Two cases of hantavirus have been confirmed after three people died and another was left seriously ill in a suspected outbreak onboard a cruise ship crossing the Atlantic Ocean.
A Dutch husband and wife and a German passenger died, with the Dutch woman testing positive for the virus, the MV Hondius ship's operator Oceanwide Expeditions said.
The rare virus was also confirmed in a 69-year-old British national who was evacuated to a hospital in South Africa for intensive care.
Two crew members - one British and one Dutch - with unconfirmed cases and a close contact of the German national are due to be evacuated from the ship, which is currently anchored near Cape Verde, off Africa's west coast.
The vessel was due to dock in Cape Verde on Tuesday but local authorities have barred passengers from disembarking on safety grounds.
Some 149 people from 23 countries remain on the MV Hondius under "strict precautionary measures", including isolation and hygiene protocols, Oceanwide Expeditions said.
They have travelled more than 6,000 miles at sea after setting sail from Argentina around a month ago.
Hantavirus is usually spread from rodents via their faeces, saliva or urine. It can cause severe respiratory illness. It is rarely transmitted between people.
So far, it has only been confirmed in the case of the Dutch woman who died and the UK national in hospital in Johannesburg. The cause of death of the other passengers is being investigated.
The Dutch man died while the ship was at sea last month. His wife later passed away upon arrival at a hospital in Johannesburg, having flown there from the island of St Helena where she disembarked the vessel.
The World Health Organisation (WHO) is in talks with Spanish authorities about continuing the ship's journey to the Canary Islands, its director of epidemic and pandemic preparedness told the BBC.
Dr Maria van Kerkhove said the plan was for a "proper risk assessment" to be carried out there with all passengers remaining on board.
Oceanwide Expeditions said the option of sailing to Las Palmas or Tenerife was being considered "to be the gateway for disembarkation, where further medical screening and handling could take place".
Van Kerkohve said that the WHO's "top priority" remained treating the two crew members who were on board with respiratory symptoms.
They are due to be medically evacuated via aircraft to the Netherlands, alongside a person "associated" with the German national who died, according to Oceanwide Expeditions.
Nobody else on the ship has shown symptoms.
Though human-to-human transmission of hantavirus is rare, van Kerkhove told BBC Breakfast some could have taken place between close contacts sharing cabins.
Hantavirus first could have been contracted from rodents before the ship embarked from Ushuaia in Argentina, or on one of its stops along the way, she added.
She noted the cruise went to many different islands, some of which have rodents.
"Our working hypothesis is that there's probably a couple of different types of transmission that might be happening," van Kerkove said.
One passenger, who asked to remain anonymous, told the BBC: "The latest word is that a plane is on its way and once it gets here three people will be evacuated from the ship and flown straight to Europe.
"Then the rest of us will almost certainly sail to the Canary Islands.
"The Cape Verde authorities clearly want nothing to do with us. This is what we're hearing from the captain and staff. From what I can see the mood (on the ship) is pretty good.
"Only one person has been tested (the one now in South Africa) and he tested positive for hantavirus. So, we don't actually know yet if the other cases are that or something unrelated.
"If they are all hantavirus then the transmission is a bit mysterious. We've been informed that there are no rodents on board, and person-to-person transmission is difficult/rare.
"Hopefully the other patients on board will be tested soon and then we'll know better what's going on."
Another passenger, travel vlogger Jake Rosmarin, posted a video from the ship to social media on Monday, saying: "What's happening right now is very real for all of us here."
He said that the passengers on board were "people with lives, with families, with people waiting for us at home".
"There's a lot of uncertainty, and that's the hardest part," he added. "All we want right now is to feel safe, to have clarity and to get home."
Outlining a timeline, Oceanwide Expeditions said a passenger had become unwell while aboard and died on 11 April.
His cause of death could not be determined, and his body was taken off the ship after it docked at St Helena on 24 April.
The passenger's wife also disembarked on St Helena and the firm said it was told she had become unwell during the return journey and later died.
On 27 April, the firm said, another passenger - the British national - became seriously ill and was "medically evacuated" to South Africa.
The 69-year-old remains in a critical but stable condition in Johannesburg after it was confirmed a variant of hantavirus had been identified.
The firm added that on Saturday, a third passenger onboard MV Hondius died.
The cause of death has not been established, Oceanwide Expeditions said. It confirmed the passenger was German.
Dr Aaron Motsoaledi, South Africa's minister of health, said of the British patient that he was critical and had been admitted to a private facililty.
"He's being taken care of. As you know, hantavirus, like all viruses, don't have any specific treatment, so they are giving symptomatic treatment and support as much as they could."
He said health workers and anyone who had contact with the patient would now be traced and tested.
A UK government spokesperson said the Foreign Office was in touch with the man's family and that consular teams were operating in the UK, South Africa, Spain and Portugal to support British nationals.
The WHO said it was "acting with urgency" to support the MV Hondius, and thanked South African authorities for taking care of the British patient.
WHO's regional director for Europe, Dr Hans Henri P Kluge, said "hantavirus infections are uncommon".
"While severe in some cases, it is not easily transmitted between people. The risk to the wider public remains low. There is no need for panic or travel restrictions."
MV Hondius is described as a 107.6m (353ft) polar cruise ship, with space for 170 passengers in 80 cabins, along with 57 crew members, 13 guides and one doctor.
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