As officials race to get passengers off a cruise ship hit by a hantavirus outbreak, health experts have revealed how the deadly rat-borne virus could have taken over the ship.

Hazmat-clad medical workers descended on the luxury cruise liner MV Hondius Wednesday in scenes reminiscent of the Covid pandemic, in a desperate attempt to get three patients sickened by hantavirus flown out for treatment. 

The outbreak has already claimed three lives and sickened at least seven people. 

On Tuesday, three patients were flown to Europe for treatment. A fourth passenger, who was also infected, remains in South Africa in a critical condition.

Passengers are mostly Europeans, but Americans are also reported to be on board - including a travel blogger who posted a tearful update - raising the prospect that the outbreak could reach US shores.

Hantavirus is typically spread via breathing in dust from the droppings of infected rodents, which may be disturbed during sweeping or cleaning. 

But the World Health Organization (WHO) has warned about the possibility of rare human-to-human transmission of the virus taking place on the ship. 

The hantavirus strain behind the outbreak is the Andes strain - which has been linked to previous outbreaks where the virus has spread between people.

'Out of all the hantaviruses we know about, only one (the Andes virus) has ever been proven to spread from person to person,' Dr Zaid Fadul, a physician and CEO of Bespoke Concierge MD, told the Daily Mail.

'Every other hantavirus strain stays in its rodent host and only jumps to humans when we breathe in aerosolized particles from their droppings, urine, or saliva. Andes virus is the exception.'

Dr Maximo Brito, infectious diseases specialist at the University of Illinois and vice president of the Infectious Diseases Society of America, told the Daily Mail that while person-to-person spread 'is not a common way of transmission of this virus,' if there are no rats detected on the ship, 'you would have to postulate that person-to-person is the way of transmission.' 

Argentine officials have noted that a Dutch couple who later boarded the MV Hondius had visited a landfill site to take pictures of birds in the city of Ushuaia, which may have exposed them to rodents carrying hantavirus.

Fadul explains human-to-human spread of the Andes strain can occur when someone is in the 'prodromal' phase of illness, which means they have early symptoms like fever, muscle aches and fatigue. 

'During this window, the virus is actively replicating in their lungs and salivary glands, and it can be spread through respiratory droplets, saliva and close contact,' he said. 

'What's genuinely surprising is that viral shedding begins up to two weeks before a person feels any symptoms at all. That pre-symptomatic window is part of why this virus is so hard to contain.'

In person-to-person hantavirus, 'close contact' is considered having prolonged, repeated exposure to a person's respiratory droplets or saliva. 

'Hantavirus exists in rodent saliva, so there could be transmission via saliva and droplets. For people, that would include coughing, kissing, or prolonged close person to person contact,' Dr Carrie Horn, chief medical officer at National Jewish Health in Colorado, told the Daily Mail.  

Buffets are also common on cruise ships with shared utensils and potentially contaminated surfaces that many passengers touch at once, raising the risk of illness. 

'If you touch something that's contaminated with the virus and then you touch your face or your nose, you could become infected that way,' Dr Nicole Iovine, chief epidemiologist and an infectious disease expert at the University of Florida Health Shands Hospital, told the Daily Mail. 

'Or it could be that you're breathing air that contains the virus, and that makes it pretty difficult when there's a organism that is spread in the air.'

An outbreak linked to the Andes strain in 2018 in Argentina led to 34 cases and 11 deaths, some of which were triggered by human-to-human transmission, but a hantavirus outbreak has not previously been recorded on a cruise ship. 

'If there's a disease such as this that has an ineffective mode of transmission - person-to-person - if we're ever going to see a transmission, it's going to be in environments like this, where there's close quarters,' Brito said.   

However, Brito does not anticipate hantavirus outbreaks becoming more common on other cruise lines, as Andes hantavirus is primarily found in South America, Argentina and Chile. 'I think this is an isolated occurrence,' he told the Daily Mail. 

'If there is more disease activity going on in those countries, then obviously that spills over to cruises that originate there. But I'm not sure that this is what's occurring now. 

'I expect this to not turn into a problem for other cruises.'  

Dr Syra Madad, infectious disease epidemiologist at Harvard’s Belfer Center and chief biopreparedness officer for New York City's public hospitals, told the Daily Mail that cruise ships 'are not inherently unsafe' due to strict sanitation programs, medical teams, surveillance and cleaning measures. 

'That said, ships can still be efficient “mixing vessels,” as passengers and crew from many places live, dine, socialize and work in close quarters. So, the risk is manageable depending on the type of infectious disease, but not zero,' she said. 

Brito also notes that while 17 Americans are onboard MV Hondius, there is likely no risk to the US population as a whole. He suspects if an American passenger shows symptoms of hantavirus, they will likely be isolated and treated onboard the ship rather than being transferred back to land. 

Even for asymptomatic passengers, he suspects they will tested before disembarking. 'I think the way this outbreak is going, they're probably going to be tested before being transferred to the US,' Brito said. 'Even if they are transferred with all the precautions, they will pose very little risk to general populations because they will be in isolation.'

'It's concerning for those onboard and exposed, but not a broad public-health threat at this time,' Madad said. 

'We might not see the virus again, but it's really hard to predict that,' Iovine said.  

Fadul urges those onboard the ship or who believe they were exposed to hantavirus to monitor early symptoms.

'The classic early picture is fever over 101 degrees Fahrenheit, severe muscle aches - especially in the thighs, hips and back - headache, and sometimes abdominal pain, nausea and vomiting,' he said. 

'It can look exactly like the flu.'

But hantavirus has a 40 percent mortality rate, primarily due to Hantavirus Pulmonary Syndrome (HPS), a severe respiratory condition where blood vessels in the lungs leak, filling air sacs with fluid. This results in respiratory failure. 

Brito also notes that hantavirus is a particular concern for the elderly, a group that makes up about one in three cruise passengers in general. Old age naturally weakens the immune system, increasing the risk of severe complications. 

'It seems that the older you are, the greater your risk for severe disease or to die from HPS, so it certainly is concerning if the cruise ship population on that particular boat does have more people who are older,' Iovine said. 

There is also no specific treatment for hantavirus, so early medical attention is key for preventing severe illness. Iovine emphasizes that 'your best bet is to wash your hands frequently with soap and water, and certainly before eating' to reduce the risk of contracting any illness on a cruise ship, including hantavirus. 

'When going off of the ship, be aware of the environment and minimize interactions with the local wildlife,' Horn notes.  

Fadul urges people with these symptoms to immediately take a flu and COVID test to rule those conditions out. 

'If both come back negative and you still feel sick, don't wait it out. Go to the emergency room and say these exact words: "I have possible hantavirus exposure." Those specific words get the right lab tests ordered quickly,' he told the Daily Mail. 

'Andes virus can progress from flu-like symptoms to life-threatening respiratory failure in as little as 24 hours. Early ICU support saves lives. Sleeping it off does not.'