of removing them are to wait for them to fall away naturally, using a fingernail to break the seal of its oral sucker - or touching it with the end of a lighted cigarette can also be successful. It usually takes around 20 minutes for a leech to fill itself with blood and fall away from the skin.