Pneumocystis jirovecii pneumonia
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- Causative organism of pneumocystis pneumonia (PCP) is the fungus called Pneumocystis jiroveci.
- Closest relative is Taphrina deformans, a fungal pathogen of peach trees.
- There are multiple species within the genus Pneumocystis
- P. jiroveci (formerly identified as P. carinii and pronounced "yee row vet zee”) cause of disease in humans.
- P. carinii and P. wakefieldiae infect rats, and P. murina infects mice.
- Other species infect a range of animals (e.g. rabbits, sheep, monkeys, aquatic mammals).
- Morphological forms:
- Trophic form:
- Predominant forms (>90% in the lungs), multiply through binary fission, heterogeneous in shape, about ∼2 µm at greatest diameter, have a cell membrane and a fragile (not rigid) cell wall.
- Asci or spore form (also called cyst):
- Formed by conjugation of two opposite mating type trophic forms (sexual reproduction), more uniform in shape, about 8–10 µm in greatest diameter.
- Have a rigid cell wall that is important for protecting the organism from environmental conditions when outside of the host (during airborne transmission) and contains beta-glucans.
- Mature cysts contain 8 intracystic bodies, which can be released to become trophic forms.
- Trophic form:
- Important biochemical properties:
- Cell membrane lacks ergosterol and hence antifungal agents such as azoles and amphotericin B products are not active against Pneumocystis.
- The fungus must synthesize its own folic acid and hence this is a typical target for treatment (e.g TMP/SMX).
- Cyst cell wall contains beta-glucans, which can be helpful for diagnosis and perhaps treatment.
- Ecological niches and transmission
- Specific environmental reservoirs are not well understood.
- The organism grows well on epithelial cells within alveoli and fungal burden correlates with the degree of immune dysfunction.
- P. jiroveci may be present at low levels in healthy humans, who may serve as a source of transmission.
- Transmission is via airborne asci (cyst) forms passed from person to person.
- There are constant inhalation and acquisition (transient or long term) of the organism. Clinical disease may be due to new acquisition of the fungus by a susceptible patient or from the transformation of long-term carriage into active disease.