In the second such instance this week, a cruise ship returned to its home port early after a significant number of passengers came down with a gastrointestinal illness. Princess Cruises' Caribbean Princess arrived back in Houston late Thursday, after more than 160 of the ship's 3,104 passengers experienced symptoms consistent with a norovirus infection, according to USA Today.

In a statement issued late today, the cruise line said the reason for bringing the Caribbean Princess back early was not the norovirus outbreak — "despite some media reports" — but an impending "fog event" that would likely close the port of Houston for much of the weekend. Princess Cruises' statement further explained:

"Regarding the illnesses onboard this past cruise, Caribbean Princess experienced an increase in the number of reported cases of gastroenteritis among passengers, which was confirmed to be caused by norovirus, a common gastrointestinal illness which is currently widely circulating throughout North America. In response, we immediately implemented aggressive and comprehensive disinfection measures developed in conjunction with the U.S. Centers for Disease Control and Prevention (CDC). As a result of our actions, case numbers declined significantly and by the end of the cruise there were no passengers with active symptoms. Over the course of the cruise, 178 passengers (5.7%) and 11 crew (1%) reported ill to the Medical Center.

"We notified the CDC, who boarded Caribbean Princess this morning to oversee the extensive sanitation program planned over two days. The next cruise will depart tomorrow as scheduled, Feb. 1."

In its article, USA Today noted: "Princess says officials from the CDC will be boarding the Caribbean Princess today to monitor an extensive sanitizing of the vessel before its next scheduled sailing on Saturday. Passengers on the cruise that ended early will receive a future cruise credit of 20% of their fare, as well as a one day per diem to help offset ancillary expenses such as meals, the line adds."

As USA Today explained, cruise ships arriving in U.S. ports must report all cases of gastrointestinal illness treated by on-board medical staff to the CDC's Vessel Sanitation Program division, and a separate notification is required when the number of cases exceeds 2% of passengers and crew. When the number of cases exceeds 3% of passengers and crew, the CDC issues a public report.

On Jan. 27, the CDC reported that a ship owned by Royal Caribbean Cruise Line had prematurely ended a 10-day Caribbean cruise the previous day, after 577 passengers and 49 crew members came down with gastrointestinal illness symptoms. The cause of the outbreak was deemed "yet to be determined." In an update today, the CDC revised the numbers of those affected to 634 passengers and 55 crew members, and confirmed that norovirus was, in fact, the cause of that outbreak.

Editor's Note: For information that may help put these latest norovirus outbreaks into context, see the CDC's Norovirus Trends and Outbreaks page and its Outbreak Updates for International Cruise Ships page.