You can't easily see what's inside, but if you peer behind the white cover, you will catch a glimpse of dozens of squirming little creatures.

They haven't eaten for months, perhaps even a year. And they are out for blood.

Here in this corner, is Victoria's only colony of medicinal leeches, a relic of our ancient medical past that has, somehow, survived into the 21st century.

Believe it or not, leeches are still an irreplaceable tool for doctors trying to repair amputations – fingers lost through workplace injuries, assaults or road accidents or sometimes a whole hand, arm or foot.

They can also be used when different body parts are moved around the body during reconstructive surgery.

Ramin Shayan, a microsurgeon and O'Brien Institute director, said leeches are used in about five cases every year at St Vincent's.

Sometimes veins are damaged in an amputation or accident, preventing blood draining properly out of a replanted body part. Leeches take on this draining job as the veins regrow, and help prevent congestion and clotting of the blood.

 "They actually have 60 unique proteins that are produced by their saliva," Dr Shayan said.

"One of them is a compound called hirudin which is much like what you use with patients with clotting disorders. It thins the blood."

So, a few times a year, patients are faced with spending a few days with leeches attached to their bloodied  body parts – a confronting sight, but one that could be the difference between losing a finger, or keeping it.

In order to get the leeches to latch on, staff have to put them on the injured digit using forceps, or sometimes they are placed into a syringe with its stopper removed.

On occasions, the leeches have been known to reject a meal.

"If the tissue is not going to pull through, they are the first and most accurate detectors of that. They are not interested in dying tissues," Dr Shayan said.

The leeches are surprisingly expensive – costing about $50 or $60 each and sourced from farmers in a secret location in Victoria.

They can only be used once, for safety reasons

Dr Shayan said that, as "the legislation deems the leeches can feel pain", they are first anaesthetised with alcohol, before being discarded in an incinerator.

The leeches at St Vincent's are part of a medical history that spans centuries. Leeches have been used by physicians for at least 2500 years and enjoyed a golden era in Europe in the late 18th and early 19th century when millions were used in hospitals. It was believed various ailments were caused by excess blood.

In recent years they have popped up in reports of "extreme facials" and beauty treatments used by celebrities.

Dr Shayan said although medicine continued to evolve, leeches would always have a role. He said developments may be able to hasten the processing of regrowing blood vessels and veins in the future.

"But I don't see a time when it can be done instantaneously and until that time, the leech will always have a place in plastic surgery."

- The Age