~A disease of the alveoli

~Aetiology – Pneumocystis jiroveci

HISTORY

1909 - Carlos Chagas – Discovered the organism in the lungs of guinea pigs while investigating the etiology of a new disease affecting Brazilian Railway workers.

1910 - Antonio Carini – Observed in the lungs of rats but mistakened as T. cruzii.

1912 – Delanoe & Delanoe (Pasteur Institut, Paris) identified and named it as Pneumocystis carinii.

Pneumo = Lung, Cyst = cyst-like structure and carinii was designated in honor of Antonio carini.

TAXONOMY

~P. carinii is considered to be a fungus and is phylogenetically classified under ASCOMYCETES.

~Cyst-wall has β-(1, 3)-D-glucan, a compound found in the wall of fungi is responsible for staining by GMS stain.

~Does not grow on fungal culture media but require tissue culture/cell lines. (WI-38)

~Susceptible to anti-protozoal agents like pentamidine and TMP-SMX and not to antifungals.

~Produces chitin at all stages of lifecycle.

~Lacks ergosterol in the cytoplasmic membrane, but has cholesterol. Therefore, antifungals that target ergosterol are inefficient against P. carinii.

~Elongation factor-3 (EF-3) & Thymidylate synthase are homologous to that of ascomycetes.

~P. carinii and fungi have similar cyst-wall ultrastructures (Fungal mitochondria with lamellar cristae and not protozoal mitochondria that has vesicular or tubular cristae).

~rRNA studies reveal that 16S-like RNA shares substantial homology with Ascomycota.

INTRAPULMONARY LIFE CYCLE

Unicellular eukaryote having 3 stages in sexual phase.

i) Trophozoite

ii) Cyst

iii) Sporozoite

The transition phase between trophozoite and cyst stages is called as precyst or sporocyst.

1. TROPHOZOITE

~Are the structures that fill the alveoli of the involved lung.

~Pleomorphic, tiny bodies, 2-3 μm in size and exist in clusters.

~Covered with tubular projections and helps in

• adherence to epithelial cells
• increasing the absorptive surface

Trophozoites have thin, fragile and flexible external wall.

2. SPOROCYST (Precyst)

Intermediate stage in the sexual phase of reproduction (leading to cyst formation).

3. CYST

The frequently observed stage of the organism in clinical specimens.

Large, disc-like structures, 4-6 μm, oval, thick walled

Possess up to 8 sporozoites (intracystic bodies).

4. SPOROZOITES

Oval, amoeboid or peach-shaped & 1-2 μm long.

Best demonstrated by Giemsa staining – Show basophilic cytoplasm & eosinophilic nuclei

Sporozoites are extruded through cyst wall after division of mature cyst and are subsequently converted to trophozoites.

Empty cysts are seen as NAVICULAR structures upon staining (GMS or TBO).

Asexual phase of life cycle

Trophic forms multiply by binary fission as seen in some fungi and never by budding.

The environmental form has never been identified.

PATHOGENESIS & PATHOLOGY

Extracellular pathogen

Initiates infection within the alveoli via preferential attachment of the trophozoites to the alveolar epithelium.

Once the organism is inhaled, it escapes the defenses of the URT and is deposited in the alveoli.

The trophozoites attach to the TYPE I ALVEOLAR EPITHELIAL CELLS and proliferate causing pneumonitis.

Alveolar cell hypertrophy

Macrophage infiltrates

Filling of alveolar spaces with foamy eosinophilic material – ‘Honey-comb’ or ‘Ground-glass’ lung

AIDS patients having CD4 less than 200 cells/mm³ are at risk.

CMI – Important in containing infection.

CLINICAL FEATURES

Pathogen of both man and animals

Mostly asymptomatic

Insidious onset

Incubation period - 4-8 weeks

1. Pulmonary pneumocystosis

Non-productive cough & dyspnoea over weeks or months

Fever, tachypnoea

Tachycardia

Hemoptysis

Breathlessness

Chills

Night sweats

Cyanosis due to hypoxia

CXR – Bilateral pulmonary infiltrates, perihilar haziness or diffuse mottling (Ground-glass).

2. Extrapulmonary pneumocystosis

AIDS patients are at risk (~0.5-2.5% cases)

Patients that take no prophylaxis

Any viscera or organs involved

CNS complication due to hematogenous spread.

LABORATORY DIAGNOSIS

CD4 counts less than 200 cells/mm³

Reduced Vital capacity, total lung capacity

Reduced single breath diffusing capacity for carbon monoxide.

DIRECT EXAMINATION

Specimens – BAL, lung biopsy, induced sputum, lung fluids (aspirates)

Staining procedures – GMS, TBO, Cresyl echt violet and Gram-Weigert stains – Stains the cyst walls.

Gram-Weigert – Most sensitive single histochemical stain.

Calcofluor white also used.

Other stains to stain trophozoites and intracystic sporozoites– Wright, Giemsa, Diff-Quik, polychrome methylene blue, May-Grunwald Giemsa (MGG) and Papanicolou (PAP).

Immunofluorescence assays

PCR to detect the thymidylate synthase sequences.

Presence of P. carinii is suspected and confirmed by demonstration of typical, granular, foamy, honeycombed material by H&E stain.

Cyst wall may have folds and assume cup-shaped.

FUNGAL CULTURE

Cannot be cultured.

However, continuous cell line derived from human lung adenocarcinoma cells A-549 can be used.

W-38, a human embryonic lung fibroblast cell line can also be used.

IMMUNODIAGNOSIS

Serum antibodies can be measured by CFT, IFT and EIAs with whole organism or soluble extracts of the antigen.

Non-specific test, due to the high prevalence of serum antibodies in the general population.

Epidemiologically useful.

TREATMENT AND PROPHYLAXIS

P. jiroveci is resistant to antifungal agents due to the lack of ergosterol in cell membrane.

TMP-SMX – First line treatment/prophylaxis. (Inhibits 2 enzymes of folate metabolism).

Patients that cannot tolerate TMP-SMX – Pentamidine isothionate

Dapsone – Prophylactic drug

Primary prophylaxis – Given to patients who never had PCP if their CD4 is less than 200/mm³ or if they have oral thrush or PUO more than 100⁰F for more than 2 weeks.

Secondary prophylaxis – Those who had an episode of PCP already.

- Dr. E. M. Shankar