WHO DON: Update: Hantavirus Cluster Linked to Cruise Ship Travel, Multi-country

AI generated with Gemini based on WHO narrative

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It's been barely 40 hours since we first learned of the cluster of Hantavirus cases aboard a ship on a polar expedition cruise in the South Atlantic. The ship departed from Ushuaia, Argentina, on April 1st, and has made more than a half dozen stops (see map above), where passengers may have left the ship.

While the exact Hantavirus has not been identified, the ship's departure point suggests the Andes Virus, which is often found in rodents in South America. 

The Andes virus would be particularly concerning because it is the only Hantavirus with an established reputation for being transmissible between humans (see NEJM  “Super-Spreaders” and Person-to-Person Transmission of Andes Virus in Argentina).

Complicating matters, the incubation period for Hantavirus can run up to 8 weeks, with a 42-day monitoring period generally recommended.  Some cases can be mild, or even asymptomatic, but the CFR (case fatality rate) can reach 30%-60%. 

As of today's announcement from the WHO, there are now 2 confirmed cases, and 5 suspected cases; 3 have died, 1 remains hospitalized, and 3 remain aboard ship.

There are also another 140+ individuals aboard the ship who may have been exposed, along with an unknown number potential contacts on shore (including healthcare workers). All of which will present some significant challenges for public health agencies across multiple jurisdictions. 

In impressively rapid fashion the WHO has produced a detailed DON (excerpts below), providing us with both a risk assessment and WHO advice. While `. . . the WHO currently assesses the risk to the global population from this event as low. . . ',  this is a dynamic situation, and no one is taking it lightly. 

Some of the public health challenges ahead include:

• Exactly how, when, and where the three symptomatic cases will be medically evacuated from the ship (and any others that might subsequently fall ill)

• How disembarkation of the remaining passengers and crew - and their ongoing medical monitoring - will be handled. 

• Contact tracing - and health monitoring - of anyone that passengers may have had close contact with (now, and going forward) across multiple countries. 

I've reproduced large excerpts from the WHO DON below, but you'll want to follow the link to read it in its entirety.

Hantavirus cluster linked to cruise ship travel, Multi-country
4 May 2026

Situation at a glance

On 2 May 2026, a cluster of passengers with severe respiratory illness aboard a cruise ship was reported to the World Health Organization. The ship is carrying 147 passengers and crew. As of 4 May 2026, seven cases (two laboratory confirmed cases of hantavirus and five suspected cases) have been identified, including three deaths, one critically ill patient and three individuals reporting mild symptoms.

Illness onset occurred between 6 and 28 April 2026 and was characterized by fever, gastrointestinal symptoms, rapid progression to pneumonia, acute respiratory distress syndrome and shock. Further investigations are ongoing. The outbreak is being managed through coordinated international response, and includes in-depth investigations, case isolation and care, medical evacuation and laboratory investigations. 

Human hantavirus infection is primarily acquired through contact with the urine, faeces, or saliva of infected rodents. It is a rare but severe disease that can be deadly. Although uncommon, limited human to human transmission has been reported in previous outbreaks of Andes virus (a specific species of hantavirus). WHO currently assesses the risk to the global population from this event as low and will continue to monitor the epidemiological situation and update the risk assessment.

Description of the situation

On 2 May 2026, WHO received notification from the National International Health Regulations (2005) (IHR) Focal Point of the United Kingdom of Great Britain and Northern Ireland (hereafter referred to as the United Kingdom) regarding a cluster of severe acute respiratory illness, including two deaths and one critically ill passenger, aboard a Dutch-flagged cruise ship. On 2 May 2026, laboratory testing conducted in South Africa confirmed hantavirus infection in one patient who is critically ill and in intensive care. On 3 May, one additional death was reported. A further three suspected cases remain on board. As of 4 May, a total of seven (two confirmed and five suspected) cases, including three deaths, have been reported.

The vessel departed Ushuaia, Argentina, on 1 April 2026 and followed an itinerary across the South Atlantic, with multiple stops in remote and ecologically diverse regions, including mainland Antarctica, South Georgia, Nightingale Island, Tristan da Cunha, Saint Helena, and Ascension Island. The extent of passenger contact with local wildlife during the voyage, or prior to boarding in Ushuaia remains undetermined. The vessel carries a total of 147 individuals, including 88 passengers and 59 crew members. Onboard passengers and crew represent 23 nationalities. As of 4 May 2026, the vessel is moored off the coast of Cabo Verde.

Summary of cases:

Case 1: An adult male developed symptoms of fever, headache, and mild diarrhoea on 6 April 2026 while on board the ship. By 11 April, the case developed respiratory distress and died on board on the same day. No microbiological tests were performed. The body of the passenger was removed from the vessel to Saint Helena (a British Overseas Territory) on 24 April.

Case 2: An adult female, who was a close contact of case 1, went ashore at Saint Helena on 24 April 2026 with gastrointestinal symptoms. She subsequently deteriorated during a flight to Johannesburg, South Africa, on 25 April. She later died upon arrival at the emergency department on 26 April. On 4 May, the case was subsequently confirmed by PCR with hantavirus infection. Contact tracing for passengers on the flight has been initiated.

Cases 1 and 2, had travelled in South America, including Argentina, before they boarded the cruise ship on 1 April 2026.

Case 3: An adult male presented to the ship's doctor on 24 April 2026 with febrile illness, shortness of breath and signs of pneumonia. On 26 April, his condition worsened. He was medically evacuated from Ascension to South Africa on 27 April, where he is currently hospitalised in an Intensive Care Unit (ICU). Laboratory testing on an extensive respiratory pathogen panel was negative; however, polymerase chain reaction (PCR) testing confirmed hantavirus infection on 2 May 2026. Serology, sequencing and metagenomics are ongoing.

Case 4: An adult female, with presentation of pneumonia, died on 2 May 2026. The onset of symptoms was on 28 April, with fever and a general feeling of being unwell.Three suspected cases have reported high fever and/or gastrointestinal symptoms and remain on board. Medical teams in Cabo Verde are evaluating the patients and collecting additional specimens for testing.

Public health response

Authorities from States Parties involved in the management of the event to date – Cabo Verde, the Netherlands, Spain, South Africa and the United Kingdom - have initiated coordinated response measures including:

• Ongoing engagement between WHO and the National IHR Focal Points of Cabo Verde, the Netherlands, South Africa, Spain and the United Kingdom, to ensure timely information sharing and coordination of response actions.
• WHO shared information about the events with National IHR Focal Points globally.
• Passengers onboard have been advised to practice maximal physical distancing and remain in their cabins where possible.
• Epidemiological investigations are underway to determine the source of exposure.
• The National IHR Focal Point of Argentina shared the passenger and crew lists with the National IHR Focal Points of the respective countries, according to each person’s nationality.
• In line with the Working Arrangement between the WHO Emergency Medical Team (EMT) Secretariat and the EU Emergency Response Coordination Centre (ERCC), the EMT Secretariat has launched formal discussions to support the clinical management and medical evacuation of symptomatic passengers.
• Logistic support has been provided, including sample collection items.
• Laboratory testing and confirmation of hantavirus infection have been conducted at the National Institute for Communicable Diseases (NICD) of South Africa. Serology, sequencing and metagenomics are ongoing.
• Additional laboratory samples from symptomatic passengers are being sent, with WHO support, to the Institut Pasteur de Dakar, Senegal, for testing.
• WHO has activated three-level coordination and is supporting national authorities in implementing risk-based, evidence-informed public health measures in accordance with the provisions of the IHR and related WHO technical guidance documents.

WHO risk assessment

Hantavirus cardiopulmonary syndrome (HCPS), also known as hantavirus pulmonary syndrome (HPS), is a zoonotic, viral respiratory disease caused by hantaviruses of the genus Orthohantavirus, family Hantaviridae, order Bunyavirales. More than 20 viral species have been identified within this genus. In the Americas, Sin Nombre virus is the predominant cause of HPS in North America, while Orthohantavirus andesense is responsible for most cases in South America.

Human Hantavirus infection is primarily acquired through contact with the urine, faeces, or saliva of infected rodents or by touching contaminated surfaces. Exposure typically occurs during activities such as cleaning buildings with rodent infestations, though it may also occur during routine activities in heavily infested areas. Human cases are most commonly reported in rural settings, such as forests, fields, and farms, where rodents are present, and opportunities for exposure are greater. HPS is characterized by headache, dizziness, chills, fever, myalgia, and gastrointestinal problems, such as nausea, vomiting, diarrhoea, and abdominal pain, followed by sudden onset of respiratory distress and hypotension. Symptoms of HPS typically occur from 2-4 weeks after initial exposure to the virus. However, symptoms may appear as early as one week and as late as eight weeks following exposure.

Hantavirus infections are relatively uncommon globally. In 2025 (as of epidemiological week 47), in the Region of the Americas, eight countries reported 229 cases and 59 deaths with a CFR of 25.7%. [1] In the European Region, 1885 hantavirus infection reported in 2023 (0.4 per 100,000), marking the lowest rate observed between 2019 and 2023.[2] In East Asia, particularly China and the Republic of Korea, Hantavirus haemorrhagic fever with renal syndrome (HFRS) continues to account for many thousands of cases annually, although incidence has declined in recent decades.

Hantavirus infections are associated with a case fatality rate of <1–15% in Asia and Europe and up to 50% in the Americas. While there are no specific treatment nor vaccines for hantavirus infections, early supportive care and immediate referral to a facility with a complete ICU can improve survival.

Environmental and ecological factors affecting rodent populations can influence disease trends seasonally. Since hantavirus reservoirs are sylvatic rodents, transmission can occur when people come into contact with rodent habitats.

Although uncommon, limited human‑to‑human transmission of HPS due to Andes virus has been reported in community settings involving close and prolonged contact. Secondary infections among healthcare workers have been previously documented in healthcare facilities, though remain rare.

WHO currently assesses the risk to the global population from this event as low and will continue to monitor the epidemiological situation and update the risk assessment as more information becomes available.

(Continue . . . )

MOH Statements On Cruise Ship With Hantavirus Outbreak

 

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Over the past hour the ever diligent newshounds at FluTrackers have posted several new statements from Ministries of Health along the route of the m/v Hondious - which is dealing with an unusual outbreak of Hantavirus among its passengers (see earlier blogs here, and here).

As I mentioned in my last blog, the ship is currently positioned off the port of Praia, Cabo Verde, and local authorities are not permitting passengers to disembark.  There are reportedly two passengers in need of medical attention.

The (translated) statement from their MOH follows:

Health authorities are monitoring the situation of the cruise ship MV/NV Hondius with an outbreak of Hantavirus on board

The Ministry of Health informs that it has been following, from the very first moment, the situation of the cruise ship MV/NV Hondius, which entered the waters of Cape Verde on May 3, following the notification by international health entities of an outbreak of respiratory illness on board, with the occurrence of serious cases and deaths.

Following technical and epidemiological evaluation, the national health authorities decided not to allow the docking of the vessel in the Port of Praia, in application of the principle of precaution and in accordance with the International Health Regulation, in order to protect national public health.

The vessel carries 147 people including passengers and crew. Of this total, three people present symptoms and were properly evaluated and assisted by a health care team, currently being clinically stable.

Since then, the ship remains on the high seas under constant supervision from the health authorities. The necessary medical assistance is being provided by a team highlighted for the effect, consisting of specialist doctors, nurses, laboratory technicians and equally prepared measures for hospital responses for possible need for differentiated care Hospital Dr. Agostinho Neto.

The situation is being properly monitored through coordinated work between the National Health Directorate, local health facilities, INSP, maritime and port authorities, with support from WHO Cape Verde/AFRO, RSI Focal Points and authorities from the Netherlands and the UK.

This articulation has allowed for a swift, safe and technically appropriate response, ensuring patient clinical follow-up and preparation of all necessary precautionary measures, including possible medical evacuation by air ambulance of patients afterwards.

It is ensured that the situation is under control, there is no risk to the population on earth, so far. We remain vigilant and in close coordination with national and international entities, ready to take any additional measures that may appear necessary.

Information will be updated as the situation develops.

The National Health Directorate calls for serenity and reaffirms its commitment to transparency, safety and protection of health for all.

What is hantavirus ?

Hantavirus is an acute and serious infectious disease, transmitted by wild rodents (rats). Caused by the virus of the genus Orthohantavirus, the infection occurs mainly by the inhalation of urine particles, feces or saliva from those rats, which can develop into cardiopulmonary syndrome (severe shortage of air) or, less commonly, kidney syndromes.

Inhalation of contaminated particles — for example, during cleaning spaces with the presence of mice — is one of the most common forms of infection.

The other forms of transmission to the human species are:

👉Percutanea, by means of skin abrasions or rodent bites;

👉Contact of the virus with mucosa (conjunctival, mouth or nose), through hands contaminated with rodent excrements;

👉 Person-to-person transmission, reported, sporadically, in Argentina and Chile, always associated with the Andes hantaviruses.

The period of transmittibility of hantavirus in humans is unknown. Studies suggest that the period of maximum coming would be a few days before the onset of signs/symptoms.

Already the incubation period of the virus, i.e. the period when the first symptoms begin to appear from the infection, is, on average, from 1 to 5 weeks, with variation from 3 to 60 days.

The first signs of infection are often mistaken for flu: fatigue, fever, and muscle aches. In some cases headaches, dizziness, chills and abdominal problems also occur.

However, the disease can progress quickly. Within a few days, more serious symptoms such as coughing, shortness of air and fluid accumulation in the lungs appear, which can lead to severe breathing difficulties.

Meanwhile, the government of St. Helena, a small island (pop. 4900) in the South Atlantic where the ship stopped in late April, has issued the following statement:

Suspected Hantavirus on MV Hondius
May 4th, 2006 

The public are advised that the Health and Social Care Portfolio have become aware of an evolving situation regarding a severe illness, affecting several passengers on the expedition vessel MV Hondius which recently visited St Helena between 22-24 April 2026.

The suspected cause is hantavirus, which is usually spread through contact with infected rodents, but may eventually pass from person-to-person. Symptoms can include fever, extreme fatigue (feeling more tired than usual), muscle aches, stomach pain, nausea, vomiting, diarrhoea or shortness of breath. Some people may develop severe breathing difficulty requiring hospital care.

Two passengers with minor symptoms came ashore and may have had some contact with members of our local community.

While the virus can be serious, no cases of this illness have been identified in St Helena and there is no significant cause for concern on the island at this time.

Public Health is working closely with the United Kingdom Health Security Agency (UKHSA) and other international partners to assess and manage the situation. A small number of people who travelled to St Helena on the MV Hondius or had very close contact with those who were showing symptoms, are being advised by Public Health to undertake a period of self-isolation as a precaution. St Helena Government (SHG) will provide full support to those who are asked to do so. A full risk-based contact tracing process is underway to identify and notify such persons.

The risk to the wider community is low and no additional precautions are necessary at this time. However out of the abundance of caution, should you experience a significant fever (>38°C), then you should contact the hospital on tele No 22500 for advice, but please DO NOT attend in person at the first instance.

For frequently asked questions about hantavirus, please visit: Hantavirus FAQ’s

The public are thanked for their support and cooperation during this time. Further updates will be provided on a regular basis.

#StHelena #Hantavirus #MVHondius

Lastly, South Africa MOH has issued this statement:

While the risks of transmission in these ports of call are low, they are not zero. And given the lengthy incubation period (1-5 weeks) of the virus, this is a going to require a protracted public health response.

WHO Statement On Hantavirus Cases Aboard Cruise Ship

 

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While we await further details on the (suspected) Hantavirus outbreak aboard a cruise ship in the Atlantic (see yesterday's blog), we have the following brief statement by the World Health Organization.

As of this posting, no DON has been published on this event, and that may not come for several days.  Sequencing of the virus will not only tell us more about its origin (is it the Andes strain?), it could also help define transmission dynamics aboard ship. 

While the WHO's messaging remains low-key, they are obviously taking this outbreak seriously, as they have notified their NFPs (National Focal Points) according to the IHR regulations. 

The BBC this morning is reporting that local authorities will not permit passengers to leave the ship in Cape Verde in order `to protect the public'.  It is not clear where or when the passengers will be allowed to disembark. 

Although human-to-human spread of the Andes virus has been documented (see EID Journal), the Andes Virus is not believed to transmit as easily as COVID, influenza, and many other respiratory viruses. 

Transmission likely requires close, prolonged exposure to respiratory droplets or other bodily fluids.

But there is still much we don't know about this virus, including its ability to spread asymptomatically. 

Studies have suggested (see Serological Evidence of Hantavirus Infection in Apparently Healthy People from Rural and Slum Communities in Southern Chile) that at least some infections are mild or asymptomatic.

A more recent 2025 study (see Virological characterization of a new isolated strain of Andes virus . . .), published in PloS NTD reported:

In this work, we described the isolation of the strain responsible for the largest ANDV PTP transmission outbreak, which occurred in the small town of Epuyén and began on November 2, 2018. This strain, ARG-Epuyén, exhibited a high capacity for PTP transmission, necessitating the implementation of quarantine measures to curtail further spread [8].

The median reproductive number (the mean number of secondary cases caused by an infected person) was 2.12 before control measures were implemented and subsequently dropped to below 1.0 by late January.

Early intervention allowed for the collection of samples leading to the isolation of this new ANDV strain from an asymptomatic case. An early passage of this strain was sequenced, revealing only one amino acid difference from the virus recovered from the patient. Like the Andes/ARG strain, this strain was able to grow in a new host without needing adaptation [26].

All reasons why public health officials will want to proceed cautiously as they work to contain this outbreak. 

Brief Background on the Suspected Hantavirus Outbreak On A Cruise Ship (ex Argentina)

 

Note: The newshounds at FluTrackers are following media reports (see thread) of a hantavirus outbreak aboard a cruise ship (MV Hondius, bound from Ushuaia in Argentina to Cape Verde), where 2 or 3 passengers have reportedly died, and several more are apparently infected. 

While details are scarce, I've prepared a backgrounder on the virus, and what we currently know about today's events.  

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Hantaviruses - which are carried by rodents in many parts of the world - are contracted by humans primarily through the inhalation or ingestion of aerosolized rodent feces, urine, saliva.  While relatively uncommon, some of these viruses have demonstrated limited human-to-human transmission.

Some - like Europe's Puumala Virus - produce relatively mild illness, while others like North America's Sin Nombre Virus and the South American Andes Virus can carry a high fatality rate.

In 2019 we closely followed an unusually large outbreak in Argentina where over a period of 3 months, a total of 29 laboratory-confirmed cases of Hanta Pulmonary Syndrome (HPS), including 11 deaths - were reported in Epuyén, Chubut Province. 

The WHO DON report stated:

The index case had environmental exposure prior to symptom onset on 2 November, and subsequently attended a party on 3 November. Six cases who also attended the party experienced the onset of symptoms between 20-27 November 2018.

An additional 17 cases, all of whom were epidemiologically-linked to previously confirmed cases, experienced symptom onset between 7 December 2018 and 3 January 2019 (Figure 1). Potential human-to-human transmission is currently under investigation.

In that outbreak, the incubation period ran from 8 to 31 days.  The CDC notes incubation can run anywhere from 1 to 5 weeks, and some studies have suggested up to 8 weeks. 

Hantavirus in humans is almost always a dead-end infection, but in recent years the Andes virus (ANDV) has garnered a reputation for being a bit of an outlier.  

Following the above outbreak, the NEJM published “Super-Spreaders” and Person-to-Person Transmission of Andes Virus in Argentina, which warned: ANDV Epuyén/18−19 strain shows a facility (R>2) for sustaining continuous chains of transmission if no control measures are enforced. 

While we don't know if this hantavirus is the ANDV Epuyén/18−19 strain, the fact that it appears to have infected 5 or 6 passengers is a concern. 

The ship in question (HV Honidus) reportedly carries up to 174 passengers, and 74 crew, and departed Argentina in early April. This ship appears to have been on a `polar exploration cruise', then proceeding on to South Africa and Cape Verde.

While we don't have a solid timeline, according to media reports, in late April a 70 y.o. passenger died on board, and his 69 y.o. wife subsequently fell ill (she reportedly died in a Johannesburg hospital).

A third passenger has reportedly died (unconfirmed), and 3 more passengers are supposedly sick (1 hospitalized in Johannesburg),while two are in isolation awaiting the next port.

Given this is Sunday, there's very little in the way of official confirmation of these reports.  Hopefully we'll get more details in the next day or two. 

While more cases are possible - with appropriate control measures, this outbreak should be manageable - although the long incubation period will certainly complicate matters. 

For more on hantaviruses, you may wish to revisit:

Two Recent Studies On the Host Range of Hantaviruses In the United States

California: Mono County Reports 3rd Hantavirus Death

 EID Journal: Experimental Infection of Peromyscus Species Rodents with Sin Nombre Virus

MMWR: A Little Bit Of Seoul (Virus)

WHO Mpox SitRep #65 : Report of 3rd Recombinant Mpox Virus With Genomic Elements of Clades Ib and IIb

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While details were scant, just short of 5 months ago (Dec 8th) the UKHSA Identified a New Recombinant Strain of Mpox Virus in a traveler recently returning from an (undisclosed) country in South-East Asia.

For the very first time, genomic sequencing showed an emerging mpox strain with elements from both clade Ib and IIb mpox. 

Two months later the WHO announced a 2nd case, describing the two cases as:

The first case was detected in the United Kingdom of Great Britain and Northern Ireland (hereafter “United Kingdom”), with travel history to a country in South-East Asia, and the second in India, with travel history to a country in the Arabian Peninsula. 

Detailed analysis of the virus genomes shows that the two individuals fell ill several weeks apart with the same recombinant strain, suggesting that there may be further cases than are currently reported. Both cases had similar clinical presentation to that observed for other clades. Neither patient experienced severe outcomes. Contact tracing for both cases in the reporting countries has been completed; no secondary cases were detected. 

With little fanfare, 72 hours ago the WHO announced the third detection of a recombinant strain in their latest SitRep (#65), although they buried the lede somewhat, putting the first mention at the very bottom of their list of highlights. 

Highlights

• Transmission of mpox continues mostly within sexual networks, affecting men and women, often followed by household transmission and, in some areas, affecting all age groups. All clades of monkeypox virus (MPXV)continue to circulate. Rapid containment of mpox outbreaks is essential to prevent community transmission in any setting.

• In March 2026, 48 countries across all WHO regions reported a total of 1235 confirmed mpox cases, including five deaths (case fatality ratio [CFR] 0.4%). Of these cases, 70.4% were reported in the WHO African Region.

• Four WHO regions – the Region of the Americas, African, European, and South-East Asian regions – reported a decline in confirmed cases in March, compared to February 2026, while the Eastern Mediterranean and Western Pacific Region reported an increase in confirmed cases.

• Sixteen countries in Africa reported active transmission of mpox in the last six weeks (9 March – 19 April 2026),with 969 confirmed cases, including three deaths (CFR 0.3%). Madagascar, the Democratic Republic of the Congo,Guinea, Kenya, and Burundi reported the highest number of cases during this period.

• Four countries, Colombia, Denmark, Ecuador and Singapore, reported mpox due to clade Ib MPXV for the first time. Poland and Slovakia reported mpox due to clade I MPXV for the first time, pending subclade identification.

• Outside Africa, Argentina, Denmark, Germany, Pakistan, Portugal, Singapore, Spain, and the United Kingdom of Great Britain and Northern Ireland reported community transmission of clade Ib MPXV, including among men who have sex with men.

• Pakistan has reported a healthcare-associated mpox outbreak in Sindh province, involving neonates, infants, and adults. In the past month, the province has reported 29 confirmed cases, including eight deaths (case fatality ratio: 28%), and one additional death in a suspected case. About half of cases, and all deaths were reported among infants younger than six months of age.

• Qatar has reported a travel-related case of mpox with a clade Ib/IIb recombinant MPXV strain, the third such reported case globally, following previous case reports in India and the United Kingdom of Great Britain and Northern Ireland. The patient has recovered and no secondary cases have been reported to date.

While there is admittedly a lot going on in the world right now, this latest case appears to have escaped the notice of the mainstream press. The WHO report goes on to state (on page 8):

Detection of recombinant MPXV strain in Qatar

On 24 March 2026, Qatar notified WHO of a laboratory-confirmed mpox case infected with a recombinant MPXVstrain containing genomic elements of clade Ib and IIb MPXV. The case is an adult male resident in Qatar, who developed general and genital mpox symptoms on 10 March and was confirmed to have mpox on 21 March.

Genomic sequencing on 2 April confirmed infection with a recombinant clade Ib/IIb MPXV strain.

The individual reported short travel to Saudi Arabia during the incubation period, but no contact with a known mpox case. He did not report any sexual or other known high-risk exposures in either Saudi Arabia or Qatar, and to date, the source of infection for this case remains unknown. Following the initial diagnosis, the case was isolated in a facility and recovered fully. 

Contact tracing was completed with no secondary cases identified.

This represents the third known detection of this MPXV recombinant strain globally, following travel-related detections in India and the United Kingdom of Great Britain and Northern Ireland. Consistent with previous detections, no differences in clinical presentation have been observed compared with infection with non-recombinant MPXV strains. 

While the public health risk associated with this event in Qatar and globally is assessed as low, it highlights the potential for undetected spread of this recombinant strain and the risk of continued emergence of recombinant strains in the context of co-circulation of multiple MPXV clades. Ongoing surveillance, genomic sequencing, and case investigation remain critical.

 
Previously the WHO reported on the first 2 cases:

Detailed analysis of the virus genomes shows that the two individuals fell ill several weeks apart with the same recombinant strain, suggesting that there may be further cases than are currently reported.

But that same report, also said:

After classification of this case and posting in a public database as a novel MPXV recombinant strain, a case of mpox detected in India in September 2025 was retrospectively reclassified as a closely-related recombinant strain based on sequencing data.

This latest report states - `This represents the third known detection of this MPXV recombinant strain globally. . . ' - which strongly suggests all 3 recombinants are genetically similar.

But without more detailed information, it is difficult to say whether all 3 cases stem from a single recombination event. 

As recently as 5 years ago, there were only 2 recognized clades of Mpox (then Monkeypox): The milder West African (now clade IIa) and the more severe Central African clade (now Clade Ia).

But like all viruses, Mpox continues to evolve and diversify, as discussed in the 2014 EID Journal article Genomic Variability of Monkeypox Virus among Humans, Democratic Republic of the Congo, where the authors cautioned:

Small genetic changes could favor adaptation to a human host, and this potential is greatest for pathogens with moderate transmission rates (such as MPXV) (40). The ability to spread rapidly and efficiently from human to human could enhance spread by travelers to new regions.

In recent years we've seen an explosion in the number of Mpox strains, with multiple lineages of clade IIb spreading globally in 2022 - followed in 2023 by the emergence of a new clade Ib, which is slowing spreading internationally as well. 

The detection of 3 (reportedly similar) recombinant strains in 3 different countries raises the possibility that a new - genetically distinct strain - may be emerging.  

While RT-PCR can detect this recombinant a Mpox positive, it doesn't flag it as being `different'. For that, WGS (Whole Gene Sequencing) is needed, and sadly, only a small fraction of samples are sequenced. 

Which means it may be some time before we know if these 3 cases are merely a flash in the pan, or a stronger, more worrisome, signal. But as Ian Fleming once famously wrote; 

"Once is happenstance, twice is coincidence, three times is enemy action"

So we'll be watching this evolving situation closely. 

EID Journal Dispatch: Replication Efficiency of Contemporary Highly Pathogenic Avian Influenza A(H5N1) Virus Isolates in Human Nasal Epithelium Model

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In the past, H5N1 transmission in humans has been thought to be hampered by the generally hostile environment found in the human upper airway; cooler temperatures, the virus's preference for avian (α2,3-linked SA) receptors, and the cytokine signaling defenses of the innate immune system.

While the evidence continues to suggest that HPAI H5N1 viruses are not transmitting efficiently (or often) among humans, it is equally clear that some mild (or asymptomatic) infections have flown under the radar (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers).

Over the past year, we've seen growing evidence that contemporary H5 viruses may be evolving towards overcoming some of these obstacles. Some recent blogs include: 

• Last month, in EID Journal: Tropism and Replication Competence of Cattle Influenza A(H5N1) Genotype B3.13 Virus in Human Bronchus and Lung Tissue, we saw a report that the `Bovine' H5N1 virus showed partial adaptation to human respiratory tissue, and replicated in the lung tissue on par with the 2009 H1N1pdm virus. 

• Last January, in Preprint: Bovine-derived Influenza A virus (H5N1) Shows Efficient Replication in Well-differentiated Human Nasal Epithelial Cells Without Requiring Genetic Adaptation, we saw another study - this time on the upper airway - which found H5N1Tex/24 crucially replicated effectively at 33 degrees Celsius, whereas earlier avian H5N1 strains require 37 degrees Celsius.

• And six months ago, in J.I.D.: Avian influenza virus A(H5N1) genotype D1.1 is better adapted to human nasal and airway organoids than genotype B3.13, researchers demonstrated that D1.1 genotype replicates better in lab-grown nasal and lung tissues than the bovine B3.13 strain, and it binds more tightly to human‑type (α2,6-linked SA) receptors.  

Yesterday the CDC's EID Journal published another in this growing list of studies suggesting that recently emerged strains of HPAI H5N1 clade 2.3.4.4b are becoming better adapted to the human upper respiratory system than older strains.  

This NIAID study compares several HPAI and seasonal flu strains and finds that some strains of HPAI H5 (B3.13 & D1.1) have become better adapted to  human physiology; including tolerance to lower temperatures and the ability to evade some of our innate immune responses. 

While these are incremental changes, and the virus still has a ways to go, this is a worrying trajectory.  Due to its technical nature, I've only posted some excerpts from the dispatch.  Follow the link to read it in its entirety. 

Replication Efficiency of Contemporary Highly Pathogenic Avian Influenza A(H5N1) Virus Isolates in Human Nasal Epithelium Model

Meaghan Flagg1, Christopher J. Winski1, Bridget G. Brackney, Tessa R. Lutterman, Johan A. Ortiz-Morales2, Brandi N. Williamson, and Emmie de Wit 

Abstract

Replication of influenza A virus in human nasal epithelium affects transmissibility and disease. We compared virus replication and immune responses in human nasal epithelium infected with seasonal and highly pathogenic avian influenza A(H5N1) viruses. Contemporary H5N1 viruses replicated better than the historical isolate; however, interferon response to B3.13 genotype viruses was dampened.

Since March 2024, a total of 70 human cases of highly pathogenic avian influenza (HPAI) A(H5N1) have been reported in the United States as a result of sporadic spillover events from poultry and dairy cattle (1). HPAI H5N1 clade 2.3.4.4b genotype B3.13 was responsible for many of the early cases (2).

On January 31, 2025, HPAI H5N1 clade 2.3.4.4b genotype D1.1 was detected in dairy cattle (https://www.aphis.usda.gov/news/program-update/aphis-confirms-d11-genotype-dairy-cattle-nevada-0External Link); D1.1 was later identified in humans (1). Those spillover events sparked global health concerns about the potential for large-scale spread of clade 2.3.4.4b HPAI H5N1 viruses and their risk to human and animal health.

Seasonal influenza A and HPAI H5N1 viruses both cause severe respiratory disease despite different tissue tropisms. Seasonal influenza A viruses primarily infect the upper respiratory tract (URT), whereas HPAI H5N1 viruses preferentially replicate in the lower respiratory tract (LRT). This contrast in tissue affinity is explained by differences in receptor specificity and has been implicated in transmission efficiency (3). Specifically, the URT predominantly expresses sialic acids linked to galactose by an α-2,6 linkage; the LRT expresses sialic acid linked to galactose via α-2,3. 

Despite the inefficient human-to-human transmission of HPAI H5N1 viruses, recent emergence and circulation of new genotypes in mammals emphasize the need to characterize these novel viruses in relevant respiratory tract models.

Here, we compare the replication kinetics and host innate immune responses in human nasal epithelium of several seasonal influenza A virus isolates and historical and contemporary HPAI H5N1 virus isolates of 3 different genotypes.

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HPAI H5N1 isolate A/Texas/37/2024 (B3.13 genotype) replicated most efficiently in nasal tissue even when compared with seasonal isolates (Figure 1). Although we noted differences in replication kinetics between viruses of the same genotype, the B3.13 and D1.1 genotype isolates replicated more efficiently than the historical HPAI H5N1 isolate A/Vietnam/1203/2004. The B3.6 HPAI H5N1 isolate A/mountain lion/MT/1/2024 replicated least efficiently (Figure 1). Presence of known mammalian adaptations of polymerase basic (PB) 2 E627K, PB2 D701N, and PB2 M631L was associated with more efficient virus replication (Table; Figure 1).

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HPAI H5N1 virus replication can be affected by the physiologic temperature of the human nasal mucosa, for which the reference is 33°C (11,12). To address the potential effect of temperature on virus replication, we quantified virus replication kinetics at 37°C and 33°C in MDCK cells.

At 8 hours postinoculation, the titers of HPAI H5N1 virus isolates were lower at 33°C than at 37°C (Appendix Figure 2). However, all viruses reached the same maximum titer at 33°C and 37°C. In addition, the relative difference observed between viruses at 37°C were similar at 33°C, suggesting that adjusting the temperature in the nasal epithelial cultures to 33°C would not have substantially altered our results.

Conclusions

Our results reveal that contemporary HPAI H5N1 isolates with known mammalian adaptations replicate more efficiently than historical HPAI H5N1 virus used. Despite high levels of virus replication, ISG induction was limited in response to B3.13 genotype virus infection. Additional studies are needed to further understand how virus replication efficiency and innate immune responses affect mammalian transmission efficiency. Existing immunity to other influenza A viruses might protect against contemporary H5N1 infection and onward transmission.

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While it will likely take more than just adapting to the human upper airway to make H5N1 a pandemic contender, increasingly breaching our first line of defense is an important milestone.