A dangerous experiment is playing out on a cruise ship with hantavirus

The tragic and fatal outbreak onboard a luxury cruise ship highlights the gaps in research and treatments for the rare and mysterious disease—including its spread among people

By Adam Kovac edited by Claire Cameron

The fatal outbreak of hantavirus on a cruise ship is a tragic case study in how the rare and mysterious virus spreads among people. Little is known about the mechanisms of human to human transmission, experts say, largely because of how rare such outbreaks are.

“This is kind a unique opportunity to try to understand what the virus is doing,” says Alison Kell, a virologist at the University of New Mexico. The World Health Organization says it suspects human to human transmission is driving the outbreak, which has seen at least eight people onboard the M/S Hondius sickened or died with suspected or confirmed cases of hantavirus. Several of these have been identified as being infected with the Andes variant of the virus, which is the only kind of hantavirus known to be capable of human to human transmission.

Hantavirus typically spreads when humans come into contact with infected rodents and their urine or feces. The outbreak currently plaguing the M/S Hondius is only the second confirmed outbreak where hantavirus was transmitted from person to person. In 2018, 34 people were infected and 11 died in an outbreak that started at a party in Argentina. Prior to that, there had been reports of human to human spread, but researchers couldn’t rule out that they had both been exposed to rodents, says Kell.

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Three people have died as a result of the infection, which can cause fluid build up in the lungs and hemmorhagic fever among other symptoms. Another three, including the ship's doctor, have been medically evacuated to receive treatment in the Netherlands. And a passenger who was on the ship and who disembarked in Saint Helena has also tested positive for the virus and is receiving treatment at the University Hospital Zurich in Switzerland.

Access to the ship is limited and the ship is experiencing trouble docking anywhere. Passengers still onboard have been taking isolation precautions to try and prevent further spread, but Kell says researchers will want to gather samples of the virus from the ship in order to sequence them. That would allow them to determine if “this strain of Andes Virus might be different from other hantaviruses, or other strains of Andes hantavirus that don't appear to cause human human transmission,” she says.

Any additional knowledge would be a boon for understanding a virus that’s largely a mystery, at least in part due to its rarity. While the mortality rate from hantavirus can be as high as 50%, fewer than 1,000 cases were recorded in the U.S. between 1993 and 2023, according to the U.S. Centers of Disease Control and Prevention.

While there’s reason to believe this particular strain of the Andes hantavirus can spread through exposure to saliva, it’s unclear if that extends to airborne saliva droplets, such as those spread through a cough or a sneeze. Virologists have long assumed “that the virus just is not very efficiently transmitted human to human, because the small outbreaks that have occurred in the past have always involved either family members or healthcare workers who have prolonged, very close contact with infected patients,” says Scott Weaver, a professor of human infections and immunity at the University of Texas Medical Branch and director of the Global Virus Network Center of Excellence.

There’s not much direct evidence of airborne spread, Weaver says, but the limited data and difficulty of studying an ongoing outbreak such as this one means it can’t be fully ruled out. Public health officials are already responding to that uncertainty. South African Health Minister Aaron Motsoaledi said in a press release that 62 other people who may have been exposed to the virus have been identified, including medical workers in several cities where affected passengers received care.

“The total number of people who were traced and who could have come in contact with them were 62. [Some] 42 of them have already been traced and they are being observed. The work is ongoing,” Motsoaledi said.

The ship’s other passengers remain in limbo and have been unable to leave. Plans are being worked out for the Hondius to sail to the Canary Islands to berth, but the exact point of arrival and any quarantine regimen for the passengers isn't yet clear.

The precautions are necessary, says Colleen Jonsson, a virologist at the University of Tennessee Health Science Center, because of the possibility of the virus mutating to become more virulent. As witnessed during the COVID pandemic, viruses can mutate as they pass from person to person, and those changes to their genetic structure can make them more dangerous. Indeed, the high mortality rate of hantavirus has led some public health researchers to raise the pathogen as a candidate to watch, as if it was more easily spread, it could spark a particularly horrific pandemic.

Still, Kell says she doesn't think the passengers on the cruise ship are likely to cause a global incident. While biology can always surprise, “because there are so limited cases, there are more limited opportunities for mutation and subsequent spread,” she says. “I'm not going to discount the possibility that that could happen. I would mark it, from my personal opinion, as relatively low risk, but something to watch for sure.”