Four Christchurch firms had legionnaires' bug

The cooling towers of four Christchurch businesses have tested positive for the bacteria that cause deadly legionnaires' disease but health officials will not name them. They say more positive results are likely.

Three Christchurch people have died and 16 others have contracted the disease since June.

The Canterbury District Health Board and Christchurch City Council are investigating the source of the contamination.

A study of those affected has confirmed that elderly and/or debilitated people living in certain parts of Christchurch have been more at risk, but health officials refuse to name the areas.

Canterbury Medical Officer of Health Dr Mel Brieseman said the four positive tests came from across the industry sector and different parts of the city.

It was not yet known if any of the people who had contracted the disease had been in those buildings.

"Although all companies with cooling towers in the city haven't yet completed their checks and we are still waiting for results, we are treating these first results very seriously," he said.

So far, results for about 40 of the city's 130 cooling towers had been returned. Overseas research indicated about 10 per cent of cooling towers tested positive for the bacteria, which meant it was possible that the number of positive tests could reach as high as 13.

Dr Brieseman said that given the widespread nature of the legionella organism, some positive results were not surprising.