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

Key Features

Essentials of Diagnosis

  • Onset of fevers, chills, malaise, cough, arthralgias, and myalgias

  • Although sporadic cases do occur, most cases of influenza occur as part of epidemics or pandemics, usually in the fall or winter seasons

General Considerations

  • An orthomyxovirus transmitted by respiratory droplets

  • Three antigenic subtypes that infect humans

    • Types A and B produce identical clinical symptoms

    • Type C produces milder disease

  • Pandemics usually due to type A infections with major antigenic shift (large genetic reassortment of the virus)

  • Influenza is difficult to diagnose in the absence of the epidemic because it resembles other viral illnesses


  • Annual epidemics and rare pandemics appear at varying intervals, usually in the fall or winter affecting 10–20% of the global population on average each year

  • Incidence highest in school-age children and young adults, students, prisoners, day care and health care workers; persons with asthma are at particular risk

  • Complications occur most often in elderly, immunocompromised individuals

Clinical Findings

Symptoms and Signs

  • Abrupt onset

  • Systemic symptoms

    • Fever

    • Chills

    • Headache

    • Malaise

    • Myalgias

  • Respiratory symptoms

    • Rhinorrhea

    • Congestion

    • Pharyngitis

    • Hoarseness

    • Nonproductive cough

    • Substernal soreness

  • Gastrointestinal symptoms and signs may occur, particularly among young children with influenza B virus infections

  • Fever typically lasts 3–5 days (range, 1–7 days)

  • Leukocytosis may be a marker of secondary complications

Differential Diagnosis

  • Common cold

  • Primary bacterial pneumonia

  • Respiratory syncytial virus

  • Mycoplasma infection

  • Pertussis

  • Legionnaires disease

  • Parainfluenza infections

  • Adenovirus

  • Enterovirus

  • Coronavirus

  • Flavivirus

  • Acute HIV infection

  • Cytomegalovirus

  • Epstein-Barr virus

  • Atypical dengue


Laboratory Tests

  • Rapid influenza diagnostic tests (RIDT) for detection of influenza antigens from nasal or throat swabs are widely available, highly specific and produce fast results but have low sensitivity leading to high false negative results. Not all commercial RIDT can differentiate between influenza A and influenza B, and none of the available RIDT can provide information on influenza A subtypes.

  • Digital immunoassays and rapid nucleic acid amplification tests are more sensitive than traditional rapid influenza diagnostic tests, however the sensitivity of newer PCR techniques is compromised early in the season during low prevalence periods. A nasopharyngeal swab, nasal aspirate, combined nasopharyngeal swab with oropharyngeal swab, or material from a bronchoalveolar lavage can be tested for any influenza strain.

  • When influenza pneumonia is suspected, lower respiratory tract specimens should be collected and tested for influenza viruses by reverse transcription polymerase chain reaction (RT-PCR) or digital immunoassays or rapid diagnostic tests.



  • Three neuraminidase inhibitors are FDA approved for treatment of Influenza A and B:

    • Oral oseltamivir

    • Inhaled zanamivir

    • Intravenous ...

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