Synopsis
Differentiate exposure-linked intracellular pneumonias by organism, test limitations and public-health consequence, then choose a severity-appropriate intracellular-active regimen without treating a nonspecific 'atypical' phenotype as a diagnosis.
- The clinically useful group includes Legionella species, Mycoplasma pneumoniae, Chlamydia psittaci and Coxiella burnetii, but ordinary bacteria and respiratory viruses can produce the same radiological pattern.
- Translate exposure into a dated hypothesis: water aerosols, hotels, spas or healthcare plumbing for Legionella; birds and dried droppings for psittacosis; ruminants and birth products for Q fever; close-contact outbreaks for Mycoplasma.
- Diarrhoea, confusion, hyponatraemia or liver and renal abnormalities increase suspicion for Legionella but cannot confirm it or justify excluding competing emergencies.
- Legionella urinary antigen is rapid yet principally detects L pneumophila serogroup 1; obtain lower-respiratory PCR and culture in severe, antigen-negative high-probability or epidemiologically important disease.
- A beta-lactam alone has no reliable activity for these intracellular or cell-wall-deficient pathogens; add or select an intracellular-active agent when probability and severity warrant it.
- Use current CAP guidance for the empirical phase, then revise agent and duration with microbiology once the organism, host, pregnancy status and complications are known.
- Fluoroquinolones are restricted because disabling and potentially irreversible adverse effects occur; use them only when common recommended alternatives are inappropriate and document the reason.
- Legionnaires' disease requires urgent public-health notification in England, while suspected zoonotic clusters or occupational hazards should be discussed promptly even when the individual disease is not routinely notifiable.
Key red flags
Severe pneumonia follows hotel, spa, cooling, healthcare or domestic water-aerosol exposure and may include gastrointestinal symptoms, confusion, low sodium or organ injury.
High fever, headache and pneumonia after aviary, poultry, pigeon or veterinary exposure can progress to respiratory failure, hepatitis, myocarditis or encephalopathy.
Shock, profound hypoxaemia, multilobar change or organ failure requires prompt broad CAP treatment including credible intracellular pathogens before exposure testing is complete.
Investigation priorities
Separate plausible sources and supply actionable information for reference testing or public-health investigation.
Management branches
Pneumonia has epidemiology or extrapulmonary features suggesting an intracellular pathogen.
- Stabilise ABCDE, grade CAP severity and take routine cultures and imaging without postponing time-critical treatment.
- Build a date-and-place exposure timeline and contact microbiology early for respiratory, reference or paired specimens.
- Start the current severity-based CAP regimen with intracellular activity when the probability is clinically material; document pregnancy, QT, renal and interaction constraints.