Billie the sniffer dog trained by police to catch rapists via semen

Police in Derbyshire have added a new weapon in their fight against sex offenders by training a sniffer dog to track semen left by rapists.

As his fellow police canines are busy searching for a whiff of drugs or explosives, Billie the cocker spaniel has been given the unenviable task of picking up the scent of sperm deposited at crime scenes.

The dog's specially trained nose is so acute it can reportedly detect as little as a millimetre of human sperm more than a year after being ejaculated indoors.

Billie can even sniff out eight-week old traces left by a potential sex offender outdoors as long as it is "protected from the worst of the weather", a police briefing leaked to The Times disclosed.

Derbyshire Police says the dog has been used in 18 live searches since March of this year.

In one case, involving a suspected voyeur, the investigating officers found nothing at the crime scene but were given a potential breakthrough after Billie pointed to some boards recovered from a loft.

"Despite CSI not being able to see any stains via the crime light, the dog indicated several areas on one of the boards. That board has now been seized for laboratory analysis," the police briefing read.

A separate investigation saw Billie find a single spot of seminal fluid among "literally hundreds" of vomit and urine stains on a flight of concrete steps in a town centre.

The find led to DNA analysis and a complete profile of the alleged rapist, who was already on bail for rape in another country. He has since been arrested.

Billie also provided crucial evidence to an investigation into claims of assault in a care home.

Superintendent Dick Hargreaves, of Derbyshire Constabulary, said Billie had become "a real asset".

"He's already been used in numerous investigations and has found further evidence that has resulted in crime scene investigators identifying DNA profiles," he said. "We are always looking for innovative ways to improve our service and we're hoping that Billie's skills will lead to more convictions."

The typical dog is believed to have a sense of smell that is about 10,000 times more acute than that of humans, according to research by James Walker, former director of Florida State University's Sensory Research Institute. Their noses contain several hundred million olfactory receptors compared to about six million in humans.

Walker said: "If you make the analogy to vision, what you and I can see at a third of a mile, a dog could see more than 3,000 miles away and still see well."