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Volume 32, Number 10—October 2026
Research
Experimental Highly Pathogenic Avian Influenza A(H5N1) Clade 2.3.4.4b Virus Infection in Alpacas, 2026
, Angele Breithaupt, Nico Joel Halwe, Maxi Hertel, Ann Kathrin Ahrens, Andrea Aebischer, Donata Hoffmann, and Martin Beer
Figure 2
![Histopathologic detection in nasopharyngeal tonsil and nasal conchae from experimental highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus infection in alpacas, 2026. In situ hybridization (ISH) performed by using Fast Red chromogen; immunohistochemistry (IHC) performed by using 3-amino-9-ethylcarbazole substrate for red-brown chromogen; Mayer’s hematoxylin counterstain used for both ISH and IHC. A, B) ISH showing strong cytoplasmic RNA labeling (arrow in A [scale bar indicates 250 µm], detail in B [scale bar indicates 100 µm]) from nasopharyngeal tonsil of alpaca 3. C) IHC of nasopharyngeal tonsil from alpaca 3 showing weak antigen signals within follicular macrophages and dendritic cells; scale bar indicates 100 µm. D–F) Tonsillar epithelium from alpaca 2. D, E) ISH showing viral RNA (arrow in D [scale bar indicates 250 µm], detail in E [scale bar indicates 100 µm]); F) IHC showing antigen (arrows) in the nucleus and cytoplasm. Scale bar indicates 100 µm. G–I) ISH showing abundant detection of viral RNA within cellular debris on left cranial nasal cavity tissue from alpaca 1 (G; scale bar indicates 250 µm), nasal epithelium of alpaca 1 at 4 days postinfection (H), and alpaca 4 at 20 days postinfection (I) (arrows). Scale bars indicate 50 µm.](/eid/images/26-0491-F2.jpg)
Figure 2. Histopathologic detection in nasopharyngeal tonsil and nasal conchae from experimental highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus infection in alpacas, 2026. In situ hybridization (ISH) performed by using Fast Red chromogen; immunohistochemistry (IHC) performed by using 3-amino-9-ethylcarbazole substrate for red-brown chromogen; Mayer’s hematoxylin counterstain used for both ISH and IHC. A, B) ISH showing strong cytoplasmic RNA labeling (arrow in A [scale bar indicates 250 µm], detail in B [scale bar indicates 100 µm]) from nasopharyngeal tonsil of alpaca 3. C) IHC of nasopharyngeal tonsil from alpaca 3 showing weak antigen signals within follicular macrophages and dendritic cells; scale bar indicates 100 µm. D–F) Tonsillar epithelium from alpaca 2. D, E) ISH showing viral RNA (arrow in D [scale bar indicates 250 µm], detail in E [scale bar indicates 100 µm]); F) IHC showing antigen (arrows) in the nucleus and cytoplasm. Scale bar indicates 100 µm. G–I) ISH showing abundant detection of viral RNA within cellular debris on left cranial nasal cavity tissue from alpaca 1 (G; scale bar indicates 250 µm), nasal epithelium of alpaca 1 at 4 days postinfection (H), and alpaca 4 at 20 days postinfection (I) (arrows). Scale bars indicate 50 µm.