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For further information, see CMDT Part 32-06: Common Viral Respiratory Infections

Key Features

Essentials of Diagnosis

  • Most human cases occur after exposure to infected poultry

  • Clinically indistinguishable from seasonal influenza

  • Epidemiologic factors assist in diagnosis

  • Rapid antigen assays confirm diagnosis but do not distinguish avian from seasonal influenza

General Considerations

  • Birds are the natural hosts

  • Avian influenza A

    • Outbreaks occur in poultry occasionally

    • Virus has become endemic in poultry in some countries, mostly in Southeast Asia and Egypt

  • Occasionally, avian influenza viruses may infect humans or other mammals, including domestic cats and dogs

  • Illness in humans ranges from mild disease to rapid progressive severe disease and death depending on the subtype

  • Risk factors for human infection

    • Direct or indirect exposure to infected live or dead poultry or contaminated environments, such as live bird markets

    • Slaughtering and handling carcasses of infected poultry

  • The emergence of H5, H7, and H9 avian influenza virus subtypes in humans raises concern that the virus may undergo genetic re-assortment or mutations in some of the genes and develop greater human-to-human transmissibility with the potential to produce a global pandemic


  • Human infections with H5N1 viruses have been reported to the World Health Organization from 16 countries; the first report in the Americas was in Canada in 2014, and approximately 60% of the cases died

  • Average case fatality rate of 40%

  • Infections with other H7 avian influenza viruses (H7N2, H7N3, and H7N7) have occurred sporadically around the world

  • Rare human cases of influenza H9N2 are also reported

Clinical Findings

Symptoms and Signs

  • Fever

  • Lower respiratory symptoms (cough and dyspnea); upper respiratory tract symptoms are less common

  • Gastrointestinal symptoms are reported more frequently in H5N1 infections

  • Conjunctivitis is reported in influenza H7 infections

  • Other systems can also be involved leading to neurologic manifestations (encephalopathy, seizure) and liver impairment

  • Prolonged febrile states and generalized malaise are common

  • Bacterial superinfection is reported

Differential Diagnosis

  • Typical influenza (clinically indistinguishable)

  • Common cold

  • Primary bacterial pneumonia

  • Infectious mononucleosis

  • Mycoplasma infection

  • Early Legionnaires disease

  • Chlamydophila pneumoniae infection (TWAR)

  • Acute HIV infection

  • Meningitis

  • In returning tropical traveler: malaria, dengue, typhoid


Laboratory Tests

  • Commercial rapid antigen tests are not optimally sensitive or specific for detection of H5N1 influenza and should not be the definitive test for influenza

  • Diagnostic yield can be improved by early collection of samples, preferably within 7 days of illness onset

  • More sensitive RT-PCR assays are available through many hospitals and state health departments

  • An initial negative result in the right clinical setting warrants retesting

  • Throat swabs or lower respiratory specimens (such as tracheal aspirate or bronchoalveolar lavage fluid) may provide higher yield of detection than ...

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