The hantavirus outbreak aboard the MV Hondius cruise ship has created a global public health crisis. But the driver of it is a rodent that weighs about an ounce, and climate shifts this year that have helped increase the odds of transmission.

Across the Southern Cone, researchers have long associated wetter years with explosive rodent population booms—known locally as ratadas—that can amplify hantavirus transmission. This year’s boom reflects a broader pattern of disease outbreaks shaped by climate change, environmental disruption, and a hyperconnected world.

“These are emerging diseases because the distribution of both the reservoirs and the viruses is expanding,” says Karina Hodara, a researcher at the Faculty of Agronomy at the University of Buenos Aires who studies hantavirus ecology. “Humans travel across continents in a matter of hours.”

The long-tailed pygmy rice rat is the common name for several species that live in Chile and Argentina that can harbor hantavirus. Each species is associated with different hantaviruses depending on geography.

It’s still unclear where the first passengers that got sick with the Andes virus contracted it. But the Patagonian long-tailed pygmy rice rat (Oligoryzomys longicaudatus), which inhabits southern Argentina and the woods and shrublands in Chile and weighs about one ounce, is the main reservoir of the only known hantavirus capable of spreading from rodents to humans and between humans. This person-to-person transmission “is precisely what makes outbreaks possible,” adds Raúl González Ittig, an expert in population genetics and evolution at the National University of Córdoba.

But other rodents, including the Pampas long-tailed pygmy rice rat (Oligoryzomys flavescens), can transmit the virus to humans. The virus’s spread is driven in part by changing ecological conditions. When food becomes abundant—following events such as the mass flowering of Patagonian bamboo (Chusquea culeou) or periods of increased fruit production from shrubs such as rosehip and blackberry—rodent populations can expand rapidly. “They eat without limits,” Hodara says. “And then they begin reproducing very quickly.”

As more rodents compete more intensely for territory, food, and reproductive access, aggressive encounters between males increase. That, in turn, can increase transmission of the virus through bites or saliva. Once infected, rodents shed the virus into the environment through urine, feces, and saliva.

“Long-tailed pygmy rice rats are climbers and can move more than 2 meters high in trees. That has both positive and negative effects,” explains Isabel Gómez Villafañe, a researcher at the Institute of Ecology, Genetics and Evolution at the University of Buenos Aires. On one hand, contaminated urine or feces deposited higher up are more exposed to ultraviolet radiation, which deactivates the virus. On the other hand, in enclosed environments—such as sheds, cabins, or houses—the virus may persist longer. And as people move through these landscapes, especially during warmer months, contact with contaminated surfaces becomes more likely.

Climate variability is one of the main factors shaping the population dynamics of Oligoryzomys species. During dry years, there’s less food available for rodents, which can lower the population, while the opposite is true in wetter years, upping the odds for more viral transmission.

According to González Ittig, this is the factor that best explains the increase in hantavirus cases recorded since last June.

Health officials have reported 101 confirmed cases, most concentrated in central Argentina and associated with the Lechiguanas strain of the virus transmitted by Oligoryzomys flavescens—double the amount of the previous 12-month period.

“We had been coming from years of intense drought, and then in 2025 a wetter cycle began with the arrival of El Niño,” González Ittig says. Central Argentina saw above-average rainfall, according to the nation’s weather service, after years of drought. Patagonia, however, experienced a more uneven pattern, with wetter conditions in some Andean areas but persistent precipitation deficits elsewhere. Researchers say those shifts likely boosted vegetation growth and expanded food availability for rodents.

Scientists say these changes are part of a broader climate trend already reshaping rainfall patterns across the Southern Cone, making some regions wetter and others drier. In the Pampas region—including central Argentina and parts of Uruguay—rising humidity, milder winters, longer warm seasons, and heavier rainfall are creating conditions that favor rodent survival, reproduction, and the spread of hantavirus.

At the same time, the geography of risk also appears to be changing. In recent years, cases have begun to emerge in new parts of Argentina, including some fatal infections. Researchers believe this may reflect a broader reconfiguration of hantavirus risk, linked to environmental transformation and expanding human activity.

“We are constantly invading natural environments,” Hodara says. “We move into forests and mountains, lakes and wetlands, we build gated communities. And that increases the chances of contact for us.”

Land-use change and the rodents’ remarkable adaptability may also be playing a role. Scientists are increasingly finding Oligoryzomys rodents in landscapes heavily altered by agriculture and human activity. “I captured Oligoryzomys right in the middle of a wheat field in Córdoba, a province in central Argentina,” González Ittig says. “These rodents tolerate modifications of the natural environment surprisingly well, and that may explain why cases are appearing outside traditional areas.”

Gómez Villafañe also notes that researchers in recent years have identified hantavirus variants in Argentina that had previously only been documented in neighboring countries, including the Alto Paraguay strain, associated not with Oligoryzomys rodents but with the Chacoan marsh rat (Holochilus chacarius).

In that context, episodes such as the MV Hondius outbreak no longer appear as isolated anomalies, but as part of an increasingly intense interaction between climate, wildlife, and human activity. Tracking viral circulation before human cases appear and expanding prevention campaigns in regions outside Patagonia—where hantavirus has traditionally received far less public attention and surveillance—can help reduce the spread of the disease.

“There is no vaccine for the strains of hantavirus in the Americas,” González Ittig says. “So the key is prevention and epidemiological surveillance.”