Microbiology · Intracellular and atypical bacteria
Chlamydia pneumoniae / Chlamydia psittaci !
- Bacterium
- Gram-negative
- Exam priority: medium
Two chlamydiae that cause atypical pneumonia: one passes between people by coughing; the other comes from birds such as parrots and pigeons.
Updated 2026-11-01
Also called: respiratory chlamydiae, psittacosis
At a glance
- Gram
- Gram-negative
- Shape
- Small coccobacilli
- Oxygen
- Obligate intracellular
- Family
- Chlamydiaceae
- Stain
- Not seen on Gram; molecular or serological diagnosis
Morphology
- Same elementary body–reticulate body cycle as C. trachomatis
- Inclusions without glycogen (do not stain with iodine)
Transmission
- C. pneumoniae: respiratory droplets from person to person
- C. psittaci: inhalation of dust from dried bird droppings or secretions
- Psittacosis rarely passes between people
Reservoir: C. pneumoniae: humans; C. psittaci: parrots, parakeets, pigeons, poultry
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Infect respiratory epithelial cells and macrophages
- C. psittaci spreads through blood to the liver and spleen
Diseases it causes
- Atypical community-acquired pneumonia Community-acquired pneumonia →
- Pharyngitis, sinusitis, bronchitis; psittacosis with high fever and headache
Clinical picture
- Gradual onset: sore throat, hoarseness and persistent dry cough
- Psittacosis: high fever, severe headache, dry cough, sometimes relative bradycardia and splenomegaly
- The X-ray often looks worse than the examination suggests
Diagnosis
- PCR on nasopharyngeal swab or sputum (respiratory panels)
- Serology with paired samples
- History of bird contact: the key to psittacosis
Treatment
Tetracyclines (doxycycline) or macrolides (azithromycin); respiratory fluoroquinolones are an alternative. Beta-lactams do not work.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Handwashing and cough hygiene
- Wet cleaning of cages, respiratory protection when handling sick birds
Test yourself
Think of the answer, then tap the question to see it.
How is C. psittaci acquired?
By inhaling dust from dried droppings or secretions of infected birds.
Which antibiotic classes treat chlamydial pneumonia?
Tetracyclines such as doxycycline and macrolides such as azithromycin.
How is C. pneumoniae transmitted?
From person to person by respiratory droplets when coughing or sneezing.
Related
Related conditions
← Back to the group: Intracellular and atypical bacteria
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: About Chlamydia pneumoniae Infection | C. pneumoniae | CDC (Centers for Disease Control and Prevention (CDC), 2024); About Psittacosis (Centers for Disease Control and Prevention (CDC), 2024).
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Representative model, for educational purposes only.