Influenza Differential Diagnoses

Updated: Aug 14, 2023
  • Author: Hien H Nguyen, MD, MS; Chief Editor: Michael Stuart Bronze, MD  more...
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Diagnostic Considerations

Accurately diagnosing influenza A or B infection solely on the basis of clinical criteria is difficult because of the overlapping symptoms caused by the various viruses associated with upper respiratory tract infection (URI). Viruses that may initially cause influenzalike symptoms include adenoviruses, enteroviruses, and paramyxoviruses.

The early presentation of mild or moderate cases of flavivirus infections (eg, dengue) may initially mimic influenza. For example, some cases of West Nile fever acquired in New York in 1999 were clinically misdiagnosed as influenza. [3]

Like influenza, URIs from these viruses are more common in the winter. As a result, during the winter, clinics and emergency department waiting rooms fill with patients who have influenza or other URIs.

Influenza pneumonia must be differentiated from other forms of viral pneumonia, bacterial pneumonia, and noninfectious causes of respiratory distress, such as heart failure, chronic obstructive pulmonary disease, pulmonary edema, and aspiration pneumonitis.

H5N1 avian influenza

Risk factors or features that should raise the index of suspicion of H5N1 avian influenza include the following [31] :

  • Recent (within the preceding 2 weeks) travel to or location in a country with known avian influenza cases in animals or humans
  • Unusual comorbidities such as encephalopathy or diarrhea
  • History of exposure to birds, especially living in close proximity to birds, contact with sick or dying birds, or consumption of incompletely cooked bird meat
  • History of exposure to individuals with known avian influenza, especially family, or to sick people in a country with known human cases of avian influenza

The situation can be complicated during outbreaks of severe respiratory disease other than avian influenza. The first case of laboratory-confirmed avian influenza infection was documented during the severe acute respiratory syndrome (SARS) outbreak of 2002-2003 and was mistakenly misdiagnosed as SARS. [32]

Cases of avian influenza in which respiratory disease was limited or not apparent (with even normal chest radiography findings) have been described, though they account for only a small percentage of cases overall. [31] The primary presenting illness has been encephalitis or diarrhea.

Differential Diagnoses