What the (rather brief) history of hantavirus reveals about its spread

Scientists think the hantavirus, the deadly pathogen that has infected 11 passengers on a Dutch cruise ship, could be as old as humans. But much of their understanding of human cases comes from a handful of outbreaks within the past century.

The first known outbreak came during the Korean War in the 1950s, when around 3,000 United Nations troops developed a mysterious illness that scientists would later recognize as hantavirus.

But it wasn’t until some 20 years later, in 1978, that scientists linked the virus to a rodent near the Hantan river in South Korea, which gave hantavirus its name.

The first cases in North America were diagnosed in 1993. And in 1996, an outbreak in Argentina marked the first confirmed instance of person-to-person transmission.

Hantavirus is rarer, but far deadlier than respiratory viruses like Covid or flu. Only one strain, which is not found locally in the U.S., is capable of passing between humans. That strain is the one involved in the cruise ship outbreak, which has forced passengers from across the world to quarantine.

The limited number of past outbreaks could offer clues about how the cruise ship passengers got sick — but doctors who have studied hantavirus said there’s more to learn.

“Because the disease is so incredibly rare, it’s been very difficult to study in the sense that we don’t really have the numbers,” said Dr. Charles Chiu, a professor of laboratory medicine and infectious diseases at University of California, San Francisco.

Fewer than 900 cases were reported in the U.S. from 1993 to 2023. Cases can also be hard to investigate because many people die quickly after developing symptoms. Up to half of hantavirus cases can be fatal, depending on the strain and method of transmission.

“The current outbreak will teach us more, because we’ve had very few of these events with more than a handful of patients,” said Dr. Gregory Mertz, emeritus professor of internal medicine at the University of New Mexico.

Here’s what we know about the major outbreaks over the last several decades.

Outbreaks in the Four Corners and Yosemite

In 1993, public health officials began investigating a mysterious cluster of illnesses in the “Four Corners” region of the United States, where Arizona, Colorado, New Mexico and Utah meet.

The first case to raise alarm was a 19-year-old long distance runner in New Mexico whose fiancée had just died of an unknown respiratory illness. The man sought medical care twice — first with a fever, muscle aches, chills, headache and fatigue, then with vomiting and diarrhea. But each time, his physical exam was normal, so he was sent home. He died after attending his fiancée’s funeral.

“There were physicians that recognized, through seeing or hearing about multiple cases, that maybe we were seeing something that we didn’t understand and we hadn’t learned about in medical school,” said Mertz, who helped investigate the outbreak.

The Centers for Disease Control and Prevention ultimately confirmed the illnesses as hantavirus based on antibody testing and autopsies.

At the time, the virus wasn’t known to infect humans in North America. But this particular strain — known as “Sin Nombre” — was linked to a deer mouse that had become more prevalent in the Four Corners after unusually heavy rain and snow that spring. By August 1993, around 30 cases had been identified in the southwestern U.S., 20 of whom died.

Most human cases of hantavirus in North America are the Sin Nombre strain and are acquired through contact with the aerosolized urine, feces or saliva of infected rodents. The strain involved in the cruise ship outbreak, the Andes strain, is not found in North America and is linked to the pygmy rice rat that’s native in parts of Argentina and Chile.

“You have to be kind of unlucky to get Sin Nombre virus … There’s this little window where the virus is still viable, but the mouse pee or the feces are dry enough that they can be stirred up into the air and people can inhale it,” said Dr. Tom Boo, the public health officer for Mono County, a rural area in California that has recorded 27 hantavirus cases since 1993 — the most in the state.

In 2012, 10 people contracted the Sin Nombre strain after staying overnight at Yosemite National Park in California. All but one camper slept in cabins that were later found to have rodents in the wall insulation. Three of the campers died.

“Some campers who maybe did not air out the cabin may have had a higher exposure to the virus, which led to the infection,” said Chiu, who helped sequence the virus and test people during the outbreak.

Hantavirus gained further attention last year after Betsy Arakawa, the wife of Oscar-winning actor Gene Hackman, died of the virus at the couple’s home in Santa Fe, New Mexico. Hackman died roughly a week later. Authorities said at the time that the actor, who had advanced Alzheimer’s, may not have been aware of his wife’s passing. A state assessment found rodents and rodent feces on the property, but not in the main living quarters.

Human-to-human transmission in Argentina

The Andes strain is the only hantavirus known to spread from person to person.

The first recognized cases occurred in 1996 across three towns in southern Argentina. Three of the 18 cases were doctors caring directly for an infected patient.

Scientists learned more about human transmission after so-called “super-spreader” events in Argentina in 2018 and 2019. The events led to 34 infections and 11 deaths.

The outbreak started after a person attended a birthday party for 90 minutes with a fever and muscle aches, and presumably infected five others.

Another infected person attended her husband’s wake while she had a fever. Ten people who had close contact with her became ill.

Public health authorities eventually contained the outbreak by isolating symptomatic patients and telling their close contacts to stay home. Before those measures were taken, one infected person was spreading the virus to about two others, on average.

A 2020 paper in the New England Journal of Medicine determined that patients with a high viral load were more likely to spread the virus, and that transmission was more likely to occur at massive social gatherings or if someone had extensive contact with an infected person.

“There is a possibility of spread in an enclosed, cramped space,” Chiu said.

The cruise ship outbreak meets that criteria, he said — especially if people shared a room with an infected person or had a drink with them at the bar for several hours.

“At least in terms of the behavior of the virus currently, how we’re seeing these cases evolve, there’s nothing that’s inconsistent with what we already know about the virus or suspect about this virus,” Chiu said.

This article was originally published on NBCNews.com