Hantavirus epidemic on board the MV Hondius cruise ship has caused a global public health crisis. But it’s caused by a rodent weighing about an ounce, and this year’s climate change has helped boost the risk of transmission.
Across the Southern Cone, researchers have long associated wetter summers with a surge in rodent populations – known locally as rated— which may boost hantavirus transmission. This year’s boom reflects a broader pattern of disease epidemics shaped by climate change, environmental disruption and a hyperconnected world.
“These are emerging diseases as the spread of both reservoirs and viruses expands,” says Karina Hodara, a researcher at the Faculty of Agriculture of the University of Buenos Aires who studies hantavirus ecology. “People travel across continents in a matter of hours.”
The common name is the long-tailed rice pygmy rat several species living in Chile and Argentina who may be carriers of hantavirus. Each species is associated with different hantaviruses depending on geographical location.
It is still unclear where the first passengers who contracted the Andean virus became infected. But the Patagonian rice pygmy rat (Oligoryzomys longicaudatus), which inhabits southern Argentina and forests and shrubs in Chile and weighs about one ounce, is the primary reservoir of the only known hantavirus capable of transmitting from rodents to humans and between humans. It is this transmission from person to person that “is what makes epidemic outbreaks possible,” he adds Raúl González So farexpert in population genetics and evolution at the National University of Cordoba.
But other rodents, including the Pampas pygmy rice rat (Oligoryzomys flavescens), can transmit the virus to humans. The spread of the virus is partly due to changing ecological conditions. When food becomes plentiful – after events such as the massive Patagonian bamboo bloom (Chusquea culeou) or periods of increased fruit production from bushes, such as rosehips and blackberries, rodent populations can expand rapidly. “They eat freely,” says Hodara. “And then they start breeding very quickly.”
As more rodents compete more intensely for territory, food and reproductive access, aggressive encounters between males boost. This, in turn, may boost 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 climb trees to heights of over 2 meters. This has both positive and negative effects,” he explains. Isabel Gómez Villafaneresearcher at the Institute of Ecology, Genetics and Evolution at the University of Buenos Aires. On the one hand, contaminated urine or feces deposited higher are more exposed to ultraviolet radiation, which deactivates the virus. On the other hand, in closed environments such as sheds, cottages or houses, the virus can survive longer. As people move through these areas, especially in the warmer months, contact with contaminated surfaces becomes more and more likely.
Climate variability is one of the main factors shaping population dynamics Oligoryzomys species. In drier years, there is less food available for rodents, which can reduce populations, while in wetter years, the opposite is true, increasing the risk of greater virus transmission.
According to González Ittig, this is the factor that best explains the boost in hantavirus cases recorded since June last year.
Sanitary services informed 101 confirmed casesmost concentrated in central Argentina and associated with the strain of Lechiguanas virus transmitted by Oligoryzomys flavescens—double the amount of the previous 12-month period.
“We were returning after years of intense drought, and in 2025, with the arrival of El Niño, a wetter cycle began,” says González Ittig. Central Argentina received above-average rainfall after years of drought, according to the country’s weather service. Patagonia, on the other hand, had a more uneven pattern, with wetter conditions in some Andean areas and persistent rainfall deficits in other areas. Scientists say these changes likely stimulated vegetation growth and increased food availability for rodents.