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^[32]:

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.^[33]

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.^[32]The primary presenting illness has been encephalitis or diarrhea.

Differential Diagnoses

Workup

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Media Gallery

Countries where avian influenza has been reported. Image courtesy of the World Health Organization.
Colorized transmission electron micrograph shows avian influenza A H5N1 viruses (gold) grown in MDCK cells (green). Image courtesy of Centers for Disease Control and Prevention.
Transmission electron micrograph (original magnification 150,000X) shows ultrastructural details of an avian influenza A (H5N1) virion, a subtype of avian influenza A. Note the stippled appearance of the roughened surface of the proteinaceous coat encasing the virion. Image courtesy of Centers for Disease Control and Prevention.

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Author

Hien H Nguyen, MD, MS Chief of Infectious Diseases, VA Northern California Health Care System; Clinical Professor, Division of Infectious Diseases and Pulmonary/Critical Care Medicine, Infectious Diseases Consultant and Hospitalist, University of California, Davis, Health System; Medical Director, Acute Infections Management Service, Antimicrobial Infusion Service

Hien H Nguyen, MD, MS is a member of the following medical societies: American College of Physicians, Infectious Diseases Society of America, Society of Critical Care Medicine

Disclosure: Nothing to disclose.

Coauthor(s)

Robert W Derlet, MD Professor of Emergency Medicine, University of California at Davis School of Medicine; Chief Emeritus, Emergency Department, University of California at Davis Health System

Robert W Derlet, MD is a member of the following medical societies: American Academy of Emergency Medicine, American Association for the Advancement of Science, Infectious Diseases Society of America, Society for Academic Emergency Medicine, Wilderness Medical Society

Disclosure: Nothing to disclose.

Specialty Editor Board

Mary L Windle, PharmD Adjunct Associate Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Nothing to disclose.

Chief Editor

Michael Stuart Bronze, MD David Ross Boyd Professor and Chairman, Department of Medicine, Stewart G Wolf Endowed Chair in Internal Medicine, Department of Medicine, University of Oklahoma Health Science Center; Master of the American College of Physicians; Fellow, Infectious Diseases Society of America; Fellow of the Royal College of Physicians, London

Michael Stuart Bronze, MD is a member of the following medical societies: Alpha Omega Alpha, American College of Physicians, American Medical Association, Association of Professors of Medicine, Infectious Diseases Society of America, Oklahoma State Medical Association, Southern Society for Clinical Investigation

Disclosure: Nothing to disclose.

Additional Contributors

Christian E Sandrock, MD, MPH, FCCP Associate Professor of Clinical Medicine, Division of Pulmonary/Critical Care Medicine, Division of Infectious Diseases, Department of Internal Medicine, University of California, Davis Medical Center

Christian E Sandrock, MD, MPH, FCCP is a member of the following medical societies: American College of Chest Physicians, American Thoracic Society, Infectious Diseases Society of America

Disclosure: Received honoraria from Pfizer for speaking and teaching; Received honoraria from Pfizer for consulting; Received honoraria from therevance for consulting; Received honoraria from GSK for speaking and teaching.

Acknowledgements

Nicholas John Bennett, MB, BCh, PhD, Assistant Professor in Pediatrics, Division of Infectious Diseases, Connecticut Children's Medical Center

Nicholas John Bennett, MB, BCh, PhD, is a member of the following medical societies: Alpha Omega Alpha and American Academy of Pediatrics