How worried should we be about hantavirus?

The outbreak of hantavirus on a cruise ship carrying passengers from across the world is a situation that authorities say they are taking incredibly seriously.

Three passengers have died either on board or after travelling on the ship, which set sail from Argentina a month ago.

Three others were evacuated from the ship for treatment.

A huge operation is in place to trace people potentially exposed to the virus who have already taken flights home to numerous countries, including the UK, South Africa, the Netherlands, the US and Switzerland.

Health experts stress the risk to the general public is low. So how worried should we be?

The contact-tracing work has been "quite a mammoth effort", Prof Robin May, chief scientific officer at the UK Health Security Agency (UKHSA) told BBC Breakfast, and one "we will continue to do... for some time".

In an update on Thursday, operator Oceanwide Expeditions said 30 passengers, including seven Britons, had disembarked when the ship docked at St Helena on 24 April.

The company said it had contacted all of those who had already disembarked.

The UKHSA said two Britons who had left the ship at St Helena had contacted health officials when they heard about the cases after flying home from Johannesburg and are now voluntarily self-isolating in the UK. They do not have symptoms.

In the US, health officials in Georgia and Arizona have confirmed to the BBC that they are monitoring three passengers who had returned to the US after disembarking. None currently have symptoms.

Human-to-human spread is possible with the specific Andes strain that has been detected, say experts.

But the World Health Organization (WHO) says the risk of infections globally remains low because this is not a disease like Covid or flu that spreads easily.

In its latest update, it says eight cases of hantavirus - three confirmed and five suspected - have been identified in people who were on the ship.

It is still not clear how the outbreak started.

Hantavirus typically spreads from rodents, with people infected by breathing in air contaminated with virus particles from rodent urine, droppings, or saliva.

The cruise had been visiting remote wildlife areas, so a passenger could have come into contact with the virus then, or before boarding the ship.

Health experts believe that some of the infections on board MV Hondius may have passed between people that were in very close contact.

Even luxury cruise ships have relatively cramped or restricted living conditions, with people sharing cabins and dining areas - places where infections could spread.

The Andes strain of the virus has been confirmed in two of the ship's passengers, according to South Africa's health minister, and experts have observed the strain spreading between human patients in previous outbreaks, through very close contact.

Unlike diseases such as measles, which are highly contagious and spread easily, the Andes strain of hantavirus is not that infectious.

People can catch it from someone that they spend prolonged time with in close physical proximity.

The three deaths include a Dutch woman who left the MV Hondius when it stopped at the island of St Helena on 24 April. She had been sharing a cabin with her husband who previously died on board on 11 April, but is not a confirmed case of hantavirus.

Hantavirus is not spread in the outside world through everyday social contact like walking in public spaces, shops, workplaces, or schools, the UKHSA says.

Symptoms usually appear between two to four weeks after being exposed to the virus, but can occur more than a month later.

People ill with the Andes strain can have symptoms similar to flu - a fever, fatigue, muscle aches. They may also get shortness of breath, stomach pain, nausea and vomiting or diarrhoea.

There is no specific treatment, but early medical support in hospital can improve survival. Treatment is for the symptoms displayed.

Any British passengers returning from the cruise ship will be asked to self-isolate in the UK for 45 days, the UKHSA says.

For the broader public, not directly involved in this cruise ship, "the risk here is really negligible", Prof May adds.

Some 19 British nationals were listed as passengers on the MV Hondius, which was sailing from Argentina to Cape Verde, with four British crew members.

The UKHSA said officials were working to co-ordinate the arrival of British nationals to the UK from MV Hondius.

Dr Meera Chand, deputy director for epidemic and emerging infections at UKHSA, said: "It's important to reassure people that the risk to the general public remains very low.

"We are contact tracing anyone who may have been in contact with the ship or the hantavirus cases to limit the risk of onward transmission."

The MV Hondius is now sailing towards Spain's Canary Islands after being anchored for three days near Cape Verde, an archipelago nation off the West African coast.

The plan is for the remaining passengers and crew to fly to their home countries.

Oceanwide Expeditions said on Thursday that none of the people still on board were displaying symptoms and local health authorities have visited the vessel and assessed the situation.

Passengers have been isolating on board and the ship has been deep cleaned by professionals ahead of the planned evacuation.