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Author affiliation: National Research Centre, Giza, Egypt (M. Gomaa, R. El-Shesheny, A. El Taweel, O. Kutkat, H. Emam, B. Helal, A. Swelam, M. GabAllah, A. Kandeil, M.A. Ali); St. Jude Children’s Research Hospital, Memphis, Tennessee, USA (A. Kandeil, P.P. McKenzie, R.J. Webby); Human Link, Baabda, Lebanon (G. Kayali)
The emergence of highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus in dairy cattle in the United States represented a major shift in influenza virus ecology, marking the first sustained transmission of H5N1 virus in a mammalian livestock species (1). Genotype B3.13 was reported in dairy cows in early 2024 in several US states; data indicated a spillover event from wild birds to cattle and then interstate transmission through cow movement (1). A defining feature of H5N1 virus infection in dairy cattle is its marked tropism for the mammary gland, resulting in extremely high viral titers in milk and relatively lower viral loads in respiratory samples (2). Mechanical transmission via the milking process is believed to play a major role in cow-to-cow transmission, particularly in large dairy operations. Cow infection was not limited to genotype B3.13; European genotype euDG was shown to be able to cause infection in an experimental setting (3), and North American genotype D1.1 was detected in cattle in the United States (3). Infection of other livestock species, including goats, sheep, swine, and alpacas, was reported in mixed-species backyards and farms in the United States and United Kingdom (4,5). In 2026, a farm in the Netherlands had a cat infected with H5N1 virus; a cow at that farm had antibodies against the virus (6).
Clade 2.3.4.4b H5N1 virus and low-pathogenicity avian influenza A(H9N2) virus are widely and endemically circulating in poultry in Egypt on farms and backyards and at live bird markets (7). Subtype H5N1 was detected in feral rats in poultry production areas (8). In addition, reassortment events between the H5N1 and H9N2 subtypes led to the emergence of novel clade 2.3.4.4b influenza A(H5N2) viruses (9). To determine whether those viruses are spilling over to livestock in Egypt, particularly in mixed-species settings, we conducted a serosurveillance study in northern Egypt.
During October 2024–January 2026, we collected 444 serum samples from 33 conveniently selected small farms (≈10 cattle, no mixed species) and backyards housing cattle (1–3 cattle with mixed species) in 3 governorates (n = 155, n = 59, and n = 230) in northern Egypt, representing common practices in Egypt. Sheep and goats were also sampled if housed in the same backyards as cattle. All sampled animals were apparently healthy adults. We tested serum samples for antibodies against a 2025 Egyptian clade 2.3.4.4b H5N1 (A/duck/Egypt/BA20361C/2022) virus using hemagglutination inhibition (HI) and microneutralization (MN) assays and against a G1-like Egypt influenza A(H9N2) (A/chicken/Egypt/RA20191OP/2022) virus by HI in accordance with World Health Organization protocols (10). We considered an antibody titer >1:40 as positive for both assays. Data on the number of cows, presence of other mammals (e.g., donkeys and horses), and presence of avian species (poultry and/or wild birds) were available for 271 samples. We performed statistical comparisons with χ2 or Fisher exact tests; we considered p
Overall, 11 (2.5%) serum specimens tested positive for influenza A(H9N2) antibodies, 20 (4.5%) were positive for influenza A(H5N1) antibodies by HI, and 13 (2.9%) were positive for influenza A(H5N1) antibodies by MN (Figure, panel A). Positive titers were 40–160 (Figure, panel B). None of the samples tested positive for antibodies against both viruses. Of the 20 H5N1 virus specimens positive by HI, 12 tested positive by MN; 1 specimen was negative by HI and positive by MN. All influenza H9N2-positive specimens were from 3 locations in the same governorate (p = 0.005). Influenza A(H5N1) antibodies were detected in 2 governorates; the differences in detection rates by governorate were statistically significant (0%, 1.3%, and 4.8%; p = 0.049). All H5N1 virus–seropositive specimens were from cattle (Table). We detected H9N2 antibodies in 6/374 (1.6%) of cattle specimens, 1/31 (3.2%) of goat specimens, and 4/39 (10.3%) of sheep specimens (p = 0.004). Presence of other animals did not predict seropositivity. Small farms had a significantly higher proportion of H5N1 virus–positive animals tested by MN (p = 0.039).
Our findings indicate that spillover of avian influenza viruses to cattle and other livestock is not geographically restricted or caused by certain genotypes of H5N1 virus. Clade 2.3.4.4b H5N1 virus of the EA-2021-AB genotype and G1-like H9N2 virus appear to be spilling over into livestock in Egypt; however, because our evidence is serologic, we could not ascertain whether true infection is occurring and whether the same signs seen in the United States are common in cattle in Egypt. Even so, the evidence is a strong indicator that spillover is occurring, similar to that reported in Europe (4,6). Furthermore, spillover is not restricted to cattle; some sheep and goats tested in our study were seropositive for influenza A(H9N2) virus. Surveillance for avian influenza viruses in Egypt and other countries, especially those in which avian influenza is endemic, will verify infection and provide data on frequency, routes of transmission, and factors affecting infection of and transmission to livestock.
Dr. Gomaa is a virologist at the National Research Centre, Giza, and a member of the Center of Scientific Excellence for virus research. His work focuses on emerging and reemerging viruses, avian influenza, coronaviruses, vaccine development, viral immunology, and pandemic preparedness.
Animal sampling was approved by the St. Jude Children’s Research Hospital Institutional Animal Care and Use Committee, Memphis, Tennessee, USA (approval no. 3130) and by the Research Ethics Committee of the National Research Center, Egypt (approval no. 06210123).
Sample collection was funded in part by the National Institute of Allergy and Infectious Diseases, National Institutes of Health, US Department of Health and Human Services (contract no. 75N93021C00016).
Author contributions: conceptualization: R.E.S., A.K., M.A.A., G.K.; data curation: M.G., A.E.T., O.K., H.E., B.H., A.S., M.G.; formal analysis: R.E.S., G.K.; funding acquisition: R.J.W., G.K.; investigation: M.G., A.E.T., O.K., H.E., B.H., A.S., M.G.; methodology: R.E.S., A.K., G.K.; project administration: R.E.S., P.P.M., M.A.A., G.K.; supervision: R.E.S., G.K., M.A.A.; writing—original draft: G.K.; writing—review and editing: R.E.S., A.K., M.A.A., G.K. All authors reviewed and approved the final version of the manuscript.