Federal health officials confirmed on Friday that norovirus was the culprit that sickened nearly 700 people on a cruise ship this week, and said it was one of the biggest norovirus outbreaks in 20 years.
But the source of the outbreak on the Royal Caribbean ship Explorer of the Seas, which returned early to New Jersey on Wednesday, may never be known, the Centers for Disease Control and Prevention said.
"CDC has been investigating the outbreak since last Sunday but no particular source has been identified and it’s quite possible a source won’t be identified," the CDC said in a statement.
The report comes after passengers streamed off the Caribbean Princess Friday morning, the second cruise cut short this week amid reports of illness on board.
The ship, operated by Princess Cruises, returned to Houston a day early with a confirmed outbreak of norovirus. "The ship was forced to return to Houston one day early because we were informed that dense fog was expected to close the port for much of the weekend," the company said in a statement.
"The ship did not return early because of the increased incidence of norovirus on board, despite some media reports."
At least 178 people on board became ill during the cruise, according to the cruise line and the CDC. Sick patients were quarantined to their rooms, and other passengers said they no longer had access to buffet tongs as crew members handed out hand sanitizer.
CDC health officials met the Caribbean Princess at the Bayport Cruise Terminal in Pasadena, Texas. The vessel launched on a seven-day cruise to the western Caribbean on Jan. 25 and had been scheduled to return on Saturday.
Princess Cruises said the outbreak was over by the time the ship returned to Houston. "As a result of our actions, case numbers declined significantly and by the end of the cruise there were no passengers with active symptoms," the company said. "Over the course of the 178 passengers (5.7 percent) and 11 crew (1 percent) reported ill to the Medical Center."
CDC officials also helped Royal Caribbean clean up the Explorer of the Seas, and said it had been approved to go back out again with a new batch of passengers Friday afternoon. Royal Caribbean officials say they cleaned the ship, which carried more than 3,000 passengers, three times.
It's the third cruise ship outbreak to occur this year. A Norwegian Cruise Line ship, the Norwegian Star, reported that 130 passengers and 12 crew members became ill on two-week cruise that launched Jan. 5 from Miami.
About 20 million passengers take cruises in the U.S. each year, fueling a $37.8 billion annual industry, according to the American Association of Port Authorities. There were nine vessel outbreaks in 2013 and 16 in 2012, according to the CDC.
Norovirus is a common culprit in outbreaks on cruise ships, in nursing homes and other confined places. It is a fast-moving gut bug typically spread by infected people or contaminated food or water. Norovirus is the most common cause of acute gastroenteritis in the U.S., resulting in about 21 million illnesses, between 56,000 and 71,000 hospitalizations and as many as 800 deaths, the CDC says.
The virus lingers on surfaces and spreads very easily. Thorough handwashing with hot water and soap and meticulous environmental cleaning can help stop the spread.
CDC says it's the season for norovirus. "Norovirus outbreaks wit high attack rates are common during this time of year," the agency said. "Most outbreaks occur between January and April."
KPRC’s Amanda Perez and The Associated Press contributed to this report.