Health officials with the Centers for Disease Control and Prevention are conducting an environmental health assessment on a cruise ship docked in Baltimore after more than 150 passengers reported falling ill with vomiting and diarrhea.
The CDC said the cause was norovirus, which causes inflammation in the stomach and intestines and is considered highly contagious.
The virus is capable of clinging to hard surfaces and has a history of spreading on cruise ships, which feature confined spaces and shared eating areas, like buffet lines. Norovirus is the leading cause of food-borne illness in the United States, according to the CDC.
Of the 917 passengers on the ship, the Balmoral, 153 reported feeling ill, the CDC said. Six of 518 crew members were ill, it said.
The CDC said specimens collected on the ship and tested onboard using "norovirus rapid tests" came back showing norovirus, but that the specimens would be further tested by the CDC. The health agency was working with officials at Fred. Olsen Cruise Lines, which owns the Balmoral, to ensure a proper response.
The illnesses were reported while the ship was in Norfolk, but it has since arrived in Baltimore. The ship is on a 34-night "Old England to New England" cruise, which departed from Southampton in England on April 16.
Officials with Fred. Olsen Cruise Lines said they are "undertaking extensive sanitisation measures and cleaning of the ship, in accordance with its strict illness containment and prevention plan."
The cruise line said just seven guests were under isolation. The CDC requires 24-hour isolation, though Fred. Olsen requires 48-hour isolation. The cruise line said "incidences have substantially reduced" since the illnesses began.
Most onboard the ship are Britons; two are from the United States.
"Fred. Olsen is co-operating fully with all the necessary maritime agencies and authorities, as Balmoral continues on her cruise, and will continue to make every effort possible to ensure the safety and well-being of all its guests and crew on board, which is of paramount importance," the cruise line said.
This is not the first time norovirus has caused illnesses on board ships that have passed through Baltimore. More than 80 percent of the nation's outbreaks occur from November to April.
Along with two environmental health officers, the CDC said an epidemiologist would board the Balmoral.
Fred. Olsen has asked guests affected to remain in their rooms, "with complimentary room service and in-room entertainment."