PNEUMONIA | Viral

Hui, D. S.; Chan, P.K.S.

Viral infection accounts for a substantial proportion of cases of acute pneumonia especially among young children and the elderly, the immunocompromised, and those with comorbidities. Influenza A and respiratory syncytial virus are by far the most common causes of viral pneumonia followed by adenovirus, parainfluenza virus types 1, 2 and 3, and influenza B. Other less common agents include picornaviruses, varicella-zoster virus, herpes simplex virus, cytomegalovirus, and hantavirus. The newly identified human metapneumovirus also plays a role. Zoonotic infections caused by severe acute respiratory syndrome-associated coronavirus and avian influenza A/H5N1 are examples of acute ‘atypical’ pneumonia with epidemic and pandemic potentials. In general, there are no reliable clinical or radiological features to distinguish viral from other causes of pneumonia. Respiratory viruses often show seasonality and a predilection for certain host groups. These epidemiological features are helpful in gauging the differential diagnoses. Confirmation of infection relies on laboratory investigations based on conventional approaches including direct viral antigen detection by specific monoclonal antibodies, virus isolation, and serology, as well as modern molecular approaches to amplify viral nucleic acid present in clinical specimens. The available spectrum of antiviral agents and the window for effective application are narrow. Treatment of viral pneumonia is primarily supportive. Vaccines for general use are only available for influenza A and B.