New York City Mayor Bill de Blasio holds up a...

New York City Mayor Bill de Blasio holds up a chart documenting cases of Legionnaires' disease while speaking about the outbreak during a news conference at Lincoln Hospital in the South Bronx on Tuesday, Aug. 4, 2015. Credit: AP

The toll death from Legionnaires' disease in New York City has climbed to 10, Mayor Bill de Blasio and his deputies said Thursday, adding that they don't know the locations and total number of city cooling towers like those believed to be the origin of the flare-up.

The structures emit water mist, which transmits Legionella bacteria. The outbreak in the South Bronx -- the largest-ever in the city -- has sickened 100 people with the noncontagious form of pneumonia, de Blasio said at City Hall. Fifty-three victims have been treated and discharged, the mayor said.

City officials again said they believe they have contained the outbreak, noting that the rate of infections is ebbing and the city Health Department's "disease detectives" have traced the illnesses to cooling towers in the South Bronx.

But as a precaution, city Health Commissioner Dr. Mary T. Bassett Thursday issued an order to owners and managers of buildings citywide with cooling towers to test and clean the equipment within 14 days. Failure to comply will result in a misdemeanor charge, she said.

De Blasio also said his team remained in close contact with Gov. Andrew M. Cuomo's office about the outbreak.

The number of cooling towers covered by Bassett's order won't be known until the count is finished, de Blasio said.

About 2,500 buildings with cooling towers have been identified, including one at City Hall, but more are uncounted and city workers are "actively" adding to the list, said Bassett and officials in de Blasio's office.

The mayor said the city will introduce legislation requiring building owners to register their cooling towers.

He defended his administration against criticism that such a registry isn't already in place to protect from Legionnaires'.

"We've never seen an outbreak like this," he said. "It stands to reason that there is no such trigger. . . . If cooling towers in general were a problem, you would be having this problem all over the country."

In a statement, Cuomo said the state Health Department will offer free Legionella testing to eligible buildings. Cuomo spokeswoman Melissa DeRosa said a team from the U.S. Centers for Disease Control will meet Friday with local and state officials at the governor's midtown Manhattan office and in the affected area of the Bronx.

"This is primarily a health crisis and must be handled as such," Cuomo said. "But at the same time, we must address the needs and fears of our citizens to make sure they understand that the matter is under control."

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