Viral Pneumonia Differential Diagnoses

Updated: Dec 19, 2017
  • Author: Zab Mosenifar, MD, FACP, FCCP; Chief Editor: Ryland P Byrd, Jr, MD  more...
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DDx

Diagnostic Considerations

Studies document that patients are frequently infected with both bacterial and viral pathogens, making it impossible to rule out bacterial disease even when rapid viral test results are positive. Additionally, no unique identifying clinical characteristics are present that allow the physician to differentiate viral disease from bacterial disease in the emergency department (ED).

Although the most common cause of community-acquired pneumonia remains Streptococcus pneumoniae (a fact that may change with the increasing use of pneumococcal vaccines), in as many as 40-60% of patients with community-acquired pneumonia, the etiologic agent is not identified. Furthermore, convincing associations between the patient's symptoms, physical findings, laboratory test results, and specific etiologies are lacking.

Therefore, no way of accurately determining the etiology of pneumonia during the initial visit to the ED exists. Obtaining a chest radiograph in patients with suspected pneumonia is recommended, both to find complications, such as pleural effusions, and to discourage the use of antibiotics in healthy patients with bronchitis rather than pneumonia.

Influenza viruses are the most common cause of viral pneumonia in civilian adults. Influenza usually is seen in epidemics and pandemics. Influenza A is the serotype most frequently responsible for major epidemics and pandemics; it is the most frequent cause of viral pneumonia in adults.

Influenza epidemics occur during the winter months and are associated with significant morbidity and mortality. Patients with chronic obstructive pulmonary disease (COPD), heart failure, hemoglobinopathies, and immunosuppression are at increased risk for severe disease, which can lead to death. Influenza virus is transmitted from person to person primarily by droplet and aerosol exposure to the virus. The incubation period is 1-5 days after exposure.

Patients with high-grade fever, myalgias, and cough during the winter months should be suspected to have influenza. If test results are negative for influenza, RSV pneumonia should also be suspected during the winter in patients with coryza, wheezing, low-grade fever, and patchy infiltrates on chest radiographs. Because clinical features and periods of activity for many viruses overlap, laboratory confirmation of influenza is recommended for cases involving patients who are seriously ill or institutionalized.

Viral pathogens and host factors

Patient age and immune status can offer a clue to the possible etiology of viral pneumonia. The following viruses commonly cause pneumonia in children:

  • Respiratory syncytial virus
  • Influenza viruses A and B
  • Parainfluenza virus
  • Adenovirus
  • Human metapneumovirus
  • Coronavirus
  • Measles virus (in unvaccinated children)

The following viruses commonly cause pneumonia in immunocompetent adults:

  • Influenza viruses A and B
  • Adenovirus
  • Respiratory syncytial virus
  • Parainfluenza virus
  • Coronavirus
  • Varicella-zoster virus

The following viruses commonly cause pneumonia in immunocompromised hosts:

  • Cytomegalovirus
  • Herpes simplex virus
  • Influenza
  • Respiratory syncytial virus
  • Parainfluenza virus
  • Adenovirus
  • Varicella-zoster virus

Differential Diagnoses