Cruise outbreaks are making headlines. Are ships a breeding ground?

Oceanwide Expeditions’ MV Hondius was grappling with hantavirus cases that resulted in three deaths when more than 110 guests and crew members reported being ill in a norovirus outbreak aboard Princess Cruises’ Caribbean Princess. The latter marked the fourth outbreak of gastrointestinal illness on cruises to meet the Centers for Disease Control and Prevention’s threshold for public notification this year.

The timing may fuel a perception that cruise ships are frequently at the center of health crises. However, while certain features of the vessels are conducive to the spread of illness, experts said the events shouldn't necessarily dissuade passengers from setting sail.

Here’s what to know.

Why are cruises at the center of so many outbreaks?

The MV Hondius outbreak, which prompted a coordinated response from the World Health Organization and other international authorities, evoked aspects of the early days of the COVID-19 pandemic. In 2020, cruise ships became early sites of high-profile outbreaks, including aboard Princess Cruises’ Diamond Princess.

Unlike land-based hotels, where guests are coming and going all day, ships are a “closed environment,” according to Dr. Ian Lipkin, John Snow Professor of Epidemiology and Director of the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health.

“If somebody is ill, whether that be somebody who's a member of a crew or staff or somebody who's a passenger, the probability that they're going to run into somebody else and communicate that is much higher than if it's only a sporadic intersection of these individuals,” he said.

In addition to spending an extended period of time with other people “from different places with different exposures,” the facilities tend to be more confined, added Dr. Emily Abdoler, an infectious diseases specialist and Clinical Associate Professor of Medicine at the University of Michigan Medical School. The closer the proximity, the more likely travelers could spread illness to one another via respiratory droplets or surfaces.

“If people took weeklong airplane rides, I imagine we'd be talking about the same thing,” she said.

Infectious diseases spread in plenty of other settings, too. Norovirus outbreaks on cruises, for example, account for just 1% of all reported cases, according to the CDC.

“Part of it is, like, 'Oh, you're in the middle of the ocean, and this percentage of the passengers are having explosive diarrhea,' and it's easy to say what the problem is, whereas, if it is something that's from a public pool or a restaurant, it's just going to be harder to trace,” Abdoler said.

Cruise Lines International Association the industry’s leading trade group, said its member lines “operate under strict requirements that include advanced cleaning and disinfection procedures, continuous monitoring for illness and rapid response measures when health concerns arise.”

“Cruise ships are also subject to oversight and inspection by public health authorities in key jurisdictions, and CLIA-member oceangoing cruise lines are required to operate with onboard medical facilities and trained personnel equipped to manage a wide range of health situations,” the organization said in a statement. Oceanwide Expeditions is not a CLIA member.

How is the hantavirus outbreak different from others on cruises?

While outbreaks of gastrointestinal illness at sea are nothing new — there were 23 last year — this appears to be the first documented hantavirus outbreak tied to a cruise ship.

Hantavirus is primarily spread through exposure to the urine, droppings or saliva of infected rodents. And while officials said the confirmed cases from MV Hondius were the Andes virus, which can be transmitted from person to person, they are still working to determine the origin.

WHO official Maria Van Kerkhove previously said the organization believes the first guest to show symptoms and his wife — both of whom died — were infected on land, but that there also might have been human-to-human transmission among “really” close contacts. Officials have emphasized that the risk to the general public is low.

Some viruses, like norovirus, are “hardy,” and a very small amount can make someone sick, Lipkin says.

“That's not the case with hantaviruses,” he said. “People probably get a large dose that they've inhaled that's responsible for triggering this disease. Noroviruses, you know, a little dab will do you.”

Abdoler recommends visiting a travel medicine clinic at least several weeks before a cruise, which can provide destination-specific guidance, pre-travel vaccines and more.

Abdoler notes, though, that there is a much higher risk of catching other kinds of viruses, like influenza, while sailing. “I don't think this particular situation should change anyone's calculation than what they've had before.”