Three people die in connection with a possible hantavirus outbreak on a cruise ship that departed from Argentina

What do the latest WHO data suggest? Are there still uncertainties?

“The WHO has identified seven cases, including two laboratory-confirmed hantavirus infections and five suspected cases, with onset of illness between April 6 and 28.

Key uncertainties remain: the source of infection on board the ship has not been precisely determined (hantavirus is normally transmitted through contact with rodents, so its emergence in a cruise ship environment raises questions about routes of contamination), and the full extent of human-to-human transmission remains unclear. Previously, only limited human-to-human spread had been documented with the Andes virus. The incubation period and rates of disease progression in this outbreak warrant close monitoring.”

Why is travel history in South America relevant?

“Cases 1 and 2 traveled through South America, including Argentina, before boarding the cruise ship. This is crucial epidemiological information, as it suggests that the infection was contracted in a hantavirus-endemic region and brought on board, rather than originating from conditions on the ship. Argentina is where the Andes virus circulates, so identifying which strain of the virus is responsible is essential to assessing the current risks associated with the outbreak.

The travel histories of other passengers and crew members are still being investigated, so it remains unclear whether other individuals may have been exposed to hantavirus sources in endemic areas before boarding or during port calls. However, most passengers and crew will have been in close quarters throughout the virus's incubation period, increasing the possibility of secondary cases.”

How concerning is this?

“The WHO currently considers the risk to the global population from this event to be low, and there is no evidence of spread beyond the ship and those who had direct contact with the cases. However, the situation remains serious for those on board and their contacts: seven cases have been identified, with three deaths, one patient in critical condition, and three mild cases. The real concern is whether additional cases will emerge among the more than 140 remaining passengers and crew during this critical period. At present, appropriate priority must be given to rapid isolation, medical evacuation, and contact tracing so that affected patients receive the full and comprehensive medical treatment they need.”

Hantaviruses are not a new virus (they have been known since the 1950s – I believe since the Korean War – and were identified in the 1970s). It is not a virus that spreads through community transmission (it has nothing to do with covid-19) and is not transmitted between people (it could happen in certain circumstances, but it is very unlikely).

It is a zoonosis. It circulates commonly in mice (and voles and similar animals; these are its natural reservoir) and can infect humans when particles (usually dust) contaminated with their urine or faeces are inhaled.

There are two variants. In Europe, the most common is the Puumala virus, a hantavirus that typically causes symptoms including fever and kidney involvement, rarely severe and which, in many cases, go unnoticed. In South America, the variant is more aggressive and causes a cardiopulmonary syndrome with rapid respiratory failure, haemorrhages and a significant mortality rate.

It is a rare zoonosis that usually results in isolated cases in rural areas (or among hikers in rural areas). The outbreak on the cruise ship is striking due to the context (a cruise ship with several cases) and the severity (suggesting the South American hantavirus; also the origin of the cruise).

The large number of passengers suggests a common source of exposure: either before boarding the ship (the hantavirus has an incubation period of 2–4 weeks between infection and the onset of symptoms) or in some area of the ship contaminated by mouse droppings.

There is no general risk to the population wherever the ship goes. Passengers will need to be monitored for as long as it takes for symptoms to develop (not because they might infect others, but in case they fall ill) and the ship will need to be cleaned and treated for rodents.

Hantaviruses are a type of virus that are carried by wild rodents like mice or rats and have been recognised since the 1980s.  They are transmitted to humans through breathing in virus in aerosol that has been shed from rodent droppings, but they do not transmit well and so most outbreaks happen among people who have a very high amount of exposure to droppings like farm workers in parts of the world where farmland overlaps with wild habitat for many rodents.

Hantaviruses can be found in wild rodents over many parts of the globe including North and South America, Asia, and Europe which includes wildlife of the United Kingdom although human infection with these viruses is exceptionally rare.

In terms of disease, there are two main types of hantavirus – one can cause disease in the kidneys with intensive fever (“Haemorrhagic Fever with Renal Syndrome” or “HFRS”), the other can cause disease in the lungs (“Hantavirus Pulmonary Syndrome” or “HPS”).  While these are life threatening, most hantavirus infections do not progress to this stage of disease and they are more likely to do so in people with underlying health conditions.  Certain specific hantaviruses are also milder than others, though it is currently hard to speculate as we don’t yet know which of these diseases or which specific hantaviruses affected the people on the MV Hondius.  So far only one case has so far been confirmed positive for hantavirus at all.

Importantly, hantaviruses are not transmissible from person to person except in extremely rare circumstances and only for one specific type of hantavirus called Andes virus, under very intensive close contact (e.g., between sexual partners or from hospital patient to hospital staff).  It’s therefore extremely likely that these cases have resulted from the same single point of exposure to rodents.  The long incubation period means that it will be challenging to pinpoint exactly when and where this occurred.

Hantavirus describes a family of viruses that are zoonotic, i.e. transmitted from animals to humans.  There are two major lineages of hantavirus: Old World Hantaviruses and New World Hantaviruses.  Old World Hantaviruses are found in Europe and Asia, New World Hantaviruses are found in the Americas.  In the current case we are most likely talking about Andes virus, which is a New World Hantavirus found in Argentina.

The danger with New World Hantaviruses is that they are both unspecific in early presentation which resembles flu-like diseases, and the progression to Hantavirus Pulmonary Syndrome which can be very severe with a case fatality rate of up to 40%.  Treatment is mainly supportive.  Given that the incubation period is one to eight weeks, more cases are still possible.

The most common route of Hantavirus spread is via rodents and their droppings (saliva, faeces or urine which can be aerosolised for example when cleaning, less commonly through bites and scratches), human-to-human transmission is possibly but very rare.  Given the timeline, there are multiple possible scenarios in this case: it is not entirely uncommon for rodents to hitch a ride on a ship which would be one possibility, people having been infected when the ship last made port in Argentina is another possibility especially given that incubation periods of up to eight weeks, and the last possibility would be human-to-human transmission which particularly at scale would be very unlikely.

Hantavirus is a viral infection that is acquired from contact with infected rodents (mice and rats).  The virus is typically spread from rodents to humans through airborne aerosols of faeces and urine.  There have been some suggestion that person-to-person spread may occur but there is still no agreement on this and others have concluded that evidence of person-to-person spread is not confirmed.  If person-to-person spread happens at all it is rare.

There are several different types of Hantavirus each associated with a different rodent.  The infection can present as hantavirus pulmonary syndrome (HPS) and haemorrhagic fever with renal syndrome (HFRS).  The different types tend to cause one or other of the clinical syndromes.  Mortality rate in diagnosed infections is high, in HPS this can be about 30% and in HFRS about 10%.  Mortality rates are generally higher in older people, and often people on cruises tend to be older.  There is no specific antiviral treatment available.  Whilst some vaccines are available they do not affect all types of the virus.

It is too early to speculate on how the affected people became infected.  But it is very unlikely that this outbreak would lead to an increased risk in the UK or elsewhere in Europe.

When hearing about outbreaks on cruise ships, we tend to think of other diseases like COVID-19 or norovirus.  Hantavirus is rarely associated with this setting, and person-to-person spread of the virus is also unusual.

Given the ship came from South America, it’s plausible that the Andes strain of the virus may be responsible for this outbreak.  There is previous reporting of person-to-person transmission from the Andes strain, though it’s not yet certain whether that’s the case here.  PCR testing would be needed to confirm whether this is the Andes strain.