The fast-moving stomach bug that affected more than 700 people aboard the Royal Caribbean Explorer, causing the ship to return two days early, has been identified as a new strain of norovirus.
According to NBC Health, the Centers for Disease Control and Prevention confirmed on Thursday that the virus responsible for the chaos overseas was identified as the GII.4 Sydney strain, a contagious germ detected two years ago. The virus is said to have originated in Australia.
"This particular strain emerged in late 2012 and quickly became the predominant strain causing outbreaks not only on cruise ships but also in land-based outbreaks," Bernadette Burden, a spokeswoman for the Centers for Disease Control and Prevention told NBC Health.
The GII.4 Sydney strain is reportedly to blame for the early return of two ships that carried both infected passengers and crew members.
Earlier this month, NBC reported that the Royal Caribbean's Explorer of the Seas cut its 10-day trip short and returned to the New Jersey docks after 634 passengers and 55 crew members became infected with the highly contagious virus. The 1,000-foot cruise ship was reportedly carrying 3,071 passengers and 1,116 crew members.
The recent incident on the Royal Caribbean's Explorer of the Seas has been reported as the largest norovirus outbreak on a cruise ship in the last 20 years.
The Caribbean Princess was also forced to cut short its seven-day trip and return two days early to the Bayport Cruise Terminal in Houston, Texas, after health officials reported a norovirus outbreak. The virus reportedly infected 178 passengers and 11 crew members aboard the ship carrying more than 4,200 people.
According to the CDC, Norovirus often causes nausea, vomiting and diarrhea. CDC reported that the GII.4 Sydney strain is the most common norovirus of about 20 different strains nationally found.
"The norovirus is a very democratic virus, it affects everybody," said Jan Vinje, the head virologist who tested the Royal Caribbean germ samples in the CDC's lab. "If we look at the land outbreaks, the cruise ships make up less than one percent [of the cases]."
Vinje said, like the flu, the norovirus mutates quickly and develops new strains every few years.
"Our Australian friends detected the new strain six months before it came here," Vinje said. "Three years before that, we discovered one in the U.S. It was an event in New Orleans."
The GII.4 Sydney strain has also been found in several nursing homes. Vinje said that the virus is often carried aboard cruise ships by a passenger and then is spread through contact. Despite the heavy cleaning, the virus can reportedly remain intact.
"The virus is not killed by alcohol sanitizers. It survives and can be infectious on surfaces," Dr. Richard Besser, ABC News' chief medical correspondent, said. "People who are infected will shed the virus for days after their symptoms resolve. This is a real problem for food workers on cruise ships. Lastly, if you go into a bathroom that has been used by someone who has the norovirus, you can get sick."
Norovirus is the leading cause of gastrointestinal in the United States. It reportedly has caused 21 million infections and about 800 deaths each year, according to the CDC. Norovirus is often spread by infected people or contaminated food or water.