New research found H5N1 and H5N5 can infect sheep mammary tissue and spread through milk to lambs, raising biosecurity concerns for mixed-species farms. USDA scaled back dairy cattle testing in “unaffected” states, while Wisconsin moved to every-other-month surveillance. Poland recorded 26 May H5N1 poultry outbreaks and nearly 9.65 million birds affected in 2026, while Indiana remained the leading U.S. hotspot with 43 commercial flocks affected. CDC also documented backyard flock owner knowledge gaps, and Norway confirmed Europe’s first known H5N5 detection in a polar bear.

CDC Survey Finds Major Knowledge Gaps Among Backyard Flock Owners

A new CDC survey of 638 U.S. backyard flock owners found that while 94% had heard of avian influenza, roughly one third did not know the signs of infection in birds or people. The survey, conducted from July through December 2025 with state health and agricultural partners, found that 54% of respondents said wild birds could access their flock or its food or water, and 71% did not have a veterinarian they consulted about their birds. Although about 90% knew recommended precautions for handling sick or dead birds, knowledge gaps were tied to lower intent to use personal protective equipment (Rolfes et al., 2026). The findings matter because three U.S. human H5 cases since 2024 have involved backyard flock owners, and 58% of respondents reported at least one household member at higher risk for flu complications, underscoring the need for clearer outreach on early detection, PPE, and separating backyard birds from wild-bird exposure (Van Beusekom, 2026).

Study Finds H5N1 and H5N5 Can Infect Sheep Mammary Tissue and Spread Through Milk

A new Science Advances study published May 8 found that H5Nx clade 2.3.4.4b viruses can infect sheep through both mammary and respiratory routes, expanding concern beyond poultry and dairy cattle to small ruminants, such as sheep and goats. Researchers exposed lactating and nonlactating sheep to H5N1 genotype D1.1 and H5N5 genotype A6 viruses. In lactating sheep, mammary exposure caused clinical mastitis, high viral loads in milk, and transmission to suckling lambs, which appeared to help spread infection to initially uninfected mammary glands. In nonlactating sheep, aerosol exposure caused only mild, transient respiratory infection, with low-level viral replication and antibody development. The findings matter for producers because they suggest sheep and potentially other small ruminants could serve as overlooked hosts or transmission bridges on mixed-species farms, particularly where lactating animals and young stock are cohoused or exposed to wild-bird contamination (Coston, 2026). The study also raises biosecurity concerns around raw milk, suckling transmission, and shared farm environments as H5 viruses continue expanding across mammalian hosts (Alkie et al., 2026).

Indiana Remains U.S. HPAI Hotspot as Outbreak Activity Picks Back Up

After a relative lull in late April, U.S. avian influenza activity increased again during this reporting period, with USDA APHIS reporting eight new outbreaks affecting roughly 130,000 birds nationwide (United States Department of Agriculture, n.d.). Indiana remains the clearest hotspot: APHIS confirmed HPAI in 7,300 commercial meat ducks in Elkhart County on May 18 and 4,400 commercial breeding ducks in LaGrange County on May 19, bringing the state to 43 affected commercial poultry flocks so far in 2026. The new Indiana detections follow a quieter prior stretch, when APHIS reported only one new poultry detection nationally in late April, though the agency still tracked 15 commercial and eight backyard flock detections over the preceding 30 days (Soucheray, 2026). The latest data suggest that while outbreak intensity has eased from the February peak, HPAI continues to resurface in commercial poultry, particularly in Indiana’s duck-producing corridor (Graber, 2026).

USDA Scales Back Dairy Herd Bird Flu Testing as Surveillance Concerns Persist

USDA has eased avian flu testing requirements for dairy cattle in states considered “unaffected” under its National Milk Testing Strategy, meaning lactating cows moving between Wisconsin and 41 other unaffected states no longer need pre-movement testing. Wisconsin also rescinded its in-state testing requirement for lactating cows moving to fairs and exhibitions and will shift from monthly to every-other-month dairy farm testing, citing lower dairy-case activity since H5N1 was first confirmed in cattle in March 2024 and only one confirmed Wisconsin dairy infection to date. State officials said the change reduces logistical and financial burdens for USDA and producers, but Wisconsin Veterinary Diagnostic Laboratory director Keith Poulsen warned that reduced testing could weaken surveillance if federal staffing cuts or funding uncertainty limit the National Milk Testing Strategy. The update matters for producers because it reflects a shift from emergency-level monitoring toward lower-intensity surveillance, even as wild-bird exposure and persistent farm-level risk remain unresolved (Kirwan, 2026).

Poland Drives Europe’s Latest Poultry HPAI Activity as France Reports More Vaccinated Duck Infections

Poland remains the center of Europe’s current commercial poultry HPAI activity, with 26 H5N1 outbreaks confirmed in May as of May 19 and 140 outbreaks recorded so far in 2026, affecting nearly 9.65 million commercial birds. Ten new Polish farm outbreaks were reported since the prior weekly update, involving flocks of 4,300 to 65,000 birds across meat turkeys, general-purpose chickens, ducks, and geese. Across Europe, the European Commission recorded 303 commercial poultry outbreaks in 16 countries as of May 20, 2026, with Poland accounting for the largest share. France also reported another H5N1 outbreak in a vaccinated commercial duck flock of roughly 12,000 birds in Haute-Garonne, following two earlier spring infections in vaccinated ducks, while the Netherlands confirmed a new outbreak in about 51,600 laying hens after its last case in March (Linden, 2026).

First Known H5N5 Detection in European Polar Bear Confirmed in Arctic Svalbard

Norwegian veterinary officials confirmed highly pathogenic avian influenza in a dead polar bear from the Arctic Svalbard archipelago, marking the first known detection of bird flu in the species in Europe. A dead walrus from the same region also tested positive. The virus was identified as H5N5, a subtype previously detected in Svalbard birds, Arctic foxes, and a walrus. Officials said the findings reflect a broader pattern of HPAI detections in European mammals and show continued spread into Arctic ecosystems, where vulnerable wildlife populations may face added pressure. The Norwegian Veterinary Institute is investigating whether the virus found in the polar bear and walrus shows signs of mammalian adaptation (Ringstrom, 2026).

References

Alkie, T. N., Embury-Hyatt, C., Signore, A. V., Moffat, E., Ramos, D., Raj, S., Hisanaga, T., Ayilara, I., Sullivan, D. S., Lloyd, P., Pama, L., Erdelyan, C. N. G., Lang, A. S., Kobasa, D., Ojkic, D., Matheson, L., Lung, O., Poljak, Z., & Berhane, Y. (2026). Mammary and respiratory infection of sheep with H5Nx clade 2.3.4.4b viruses with milk-mediated transmission to lambs. Science Advances, 12(19). https://doi.org/10.1126/sciadv.aed1287

Coston, M. (2026, May 22). Sci Adv: Mammary and Respiratory Infection of Sheep with H5Nx clade 2.3.4.4b Viruses with Milk-mediated Transmission to Lambs. Blogspot.com. https://afludiary.blogspot.com/2026/05/sci-adv-mammary-and-respiratory.html

Graber, R. (2026, May 20). Avian influenza infections continue in Indiana, Alberta. WATTPoultry.com. https://www.wattagnet.com/poultry-meat/diseases-health/avian-influenza/news/15825657/avian-influenza-infections-continue-in-indiana-alberta

Kirwan, H. (2026, May 10). Why are USDA officials scaling back to bi‑monthly bird flu testing in dairy herds? Wisconsin State Farmer. https://www.wisfarmer.com/story/news/2026/05/10/as-testing-drops-questions-linger-over-dairy-bird-flu-monitoring/90000955007/

Linden, J. (2026, May 21). Poland records ongoing spike in poultry avian flu outbreaks. WATTPoultry.com. https://www.wattagnet.com/poultry-meat/diseases-health/avian-influenza/news/15825789/poland-records-ongoing-spike-in-poultry-avian-flu-outbreaks

Ringstrom, A. (2026, May 19). Bird flu detected in dead Arctic polar bear in European first. The Independent. https://www.independent.co.uk/news/world/europe/bird-flu-polar-bear-europe-b2979704.html

Rolfes, M. A., Bauck, L., Lipton, B. A., Margrey, S. F., Campagna, R. A., Harker, E., Basler, C. A., Dewart, C. M., Ellington, R., Holzbauer, S. M., Ireland, M. J., Kuo, J. W., Szablewski, C. M., Durand, L. O., & Reed, C. (2026). Knowledge, Attitudes, and Practices Regarding Avian Influenza Among Owners of Backyard Flocks — United States, July–December 2025. MMWR. Morbidity and Mortality Weekly Report, 75. https://doi.org/10.15585/

Soucheray, S. (2026, April 30). Center for Infectious Disease Research and Policy. Umn.edu. https://www.cidrap.umn.edu/avian-influenza-bird-flu/avian-flu-detections-drop-across-us

United States Department of Agriculture. (n.d.). HPAI Confirmations in Commercial and Backyard Flocks. Www.aphis.usda.gov. https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/commercial-backyard-flocks

Van Beusekom, M. (2026, May 15). Center for Infectious Disease Research and Policy. Umn.edu. https://www.cidrap.umn.edu/avian-influenza-bird-flu/survey-third-us-backyard-flock-owners-don-t-know-signs-symptoms-avian-flu