Health authorities say a rare and highly dangerous strain of hantavirus – capable of spreading between people and carrying a mortality rate of about 40% – is likely behind the outbreak aboard the MV Hondius cruise ship, where three passengers have died and seven others have fallen ill.

While hantavirus is typically transmitted through contact with rodent waste, officials warned that this unusual strain appears to break that pattern – a possibility the World Health Organization said Tuesday “can’t be ruled out.”

Authorities in Cape Verde have refused to allow the Dutch-flagged vessel to dock, explaining the decision was made with the “aim of protecting national public health.”

The outbreak emerged during the ship’s voyage from Argentina to Cape Verde, and no passengers have been permitted to disembark so far, according to the cruise operator.

“We do believe that there may be some human-to-human transmission that is happening among the really close contacts,” said Maria Van Kerkhove, the World Health Organization’s epidemic and pandemic preparedness and prevention director.

The suspected strain, known as the Andes virus, is typically found in mountainous regions of Argentina and Chile. Although the MV Hondius departed Argentina on March 20, it did not originate from an area where the strain is commonly detected.

With a fatality rate approaching 40%, the Andes strain poses a severe risk, particularly in a confined setting like the 80-cabin vessel carrying roughly 150 people, where infections could escalate rapidly.

Concerns about this strain intensified after the WHO reported no evidence of rodents aboard the ship, which remained anchored off Cape Verde on Tuesday.

Medical teams in full protective gear have boarded the vessel to assist with suspected cases involving two crew members – one British and one Dutch – while also testing other passengers and crew showing symptoms.

The WHO believes the first infected passenger may have contracted the virus before boarding, Van Kerkhove said.

“Our working hypothesis is that there’s probably a couple of different types of transmission that might be happening,” she told BBC Breakfast Tuesday, noting that passengers visited multiple islands where exposure to infected rodents – through waste or saliva – could have occurred.

Spain has reportedly granted the ship permission to dock in the Canary Islands, though its health ministry pushed back on claims that a final decision has been made.

“Depending on epidemiological data gathered from the boat as it travels past Cape Verde, the most appropriate next stop for it will be decided,” the ministry said in a statement.

“Until then, the health ministry will not take a decision, as we have explained to the WHO.”