I’ve investigated a hantavirus outbreak. Here’s what I can tell you about the cruise ship cluster

The cruise ship cluster of hantavirus cases continues to grow. The World Health Organization reports that as of May 6 there were eight cases, three of whom are confirmed by laboratory testing as hantavirus. In recent days, we heard three passengers had died.

Now some passengers are being medically evacuated from the cruise ship MV Hondius. Other passengers have disembarked and are returning home. Swiss authorities have confirmed a passenger on the ship is now a confirmed case and is receiving care in a Zurich hospital.

I’m a public health physician with a special interest in respiratory diseases. I’ve also investigated a hantavirus outbreak.

Here’s what investigators want to know about the current cluster of cases. This includes gathering evidence to see if the virus is transmitting from person to person.

Back in 1993, there was an unknown pathogen

In 1993, I was a young epidemic intelligence service officer working at the United States Centers for Disease Control. I was deployed to the deserts of the south-western US to help investigate a frightening outbreak, mainly among Navajo people.

Adults in their 20s and 30s were becoming suddenly unwell. They would develop a fever and cough, then rapidly progress to severe respiratory failure as fluid leaked into their lungs. Some appeared well enough to be dancing in the evening and were dead within hours.

The investigation team was nervous. We did not yet know the pathogen, how it was spreading, or whether we were at risk.

One of the first recognised cases was a well-known runner, so we initially wondered whether infection might be linked to inhaling something stirred up in desert dust. A leak from a remote military biowarfare laboratory was also considered, as was plague that was endemic to the area.

After laboratory testing, the cause was identified as a new hantavirus, later known as Sin Nombre virus. The virus attacked the small blood vessels of the lungs and was linked to exposure to the urine, faeces and saliva of infected deer mice. Mice numbers had increased dramatically and were entering homes and workplaces across affected communities.

A crucial finding was that, like most hantaviruses, Sin Nombre virus did not appear to spread from person to person. Family clusters were explained by shared exposure to rodents or rodent-contaminated environments, especially during cleaning or other close contact with contaminated objects or dust.

That is why many of us were surprised years later when Andes virus, a South American hantavirus, was shown to spread occasionally from person to person.

This remains uncommon, but it has been documented, including in outbreaks in Argentina – the country from which the MV Hondius departed before the current suspected outbreak.

What would a disease detective do now?

The first step in any outbreak investigation is to confirm the diagnosis. At this stage, the difference between a “suspected” and “confirmed” case still matters.

Investigators need to know whether all severe respiratory illnesses in the cluster are due to hantavirus, or whether confirmed cases are occurring against a background of another infection, such as influenza or COVID.

The next step is to build a timeline. The timing of when symptoms started is often the first clue to where and how people were exposed.

According to WHO, the ship departed Ushuaia, Argentina, on April 1 2026. The first known case developed symptoms on April 6. Other cases developed symptoms later in April.

Let’s focus our attention on the first three cases.

Hantavirus pulmonary syndrome describes the respiratory symptoms that follow after the type of hantavirus infection that mainly attacks the lungs. These typically develop two to four weeks after exposure. However, illness can appear as early as one week and as late as eight weeks after infection.

That makes the first case difficult to explain as an exposure acquired on the ship after departure. Symptoms started on April 6, only five days after leaving Argentina. That’s shorter than the usual incubation period (the period from infection to showing symptoms) and even shorter than the lower end commonly cited.

So for that case, it’s more plausible for that person to have been exposed in Argentina before boarding. There are emerging reports of a bird-watching activity that might have led to rodent exposure.

The later cases are more ambiguous. They could have been exposed before departure, or during shore activities in Argentina, or elsewhere. But their timing also raises another possibility: transmission from the first case to close contacts on board.

This is where the epidemiology becomes interesting.

Did the virus spread from person to person?

The second case was a close contact of the first. This creates two plausible explanations. They may have both been exposed to the same infected rodent (or its urine or droppings, for example). Alternatively, it’s very likely the second case contracted the infection from the first case.

The third case was not part of that same close family unit. If investigators find this person shared the same excursions in Argentina as the first two, the outbreak may still be explained by a common source. But if there was no shared rodent exposure, suspicion of person-to-person transmission increases.

This does not mean person-to-person transmission is proven. It means it becomes one of the leading hypotheses to test.

If human-to-human transmission is not the explanation, investigators would need to consider a less tidy chain of events.

The first case would have had a pre-boarding exposure with a short incubation period. The second case would need either the same exposure with a longer incubation period, or infection from the first case.

The third case would need either an independent exposure to infected rodents before boarding, or another exposure during the voyage. None of these is impossible. But as more cases appear, and if they cluster in time around contact with earlier cases, the human-to-human hypothesis becomes harder to dismiss.

The approximate gap between the first case’s illness and the later cases is also important. If person-to-person transmission is occurring, severe hantavirus illness is likely to coincide with a higher risk of being infectious and infecting others. So we would expect symptoms that start two to three weeks after close contact with an earlier severe case, and this is what we’re seeing from the cruise ship.

What are the public health implications?

The practical public health response must therefore cover both possibilities: a common environmental source and limited person-to-person spread.

That means detailed interviews about pre-boarding travel, shore excursions, wildlife exposure, rodent sightings, cabin locations, cleaning activities, shared dining, shared transport, and close contact with ill passengers.

It also means laboratory confirmation in multiple cases, sequencing of viral samples where possible, and careful reconstruction of who had contact with whom, and when.

Genetic fingerprinting can explore if the virus has the same historical mutation that allowed human-to-human transmission to emerge in previous outbreaks (which were easily controlled with basic isolation and infection control). If a new mutation was found, this would raise concerns of greater transmission risks.

For the public and health authorities considering receiving the passengers from the quarantined ship, the key message is not to panic.

Most hantaviruses are not spread between people. Even with Andes virus, person-to-person transmission is uncommon and usually requires close or prolonged contact. WHO currently assesses the risk to the global population as low. This virus does not spread like influenza or COVID.

But for outbreak investigators, this is exactly the sort of cluster that demands disciplined shoe-leather epidemiology: confirm the diagnosis, build the timeline, test the competing hypotheses, and let the pattern of exposure, illness and laboratory evidence tell the story.