MELBOURNE (Reuters) - Australian health authorities issued a worldwide alert on Thursday warning tourists who visited a Melbourne aquarium they could be at risk from a deadly Legionnaire's disease outbreak that has killed two people.

Nine are in critical condition in hospital.

International tourists were among the thousands of people to visit the center last month and are believed to have been exposed to the disease through the aquarium's air conditioning system.

Victoria state health officials said on Thursday test results had proved the air conditioning unit was the source of the outbreak and that all the confirmed victims had been in or near the aquarium between April 11 and 25. The disease can incubate for up to 10 days.

State health officials said the confirmed number of cases of the flu-like disease had risen by eight in the past day to 66, including two elderly women who died last week.

All the confirmed cases are Australian. Another 41 people are thought to have the disease after developing respiratory illnesses and are waiting for test results.

That group includes two people from New Zealand, one from Britain and one from the United States.

In its international alert, the Communicable Disease Network Australia and New Zealand (CDNANZ) said all tourists who went to the aquarium must see their doctors immediately if they develop respiratory illness.

``We are concerned there may be more travelers who are at risk,'' CDNANZ spokesman Dr John Carnie said in a statement.

Few details are available about the overseas tourists thought to have the disease. New Zealand officials said a 16-year-old girl was thought to have been infected while visiting the center.

The warning comes as Victoria faced a second health scare on Thursday after the Royal Melbourne Hospital said nine of its patients were possibly exposed to the degenerative brain disease Creutzfeldt-Jakob Disease after undergoing surgery with potentially contaminated instruments.

Possibly Thousands Exposed

Victoria Human Services Department officials have said up to 4,500 people visited the aquarium a day in mid-April but it was impossible to know how many of those were from overseas.

The department's Web site is www.dhs.vic.gov.au

The Human Services Department said tests had confirmed the presence of legionella bacteria in samples taken from the cooling towers of the aquarium.

The aquarium's air conditioning unit has been disinfected and the center remains open.

About 2,500 people have been tested for the disease since last Thursday.

``We would expect the number of cases from this outbreak to start tapering off now,'' Carnie said. ``The incubation period for this illness is from two to 10 days, so most people who could have been infected should have shown up by now.''

State health official Bram Alexander said another woman had been found suffering from Legionnaire's but she had not been to the aquarium and her case was regarded as an isolated incident.

He told Reuters 46 other ``sporadic'' Legionnaire's cases had been detected in Australia this year.

Legionnaire's flu-like symptoms include headaches, fevers, chills and muscle aches, followed by respiratory problems and pneumonia. The disease is fatal in about 10 percent of cases.

Infection is not easily spread from person to person and is usually acquired through breathing in very fine droplets of water which contain the bacteria, such as spray drifts which are vented off from air conditioning towers.

Australia's worst outbreak of the disease was in the New South Wales town of Wollongong in 1987, when 10 people died among 44 confirmed cases.

The disease was first detected in the United States in 1976.