What is viral pneumonia?

What is Viral Pneumonia

Viral pneumonia is a lung infection caused by respiratory viruses that invade respiratory epithelial cells, triggering inflammatory responses and lung damage, and represents an important and often underappreciated cause of nosocomial and community-acquired pneumonia, accounting for approximately 20% of pneumonia cases in hospitalized patients. 1

Causative Pathogens

The most common viral pathogens causing pneumonia include:

• Influenza virus, respiratory syncytial virus (RSV), parainfluenza viruses, and adenoviruses account for approximately 70% of all viral pneumonias 1
• Additional causative agents include rhinoviruses, measles virus, varicella-zoster virus, human metapneumovirus, and coronaviruses (including SARS-CoV-2) 1, 2, 3
• The rate of viral pneumonia diagnosis in children with community-acquired pneumonia is approaching 60%, while approximately 10-23% of immunocompetent adults hospitalized with community-acquired pneumonia have evidence of viral infection 1, 2

Clinical Patterns and Presentations

Viral pneumonia manifests in three distinct clinical patterns, each with different mortality implications:

Primary Viral Pneumonia

• Patients develop breathlessness within the first 48 hours of fever onset, with initially dry cough that may become productive of blood-stained sputum 1
• Physical examination reveals cyanosis, tachypnea, bilateral crepitations, and wheeze, with leucocytosis commonly present 1
• Chest radiography demonstrates bilateral interstitial infiltrates predominantly in the mid-zones, though focal consolidation is also recognized 1
• Mortality exceeds 40% in hospitalized patients despite maximal intensive care support, with death typically occurring within 7 days of hospital admission 1, 4

Secondary Bacterial Pneumonia

• This pattern is more common than primary viral pneumonia (up to 4 times more frequent) and develops during early convalescence, typically 4-5 days after initial symptom onset 1, 4
• Chest radiography usually shows lobar consolidation rather than diffuse infiltrates 1
• Key bacterial pathogens include Streptococcus pneumoniae, Staphylococcus aureus, and Haemophilus influenzae 1, 4
• Mortality ranges from 7-24%, substantially lower than primary viral pneumonia, though Staphylococcus aureus co-infection carries worse prognosis (47% mortality) with higher abscess formation rates (14% vs 2%) 1, 5

Mixed Viral-Bacterial Pneumonia

• Bacterial and viral pneumonia occur concurrently, with chest radiography showing lobar consolidation superimposed on bilateral diffuse infiltrates 1
• Mortality exceeds 40%, similar to primary viral pneumonia 1, 4

Epidemiological Characteristics

• Nosocomial respiratory viral infections typically follow community outbreaks occurring during particular periods each year (December through March for RSV), lasting 3-5 months 1
• These infections confer only short-term immunity and affect both healthy and ill persons 1
• Patients with chronic cardiac or pulmonary diseases, immunocompromised individuals, the elderly, and children with chronic conditions face substantially elevated risk of severe disease and mortality 1, 6

Diagnostic Challenges

• No clinical or radiographic criteria reliably distinguish viral pneumonia from bacterial infection, making empirical diagnosis difficult 1, 6
• Rapid antigen detection tests for influenza have sensitivity of only 50-70% in adults, so negative results do not exclude diagnosis 1
• RSV antigen tests are highly insensitive (<15%) when using upper respiratory samples from adults 1
• Viral cultures and serologic studies are typically too slow for individual patient treatment decisions 1
• Accessory symptoms such as anosmia or ageusia alongside respiratory symptoms suggest COVID-19 specifically 2

Treatment Limitations

• No antiviral agent has established efficacy for treating adults with pneumonia caused by parainfluenza virus, RSV, adenovirus, metapneumovirus, SARS, or Hantavirus 1, 6
• Varicella-zoster virus (VZV) or herpes simplex virus (HSV) pneumonia should be treated with parenteral acyclovir 1
• When antivirals are used, they should be initiated as early as possible in the disease course, though evidence from randomized controlled trials remains limited 4
• Neuraminidase inhibitors for influenza have been proven to reduce ventilatory support needs and mortality 2

Common Pitfalls

The major clinical pitfall is assuming that viral pneumonia can be reliably distinguished from bacterial pneumonia based on clinical presentation alone—this is not possible, necessitating a low threshold for empirical antibiotics in severe cases despite suspected viral etiology 1, 6. Additionally, clinicians often underappreciate that viral pneumonia accounts for a substantial proportion (20%) of nosocomial pneumonia cases and can cause severe, fatal disease even in previously healthy adults 1.