What we know about the hantavirus outbreak

Hantavirus, a rare family of rodent-borne viruses, is responsible for a deadly outbreak aboard a cruise ship sailing in the Atlantic Ocean.

According to the World Health Organization (WHO), the number of people affected on the Dutch-flagged luxury cruise ship MV Hondius — currently anchored off the coast of Cape Verde with 147 people on board — has risen to seven. Three people have died, one patient is in a critical condition, and three have mild symptoms.

Two of the seven cases have been confirmed as hantavirus in the laboratory, and the rest are considered suspected cases. The WHO plans to evacuate the sick from the ship and allow it to continue its planned route to Spain’s Canary Islands with the remaining passengers.

Here is an overview of the scientific evidence about the hantavirus.

Not very contagious, but dangerous

The three deaths and the parallels with the cruise ships that were stranded in 2020 by coronavirus outbreaks have created a degree of panic, but the actual risk is low. Hans Kluge, WHO Regional Director for Europe, confirmed this on Monday, adding that “there is no need to panic or impose travel restrictions.”

The key difference is that the coronavirus was highly contagious. In Europe, it had a basic reproduction number (R₀) close to three — that is, each infected person typically passed the virus on to three others. By contrast, the basic reproduction number of hantavirus is virtually zero.

The hantavirus is usually transmitted through the inhalation of particles from the saliva, feces, or urine of infected rodents, particularly when entering enclosed spaces where large numbers of infected animals have been present.

Person‑to‑person transmission is extremely rare. It has only been documented in a handful of cases and in just one of the 24 hantavirus strains identified in humans. This is the so‑called Andes virus, a variant first detected in Argentina in 1996, where it predominates in certain regions such as Patagonia. That is precisely where the cruise ship departed.

Estanislao Nistal Villán, a researcher with the virology group at CEU San Pablo University, downplays the alarm. “In these cases, close and prolonged contact with the bodily fluids of an infected person is required.” Scientific evidence points to hospital or sexual contexts.

With initial symptoms similar to those of the flu, hantavirus infection is usually mild. Its mortality rate, however, is highly variable. In Argentina — one of the countries where the virus is most prevalent — the average fatality rate in recent years has been around 18.5%. As the infection progresses, it can develop into conditions such as hantavirus pulmonary syndrome, which raises the risk of death to almost 40%, or hemorrhagic fever with renal syndrome, which is also potentially fatal.

European and American: A matter of local fauna

The hantavirus is a highly variable virus, with up to 24 different strains found in humans. Rafael Medina Silva, a professor of pathology at Emory University School of Medicine, has been studying its variability for years and believes it’s important to differentiate between them. “We talk about Old World hantavirus, from Europe and Asia, and New World hantavirus, from the Americas,” he explains.

The two infections produce illnesses with slightly different symptoms. In the Americas, the most severe cases develop into hantavirus pulmonary syndrome (with the aforementioned mortality rate of nearly 40%) and can involve cardiorespiratory complications. The viruses circulating in Europe and Asia are more often associated with kidney disease, although they can also develop into severe cases.

What’s the reason for this difference? Unlike the flu, humans are not the primary reservoir of this virus. We are not its natural host, so to speak; rodents are. And the rodents found in Europe, Asia, and the Americas are different species.

“They are not found in the common rats or mice that are usually seen in cities,” explains Nistal Villán. “The best-documented are the deer mouse, from the United States, and the long-tailed mouse, typical of the southern cone of America.” These animals carry the most aggressive variants of the virus.

“In Europe, and specifically in Spain, hantavirus species have been described in field mice such as voles, with very low pathogenicity,” explains the expert. In 2024, an autochthonous case was recorded in the Spanish region of Catalonia. The patient was admitted to the ICU, but subsequently recovered without any lasting effects. It is one of the few severe cases: normally, the disease presents with symptoms resembling those of a common flu.

What if the ship docks in the Canary Islands?

The WHO plans to evacuate the two sick passengers from the ship and allow it to continue its planned route to one of the Canary Island ports with the rest of the passengers, who are in good health. What would be the real risks of this plan?

“Taking precautions, and with the proper quarantine, it could be done with practically no risk,” explains Medina Silva. “The paranoia is somewhat understandable given the unusual nature of this disease, but hantavirus is not transmitted between people; therefore, the risk is much lower than with the ships carrying COVID a few years ago.”

Another concern is the possible escape of infected fauna from the ship, but all indications suggest this risk is practically nonexistent. The WHO appears to rule out the presence of rodents on board and points to a previous infection.

“Taking into account the duration of the hantavirus incubation period, which can range between one and six weeks, our hypothesis is that they were infected outside the boat,” said Maria Van Kerkhove, WHO’s Director of Epidemic and Pandemic Prevention and Preparedness.

Medina agrees with this hypothesis, noting how difficult it would be for transmission to occur on board, as it would require the presence of many rodents. “Historically, these mice maintain the virus in less than 5% of the population,” he explains. “Infection usually occurs through exposure to feces in areas with high mouse populations and abundant wildlife. I don’t believe there are large numbers of mice on the ship, nor that they were infected.”

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