(CNN) -- The outbreak of a gastrointestinal illness has spread to more passengers on a U.S. cruise ship, with nearly 450 falling sick while at sea, the cruise company said Tuesday.
The Celebrity Cruises ship Mercury departed Charleston, South Carolina, February 15 and headed for the eastern Caribbean, according to a Celebrity Cruises statement. On the way, 419 of the 1,838 passengers fell ill, along with 27 of the 849 crew members, the cruise line said. The number of affected passengers increased from the 353 reported the day before.
Their symptoms included upset stomach, vomiting and diarrhea, Celebrity Cruises spokeswoman Cynthia Martinez said. Guests who were in isolation while ill will receive compensation, she said.
The sickened passengers and crew have been administered over-the-counter medicine and are responding well, Celebrity Cruises said.
But it is still unclear how they became sick, Martinez said.
Medical samples will be sent to a lab for testing Tuesday, she added.
The ship's medical facility first started treating guests Sunday, Martinez said, and by Monday, hundreds of others were sick, too, Celebrity Cruises reported.
A doctor and two nurses joined the cruise medical staff Monday to help with the overload of patients when the ship stopped in Tortola, British Virgin Islands, according to the statement. The ship does not return to Charleston until Friday and will be at sea until then, Martinez said.
To control the outbreak, the crew has stepped up cleaning of the ship, which is advised by the Centers for Disease Control and Prevention when an outbreak of gastrointestinal illness occurs.
Norovirus commonly causes viral gastrointestinal illness outbreaks on cruise ships, the CDC says. It can spread from contact with contaminated food or drink, by touching objects infected by people who are already sickened, or through close contact with people who are infected, according to the CDC.
So far this year, three gastrointestinal illness outbreaks have occurred on cruise ships that docked at a U.S. port, according to the CDC. Norovirus was the cause of two outbreaks on the Mercury in 2009, the CDC reported. The outbreaks reported and investigated by the CDC infected at least 3 percent of the people onboard the cruises carrying at least 100 passengers for anytime between three days to three weeks.