After years of decline, norovirus outbreaks surge on cruise ships

(CNN) — Whether from Miami, New York, Seattle or more, millions of Americans pack aboard cruise ships on vacation. But when it comes to the health of those travelers, it’s not always smooth sailing.

There have been 13 outbreaks of norovirus on cruises this year, according to the US Centers for Disease Control and Prevention. That’s the highest number of norovirus outbreaks on cruises recorded since 2012, with almost half of the calendar year left to go.

The most recent norovirus outbreak occurred on a Viking Cruises voyage from Iceland that docked in New York City on June 20. About 13% of passengers and several crew members fell ill on board.

“We believe the gastrointestinal illness originated from a shoreside restaurant in Iceland where a group of guests dined during their free time,” a Viking representative told CNN.

‘An extraordinarily contagious virus’

Norovirus is a highly infectious virus that causes inflammation in the stomach and intestines, a condition called acute gastroenteritis. Often labeled a “stomach bug,” norovirus is the most common cause of nausea, vomiting, diarrhea and stomach pain, according to the CDC.

A person can get norovirus by accidentally ingesting microscopic particles of feces or vomit – which could happen when coming into contact with someone who’s infected, consuming contaminated food or water or touching contaminated surfaces. Although symptoms usually last only a few days, a person can be infectious over two weeks later.

“This is an extraordinarily contagious virus,” said Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University. “It takes but a few normal viral particles to initiate an infection in someone who’s exposed. In other words, this doesn’t take a large dose; it takes only a little bit.”

While there is no medicine to treat the illness, most people fully recover without treatment. Hydration therapy to replace lost fluids from vomiting or diarrhea is standard practice to address symptoms.

Still, to prevent the spread of norovirus, Schaffner recommends that cruise passengers take extra precautions and wash their hands thoroughly with soap and warm water. Antiseptic gels and hand sanitizers are not effective against the virus.

“As far as hand washing goes, we’ve all been doing it since we’ve been children,” said Jeffrey Fisher, an associate professor of nutrition and dietetics at Central Michigan University. “But there’ve been a lot of studies done that say we’re not doing it well enough, or as often as we should. So we want to revisit best hand washing practices.”

Surging cases

The exact cause of this year’s uptick in cruise cases remains unclear, but experts believe that soaring demand for cruises and record numbers of passengers might be behind it.

According to data from the CDC’s Vessel Sanitation Program, the number of norovirus outbreaks on cruise ships docking at U.S. ports had years of steady decline after 2015. Overall rates of acute gastroenteritis on cruise ships in the U.S. also decreased from 2006 to 2019.

In response to the Covid-19 pandemic, the CDC instituted a travel health notice that recommended against cruise ship travel over safety concerns. Cruising was limited, CDC spokesperson Kathleen Conley said, and outbreaks were few as the number of passengers on board decreased.

In fact, the program recorded no norovirus outbreaks in 2020 and 2021, likely due to limited cruising and updated sanitation protocols through the Covid-19 pandemic.

In March 2022 though, the agency lifted that risk advisory for cruise travel, and passengers are returning at record rates.

31.5 million passengers worldwide are expected to set sail this year, surpassing pre-pandemic levels and creating high-density environments that are ripe for transmission.

The CDC releases information about outbreaks on cruises that have over 100 passengers traveling between three and 21 days, and when over 3% of passengers and crew report symptoms.

Those conditions have been met 13 times this year, up from twice in 2022.

Four norovirus outbreaks, involving a total of 449 passengers and crew members, have happened aboard Royal Caribbean ships this year, the most of any individual cruise company, according to CDC data.

“The health and safety of our guests, crew and communities we visit are our top priority,” a spokesperson from Royal Caribbean International told CNN in an email. “To maintain the highest levels of health onboard our ships, we implement rigorous safety and cleaning procedures, many far exceeding public health guidelines.”

A perfect storm

Compared with the general population, norovirus outbreaks are fairly uncommon on cruise ships. It infects 19 million to 21 million people in the US each year, according to the CDC, compared with a few thousand cases aboard cruises.

Transmission often occurs in crowded environments where tiny particles can float through the air, and Schaffner says cruises can create the perfect environment for norovirus outbreaks. A large group living and eating in proximity, he said, can serve as a breeding ground for the disease.

“They’re an enclosed population: a very large, compactly packaged population that’s together for long periods of time, often in really tight quarters,” he said. “So there are just very many opportunities for people to encounter each other … making it relatively easy for this virus to be transmitted.”

Norovirus symptoms can also come on suddenly, he said. A passenger could be walking to their cabin or attending an event and abruptly begin throwing up. That vomit is aerosolized, and those floating microscopic particles can infect people nearby.

“You have this highly transmissible virus that’s introduced into an environment, the cruise ship, which is ideally designed for the rapid spread of a communicable disease,” Schaffner said.

Although symptoms like diarrhea and vomiting often fade away in a few days, they can also cause dehydration. For older passengers who may make up a disproportionate number of cruise clientele, Schaffner warned, rapid fluid loss can create the risk of more serious illness. For people with diabetes, dehydration can throw off treatment strategies.

Fisher thinks that much of the problem stems from a “knowledge gap” about the virus among the public. “People are relaxing” as the Covid-19 pandemic appears to wind down, rather than remaining vigilant about spreading disease.

“I think a lot of the public don’t even understand norovirus, let alone how to begin to protect themselves,” he said. “They’re not taking those precautions, those good protection behaviors they learned over the pandemic.”

Schaffner also suspects that people feeling ill before boarding are now more inclined to set sail. Eager for vacation after a 2½-year pandemic delay, many could be bringing norovirus with them, he suggested.

“The first thing that passengers can do is postpone their trip if they’re not feeling well,” he recommended. “Try to limit the exposure of others on the front end, and take another cruise a month later.”

Besides monitoring disease outbreaks on board, the CDC’s Vessel Sanitation Program also requires periodic illness reports from cruise ships, conducts regular unannounced inspections, and conducts training for cruise ship employees about public health.

As precautions, the CDC advises passengers to thoroughly wash their hands, avoid contaminated food and keep their hands away from their mouths. If a passenger does become ill, Schaffner and the CDC both recommend staying in their cabins and notifying the ship’s medical team immediately.

“Let them take over and care for you,” Schaffner said. “Don’t go out and about and spread the virus.”