The flesh-eating bacteria shewanella is fairly common, but there is no reason for River Murray users to fear it, an SA Health expert says.
Dr David Cunliffe, a microbiologist specialising in water quality, said Murray Bridge resident Bill Andrews had been "extremely unlucky" to be infected by shewanella algae, as The Standard reported last week.
His case was the only one ever recorded in South Australia, and one of less than 300 anywhere in the world over the past 35 years.
Figuring out whether Mr Andrews' infection came from the river or somewhere else would be difficult, Dr Cunliffe said, even at the time he was diagnosed.
But he said he would not be surprised if shewanella were found in the Murray.
"It's a fairly common organism," Dr Cunliffe said.
"Generally they don't cause any problems, (but) very, very occasionally they can cause disease.
"It's likely the organism has been in the river for many years, since people have been recreating in the Murray."
But he said he did not intend to give the impression that the river was a dangerous place for people who took simple precautions, such as covering any open wound with a band-aid.
"In most cases it's good fun, it keeps you fit, keeps you healthy and you don't end up with an infection," he said.
It's likely the (shewanella) organism has been in the river for many years.Dr David Cunliffe
He disagreed with the suggestion shewanella should be a notifiable disease, requiring a public alert every time someone became infected, because of its extreme rarity and the unlikelihood of a patient infecting anybody else.
"The reason organisms are notifiable is to allow us to investigate potential outbreaks," he said.
"In the case of this organism, the chance of an outbreak is very low."
Environment Protection Authority (EPA) science and assessment director Peter Dolan said shewanella algae was not one of the things the organisation looked for in its monitoring of the river.
"The EPA’s role is to regulate human activities which might pollute the environment, and we periodically assess the general environmental condition of lakes and rivers," he said.
"We do not have a program to monitor for naturally-occurring pathogens as that is not our role."
But even if the algae were as common as Dr Cunliffe suspected, household water supplies would remain safe.
SA Water uses a seven-step process to remove any microbiological pathogens present in mains water.
Mild amounts of chlorine and/or ammonia and ultraviolet light are all part of the process, depending where in the state a user lives.