Influenza testing is recommended for anyone hospitalized with symptoms resembling influenza during flu season or who is connected to an influenza case. For severe cases, earlier diagnosis improves patient outcome. Diagnostic methods that can identify influenza include viral cultures, antibody- and antigen-detecting tests, and nucleic acid-based tests.Viruses can be grown in a culture of mammalian cells or embryonated eggs for 3–10 days to monitor cytopathic effect. Final confirmation can then be done via antibody staining, hemadsorption using red blood cells, or immunofluorescence microscopy. Shell vial cultures, which can identify infection via immunostaining before a cytopathic effect appears, are more sensitive than traditional cultures with results in 1–3 days. Cultures can be used to characterize novel viruses, observe sensitivity to antiviral drugs, and monitor antigenic drift, but they are relatively slow and require specialized skills and equipment.Serological assays can be used to detect an antibody response to influenza after natural infection or vaccination. Common serological assays include hemagglutination inhibition assays that detect HA-specific antibodies, virus neutralization assays that check whether antibodies have neutralized the virus, and enzyme-linked immunoabsorbant assays. These methods tend to be relatively inexpensive and fast but are less reliable than nucleic-acid based tests.Direct fluorescent or immunofluorescent antibody (DFA/IFA) tests involve staining respiratory epithelial cells in samples with fluorescently-labeled influenza-specific antibodies, followed by examination under a fluorescent microscope. They can differentiate between IAV and IBV but cant subtype IAV.
The envelope is reinforced structurally by matrix proteins on the interior that enclose the RNPs, and the envelope contains HA and NA (or HEF) proteins extending outward from the exterior surface of the envelope. HA and HEF proteins have a distinct "head" and "stalk" structure. M2 proteins form proton ion channels through the viral envelope that are required for viral entry and exit. IBVs contain a surface protein named NB that is anchored in the envelope, but its function is unknown. Life cycle
The viral life cycle begins by binding to a target cell. Binding is mediated by the viral HA proteins on the surface of the evelope, which bind to cells that contain sialic acid receptors on the surface of the cell membrane. For N1 subtypes with the "G147R" mutation and N2 subtypes, the NA protein can initiate entry. Prior to binding, NA proteins promote access to target cells by degrading mucous, which helps to remove extracellular decoy receptors that would impede access to target cells. After binding, the virus is internalized into the cell by an endosome that contains the virion inside it. The endosome is acidified by cellular vATPase to have lower pH, which triggers a conformational change in HA that allows fusion of the viral envelope with the endosomal membrane. At the same time, hydrogen ions diffuse into the virion through M2 ion channels, disrupting internal protein-protein interactions to release RNPs into the host cells cytosol.