Viral pneumonia

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Editor(s)-in-Chief: C. Michael Gibson, M.S., M.D. [mailto:mgibson@perfuse.org] Phone:617-632-7753; Philip Marcus, M.D., M.P.H.[mailto:pmarcus192@aol.com]

Overview
Viral pneumonia is an inflammation of the lung caused by a virus. Viruses are the most common cause of pneumonia in children. Viral pneumonia is commonly caused by viruses such as influenza virus, respiratory syncytial virus (RSV), adenovirus, and metapneumovirus.Herpes simplex virus is a rare cause of pneumonia except in newborns. People with immune system problems are also at risk of pneumonia caused bycytomegalovirus (CMV).

Features
Symptoms of viral pneumonia include fever, non-productive cough, rhinitis, and systemic symptoms (e.g. myalgia, headache). Different viruses cause different symptoms.

Diagnosis
Findings at physical examination are similar to those of bacterial pneumonia. The diagnosis can be confirmed by isolation and identification of the pathogen through viral culture. Chest X-ray findings are usually nonspecific. Asians are particularly at risk.

Pathophysiology
Viruses must invade cells in order to reproduce. Typically, a virus will reach the lungs by traveling in droplets through the mouth and nose with inhalation. There, the virus invades the cells lining the airways and the alveoli. This invasion often leads to cell death either through direct killing by the virus or by self-destruction through apoptosis. Further damage to the lungs occurs when the immune system responds to the infection. White blood cells, in particular lymphocytes, are responsible for activating a variety of chemicals (cytokines) which cause leaking of fluid into the alveoli. The combination of cellular destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream. In addition to the effects on the lungs, many viruses affect other organs and can lead to illness affecting many different bodily functions. Viruses also make the body more susceptible to bacterial infection; for this reason, bacterial pneumonia often complicates viral pneumonia.

Etiology
Viruses that commonly cause pneumonia include:
 * Influenza virus
 * Respiratory syncytial virus (RSV)
 * Adenoviruses
 * Metapneumovirus

Viruses that rarely cause pneumonia include:
 * Herpes simplex virus (HSV), mainly in newborns
 * Varicella-zoster virus (VZV)
 * Cytomegalovirus (CMV), mainly in people with immune system problems

Treatment
In cases of viral pneumonia where influenza A or B are thought to be causative agents, patients who are seen within 48 hours of symptom onset may benefit from treatment with oseltamivir or zanamivir. Respiratory syncytial virus|RSV may be treated with ribavirin. Herpes simplex virus and varicella-zoster virus infections are usually treated with aciclovir, whilst ganciclovir is used to treat cytomegalovirus. There is no known efficacious treatment for pneumonia caused by SARS coronavirus, adenovirus, hantavirus, or parainfluenza virus; treatment is largely supportive.