Murray Bridge man Bill Andrews had never heard of shewanella bacteria until days after it entered his left leg, causing a horrifying flesh-eating infection.

"It just felt like somebody was down there with a saw going to cut my leg off," he said.

"I woke up with throbbing in the leg. I switched the light on to have a look and it was about the size of a football, my left ankle.

"I had to go to the toilet. I got only about two thirds of the way to the bedroom and I collapsed on the floor."

Mr Andrews' case of a flesh-eating shewanella infection is believed to be the first ever reported in South Australia.

He ended up losing about 20 per cent of the flesh in his calf.

Mr Andrews said he was at his local boat ramp at Hume Reserve on the River Murray waterfront when, he suspects, the bacteria made its way through a scratch in his skin.

"Worst-case scenario would have been, had it been left much longer, I would have lost my leg," he said.

"I've had three skin grafts — two of them approximately two inches wide by eight inches long. The other one is about three inches by three inches.

"I've more or less got to start learning to walk again."

Mr Andrews ended up losing about 20 per cent of the flesh in his calf. ( Supplied: Kylie Andrew-Komar )

Fewer than 300 cases in 35 years: SA Health

At first, Mr Andrews went to the Murray Bridge hospital, but it was only after blood tests were carried out at the Royal Adelaide Hospital that the infection was diagnosed.

SA Health said it was extremely rare for the bacteria, which is found in sea and fresh water as well as the soil, to cause a flesh-eating infection.

"Shewanella is fairly common but infection isn't," SA Health water quality advisor David Cunliffe said.

"There was a recent survey published and they looked at all reported cases worldwide, and they found less than 300 cases [this extreme] in the last 35 years.

"Depending on underlying conditions, it can be fatal."

SA Health said while the bacteria is common, infection is not — especially extreme cases like this one. ( Supplied: Kylie Andrew-Komar )

Dr Cunliffe said while it was an alarming example of shewanella infection, there was little need for public concern.

"If you do have a wound or skin abrasion or skin lesion, keep it covered," he said.

"But we don't want to give the message that the River Murray is a dangerous place.

"It's not sterile but [it] is a place where you can enjoy yourself and the chances of getting an infection like this are extremely low."

But Mr Andrews, who is recovering with the help of a walking frame, believes his case should serve as a warning to river users.

"I'll never go in there again unless I've got waders on."