A bird-watching tour in a region that had never recorded a hantavirus case is now at the center of an urgent investigation, as Argentine officials examine whether a Dutch couple unknowingly brought the deadly virus aboard a cruise ship after a stop at a landfill.

Argentina’s leading hypothesis is that a Dutch couple may have been exposed to rodents while visiting the landfill during the tour in the city of Ushuaia, contracting the virus before boarding the cruise ship, two Argentine officials investigating the origins of the outbreak told The Associated Press on Wednesday.

Hantavirus usually spreads by inhaling contaminated rodent droppings. The World Health Organization (WHO) said human-to-human transfer is uncommon, but possible.

Authorities previously said Ushuaia and surrounding Tierra del Fuego province had never recorded a hantavirus case.

RARE HANTAVIRUS HUMAN-TO-HUMAN TRANSMISSION SUSPECTED ON LUXURY CRUISE SHIP WHERE 3 HAVE DIED

The Dutch-flagged MV Hondius ship is at the center of an outbreak that has already killed three passengers and sickened several others. Contact tracing is underway across Europe and Africa to track possible spread among passengers who have since disembarked.

Testing in Switzerland, South Africa and Senegal has confirmed that this is the Andes strain, according to authorities. The WHO says the variant is found primarily in Argentina and Chile and can spread through close contact, though rare.

Swiss officials said a man who returned from the cruise sought treatment after developing symptoms and was immediately isolated. They said he tested positive for the Andes strain.

WHO Director-General Tedros Adhanom Ghebreyesus said Wednesday that three suspected cases were evacuated from the vessel and are being transported to the Netherlands for treatment.

“At this stage, the overall public health risk remains low,” Tedros said.

The Associated Press contributed to this report.