Medicine – Pneumonia

Pneumonia is an acute infection of the lung parenchyma involving the alveoli and surrounding tissues. Infection produces inflammation and accumulation of inflammatory exudate within the alveoli, leading to consolidation and impaired gas exchange. Pneumonia may be classified according to where it was acquired, the causative organism, or the patient’s immune status.

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1. Clinical Features

The classic presentation is an acute cough, which may be productive of purulent sputum. However, sputum production is not universal, particularly with viral or so-called atypical infections.

Patients commonly develop fever, breathlessness, malaise, and tachypnoea. Older adults and immunocompromised patients may have less typical presentations, including confusion or functional deterioration without prominent fever.

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Haemoptysis

Haemoptysis may occur because inflammation damages the respiratory mucosa and small pulmonary vessels.

Blood may appear as streaking within sputum, although significant haemoptysis should prompt consideration of complications or alternative diagnoses such as pulmonary embolism, tuberculosis, or malignancy.

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Chest Pain

Chest discomfort may be a nonspecific dull ache or sharp pleuritic chest pain.

Pleuritic pain is caused by inflammation extending to the pleural surface and typically becomes worse during deep inspiration or coughing.

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Sepsis

Severe pneumonia can produce sepsis, with systemic manifestations such as hypotension, altered mental status, tachycardia, tachypnoea, and organ dysfunction.

Sepsis associated with pneumonia requires urgent assessment and treatment.

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Community-Acquired Pneumonia

2. Definition

Community-acquired pneumonia (CAP) refers to pneumonia acquired outside a hospital or similar healthcare environment.

The exact frequency of individual organisms varies considerably with age, geography, vaccination, comorbidities, season, and diagnostic methods, so older fixed percentages should not be regarded as universally applicable.

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3. Streptococcus pneumoniae

Streptococcus pneumoniae (pneumococcus) remains a major bacterial cause of community-acquired pneumonia.

Classically, pneumococcal pneumonia produces acute fever, productive cough, pleuritic chest pain, and lobar consolidation.

Although older texts quote S. pneumoniae as causing 60–75% of CAP, contemporary studies generally find a lower and highly variable proportion.

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Atypical Pneumonias

The traditional term “atypical pneumonia” describes pneumonia caused by organisms such as Mycoplasma pneumoniae, Legionella species, Chlamydia pneumoniae, and Chlamydia psittaci.

These organisms may produce prominent systemic features and relatively little sputum production, although the clinical distinction between typical and atypical pneumonia is not always reliable.

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4. Mycoplasma pneumoniae

Mycoplasma pneumoniae commonly causes respiratory infection in children and younger adults, particularly in households, schools, universities, and other close-contact environments.

It may occur in cyclical epidemics every few years.

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Clinical Pattern of Mycoplasma

The illness may begin with a relatively long prodrome, including headache, malaise, low-grade fever, sore throat, and a persistent dry cough.

Respiratory symptoms can subsequently progress to pneumonia.

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Cold Agglutinins

Mycoplasma pneumoniae infection can stimulate production of cold agglutinins, usually IgM antibodies that react with red blood cells at lower temperatures.

In some patients this can produce haemolytic anaemia, although cold agglutinins are neither sufficiently sensitive nor specific to serve as the main diagnostic test for Mycoplasma infection.

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5. Legionella Pneumonia

Legionella pneumophila causes Legionnaires’ disease, an important cause of potentially severe community-acquired pneumonia.

The organism proliferates in water systems and infection occurs through inhalation of contaminated aerosols rather than usual person-to-person respiratory transmission.

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Environmental Sources

Potential sources include showers, cooling towers, complex plumbing systems, hot-water systems, and other aerosol-producing water sources.

The older association with “air-conditioning” is more accurately understood as an association with contaminated water-containing cooling systems, rather than ordinary air-conditioning units themselves.

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Risk Factors

Legionella infection is more likely to become severe in older adults, smokers, people with chronic lung disease, and immunocompromised patients.

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Extrapulmonary Features

Legionella may produce prominent systemic manifestations in addition to pneumonia.

Patients can develop gastrointestinal symptoms, confusion, headache, neurological abnormalities, and abnormal liver function tests. Jaundice may occasionally occur but is not a defining feature.

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Laboratory Findings

Important laboratory clues include hyponatraemia, sometimes associated with inappropriate antidiuretic hormone activity, and abnormal liver enzymes.

The total white cell count is variable, and lymphopenia may occur.

These findings can support suspicion of Legionella but are not sufficiently specific to establish the diagnosis.

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6. Chlamydial Pneumonia

Chlamydia pneumoniae can cause respiratory infections ranging from mild upper respiratory disease to pneumonia.

Chlamydia psittaci causes psittacosis, which is particularly associated with exposure to infected birds such as parrots, pigeons, and poultry.

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7. Haemophilus influenzae

Haemophilus influenzae is another cause of community-acquired pneumonia.

It is particularly important in patients with chronic respiratory disease, including COPD and bronchiectasis.

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8. Moraxella catarrhalis

Moraxella catarrhalis can cause lower respiratory tract infection and pneumonia, particularly in older adults and patients with chronic lung disease such as COPD.

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9. Viral Pneumonia

Respiratory viruses are important causes of pneumonia and may also predispose to secondary bacterial infection.

Important viruses include influenza, respiratory syncytial virus (RSV), parainfluenza viruses, SARS-CoV-2, and varicella-zoster virus in appropriate clinical circumstances.

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Staphylococcal Pneumonia

10. Staphylococcus aureus

Staphylococcus aureus can cause severe pneumonia and is particularly important following influenza infection.

Damage to the respiratory epithelium caused by influenza facilitates secondary bacterial invasion, which can result in rapidly progressive pneumonia.

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Intravenous Drug Use

People who inject drugs are at increased risk of S. aureus bacteraemia and right-sided infective endocarditis.

Septic emboli from right-sided endocarditis can travel to the lungs and produce multiple peripheral pulmonary infections, nodules, or abscesses.

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Complications

Staphylococcal pneumonia can produce extensive tissue destruction and is associated with lung abscess formation, cavitation, parapneumonic effusion, and empyema.

MRSA should be considered when appropriate epidemiological or healthcare-associated risk factors are present.

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Assessing Severity

11. CURB-65 Score

A commonly used severity assessment for community-acquired pneumonia is the CURB-65 score.

One point is assigned for each of the following:

C – Confusion

U – Urea >7 mmol/L

R – Respiratory rate ≥30/min

B – Blood pressure: systolic <90 mmHg or diastolic ≤60 mmHg

65 – Age ≥65 years

The score ranges from 0 to 5.

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Interpreting CURB-65

In general, 0–1 suggests lower mortality risk, while a score of 2 indicates increased risk and usually warrants consideration of hospital-based assessment or treatment.

A score of 3 or more indicates severe pneumonia and a substantially increased risk of mortality, requiring urgent hospital management and consideration of higher-level care depending on the overall clinical picture.

The score supports rather than replaces clinical judgement.

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Hospital-Acquired Pneumonia

12. Definition

Hospital-acquired pneumonia (HAP) generally refers to pneumonia developing 48 hours or more after hospital admission that was not already incubating when the patient entered hospital.

The spectrum of causative organisms differs from CAP because hospitalised patients may become colonised with more resistant bacteria.

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13. Causative Organisms

Important organisms include Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) in patients with appropriate risk factors.

Gram-negative organisms are also particularly important and include Klebsiella species, Escherichia coli, Enterobacterales, and Pseudomonas aeruginosa.

The likely pathogen depends strongly on the hospital environment, previous antibiotic exposure, duration of hospitalisation, local resistance patterns, and whether mechanical ventilation is present.

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Anaerobic Infection

Anaerobic bacteria are particularly relevant when there is significant aspiration, especially in patients with impaired consciousness, swallowing dysfunction, or poor dentition.

They are therefore not necessarily routine causes of every case of hospital-acquired pneumonia.

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Pneumocystis Pneumonia

14. Pneumocystis jirovecii

The organism historically called Pneumocystis carinii in humans is now called Pneumocystis jirovecii.

The disease continues to be abbreviated PCP, meaning Pneumocystis pneumonia.

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15. Risk Factors for PCP

PCP occurs predominantly in patients with impaired cell-mediated immunity.

It is classically associated with advanced HIV infection, particularly when the CD4 count is below approximately 200 cells/µL, but it can also occur with transplantation, malignancy, prolonged corticosteroid therapy, and other immunosuppressive treatments.

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16. Clinical Features of PCP

The characteristic presentation consists of progressive breathlessness, fever, and a dry or minimally productive cough.

Symptoms often develop gradually over days to weeks, particularly in patients with HIV.

Hypoxaemia can become severe and may be disproportionately marked compared with the initial chest examination.

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17. Imaging in PCP

A chest radiograph may initially be normal, particularly early in the disease.

The classic abnormality is bilateral diffuse or perihilar interstitial/ground-glass opacity.

High-resolution CT is more sensitive and commonly demonstrates widespread ground-glass opacities.

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18. Diagnosis of PCP

Because P. jirovecii cannot be diagnosed by routine bacterial culture, respiratory material is examined using specific methods.

Samples may be obtained from induced sputum or bronchoalveolar lavage, with the organism demonstrated by staining, immunofluorescence, or PCR.

PCR is highly sensitive but must be interpreted in the clinical context because detection does not always prove active disease.

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Treatment of Pneumonia

19. Community-Acquired Pneumonia

Amoxicillin remains a common first-line antibiotic for uncomplicated, lower-severity community-acquired bacterial pneumonia when pneumococcal infection is likely.

A macrolide, such as clarithromycin, may be used when atypical infection is suspected or as part of broader treatment for more severe disease.

The exact antibiotic regimen should follow local antimicrobial guidelines, disease severity, allergies, comorbidities, and local resistance patterns.

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20. Staphylococcal Pneumonia

When methicillin-sensitive S. aureus is established or strongly suspected, an appropriate antistaphylococcal beta-lactam such as flucloxacillin may be used.

Suspected or confirmed MRSA requires appropriate MRSA-active treatment, such as vancomycin or another agent selected according to local guidelines and susceptibility results.

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21. Hospital-Acquired Pneumonia

Treatment of hospital-acquired pneumonia requires broader consideration of Gram-negative organisms, Pseudomonas, resistant Enterobacterales, and MRSA.

The older universal recommendation of a third-generation cephalosporin plus metronidazole is no longer an appropriate standard regimen for all HAP.

Empirical therapy should instead be determined by local hospital antimicrobial guidelines, previous microbiology, recent antibiotic exposure, illness severity, and individual resistance risk, followed by narrowing of treatment when culture results become available.

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22. Treatment of PCP

The first-line treatment for Pneumocystis jirovecii pneumonia is generally co-trimoxazole (trimethoprim-sulfamethoxazole).

Patients with moderate-to-severe PCP and significant hypoxaemia may additionally require adjunctive corticosteroids, particularly in HIV-associated disease.

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Key Clinical Pattern

Think of pneumonia in a patient with an acute illness characterised by cough, fever, breathlessness, purulent sputum and pleuritic chest pain, with pulmonary infiltrates or consolidation supporting the diagnosis.

For CAP, S. pneumoniae remains a major bacterial pathogen. Remember the characteristic associations: Mycoplasma → younger patients and cold agglutinins; Legionella → water aerosols, hyponatraemia, abnormal LFTs and neurological/GI features; S. aureus → post-influenza pneumonia, cavitation, abscess and empyema.

For severity assessment, remember CURB-65: Confusion, Urea >7, Respiratory rate ≥30, low Blood pressure, and age ≥65. For immunocompromised patients with dry cough + fever + progressive dyspnoea + bilateral ground-glass changes, consider Pneumocystis jirovecii pneumonia.