Influenza, Avian

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

Demographics

• 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

Diagnosis

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 ...