Chicken were infected intravenously (i.v.) with either (A) CK/DE/1940/19 or (B) DK/BE/1358/19 H3N1 isolate. The clinical status was monitored over a period of ten days post inoculation (dpi). Clinical status is recorded as healthy (0), birds showing slight (1) or severe disturbances of the general condition (2). In both groups, one animal showed neurological signs including torticollis and was euthanized, as indicated with asterisk. Subsequently they were recorded as dead (3). The IVPI is the mean score per bird per observation over the 10-day period (1 observation per bird per day). In addition, virus shedding was measured and presented here as infectious virus equivalents (VE) per ml, calculated on basis of cq values from combined oro-pharyngeal swabs and correlated to a standard curve of stock virus with known infectivity titer. VE below 1 are indicated with (+) and VE of 0 are indicated with (-). Finally, antibody HI-titers of sera were determined three weeks after infection. Viral matrix protein detection in the (C) brain stem and (D) cerebral cortex after i.v. infection with CK/BE/1940/19. (E) No antigen detection following i.v. TK/BE/1358/19 infection, despite significant inflammatory infiltration. Influenza virus matrix protein was detected by immunohistochemistry, using the ABC method, AEC as chromogen (red) and counterstained by haematoxylin (blue). Bar = 50 μm (C, D) and 100 μm (E).