Across the Southern Cone, researchers have long linked wetter years to explosive population booms of the rodents – known locally as Ratada-This can amplify the transmission of Hantavirus. 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 reservoirs and viruses is expanding,” says Karina Hodara, a researcher at the University of Buenos Aires’ School of Agronomy who studies the ecology of hantaviruses. “Humans travel across continents in a matter of hours.”
The dwarf long-tailed rice rat is the common name for several species living in Chile and Argentina that can harbor the Hantavirus. Each type is associated with different hantaviruses depending on geography.
It is still unclear where the first passengers became infected with the Andean virus. But the Patagonian long-tailed dwarf rice mouse (Oligoryzomys longicaudatus), which lives in southern Argentina and the forests and shrubs of Chile and weighs about one ounce, is the main reservoir for the only known hantavirus capable of spreading from rodents to humans and between humans. Person-to-person transmission “is precisely what makes outbreaks possible,” adds Raul Gonzalez Itig, an expert in population genetics and evolution at the National University of Cordoba.
But other rodents, including the Pampas long-tailed dwarf rice mouse (Oligoryzomys flavescens) The virus can be transmitted to humans. The spread of the virus is driven in part by changing environmental conditions. When food becomes abundant – after events such as the mass flowering of Patagonian bamboo (Chuskia Coolio) or periods of increased fruit production from shrubs such as rosehips and blackberries – rodent populations can expand rapidly. “They eat without limits,” says Hodara. “And then they start multiplying very quickly.”
As more rodents compete intensely for territory, food and reproductive ability, aggressive confrontations between males increase. This, in turn, can increase transmission of the virus through bites or saliva. Once infected, rodents spread the virus into the environment through urine, feces, and saliva.
“Dwarf long-tailed rice mice are climbers, and they can move more than two meters high in trees. This has both positive and negative effects,” explains Isabel Gomez Villafañe, a researcher at the Institute of Ecology, Genetics and Evolution at the University of Buenos Aires. On the one hand, contaminated urine or feces deposited on top are more susceptible to ultraviolet rays, which inactivate the virus. On the other hand, in closed environments – such as barns, cabins, or homes – the virus may persist longer. As people move across these landscapes, especially during the warmer months, contact with contaminated surfaces becomes more likely.
Climate variability is one of the main factors shaping population dynamics Oligarchy Classify. During dry years, less food is available for rodents, which can lead to population declines, while the opposite is true in wet years, which increases the potential for further transmission of the virus.
According to Gonzalez Itig, this is the factor that best explains the increase in Hantavirus cases recorded since last June.
Health officials reported 101 confirmed cases, most concentrated in central Argentina and linked to the transmissible strain of Lichiguanas virus. Oligoryzomys flavescens– Double the amount of the previous 12-month period.
“We were coming from years of extreme drought, and then in 2025 a wetter cycle started with the arrival of El Niño,” González Ittig says. Central Argentina has seen above-average rainfall, according to the country’s meteorological service, after years of drought. However, Patagonia saw a more uneven pattern, with wetter conditions in some Andean regions but persistent rainfall deficits elsewhere. The researchers say these shifts likely enhanced vegetation growth and increased food availability for rodents.
