Synopsis
Recognise pneumonia caused by intracellular or exposure-linked pathogens without relying on an unreliable 'atypical' phenotype, choose targeted microbiology, provide intracellular-active therapy, and trigger UK public-health action where required.
- Atypical pneumonia commonly refers to Legionella species, Mycoplasma pneumoniae, Chlamydia psittaci and Coxiella burnetii; clinical features overlap substantially with ordinary bacterial and viral pneumonia.
- Ask about hotel, hospital, spa or water-aerosol exposure for Legionella; birds for psittacosis; and sheep, cattle, goats or birthing products for Q fever.
- Legionella may cause severe pneumonia with diarrhoea, confusion, hyponatraemia, renal or liver disturbance, but none of these features is sufficiently specific to diagnose it.
- Legionella urinary antigen mainly detects L. pneumophila serogroup 1; a negative result does not exclude other serogroups or species, so obtain respiratory PCR/culture in severe or epidemiologically important disease.
- Beta-lactams do not treat intracellular organisms. Moderate or high-severity CAP with suspected atypical infection generally needs macrolide-containing therapy through NICE/local guidance.
- Doxycycline is useful for Mycoplasma, psittacosis and Q fever in appropriate adults; pregnancy, age, interactions and severe disease require specialist selection.
- Legionnaires' disease is notifiable in England and Wales; identify the relevant UK jurisdiction and contact health protection promptly for suspected cases and clusters.
- Do not allow an 'atypical' label to obscure severity assessment, oxygen targets, sepsis treatment, pleural complications or an alternative diagnosis.
Key red flags
High fever, cough and severe pneumonia after travel, hotel, healthcare, spa or aerosolised-water exposure, sometimes with diarrhoea, confusion, hyponatraemia or renal and hepatic disturbance.
Flu-like illness, headache and pneumonia after handling parrots, poultry, pigeons or contaminated bird material; severe respiratory, hepatic, cardiac or neurological disease can occur.
Shock, confusion, hypoxaemia or multilobar disease should be treated for severe CAP including credible atypical pathogens before a precise organism is known.
Investigation priorities
Direct organism-specific testing and identify a reportable exposure or outbreak.
Management branches
Pneumonia with a possible atypical respiratory pathogen.
- Assess ABCDE, oxygen requirement, NEWS2 and CRB65/CURB65 where appropriate; manage shock or respiratory failure immediately and obtain chest imaging.
- Take a focused exposure history and send proportionate microbiology before antibiotics when this creates no harmful delay.
- Start the current NICE/local CAP regimen with intracellular activity when moderate/high severity or exposure makes an atypical pathogen credible.