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.
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.
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.
Key medicines
ClarithromycinFor adult CAP give 500 mg orally or intravenously twice daily for 5 days; specialist-confirmed complicated infection may need longer.
Levofloxacin when common alternatives are inappropriateFor high-severity adult CAP, NICE lists 500 mg orally or intravenously twice daily for 5 days, then review.