Officials are investigating the origin of the outbreak of hantavirus on the MV Hondius cruise ship that departed Argentina’s southernmost city, Ushuaia, last month. One theory that has gained prominence in media coverage holds that the Dutch husband and wife who were the first to show symptoms of hantavirus and later die from it picked up the virus while bird-watching at a landfill in Ushuaia before the cruise. But a closer look at the publicly available evidence reveals reasons not to put much stock in this scenario.

Here’s what we know so far. To date, there have been 11 reported cases of hantavirus from the cruise ship outbreak, nine of which have been confirmed; three people have died. Health authorities studying the cause of the outbreak are currently focused on the Dutch citizens who showed the first symptoms of hantavirus and who subsequently died: a 70-year-old man who developed symptoms on April 6 and died onboard on April 11, and his 69-year-old wife, who developed symptoms April 24 and died April 26 in a clinic in Johannesburg, South Africa, while attempting to get home to the Netherlands. These victims are being referred to as the “index cases,” the first documented cases in an outbreak.

Hantavirus usually spreads from rodents to humans. People can contract it when they are exposed to infected rodents or their feces, urine or saliva. Infection typically occurs in poorly ventilated indoor spaces located in rural settings. The classic scenario is catching it while cleaning out a rodent-infested attic or cabin. In March 2025 hantavirus made headlines when the actor Gene Hackman and his wife Betsy Arakawa, a concert pianist, were found dead in their home in Santa Fe, New Mexico, and Arakawa was found to have died from an illness caused by hantavirus.

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There are many kinds of hantaviruses. Scientists have identified the one responsible for the cruise outbreak as the Andes strain found primarily in Argentina and Chile, where it is carried mainly by the long-tailed pygmy rice rat. The Andes virus is the only hantavirus known to spread between people.

The Dutch couple had been traveling in the Southern Cone of South America on a bird-watching trip since late November before embarking on the cruise. According to the Ministry of Health of Argentina, the pair arrived in Argentina on November 27 of last year and set off on a road trip, crossing into Chile on January 7, then traveling to the province of Neuquén in western Argentina on January 31. Twelve days later they visited Chile again before returning to Argentina, where they drove from Mendoza in the western central part of the country to Misiones in the northeastern corner. On March 13 they crossed to Uruguay for two weeks, then went to Ushuaia March 27. The cruise departed Ushuaia on April 1.

On May 6 the Ministry of Health of Argentina announced that researchers from its Malbrán Institute will go to Ushuaia to “to conduct rodent capture and analysis operations in areas linked to the movements of the [index] cases and to detect the possible presence of the virus in natural reservoirs.” That same day the Associated Press reported that two Argentine officials who are investigating the origins of the outbreak and who spoke on the condition of anonymity, said the government’s leading hypothesis is that the couple contracted the virus while bird-watching in Ushuaia before the cruise. As part of their birding tour, the officials told the AP, the couple had visited a landfill where they may have been exposed to rodents.

When I heard about this theory I was curious—and skeptical. I visited that landfill in Ushuaia on a birdwatching trip in February 2025 and didn’t immediately see why it would be conducive to hantavirus transmission based on what I knew about the disease.

Surrounded by the snow-capped mountains and majestic beech forests of Tierra del Fuego, the port city of Ushuaia serves as the main gateway to Antarctica. It hosted more than 150,000 cruise passengers in 2025. Its motto is “Fin del mundo, principio de todos,” which translates to “End of the world, beginning of everything.”

The landfill, known as the relleno sanitario, sits on the outskirts of the city and is well known to bird-watchers as an avian hotspot. Landfills are often good places to look for birds because they offer abundant food resources. Same goes for sewage plants. For birders visiting Ushuaia, a trip to the relleno sanitario is a must because it regularly attracts uncommon species such as the White-throated Caracara, Black-chested Buzzard-Eagle and Andean Condor.

But birding this landfill does not mean traipsing through the trash. The landfill itself is fenced off to the public, so visitors must observe any avian activity from the side of the road, far from the garbage—and the birds. What is more, the landfill is completely open and exposed to the elements, unlike the closed environments with limited ventilation that are typically associated with hantavirus transmission.

On the rare occasions when hantavirus transmission has occurred while outside, it has been accompanied by soil or nest disturbance, and then by breathing in the virus, explains Jennifer Mullinax of the University of Maryland, who studies wildlife ecology as it relates to zoonotic disease. But “open air, rain, etc., quickly dilute virus particles, making it much less likely than typical indoor transmission,” she says.

Local health officials have protested the spotlight on Ushuaia, the capital of Tierra del Fuego province, which relies heavily on tourism for income. At a press conference on May 8, Juan Petrina, director of epidemiology for Tierra del Fuego, noted that there has never been a single recorded case of hantavirus in the province. “The scarcity of this rodent, combined with the province's historical health status and the short period of time during which this couple may have been exposed to these rodents, greatly reduces the likelihood that the infection occurred here,” Petrina remarked.

Petrina is not the only one challenging the focus on the Ushuaia garbage dump as the starting point of the cruise ship outbreak. “From an ecological perspective, it would be premature to attribute infection to a single location, such as the landfill, without considering the entirety of [the Dutch couple’s] travel history, including the sequence of sites visited and the amount of time spent in each habitat,” says Luis E. Escobar of Virginia Tech, who studies the ecology and biogeography of infectious diseases. He suggested that based on the incubation period of the virus, which ranges from four to 42 days, the couple might have gotten infected in Chile.

“The landfill alone should not be considered a strong or definitive candidate for the exposure site,” Escobar says. “Instead, the landfill is one of several possible environments that merit investigation within a broader, spatially explicit epidemiological framework.” A more robust approach to tracing the origin of the outbreak “would involve targeted surveillance of rodent populations across multiple locations along their route, particularly in areas with known hantavirus reservoirs,” he says. “Comparing viral sequences obtained from these rodent populations with those from human cases could help narrow down the most plausible sites of spillover transmission.”

Officials have yet to share the details of where the Dutch couple traveled on the pre-cruise birding tour. Earlier this week some media outlets reported the name of the 70-year-old Dutch man who was the first passenger to die. Scientific American was unable to independently verify his name. But a google search of the name on eBird, an online database where birders submit their avian observations, did turn up a number of checklists from sites in Chile, Argentina and Uruguay in the interval during which the Dutch husband and wife are known to have been on their bird-watching adventure, prompting speculation that eBird could be used to figure out where they went and how long they spent at each location. Yet it is unclear just how regularly the man submitted his observations. His name was not among the people who shared their eBird observations from the relleno sanitario.

Even if investigators are able to reconstruct the couple’s travel itinerary in detail and test rodents in all the areas they visited, the precise location of the site where hantavirus first infected the person who later unknowingly carried it onto the ill-fated cruise ship may prove elusive. “The hantavirus genomes from the Dutch couple will be closest to the hantavirus carried by rodents in the area of exposure,” says virologist Colleen Jonsson of the University of Tennessee. “Sampling the virus carried by the rodents for its genetic sequence at the site(s) where exposure was thought to occur will help narrow the probable exposure site(s). However, viruses carried by these rodents do not vary greatly across fairly large areas, so it would not pinpoint the site of exposure, only the general geographic area,” she explains.

The genetic similarity of the viruses carried by the rodents is not the only obfuscating factor. “Given that asymptomatic hantavirus infections can occur, it is possible that the Dutch couple were not the true index cases,” Escobar says. “There will likely always be some degree of uncertainty regarding the origin of this outbreak.”