A Royal Caribbean cruise ship is headed back to New Jersey two days early after an outbreak of gastrointestinal illness spiked over the weekend, the cruise line says.

A report posted online by the Centers for Disease Control and Prevention says 577 of the ship's 3,050 passengers, or 18.9%, and 49 of 1,165 crew, or 4.2%, reported symptoms such as vomiting and diarrhea during the voyage.

The cruise line announced the change in itinerary Sunday after inviting U.S. health inspectors to investigate and evaluate conditions on its Explorer of the Seas cruise ship during a port call in the U.S. Virgin Islands.

The ship launched Jan. 21 from Cape Liberty, N.J., and was scheduled to return Friday. Royal Caribbean says Explorer of the Seas instead will return to Cape Liberty on Wednesday.

Earlier this month, the CDC reported a similar outbreak on a Norwegian Cruise Line ship. Last year, it reported nine cases, including one involving a Royal Caribbean ship.

In most cases, the cause was found to be norovirus, a gastrointestinal illness that spreads swiftly when large groups of people are contained in a small area such as dormitories, nursing homes and cruise ships.

The CDC reported last year that a new strain of norovirus had reached the USA from Australia. The norovirus season typically runs from November through March and peaks in January. Norovirus usually begins suddenly and lasts one to three days. Most people recover without treatment, but some require rehydration with liquids or intravenous fluids.

Dr. Andrew Coggins, who teaches courses on travel and tourism management at Pace University, says a passenger likely brought the virus onboard after his or her symptoms subsided. "Last-minute cancellations usually result in forfeiting the cruise fare so if someone doesn't have travel insurance they are going to try to make the cruise as long as they can get from toilet to toilet," he said.

"RCI is taking the right action," Coggins said. "By returning early, they will get everyone off the ship and can do a thorough top-to-bottom disinfection. It will be very expensive in terms of lost revenue, refunds, and bad publicity, but they will protect the next sailing."

Royal Caribbean said Sunday that reports of illness peaked during the first few days of the cruise. It said cleaning products and disinfectants proven to kill norovirus were being used to clean the ship before it returned to the USA. It said a full sanitization program would be carried out after the Explorer of the Seas reached its home port.

Janet Diaz, a company spokeswoman, said the ship underwent "extensive and thorough sanitizing" during its previous port call in Puerto Rico to help prevent more people from getting sick.

"New reports of illness have decreased day-over-day, and many guests are again up and about. Nevertheless, the disruptions caused by the early wave of illness means that we were unable to deliver the vacation our guests were expecting," Royal Caribbean Cruises said in a statement.

Passengers and crew who fell ill have "responded well to over-the-counter medication being administered on board the ship," Diaz said.

On Friday, an Explorer of the Seas passenger named Arnee Dodd tweeted that she had fallen ill aboard the ship and was quarantined with other sick people. The Connecticut woman wrote that ship employees "put a lockdown on food & are constantly cleaning everything."

Royal Caribbean said it was "taking several steps" to compensate passengers for the shortened trip. It also sought to assure customers scheduled for the ship's next voyage that "all possible measures will have been taken to prevent further problems."

Contributing: Associated Press