Doctors have traced the Legionnaires' disease outbreak on Horizon to the cruise ship's hot tubs.

Up to 50 passengers are thought to have been infected during nine Bermuda-New York trips between April and July. One person died.

But fears the disease would frighten off passengers appear to have been unfounded.

Occupancy levels have been virtually the same as in past years, say the ship's owners, Miami-based Celebrity Cruises.

The investigation into the legionella outbreak was conducted by the US Centres for Disease Control and Prevention (CDC).

New measures are now likely to be taken to control the maintenance of whirlpools.

The CDC will hold an October 17 meeting with cruise lines and hot tub makers to thrash out ways of preventing such outbreaks.

Yesterday Celebrity vice-president of marketing and passenger service Mr. Al Wallack was not surprised the finger had been pointed at hot tubs.

He told The Royal Gazette the CDC had always suspected the whirlpool baths were linked with the bacteria.

"From the first day we took the baths off line, and they are still that way,'' he said.

Mr. Wallack said the CDC recently spelled out their suspicions to the Joint Committee of the Coast guard and Maritime Committee of the House of Representatives.

It was one of the CDC's first recommendations to Celebrity to put the baths out of service.

Continued Mr. Wallack: "I should point out the baths have been pinpointed as a breeding ground for the bacteria.

"The baths cannot manufacture the bacteria, and could not be described as a source of it.

"I imagine the CDC are still trying to find the source, although we will have to wait until we get their report.'' Doctors from the CDC announced yesterday they had tracked the bacteria to one contaminated filter that recirculated water for the ship's three spas.

They say lounging in the steamy water, or merely walking by the tubs, was enough to catch the illness.

Dr. Jo Hofmann, one of the investigators, said several other outbreaks of Legionnaires' have been linked to hot tubs.

But this was the first on a cruise ship.

"We have to do something about controlling the way whirlpools are maintained on ships,'' she said.

Dr. Hofmann presented the findings of the CDC investigation yesterday at a meeting of the American Society of Microbiology.

Ms Joan Brower, a spokeswoman for Celebrity Cruises, said the company had not seen the CDC report and declined to comment.

Mr. Wallack told The Royal Gazette he believed protocol and procedures would be changed following the October 17 meeting.

"The CDC will let us know what they found. Obviously I can't say what changes might take place in advance of seeing the report.'' Authorities first learned of the outbreak in July when a New Jersey doctor reported that three patients had got pneumonia after sailing on Horizon .

Tests showed their illness was caused by the Legionnaires' bacteria.

To track down the source, investigators questioned 3,322 passengers who had sailed on the ship. Eventually, they found 50 people who had got pneumonia on nine sailings.

Although they suspected that all had Legionnaires' disease, they could definitely pinpoint the cause as Legionnaires' in 16 of them.

All of them had been in or near the ship's hot tubs. The researchers calculated that sitting in the tubs increased passengers' risk of getting Legionnaires' disease 17 times, and walking past the tubs increased the risk 13 times.

The germ is spread in water drops. The doctors speculate that people strolling near the tubs caught the germ from breathing spray given off by the tubs.

Three people even got sick while eating in an outdoor restaurant on the deck below the tubs.

The investigators found the Legionnaires' germ in one of the spas' sand filters. The bug was identical to the variety of the germ isolated from one of the sick cruise passengers.

Although the tubs had been drained by the time investigators arrived on the scene, Dr. Hofmann said they were typically kept at about 100 degrees -- a perfect temperature for Legionnaires' bacteria to thrive.

She said people who were especially susceptible to catching the bacteria were smokers, diabetics, people with heart and lung disease and those whose immune systems were suppressed.

HOT TUB PERIL -- The Horizon off Bermuda this year