The guests who contracted the lung infection stayed separately in March and April at the Rio All-Suite Hotel and Casino, the Southern Nevada Health District said.
Legionella, the bacteria that causes the disease, was found in the hot-water system of one of the hotel's towers during a test after the second illness was reported in late May, said Mark Bergtholdt, the district's environmental health supervisor.
The hotel is disinfecting the system with chlorine, Bergtholdt said. Guests were relocated from rooms served by that system, and those rooms won't be occupied until the system is properly flushed, he said.
Most healthy people who are exposed to legionella don't get sick, the CDC says
. But those who do usually develop fever, cough, chills or muscle aches. Symptoms usually begin within two to 10 days after exposure to the bacteria.
"Guests who stayed at the Rio more than two weeks ago and have not developed symptoms are not at risk for disease," the health district said in a statement.
Health officials contacted the hotel after determining the Rio was the only place the two patients stayed at while in Las Vegas, Bergtholdt said.
"The company is working closely with the Southern Nevada Health District and taking aggressive remediation actions to ensure the safety of Rio's water," said Caesars Entertainment, the hotel's parent company.
Most patients recover after treatment with antibiotics. About 10% die because of complications from their illness, the CDC says
Where the bacteria can be found
The bacteria that causes Legionnaires' disease can be found in "freshwater environments, like lakes and streams" and can grow in "human-made" water systems such as hot tubs, large plumbing systems, decorative fountains and hot water tanks, the CDC says.
The disease got its name from an 1976 outbreak in Philadelphia that largely affected people attending an American Legion convention. Thirty-four people died in the ensuing weeks.
In 2011, a person who stayed at the Luxor hotel in Las Vegas died after contracting Legionnaires' disease
, the health district said.
The Rio is hosting the 48th World Series of Poker
, which started late last month and will conclude with the main event in mid-July.