It has been 11 days since UW Medicine officials installed filters, flushed water systems and took other steps in the hospital’s Cascade Tower to halt an outbreak of Legionnaires’ disease. The incubation period is two to 10 days, health officials said.

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An outbreak of Legionnaires’ disease at the University of Washington Medical Center that sickened four people, including two who died, appears to be contained, King County health officials said Saturday.

It has been 11 days since UW Medicine officials installed filters, flushed water systems and took other steps in the hospital’s Cascade Tower to prevent the spread of Legionella bacteria, which cause the type of serious pneumonia. The incubation period for the disease is two to 10 days, officials with Public Health — Seattle & King County said.

“Although it is possible additional cases will be identified, the absence of new cases suggests that the steps taken by UWMC to eliminate Legionella transmission are working,” they reported in a blog post Saturday.

An investigation is not complete, but it points so far toward the water system in Cascade Tower, not heater-cooler units used in heart surgeries, as the likely source of the outbreak, officials said.

The heater-cooler units, used to maintain patients’ temperature during surgeries, have been associated with other types of infections. Three of the 12 CardioQuip heater-coolers used at UW Medicine tested positive for the bacteria, and two of the four patients infected were exposed to the machines, including one who died.

But Dr. Jeff Duchin, King County health officer, said there is a “long list” of reasons that the heater-coolers are likely not the culprit.

No Legionella infections have previously been associated with the devices, two of the patients had no contact with the units, and those who did were on ventilators that protected their lungs from bacteria. In addition, the timing of the Legionella infections in the patients exposed to the devices was at the end or outside of the typical incubation period.

Instead, it’s more likely that the water system in Cascade Tower was contaminated with the bacteria, which were found in sinks and an ice machine, as well as the heater-cooler units.

“We assume that anything that came in contact with water in that building could have Legionella,” Duchin said.

The water and ice used in the heater-cooler machines are typically supplied by tap water from the tower system.

UW Medicine officials flushed the water system with chlorine and installed point-of-use filters on faucets, showers and other water sources in the tower.

The investigation remains active and test results are pending.

It follows reports of four cases of Legionella in UW Medicine patients. They included a 32-year-old woman reported on Aug. 26 and a man in his 50s reported on Sept. 6. He died on Sept. 8

A woman in her 50s who died Aug. 27 had a Legionella infection detected during autopsy. An infection in a man in his 40s was reported Sept. 16.

Legionnaires’ disease occurs when people breathe in mist or vapor contaminated with the bacteria. Healthy people exposed to the bacteria typically don’t get sick, but it can be dangerous, even deadly, to people older than 50, former or current smokers and those with weakened immune systems.