The World Health Organization says there are two confirmed cases and five suspected cases on the ship, carrying mostly British, American and Spanish passengers.
"As of 4 May 2026, seven cases (two laboratory confirmed cases of hantavirus and five suspected cases) have been identified, including three deaths, one critically ill patient and three individuals reporting mild symptoms," the WHO said.
About 150 people are still stuck on the vessel after a Dutch couple and a German national died, and others fell ill, including a Briton who left the vessel and was being treated in South Africa, authorities said.
Medics were working on Monday to evacuate two people with symptoms of the virus.
The Netherlands' National Institute for Public Health and the Environment (RIVM), which is assisting with the outbreak, said hantavirus had been confirmed in one of the patients showing symptoms.
Hantavirus, which can cause fatal respiratory illness, can be spread when particles from rodent droppings or urine become airborne. It does not transfer easily between humans.
There are no specific drugs to treat the disease, so treatment focuses on supportive care, including putting patients on ventilators in severe cases.
The WHO said the risk to the wider public was low and there was no need for panic or travel restrictions.
But authorities in the island nation of Cape Verde said they had not allowed Dutch-flagged MV Hondius to dock as a precaution.
"We're not just headlines: we're people with families, with lives, with people waiting for us at home," Jake Rosmarin, a US travel blogger, said in a tearful Instagram video post from the ship on Monday.
"There is a lot of uncertainty and that is the hardest part," he added.
A spokesperson for the ship's Netherlands-based operator, Oceanwide Expeditions, said, as a precaution, all passengers were instructed to remain inside their cabins to prevent any potential spread of the virus.
Although human-to-human transmission is rare, the incubation period can last several weeks, meaning some people may not yet be showing symptoms.
Oceanwide Expeditions was trying to arrange the repatriation of two crew members with symptoms of the disease - one British and one Dutch - along with the body of the German national and a "guest closely associated with the deceased" who does not have symptoms.
The company said it was looking into whether passengers could be screened and disembarked on the islands of Las Palmas and Tenerife.
Spanish authorities said they had not yet received a request for the ship to dock and disembark passengers there.
The Dutch Foreign Affairs Ministry, which Oceanwide Expeditions said would be the one making the request, did not immediately respond to a request for comment.
The Hondius left Ushuaia in southern Argentina in March, according to company documentation, on a voyage marketed as an Antarctic nature expedition, with berth prices ranging from 14,000 to 22,000 euros ($A23,000 - $A36,000).
It travelled past mainland Antarctica, the Falklands, South Georgia, Nightingale Island, Tristan, St Helena, and Ascension before reaching Cape Verdean waters on May 3.
South Africa's Health Department confirmed two of the dead were Dutch nationals: a 70-year-old man, who died on St Helena on April 11, and his wife, 69, who died in South Africa after collapsing at OR Tambo International Airport.
The British man being treated in a private clinic in Johannesburg became ill on April 27, while the German victim on the ship died on May 2, Oceanwide Expeditions said.
Hantavirus usually begins with flu-like symptoms, such as fatigue and fever, one to eight weeks after exposure.
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