Submitted to: Isolation and Identification of Avian Pathogens
Publication Type: Book / chapter
Publication Acceptance Date: 8/1/2006
Publication Date: 6/1/2008
Citation: Swayne, D.E., Senne, D.A., Suarez, D.L. 2008. Avian influenza. In: Dufour-Zavala, L., editor. Isolation, Identification, and Characterization of Avian Pathogens. Jacksonville, FL: American Association of Avian Pathologists. p. 128-134. Interpretive Summary: Not required.
Technical Abstract: Avian influenza (AI) is caused by type A influenza virus, in the family Orthomyxoviridae. Type A influenza viruses are serologically categorized into 16 hemagglutinin (H1–H16) and 9 neuraminidase (N1–N9) subtypes. All subtypes have been identified in birds. Infections by influenza virus have been reported in a variety of domestic and wild birds, but most infections are subclinical. In poultry, influenza virus infections can be clinical, but the features are variable depending on virus strain, host species and breed, host age, concurrent infections, physiologic stresses, and environmental factors. Some H5 and H7 strains are highly pathogenic for poultry, producing a systemic disease with high mortality, and all H5 and H7 strains should be considered to have the potential to mutate from low (LP) to high pathogenicity (HP). Both LP and HP H5 and H7 strains isolated from poultry are notifiable to the World Organization for Animal Health (OIE). Agent Detection or Identification. Although several diagnostic tests can be used for detection of avian influenza, the initial diagnosis is preferably made by virus isolation via inoculation of embryonating chicken eggs, demonstration of hemagglutinating activity, and verification as avian influenza virus by agar-gel immunodiffusion (AGID) or other antigen-detection tests. Once a virus has been isolated, additional biologic and molecular studies can be performed to characterize the virus that isn’t possible by other means. This includes virus subtyping, testing of pathogenicity in chickens or other species, and for H5 and H7 subtypes the determination of amino acid sequence at the hemagglutinin proteolytic cleavage site to develop the appropriate control strategies. We do have an increased ability to rapidly diagnose and characterize AI by detection of nucleic acids specific for AI viruses and potentially by rapid and sensitive immunoassays, but these tests will not replace virus isolation in our ability to characterize AI viruses. Serologic Detection in the Host. Serologic detection of infection in the host is important for moderate-to-large scale surveillance by industry or government and for early diagnostic detection programs. Detection of type A group-specific antibodies via AGID or enzyme-linked immunosorbent assay is preferred as the initial step for detection of primary infection, but confirmation and subtype determination via hemagglutination-inhibition and neuraminidase-inhibition tests are crucial adjuncts.