New Study by CDC: Avian flu, H5N1, transits through air; it’s airborne (If B.C.’s Dr. Bonnie Henry tells you to be kind and wash your hands to stay safe from it, don’t believe her).

Posted on June 6, 2025 by Jessica Ernst

ISSN: 1080-6059

Avian Influenza A(H5N1) Isolated from Dairy Farm Worker, Michigan, USA Research Letter by Nicole Brock, Joanna A. Pulit-Penaloza, Jessica A. Belser, Claudia Pappas, Xiangjie Sun, Troy J. Kieran, Hui Zeng, Juan A. De La Cruz, Yasuko Hatta, Han Di, C. Todd Davis, Terrence M. Tumpey, and Taronna R. Maines, Volume 31, Number 6—June 2025

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Abstract

Influenza A(H5N1) viruses have been detected in US dairy cow herds since 2024. We assessed the pathogenesis, transmission, and airborne release of A/Michigan/90/2024, an H5N1 isolate from a dairy farm worker in Michigan, in the ferret model. Results show this virus caused airborne transmission with moderate pathogenicity, including limited extrapulmonary spread, without lethality.

Highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b viruses have displayed unprecedented global spread among wild birds leading to numerous spillover infections in mammalian species. Of note, outbreaks in dairy cattle and gallinaceous birds have resulted in human infections in the United States during 2024–2025 (1). Increased frequency of H5N1 viruses crossing species barriers has caused concern that the avian influenza viruses are adapting to mammals. A critical component of influenza pandemic preparedness is early identification of emerging novel influenza viruses that cause disease and transmit efficiently in humans. A clade 2.3.4.4b H5N1 virus, A/Michigan/90/2024 (MI90), genotype B3.13, was isolated from a conjunctival swab specimen collected from a human patient in Michigan with conjunctivitis after exposure to infected cattle (2,3). In this article, we report the pathogenesis, transmission, and airborne exhalation of MI90 virus in ferrets, the standard animal model for influenza virus risk assessments (4).

We inoculated 18 ferrets with MI90 virus as previously described (5,6). We euthanized 3 ferrets on 3 and 5 days postinoculation (dpi) to assess virus spread in tissues. We used 6 ferrets to assess transmission in a cohoused, direct contact setting as a direct contact transmission model and through the air in the absence of direct or indirect contact as a respiratory droplet transmission model. We paired each ferret with a naive contact, as previously described (4). We observed clinical manifestations daily and collected nasal wash (NW), conjunctival, and rectal swab samples every 2 days postinoculation or postcontact. We confirmed transmission by testing for seroconversion to homologous virus in the contact animals.

Although all MI90-infected ferrets survived the 21-day study, we noted moderate disease. In inoculated ferrets, the mean maximum weight loss was 9.8%, fever (1.8°C above baseline) and lethargy were transient, and nasal and ocular discharge and sneezing were evident at 4–9 dpi (Table). We detected virus 3 dpi primarily in respiratory tract tissues; titers were highest in ethmoid turbinate samples (7.4 log10 PFU/mL) and at low levels in brain and gastrointestinal tissues. We observed similar results in tissues collected 5 dpi.

Transmission and measurement of airborne avian influenza A(H5N1) virus isolated from dairy farm worker, Michigan. A, B) For DCT and RDT testing, ferrets (n = 12) were intranasally inoculated with 106 PFU A/Michigan/90/2024 virus, isolated from the dairy worker, in 1 mL phosphate-buffered saline and were cohoused with naive ferrets in a DCT model (A) or in adjacent cages with perforated sidewalls permitting airborne virus spread but restricting contact in an RDT model (B). Each bar represents a single animal. C, D) For aerosol transmission testing, ferrets (n = 3) were inoculated intranasally with 106 PFU of MI90 virus and tested daily (C). Orange dots represent viral titers from NW in log10 PFU/mL; limit of detection 10 PFU/mL. Gray bars show average viral M gene RNA load. Error bars indicate SD. Limit of detection was 2.9 log10 RNA copies/mL. D) Aerosol samples were collected daily for 5 dpi by using a BC251 cyclone-based sampler (kindly provided by Dr. William Lindsley, National Institute for Occupational Safety and Health) and the SPOT water condensation sampler (Aerosol Devices, https://aerosoldevices.com), as described previously (8). Orange dots represent log10 PFU/mL per hour. Gray bars show average viral M gene RNA. Error bars indicate SD. Limit of detection was 2.5 log10 RNA copies/h. Ferrets were used for tissue collection on day 5. DCT, direct contact transmission; dpi, days postinoculation; NW, nasal washes; RDT, respiratory droplet transmission.

Figure. Transmission and measurement of airborne avian influenza A(H5N1) virus isolated from dairy farm worker, Michigan. A, B) For DCT and RDT testing, ferrets (n = 12) were intranasally inoculated with…

During the direct contact transmission experiment, inoculated ferrets shed virus in NW that peaked at 4.7–5.4 log10 PFU/mL at 1–5 dpi (Figure, panel A). Four of 6 cohoused contact animals had virus in NW (peak 2.5–4.9 log10 PFU/mL) at 5–7 days postcontact, whereas all 6 contact animals had viral RNA detected (3.6–7.7 log10 copies/mL) in NW (7) and seroconverted to MI90 virus, indicating that transmission was 100% (6/6 animals). In the respiratory droplet transmission experiment, NW collected from inoculated animals peaked 2.6–4.8 log10 PFU/mL at 1–3 dpi, whereas 3/6 contact ferrets had detectable virus in NW by day 7 postcontact (peak 2.6–4.8 log10 PFU/mL; days 9–11 postcontact) (Figure, panel B) as well as viral RNA (6.7–8.2 log10 copies/mL), and seroconverted, confirming transmission through the air in 50% of ferrets (3/6). We also detected infectious virus in conjunctival and rectal samples from inoculated animals, but only from 2 contact animals (Table).

To further evaluate the level of virus exhaled by MI90-inoculated ferrets and the potential for airborne transmission, we collected aerosol samples 1 time each day at 1–5 dpi for 1 hour from the 3 ferrets that were euthanized at 5 dpi. Air samples were analyzed for infectious virus and viral RNA by using the BC251 cyclone-based sampler (kindly provided by Dr. William Lindsley, National Institute for Occupational Safety and Health) and the SPOT water condensation sampler (Aerosol Devices, https://aerosoldevices.comExternal Link), as described previously (8) (Figure, panel D). The highest mean titer of virus was detected at 2 dpi in NW collected from all 3 inoculated ferrets (6.5 log10 PFU/mL) (Figure, panel C). Airborne virus was highest at 3 dpi as measured in both samplers, up to 133 and 41 PFU/hour, supporting transmission observed in both contact models within 3–5 days after exposure.

Overall, MI90 virus displayed reduced virulence in ferrets compared with another H5N1 virus isolated from a dairy farm worker in Texas (8,9); the Texas virus possesses a genetic marker in the polymerase basic 2 protein (E627K), known for enhanced replication and pathogenesis in mammals. At this position, MI90 encodes 627E, like most other viruses isolated from cattle, and contains polymerase basic 2 M631L, which is associated with mammal adaptation (3,9). In addition, polymerase acidic 142N/E has been linked to increased virulence in mice (10); the Texas virus has an E and MI90 virus has a K at this position. Both viruses have identical hemagglutinin sequences associated with receptor binding and the multi-basic cleavage site. Despite differences in virulence, both viruses transmitted in the ferret model with similar proficiency and levels of airborne virus.

Because avian H5N1 viruses cross the species barrier and adapt to dairy cattle, each associated human infection presents further opportunity for mammal adaption. This potential poses an ongoing threat to public health and requires continual surveillance and risk assessment of emerging viruses to improve our ability to predict and prepare for the next influenza pandemic.

Dr. Brock is a microbiologist in the Influenza Division, National Center for Immunization and Respiratory Diseases, at the Centers for Disease Control and Prevention. Her research interests include the pathogenicity, transmissibility, and host response associated with emerging strains of influenza virus.

Acknowledgments

We thank the Centers for Disease Control and Prevention’s Comparative Medicine Branch (Division of Scientific Resources, National Center for Emerging and Zoonotic Infectious Diseases) for providing excellent animal care and the Mid-Michigan District Health Department and the Michigan Department of Health and Human Services for access to the human samples for virus isolation. We thank William Lindsley for his extensive contributions in developing and generously providing access to BC251 cyclone samplers, which have led to risk assessments of airborne pathogens worldwide.

All animal procedures were approved by the Institutional Animal Care and Use Committee of the Centers for Disease Control and Prevention and were conducted in an AAALAC-accredited facility.

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References

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Top Cite This Article

DOI: 10.3201/eid3106.250386

Original Publication Date: May 02, 2025

Table of Contents – Volume 31, Number 6—June 2025

Refer also to:

USA: Third human case of H5N1 with unknown source of infectionAirborne transmission?

H5N1 kills first human in USA, CDC of course minimizes the risk, just like BC health authorities. H5N1 has 52% mortality rate.

H5N1: BC teen still in hospital two months later, health authorities – as usual – down playing the risks

H5N1 case report of BC teen (recovering; still no source for the infection identified): Barely survivable respiratory failure, needed plasma exchange and ECMO – lung failure so severe, oxygenation done outside the body, multi-organ failure 3 days after mild conjunctivitis.

H5N1: Louisiana resident hospitalized in critical condition; BC’s Office of Social Murder, Dr. Bonnie Henry, will not provide update on teen in ICU. California declares state of emergency; 649 dairy herds tested positive, roughly 60 per cent of state total.

H5N1 infected BC teen in intensive care: Public Health adverse to protecting health, “addicted to droplet dogma” thanks to social mass murderer Dr. Bonnie Henry.

Fur farming and highly pathogenic avian influenza, H5N1, 2023: “Dead and culled animals, which might have already been infected, were taken from the farms to be processed as feed for other fur animals.” How hideous the human species is.

Dr. Bonnie “Hitler” Henry, BC’s Social Murderer of Health. Will she ever stop lying about airborne viruses? Do you value your health? Do you value others in your community and your loved ones? Wear an N95 mask indoors in public and in crowded places outside.

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