What Argentina's scientists do and do not know about Andes hantavirus strain

The Andes hantavirus has circulated for decades in Argentine and Chilean Patagonia, spread by wild rodents. But the outbreak linked to the cruise ship MV Hondius has drawn renewed attention to one exceptional feature of this strain: its ability to spread between people.

How does the rodent that carries it behave? Are there environmental factors behind its spread? And why is the virus so difficult to study?

The rodent and the environment

The natural reservoir of the Andes virus in Patagonia is the long-tailed pygmy rice rat, Oligoryzomys longicaudatus, known locally as the “colilargo.” Initial infection occurs through exposure to saliva, urine or faeces from infected rodents, generally in enclosed environments.

For biologist Raúl González Ittig, associate professor of population genetics at the National University of Córdoba, the cases recorded in Argentina may be linked to an environmental sequence: heavy rainfall associated with El Niño, increased vegetation growth and greater food availability for rodents.

More rodents do not necessarily mean an outbreak, but they do create more opportunities for contact.

“There are more individuals and a greater likelihood that a rural worker becomes infected,” González Ittig told the AFP news agency.

By contrast, droughts and wildfires, which commonly occur during the region’s summer months, “reduce rodent populations,” the specialist explained.

In cases of person-to-person transmission, however, the only rodent responsible is the one that caused the initial infection.

In such cases, “what is known or suspected regarding the association or influence of environmental factors does not apply,” said infectious disease specialist María Ester Lázaro, a retired doctor from the Hospital Zonal de Bariloche whose doctoral thesis focused on Andes hantavirus.

Beyond the known outbreaks in Argentine Patagonia in 1996 and 2018, and now the cruise ship cluster, person-to-person transmission has only been reported very occasionally in the region.

Human transmission is not a mutation

Epidemiologist Rodrigo Bustamante, from the Bariloche hospital, stressed that person-to-person transmission of the Andes strain “is not the rule but an exceptional event requiring close contact of less than one metre for around thirty minutes.”

Nor does it behave like Covid-19 or influenza.

“It is far less transmissible,” Bustamante told AFP. “This requires much closer contact, generally between people living together.”

Scientists reject the idea that a recent mutation turned the Andes virus into a pathogen capable of spreading between humans.

“It is a very stable virus, unlike Covid-19 or influenza. Each hantavirus has evolved since ancient times alongside its rodent host without undergoing major mutations,” Lázaro said.

“What we do not know,” she continued, “is why the Andes virus, instead of producing an isolated case after infecting one person, is sometimes capable of spreading to another person and even generating transmission chains involving several links,” as seen in 1996 and 2018.

On that point, González Ittig said: “I think the virus always had that property.”

Humans probably “began occupying the environments where the mice lived. It was not a specific mutation,” he argued.

Difficult to study

“The problem with hantavirus is that there are so few cases, especially in our region, that you need a great deal of time to obtain a minimally decent number of cases to draw conclusions,” Lázaro said.

Bustamante described the same obstacle from his experience at the Bariloche hospital.

“Normally we have between two and four hantavirus cases a year,” he said.

The disease’s clinical progression also complicates research. At first, patients may appear healthy or display flu-like symptoms accompanied by diarrhoea or vomiting.

“By the fourth day, within hours, the patient can go from what seems like the flu to being on a ventilator,” Lázaro explained.

That dynamic creates difficulties “when trying to reconstruct patients’ itineraries, where they were, and also for clinical trials,” she added.

Meanwhile, in Ushuaia, from where the Hondius departed, scientists are debating whether the local rodent is the same long-tailed species or a subspecies whose possible role as a hantavirus reservoir remains under discussion.

Specialists from ANLIS-Malbrán, the national institute that studies epidemiological diseases, are due to travel to Ushuaia on Monday to carry out the investigation.

So far in the current epidemiological season – which runs for 12 months from June each year – Argentina has recorded 102 cases of different hantavirus strains, almost double the 57 registered during the same period a year earlier.