CDC HAN #00528: 2026 Multi-country Hantavirus Cluster Linked to Cruise Ship

 
WHO Epi Chart of Probable & Confirmed Cases

#19,151

While it remains to be seen whether the hantavirus outbreak aboard the m/v Hondius has `legs' , overnight the CDC published its first HAN advisory on the outbreak. 

The CDC’s Health Alert Network (HAN) is designed to ensure that communities, agencies, health care professionals, and the general public are able to receive timely information on important public health issues.

Given the extended incubation period, and the co-circulation of other respiratory and gastrointestinal illness this time of year, we are likely to hear of a lot of `suspected' cases in the days and weeks ahead. 

While most will probably turn out to be false alarms, it is imperative that physicians maintain a high index of suspicion when dealing with epidemiologically linked patients.   

While primarily of interest to clinicians, I've reproduced the CDC HAN below. Follow the link for references. 

2026 Multi-country Hantavirus Cluster Linked to Cruise Ship
May 8, 2026

Distributed via the CDC Health Alert Network
May 8, 2026
CDCHAN-00528

Summary

The Centers for Disease Control and Prevention (CDC) is issuing this Health Alert Network (HAN) Health Advisory to inform clinicians and health departments about a new cluster of hantavirus disease cases caused by infection with Andes virus. Hantavirus disease can cause severe illness and can be fatal. Clinicians should be aware of the potential for imported cases, although the risk of broad spread to the United States is considered extremely unlikely at this time. As a precaution, this Health Advisory summarizes CDC's recommendations for U.S. public health departments, clinical laboratories, and healthcare workers about hantavirus disease case identification, testing, and biosafety considerations in clinical laboratories.

Background

On May 2, 2026, the World Health Organization (WHO) was notified of a cluster of severe acute respiratory illness (SARI) among passengers and crew of a cruise ship in the Atlantic Ocean. The cluster included two deaths and one critically ill passenger, whose laboratory tests confirmed hantavirus. On May 6, 2026, WHO confirmed that the type of hantavirus responsible for this outbreak is the Andes virus. As of May 8, 2026, WHO has reported eight cases (six confirmed and two suspected), including three deaths. Investigations are ongoing to assess exposure risk of all Americans passengers on the cruise ship or who may have been exposed to an infected cruise ship passenger on an aircraft.

The cruise ship departed from Ushuaia, Argentina, on April 1, 2026, and traveled across the South Atlantic Ocean, stopping at several remote locations, including Antarctica, South Georgia Island, Tristan da Cunha, Saint Helena, and Ascension Island. It carried 147 people (86 passengers and 61 crew) from 23 different countries. The extent of their contact with wildlife before or during the expedition is unknown.

CDC is working with partners (federal government, state and local and international) on safely repatriating American passengers from the cruise ship to a facility in Nebraska with specialized medical capabilities. On May 7, 2026, CDC sent a team to meet the cruise ship in the Canary Islands, Spain following travel from Cape Verde, South Africa. The team is prepared to assess exposure risk among U.S. passengers and determine appropriate monitoring measures.

CDC is also coordinating with international partners to align public health guidance and has already issued health guidance to affected Americans via the State Department. The risk to the public's health in theUnited States is considered extremely low at this time. As a precaution, CDC is working to increase awareness of the outbreak among travelers, public health agencies, laboratories, and healthcare professionals nationwide.

Hantavirus pulmonary syndrome

Hantaviruses are a group of viruses that can cause severe illness and death. They are most commonly transmitted (spread) to humans through contact with infected rodents (e.g., urine, droppings, saliva). Rarely, infection can occur from rodent bites or scratches. From 1993 through 2023, a total of 890 laboratory-confirmed cases of hantavirus were reported in the United States.

In the Americas, hantaviruses can cause hantavirus pulmonary syndrome (HPS), a severe and potentially deadly disease that affects the lungs. HPS can be life-threatening. Among patients who have severe respiratory symptoms, the case fatality rate has been estimated at approximately 38%.

Andes virus, confirmed as the cause of this hantavirus outbreak, is the only type of hantavirus that has been documented to spread from person-to-person. Although rare, spread between people has typically required close, prolonged contact with a symptomatic person. This could include direct physical contact, prolonged time spent in close or enclosed spaces, and exposure to the infected person's saliva, respiratory secretions, or other body fluids (e.g., kissing, sharing utensils, handling contaminated bedding).

Symptoms of HPS caused by Andes virus usually appear within 4-42 days after exposure. Early symptoms can include fever, fatigue, and muscle aches, especially in large muscle groups like the thighs, hips, back, or shoulders. Early symptoms such as fever, headache, muscle aches, nausea, and fatigue can be easily confused with influenza or other viral illnesses. About half of all HPS patients have experienced headaches, dizziness, chills, and gastrointestinal symptoms, including nausea, vomiting, diarrhea, and abdominal pain. Late symptoms of HPS appear approximately 4-10 days after the initial phase of illness and can include coughing, shortness of breath, and chest tightness Individuals are generally only infectious while symptomatic.

Early diagnosis of HPS can be difficult, especially within the first 72 hours of symptoms, before the virus can be accurately detected in body secretions and excretions. Repeat diagnostic testing is often done 72 hours after symptom onset. CLIA assays for detection of New World hantavirus IgM and IgG antibodies are available at CDC, some state public health laboratories, and Quest Diagnostics.

For questions or concerns about submitting a specimen, please contact your state or local health department or CDC's Emergency Operations Center at 770-488-7100.

No specific treatment is recommended for hantavirus infection; early supportive care is critical even before the diagnosis is confirmed. Patients with suspected HPS can deteriorate rapidly, and delayed care reduces the chance of survival. In severe cases, extra-corporeal membrane oxygenation (ECMO) can significantly improve survival (up to ~80%) if started early. Usually, the critical phase of disease is fairly short, and survivors can recover quickly.

Recommendations for Healthcare Providers

• Be prepared to follow CDC's guidance under Appendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions | Infection Control | CDC.

• In healthcare settings, for patients with suspected or confirmed Andes virus infection, CDC recommends patient placement in an airborne infection isolation room and the use of a gown, gloves, eye protection, and an N95 or higher-level respirator when entering the patient's room.

• Include HPS in the differential diagnosis for an ill person who has compatible symptoms AND who has reported epidemiological risk factors, including at least one of the following, within the 42 days before symptoms onset:

• Had direct physical contact, or spent time in close or enclosed spaces, with a symptomatic person with confirmed or suspected Andes virus infection or with any objects contaminated by their body fluids.

• Had exposure to an infected person's saliva, respiratory secretions, or other body fluids (e.g., kissing, sharing utensils, handling contaminated bedding).

• Experienced a breach in infection prevention and control precautions that resulted in potential contact with body fluids of a patient with suspected or confirmed Andes virus infection.

• Consider and perform diagnostic testing for more common illnesses as well, such as COVID-19, influenza, and other common causes of gastrointestinal and febrile illnesses in an acutely ill patient with epidemiological risk factors and compatible symptoms.

For More Information

General Resources 

About Hantavirus | CDCAbout Andes Virus | CDCReported Cases of Hantavirus Disease | CDCHantavirus Prevention |CDC

Clinician Resources 

Clinical Overview of Hantavirus | CDCClinician Brief: Hantavirus Pulmonary Syndrome (HPS) | CDCClinician Brief: Hemorrhagic Fever with Renal Syndrome | CDCHantavirus Disease Trainings for Healthcare Providers | CDCAppendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions | Infection Control | CDC

Health Department Resources 

Hantavirus Case Definition and Reporting | CDC

MMWR: Fatal Human Case of HPAI A(H5N5) in a Backyard Flock Owner — Washington, November 2025

#19,150

As we've discussed often, it takes a certain amount of luck for novel flu infections to be detected, properly treated, and then reported to the CDC.  It seems likely that some number of mild illnesses go unrecognized (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers) , and even severe cases can be misdiagnosed. 

We've seen instances where patients were only diagnosed retrospectively; sometimes well after they had been treated and released from the hospital (see 2024 Missouri case).

Standard rapid influenza tests don't distinguish between seasonal and novel flu strains, and sometimes, repeated swabs of the upper airway can fail to `capture' a deep lung infection for RT-PCR testing.  

Such is the case of the fatal H5N5 case, reported last November, from Washington state, according to a detailed MMWR report published yesterday by the CDC   As the graphic at the top of this blog shows, the patient repeatedly tested negative for influenza/COVID during the first 6 days of his hospitalization. 

Not surprisingly, a large number (n=139) of hospital personnel and other close contacts were exposed to this patient before he was moved to an isolation unit. 

While no secondary transmissions were reported, it appears only those contacts reporting flu-like symptoms were tested by RT-PCR (NP swabs), and I could find no mention of follow-up serological testing.  

Given this is the first known human infection with novel H5N5, I would have thought a wider net would have been cast.  But that's just me, I guess. Perhaps there's another study in the pipeline on this. 

In any event, the takeaway here appears to be that when dealing with a respiratory infection with someone with contact with poultry or wild birds, a high index of suspicion is warranted, and taking samples from both the upper and lower respiratory tract is recommended. 

Due to it length, I've just posted the summary, abstract, and a few excerpts.  I'll have a postscript after you return. 

Fatal Human Case of Highly Pathogenic Avian Influenza A(H5N5) in a Backyard Flock Owner — Washington, November 2025

Weekly / May 7, 2026 / 75(17);221–225
 
Lynae Kibiger, MPH1; Hanna N. Oltean, PhD1,2; Lisa Leitz3; Emma Krause3; Debra Barrett3; Anna Halloran, MHPA1; Kyle Yomogida, PhD1; Beth Lipton, DVM1; Keely Paris, MPH1; Jared Keirn, MS1; Minden Buswell, DVM1; Allison Black, PhD1; Pauline Trinh, PhD1; Theresa Murray, MT1; Roberto Bonaccorso1; Leticia Banuelos1; Ethan Dieringer1; Jennifer Lenahan, MPH4; Emily Spence Davizon, MPH4; Ellyn P. Marder, DrPH2,4; Jocelyn Mullins, DVM, PhD4; Meagan Kay, DVM2,4; Eric J. Chow, MD2,4,5,6; Sandra J. Valenciano, MD4; John Lynch, MD5,7; Vanessa Makarewicz, MN7; Chloe Bryson-Cahn, MD5,7; Jennifer Hernandez7; Kyla Haggith7; Valicia Linn7; Alex L. Greninger, MD, PhD8; Stephanie Goya, PhD8; Sierra Gulla9; Jennifer Young, MPH9; Sierra Kerns-Funk, MPH10; Brianna da Silva Bhatia, MD10; Hollianne Bruce, MPH11; Krista Kniss, MPH12; Katie Reinhart, PhD12; Rachel Ohlstein13; Shannon Johnson13; Christina Schofield, MD14; Patrick Smith, DO14; Amber Itle, VMD15; Maura Gibson, DVM16; Brandi Torrevillas17; Azeza Falghoush, PhD17; Thomas B. Waltzek, DVM, PhD17; Kevin Snekvik, DVM, PhD17; Mia Torchetti, DVM, PhD16; Timothy M. Uyeki, MD12; Scott Lindquist, MD1 (VIEW AUTHOR AFFILIATIONS)View suggested citation

Summary

What is already known about this topic?

Since 2022, highly pathogenic avian influenza (HPAI) A(H5) viruses have circulated among wild birds in the United States. Seventy human cases of influenza A(H5), most with mild illness, have been reported in the United States since 2024; 14 human influenza A(H5N1) cases were previously identified in Washington.

What is added by this report?

In November 2025, Washington reported the first human case of HPAI A(H5N5) infection worldwide. A positive laboratory result was obtained from a lower respiratory sample after multiple negative upper respiratory sample results; the patient experienced respiratory failure and died 28 days after symptom onset. The public health investigation identified approximately 135 exposed persons.

What are the implications for public health practice?

Symptom management and testing of exposed persons are critical to monitoring for human-to-human transmission of novel influenza infection. Environmental and animal investigations, including genomic analysis, can identify epidemiologic risk factors.ls
Article PDF
Full Issue PDF

Abstract

Clade 2.3.4.4b influenza A(H5N1) viruses have circulated across migratory bird flyways in the United States since 2022, including in Washington, where backyard flock detections have been reported annually. In November 2025, a Washington resident died from acute respiratory failure after receiving a positive influenza A(H5) test result at a hospital laboratory. 

Washington Public Health Laboratories confirmed influenza A(H5), and genomic sequencing identified influenza A(H5N5) virus (A6 genotype). Polymerase chain reaction testing detected highly pathogenic avian influenza A(H5) virus clade 2.3.4.4b from an apparently healthy backyard flock of ducks and sediment from a watering basin on the patient’s property. Six of eight gene segments from the environmental sample and one duck sample (partial neuraminidase segment) were highly genetically similar to the patient’s virus sequence.

Although existing wild bird surveillance had not detected influenza A(H5N5) virus (A6) in the U.S. Pacific Flyway, introduction via wild birds into the environment of the backyard flock was likely the source of the patient’s exposure. 

The public health investigation identified approximately 135 exposed persons; symptom monitoring and influenza testing detected no additional cases. The overall risk for avian influenza A remains low among the general U.S. population; however, novel avian influenza A virus infection should be considered in persons with symptoms of influenza and potential exposures.

       (SNIP)

The diagnosis of influenza A(H5N5) virus infection in the patient described in this report was complicated by early and repeated negative influenza test results from upper respiratory swab specimens. Negative influenza results from initial upper respiratory specimens have been described in three similar patients with lower respiratory tract disease hospitalized with avian influenza A(H5N1) infection (4). Thus, avian influenza virus infection should not be ruled out in hospitalized patients based on negative influenza laboratory test results from upper respiratory tract specimens if the patients have lower respiratory tract disease, relevant exposures, and no confirmed etiology for their disease. If avian influenza virus infection is suspected in a patient with severe respiratory disease, both upper and lower respiratory tract specimens should be collected for influenza testing by RT-PCR at a public health laboratory (6).

Early negative influenza results delayed initiation of isolation precautions, reporting to public health authorities, and symptom monitoring. Although isolation precautions were not established consistently until the ninth day of inpatient care, no cases among HCP were identified. Likewise, no cases were detected among family members, despite lengthy exposure to both the symptomatic patient and the property. One household member reported direct contact with the ill and dead ducks but remained asymptomatic. Establishing a tiered risk assessment for HCP exposures based on setting and PPE use allowed staff members to continue working while having their symptoms monitored and limited new HCP exposures. The investigation was complicated by its occurrence during viral respiratory season and symptom development among several persons whose symptoms were being monitored. Human-to-human transmission of avian influenza A viruses has only rarely been reported globally and has not been reported in the United States (3,7).

Timely HPAI risk evaluation is important for persons with influenza symptoms requiring hospitalization to support infection prevention and control, early notification of public health authorities, and robust epidemiologic investigation, including genomic sequencing to identify possible transmission pathways. Ill or dead animals should be reported to animal health authorities for surveillance and potential testing and to reduce human exposure. Public health guidance for evaluating suspected cases of avian influenza should include immediate isolation precautions, prompt initiation of antiviral treatment, repeated influenza testing, and specimen collection from multiple sites (2,6,8). Considering the successive influenza A–negative laboratory results in the Washington patient, sampling from both upper and lower respiratory tracts in hospitalized patients should be considered to increase the likelihood of laboratory detection.

        (Continue . . . )


While the exact exposure to H5N5 remains uncertain, it is likely that unprotected (mask, eye protection, gloves) with wild backyard birds was the source of this fatal infection.  

This is remarkably similar to the fatal case reported earlier in 2025 in Louisiana. 

The CDC has released guidelines (see Backyard Flock Owners: Protect Yourself from Bird Flu) - but it is unknown how many backyard poultry owners have actually read it - or would bother to follow their - at times - stringent recommendations.

There are reportedly more than 11 million backyard poultry flocks in the United States, and tens of millions more in Europe and Asia. That's a lot of opportunities for spillovers. 

Last October, in UF/IFAS Extension: What Backyard Flock Owners Need to Know about Bird Flu (Influenza H5N1), we looked at two H5N1 related publications; one for backyard poultry owners, and another for consumers of poultry products and milk.

While there's plenty of useful guidance out there, getting people to follow it is another thing entirely. 

UKHSA Hantavirus Update: Cryptic Mention of a Suspected Case on Tristan Da Cunha

 

#19,149

The online signal-to-noise ratio regarding the hantavirus outbreak continues to degrade with several `false alarms' making the rounds yesterday, and the arrival of the highly vocal `plandemic' contingent, who view this outbreak as some sort of vast global conspiracy. 

Finding credible, informative, reports is made even more difficult by the tendency of governments to downplay risks to reassure a nervous public. 

While I still believe this outbreak should be containable - there are a lot of moving parts - many of which we probably aren't aware of.  It now appears at least 30 people left the ship before the outbreak was recognized, and some may have had opportunities to spread the virus.  

All of which brings us to the UK's HSA announcement this morning that they are aware of a `suspected' case on Tristan Da Cunha (stop #5 on the map above); a small volcanic island in the south Atlantic that is home to > 200 Brits. 

Exactly why they suspect this person is infected - or their condition - is not stated. Nor do they say how the person was likely exposed (was this a passenger, a local resident, etc.?). 

If confirmed (and that's still an `if'), this would be a significant turn of events. 

First today's statement, after which I'll have a bit more. 

Statement from the UK Health Security Agency (UKHSA), Department for Health and Social Care (DHSC), and Foreign, Commonwealth and Development Office (FCDO)


The UK government continues to work with international authorities in preparing for the arrival of British nationals to the UK from the MV Hondius cruise ship where an outbreak of Hantavirus was confirmed by the World Health Organization.

Two British nationals have confirmed hantavirus, with an additional suspected case of a British national on Tristan da Cunha. None of the British citizens onboard are currently reporting symptoms but they are being closely monitored.

The ship is expected to dock in Tenerife on Sunday, according to the latest updates from the Spanish Health Ministry. UK government staff will be on the ground ready to support the British nationals disembarking. British Passengers and ship crew not displaying any symptoms of hantavirus will be escorted by UK Government staff to an airport and given free passage back to the UK.

FCDO and UKHSA teams will be on the ground to support these arrangements, bolstered by a Rapid Deployment Team sent from the UK. The FCDO is chartering a dedicated repatriation flight for British ship passengers and crew only. This flight will be free of charge.

UKHSA is working with partners to ensure the flight operates under strict infection control measures. Public health and infectious disease specialists from UKHSA and the NHS will be on board to monitor British Nationals whilst on the flight, to ensure that preventative measures are in place and to provide any care in the unlikely event that any passengers become unwell on the flight.

All British passengers and crew on board the MV Hondius will be asked to isolate for 45 days upon returning to the UK and UKHSA will closely monitor these individuals, with testing as required.

Follow up is already underway for individuals who may have been in contact with cases and have since returned to the UK or are in in UK Overseas Territories. The UK government will ensure those self-isolating are given appropriate support.

UKHSA is aware of seven British Nationals who disembarked the ship at St Helena on 24 April.

Two of those people have returned to the UK independently and are isolating at home in the UK. Neither of these individuals is currently reporting symptoms. They are receiving advice and support from UKHSA and have been advised to self-isolate.

Four of these individuals remain in St Helena. A seventh individual has been traced outside of the UK.

The FCDO is in direct contact with the ship and British nationals on board and has stood up consular teams across multiple countries to support British nationals. UK government is working very closely with international partners in response to this incident, including the cruise ship operator and the governments of UK overseas territories which were visited by the ship. UK government teams are working at pace to get medical support to all affected Overseas Territories. The Ministry of Defence has worked with UKHSA to provide vital diagnostic supplies, including PCR tests, which were delivered to Ascension Island via a military plane on 7 May.

The risk to the general public remains very low.

Professor Robin May, Chief Scientific Officer at UKHSA, said:

This is an evolving situation, and we are working closely with partners to support British Nationals on board the MV Hondius.

The risk to the general population remains very low and the public can be reassured that established infection control measures will be put in place at every step of the journey to ensure the safe repatriation of British passengers on board.

Further information on the repatriation of British nationals will be provided in due course.

While it may turn out due more to an unfortunate series of events than anything else, the world's health agencies became aware of this outbreak weeks after it started, and are playing catch up.

Dozens of potentially infected individuals have scattered around the world, and they have had contact with hundreds - perhaps thousands - of people.

Luckily, most won't have been infected, and based on past Andes virus outbreaks, transmission should be limited. At least that's the assumption right now. 

The UK has signaled its intention to have people to `isolate' (technically quarantine) for 45 days after returning to the UK. Note: I suspect someone, somewhere, has determined that `quarantine' is a scarier word than `isolate', but I digress. 

Right now, it isn't clear whether all other countries will follow suit, or whether they'll get the cooperation they'll need from those exposed.  While unpopular, quarantine has proven its worth many times, including in halting the 2018-2019 outbreak of the Andes Virus in Argentina. 

Hopefully we'll take those lessons seriously. 

MMWR: Serologic Evidence of HPAI A(H5N1) Virus Infection in a Veterinary Professional Exposed to an Infected Domestic Cat

 

USDA list of Domestic Cats with H5N1

#19,148

We've a new report in today's MMWR that details the probable feline-to-human transmission of the H5N1 virus - during an outbreak of the virus in domestic cats in California in late 2024 - which was linked to contaminated cat food and/or raw milk. 

While long considered possible, actual evidence of  cat-to-human transmission of avian flu viruses has been pretty sparse.  Two notable exceptions being:

• In 2005, the CDC's EID Journal  Probable Tiger-to-Tiger Transmission of Avian Influenza H5N1, reported that while no humans tested positive by RT-PCR, 2 close human contacts (3.5%) tested positive via serology (microneutralization) for the virus 4 weeks later.

• In 2018 we saw evidence suggesting several employers had been infected by LPAI H7N2 during an outbreak in 2016 (see J Infect Dis: Serological Evidence Of H7N2 Infection Among Animal Shelter Workers, NYC 2016).

Admittedly, testing has been fragmented over the years, with many exposed individuals either tested too late, or not at all. And that is a challenge we find once again with today's report.

In today's example, of the 139 people exposed to infected cats, 30 reported flu-like symptoms, but of those only 18 (60%) submitted to PCR testing with 12 declined or were unavailable.  Another 15 asymptomatic cases were tested (combined n=33), meaning that  < 24% of total number of exposed individuals were tested. 

And the timing of sample collection was not ideal, as the `window' for PCR detection can be short, particularly in mild or asymptomatic infections. The authors wrote:

The median interval between the most recent exposure date and specimen collection was 8 days (range = 1–13 days). Specimens from 19 (58%) persons who received testing were collected >7 days after the last exposure.

As the above chart indicates, all PCR tests were negative for H5N1.  

Serological testing for post-infection antibodies faced similar challenges, with only 25 of the 139 exposed individuals (18%) submitting to serological tests. It isn't clear from my reading how many of those may have come from the symptomatic cohort. 

Again timing may be an issue, since they authors report `. . .  the average interval between exposure and serum collection was 104 days (range = 35–137 days)', which could have been long enough for some antibody titers to wane. 

In this case, one of the 25 samples tested positive for H5N1, that belonging to an asymptomatic veterinarian who had unprotected (no respiratory/eye protection) contact with infected cats during the outbreak. 

I've only reproduced the summary and abstract below. Follow the link to read the report in its entirety.  I'll have a brief postscript when you return. 

Serologic Evidence of Highly Pathogenic Avian Influenza A(H5N1) Virus Infection in a Veterinary Professional Exposed to an Infected Domestic Cat — Los Angeles County, California, December 2024–January 2025

Weekly / May 7, 2026 / 75(17);215–220
 
Aisling Vaughan1; Allison Joyce2; Elizabeth Traub2; Mellissa Jae3; Emily Beeler3; Erick Paiva2; Kristopher Ananian2; Crystal Holiday4; Stacie Jefferson4; Jessica Richardson2; Cortney Munna2; Cynthia Chan3; Tamerin Scott3; Noah Kojima2; Tanya Seneviratne2; Alexandra Mellis4; Sonja J. Olsen4; Nicole Green5; Matt Feaster6; Dawn Terashita2; Sharon Balter2; Min Z. Levine4; Jamie Middleton3,*; Annabelle de St. Maurice2,*  

Summary

What is already known about this topic?

Transmission of influenza A(H5N1) viruses from domestic cats to humans has not been documented.

What is added by this report?

During November 2024–January 2025, a total of 139 persons exposed to 19 A(H5N1)-infected domestic cats that consumed raw animal products were identified in Los Angeles County, California. Among 25 exposed persons who received serologic testing, one asymptomatic veterinary professional had serologic evidence of A(H5N1) infection after occupational exposure to an A(H5N1)-infected cat.

What are the implications for public health practice?

These findings provide evidence of zoonotic transmission of influenza A(H5N1) virus from domestic cats to humans. Pet owners are advised not to feed raw animal products to cats. Veterinary professionals should be aware of infection risks, use appropriate personal protective equipment, and adhere to recommended infection control practices to reduce the risk for zoonotic transmission of influenza A(H5N1).

 
Article PDF
Full Issue PDF

Abstract

Since 2021, avian influenza A(H5N1) clade 2.3.4.4b viruses have spread widely among wild birds and domesticated poultry in the United States, with sporadic spillover into mammals. During November 2024–January 2025, 19 domestic cats in Los Angeles County, California, became ill after consumption of commercially purchased raw milk, raw meat, or raw pet food; nine cats tested positive for influenza A(H5N1) virus (clade 2.3.4.4b genotype B3.13).

Overall, 139 persons were exposed to the 19 infected cats, and all were monitored for symptoms. Although 30 persons reported influenza-like illness symptoms, none received a positive influenza A(H5) reverse transcription–polymerase chain reaction (RT-PCR) test result. In April 2025, the Los Angeles County Department of Public Health and CDC invited all exposed persons to participate in an influenza A(H5N1) serosurvey to determine whether transmission of influenza A(H5N1) virus occurred, including in those without symptoms.

Sera from 25 (18%) of the 139 exposed persons were tested. Among these, antibodies specific to A(H5N1) clade 2.3.4.4.b (antigenically similar to the clade 2.3.4.4.b influenza A[H5N1] virus isolated from the infected cats) were detected in serum from one veterinary professional, who was asymptomatic. This person did not use respiratory or eye protection during the exposure, did not report influenza-like illness after the exposure, and reported no other known risk factors for A(H5N1) infection.

These findings represent serologic evidence of possible transmission of influenza A(H5N1) clade 2.3.4.4.b virus from a domestic cat to a human, highlighting concerns about potential cat-to-human transmission of influenza A(H5N1) virus and the importance of infection control practices in veterinary settings.

        (SNIP)

Limitations

The findings in this report are subject to at least two limitations.

• First, RT-PCR testing and serologic testing were not performed for all persons; therefore, some infections might have been missed.
• Second, serologic testing was performed 4–5 months after exposure, at which time antibody responses might have waned.

Collection of acute and convalescent serology specimens was not feasible in this investigation; however, this step should be considered during future influenza A(H5N1) virus outbreaks in animals.

        (Continue . . . )


We've seen previous examples where the public, or agricultural workers, have been reluctant to cooperate with public health officials or researchers (see EID Journal: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA).

Around the time this feline outbreak was occuring, we were seeing the first evidence of asymptomatic infections among dairy workers (see MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers), once again uncovered by retrospective serological testing.

Although better cooperation and participation by the public might have turned up more instances of feline-to-human H5N1 transmission - thanks to these researchers - we now have pretty good evidence that feline-to-human transmission of clade 2.3.4.4b. can occur. 

While this should be enough of a signal to convince people to take the threat of HPAI to their pets more seriously - given the ambivalence and hostility towards science I see online - I'm not particularly hopeful.  

PLos Bio: Surveillance on California dairy farms reveals multiple possible sources of H5N1 influenza virus transmission

 

#19,147

Last August, in Preprint: Surveillance on California Dairy Farms Reveals Multiple Sources of H5N1 Transmission, we looked at the testing of air, wastewater, and milk samples from 5 California farms/milking parlors, which found evidence of extensive environmental (air, water & milking equipment) contamination with H5N1.

Key findings included:

• detection of infectious H5N1 virus in milking parlor air and farm wastewater evidence of airborne transmission from exhaled cow breath

• H5N1 infection without mastitis symptoms, suggesting subclinical milk producing cows may be going undetected

• the detection of an N189D HA mutation (which may affect the receptor binding domain) in at least one air sample 

• and heterogeneous patterns of viral infection across individual udder quarters that call into question the assumed `primary role'  played by milking equipment

This week PLoS Biology has published a revised and extended (now 14 farm) study by these same authors which (again) reports multiple plausible H5N1 transmission routes on dairy farms; including milk contact, aerosols in parlors, and contaminated wastewater.

The published version provides additional evidence for infectious H5N1 detection in both air and wastewater (not just viral RNA detection), along with a much larger sampling size. 

Due to its length, I've only posted the link, abstract, and some excerpts from the conclusion.  Follow the link to read it in its entirety.  I'll have a brief postscript after you return.

RESEARCH ARTICLE
Surveillance on California dairy farms reveals multiple possible sources of H5N1 influenza virus transmission
A. J. Campbell1 , Meredith Shephard1☯, Abigail P. Paulos2☯, Matthew D. Pauly1 , Michelle N. Vu1 , Chloe Stenkamp-Strahm3 , Kaitlyn Bushfield1 , Betsy Hunter-Binns4 , Orlando Sablon2 , Emily E. Bendall5 , William J. Fitzimmons5 , Kayla Brizuela1 , Grace E. Quirk1 , Nirmal Kumar1 , Brian McCluskey3 , Nishit Shetty6 , Linsey C. Marr7 , Jenna J. Guthmiller8 , Jefferson J. S. Santos9 , Scott E. Hensley9 , Edith S. Marshall10, Kevin Abernathy4 , Adam S. Lauring5 , Blaine T. Melody11, Marlene K. Wolfe2 , Jason Lombard3 *, Seema S. Lakdawala

        Abstract

Transmission routes of highly pathogenic H5N1 between cows or to humans remain unclear due to limited data from affected dairy farms. We performed air, farm wastewater, and milk sampling on 14 H5N1-positive dairy farms across two different California regions.

Infectious virus was detected in the air in milking parlors and in wastewater streams, while viral RNA was found in exhaled breath of cows. Sequence analysis of infectious H5N1 virus from air and wastewater samples on one farm revealed viral variants relevant for potential human susceptibility. Longitudinal analysis of milk from the individual quarters of cows revealed a high prevalence of subclinical H5N1-positive cows.

Additionally, a heterogeneous distribution of infected quarters that maintained a consistent pattern over time was observed, inconsistent with shared milking equipment serving as the sole transmission mode. The presence of subclinically infected cows was further supported by detection of antibodies in the milk of animals that exhibited no clinical signs during the H5N1 outbreak on one farm. Our data highlight additional sources and potential modes of H5N1 transmission on dairy farms.

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Discussion

Elucidating the routes of transmission of H5N1 between cows is critical to defining successful mitigation strategies. In this study, we successfully detected H5N1 in the air and in reclaimed farm wastewater on separate dairy farms on multiple days. This included infectious air samples from three different milking parlors and viral RNA from the exhaled breath of rows of cows on two distinct farms. Additionally, we detected viral RNA in farm wastewater at multiple sites on various farms and infectious virus at two different sites on the same farm.

Together, these results highlight the extensive environmental contamination of H5N1 on affected dairy farms and identify additional sources of viral exposure for cows, peridomestic wildlife, and humans.

Dairy parlors, which are often enclosed spaces and where aerosolization of milk occurs, pose the greatest threat from inhalation of the virus to dairy farm workers compared to the open-air housing pens.

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Taken together, our data confirm the presence of infectious H5N1 virus in the air and reclaimed farm wastewater sites. In addition, we observed high viral loads and H5 antibodies in the milk of cows, including those without clinical signs, and heterogenous patterns of H5N1 positivity by quarter, suggesting that multiple modes of H5N1 transmission likely exist on farms.

These transmission routes could include contaminated milking equipment from an infected cow, aerosols generated within the milking parlor, and/or contact of teats with contaminated water used to clean housing pens.

Multiple mitigation strategies should therefore be implemented to reduce the risk of H5N1 spread within a herd and to humans. Respiratory and ocular personal protective equipment (PPE) for farm workers to prevent deposition of virus-laden aerosols on these sites, especially in the milking parlor. Disinfection of milking equipment between milking of each cow, such as with consistent use of backflush system, could also reduce spread of H5N1 between cows. Treatment of milk from sick cows to inactivate H5N1 prior to disposal as well as treatment of waste streams prior to their use in fields or on farms should also be considered. Finally, identification of infected cows, regardless of clinical signs, for isolation will help reduce the transmission of H5N1 on farms.

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This paper calls for a number of actions to reduce the spread of H5N1 on dairy farms, including:

• Respiratory & ocular personal protective equipment (PPE) for farm workers
• Treating waste milk before disposal
• Treating manure/wastewater/waste streams
• Isolating infected cows, including subclinical cases

While the USDA offers many similar biosecurity recommendations for dairy farms  (see APHIS Biosecurity Document) - most are not mandatory - and are left up to the discretion of the owner/operator.  

Although the number of positive herds being reported has declined, it is unclear how sensitive current testing and surveillance practices truly are. 

UKHSA Update On Hantavirus: 2 Nonsymptomatic Passengers Self-Isolating at Home

 

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The UK's Health Security Agency has published an update this afternoon on the current status of British nationals aboard the m/v Honidius, which also includes details on two passengers who apparently already left the ship, and - while showing no symptoms - are self-isolating at home. 

Given the large number of people who have been exposed aboard ship - and an unknown number of contacts offship - the use of home self-isolation (with daily phone check ins) would seem a reasonable precaution; at least until we get a better handle on the infectivity of this strain. 

Whether other jurisdictions will adopt similar measures remains to be seen. The 2-8 week incubation period of the Andes Virus - and its early flu-like presentation - is likely to present public health agencies around the globe with a formidable challenge over the weeks ahead.

UKHSA update on the hantavirus cruise ship outbreak

Latest information on cases including British nationals.
From:UK Health Security Agency Published 6 May 2026




The UK Health Security Agency (UKHSA) continues to work with the World Health Organization (WHO), Foreign, Commonwealth and Development Office (FCDO), the Department of Health and Social Care (DHSC), and other international partners to prepare for the arrival of British nationals to the UK from the MV Hondius cruise ship where an outbreak of Hantavirus was confirmed.

Three people, including one British national, with suspected hantavirus have been evacuated from the Hondius in order to receive medical care in the Netherlands in co-ordination between the Cape Verde, UK and Dutch governments. UKHSA are in close contact with medical teams providing their care.

The remaining British nationals can now be repatriated once the ship docks at its next destination if they do not develop symptoms. None of the British citizens onboard are currently reporting symptoms but they are being closely monitored. The FCDO is making arrangements for these individuals to return to the UK, where UKHSA is working with government to support them to isolate with regular testing and contact with healthcare professionals.

UKHSA is aware of two people who have returned to the UK independently having been on board the MV Hondius. Neither of these individuals is currently reporting symptoms. They are receiving advice and support from UKHSA and have been advised to self-isolate. UKHSA are supporting a small number of individuals identified as close contacts of those on the boat. They are being offered support and are also self-isolating. None are reporting any symptoms. The risk to the general public remains very low.

Dr Meera Chand, Deputy Director for Epidemic and Emerging Infections at UKHSA said:

• Our thoughts are with all those affected by the hantavirus outbreak onboard the MV Hondius.

• It’s important to reassure people that the risk to the general public remains very low. We are standing up arrangements to support, isolate and monitor British nationals from the ship on their return to the UK and we are contact tracing anyone who may have been in contact with the ship or the hantavirus cases to limit the risk of onward transmission.

• UKHSA will continue to work closely with government partners to offer all necessary support.

UKHSA is working closely with the FCDO, the Home Office, and Border Force to trace further individuals who may have been on the same flight as a confirmed case, in order to carry out public health risk assessments and ensure appropriate precautionary measures are in place.

Hantavirus is the name given to a group of viruses carried by rodents and transmitted by their droppings and urine. They can cause a range of diseases from mild, flu-like illness to severe respiratory illness. Infections in humans are rare and tend to occur in places where people and rodents coexist - most commonly in rural, agricultural settings, though the viruses can also sometimes be found in cleaning sheds, barns and holiday homes where rodents might have nested.

Most hantaviruses do not spread easily between humans, although person-to-person transmission has been observed in some cases involving particular strains. The World Health Organization is leading the international response to this incident and overseeing direction of the ship, including advising on how to minimise the risk of the disease spreading.

The FCDO is in direct contact with the ship and has stood up consular teams across multiple countries to support British nationals. UK government is working extremely closely with international partners in response to this incident, including the cruise ship operator and the governments of overseas territories which were visited by the ship.

Further information on Hantavirus can be found in the recent blog from UKHSA.