How Wet Weather in Argentina Helped Fuel the Cruise Ship Hantavirus Outbreak

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.