The hantavirus outbreak is a tragedy—and a valuable data source

May 20th 2026|3 min read

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UNCOVERING HOW a virus spreads among a population involves some tricky detective work. The schedules of all those infected have to be carefully examined in the days or weeks around the time they fell ill, in order to work out who infected whom—and, most important, where and how. The more isolated the population, the more accurately such records can be gathered. To epidemiologists, therefore, cruise ships with onboard outbreaks are the equivalent of floating Petri dishes bursting with valuable information.

The MV Hondius is the latest example. As of May 20th at least 11 cases of hantavirus were confirmed among its 147 passengers and crew; three of those infected have died. The outbreak was caused by the Andes strain of hantavirus, a bug carried by South American rodents that causes about 100-150 known human infections in Argentina and Chile each year. Human-to-human transmission can occur but such secondary cases are rarer still. The Hondius case study is, therefore, a valuable addition to this body of knowledge.

At the moment, the original source of the onboard virus is thought to be exposure to rodent droppings or urine prior to departure. The data—including genomic sequencing of the virus—suggest that the first four cases in the outbreak may have originated that way, possibly from the same source, says Thomas Hofmann from the European Centre for Disease Prevention and Control. But there is rarely complete certainty about how an infection was picked up. People cannot recall every single interaction they have with others. It is even harder to know who used the same toilet or touched the same serving utensils at a lunch buffet. Better records of passenger interactions are needed to get to the bottom of the outbreak.

The disease detectives on board were tasked with working out what sorts of interactions had occurred among the known cases, as well as between that cohort and the healthy passengers, in order to help manage the outbreak. In time, the data will also help researchers assess the odds of a particular type of contact leading to transmission of the virus (such as sharing a dinner table with someone who was already infected, or giving them a hug), which will help manage future outbreaks.

Although the pieces of the puzzle are still being put together, the results so far are consistent with what was already known about the Andes strain, says Dr Hofmann. Crucially, it is not a virus that transmits easily among people, such as those that cause covid-19 and the flu. If it did, he says, there would be far more cases on the Hondius, where passengers spent a lot of time in communal indoor lounges.

That being said, widespread transmission between people cannot be ruled out. In an outbreak that began in Argentina in 2018 one person unleashed a chain of transmission that ultimately infected 33 others. That outbreak has led researchers to think some people may be hantavirus “superspreaders”. For reasons that are unclear, they may be shedding and dispersing exceptionally high quantities of the virus.

Even so, the studies of the Andes strain (to which the Hondius data will soon be added) show that the virus does not have what it takes to be a pandemic threat. Genomic sequencing of samples taken over the years shows that it changes very little as it circulates in rodents. What is more, samples from the ship outbreak do not show the emergence of any adaptations that could make it better at transmitting between humans.

As the typical incubation period is around three weeks and transmission generally occurs when people already have symptoms, there is usually plenty of time to find and isolate close contacts. Such measures cannot contain viruses with short incubation periods (like influenza) or that are spread by asymptomatic carriers (like covid and influenza).

It remains a disease to be taken seriously: mortality can be as high as 30% even when the disease is recognised early and intensive care is available. But, thankfully, becoming infected from a rodent is rare—and from a human rarer still. ■