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MI7.{2-3,5} | Lower Respiratory Tract Infections — SDL Guide (Part 2)

Viral and Fungal LRTIs

A five-panel infographic compares viral and fungal lower respiratory tract infections, showing influenza, RSV, SARS-CoV-2, Pneumocystis jirovecii pneumonia, and pulmonary aspergillosis with key mechanisms, diagnostic clues, and treatments. — **Panel A:** Influenza pneumonia: orthomyxovirus, primary viral pneumonia, secondary bacterial superinfection, bilateral white-out infiltrates, oseltamivir within 48 hr. **Panel B:** RSV bronchiolitis: infant bronchiole, syncytia formation, airway narrowing, mucus, air trapping, wheeze, palivizumab prophylaxis. **Panel C:** SARS-CoV-2 pneumonia: spike protein, ACE-2 receptor, alveolar epithelial cell, cytokine storm, ARDS, bilateral ground glass opacities, RT-PCR nasopharyngeal swab. **Panel D:** Pneumocystis jirovecii pneumonia: HIV with CD4 <200/µL, bilateral interstitial infiltrates, BAL sample, GMS stain, helmet-shaped cysts, TMP-SMX, corticosteroids if PaO₂ <70 mmHg. **Panel E:** Pulmonary aspergillosis: Aspergillus fumigatus, septate hyphae, dichotomous 45° branching, aspergilloma, fungus ball in pre-existing lung cavity.

Viral LRTIs:

Influenza: Orthomyxovirus can cause severe pneumonia (primary viral pneumonia) or secondary bacterial superinfection (S. pneumoniae, S. aureus). ICU-requiring pneumonia with bilateral infiltrates ('white-out') — high mortality. Oseltamivir within 48 hr of onset.

RSV (Respiratory Syncytial Virus): Leading cause of bronchiolitis in infants <2 years. Syncytia formation in bronchiolar epithelium → air trapping → wheeze. Palivizumab prophylaxis for high-risk infants.

SARS-CoV-2 (COVID-19): Binds ACE-2 receptor via spike protein. Cytokine storm → ARDS; bilateral ground glass opacities on CT. RT-PCR of nasopharyngeal swab is gold standard.

Hantavirus (India — rare): Rodent-associated; Hantavirus Pulmonary Syndrome (HPS) — acute respiratory failure.

Fungal LRTIs:

Pneumocystis jirovecii Pneumonia (PCP):
- Previously called P. carinii; a fungus (despite being treated with cotrimoxazole)
- Cannot be cultured; diagnosis by GMS (Gomori Methenamine Silver) stain or immunofluorescence on BAL — cysts (trophic forms cluster as 'helmet-shaped' cysts)
- Classic: HIV patient with CD4 <200/µL; bilateral interstitial infiltrates; PaO₂ <70 mmHg
- Treatment: High-dose co-trimoxazole (TMP-SMX); add corticosteroids if PaO₂ <70
- Primary prophylaxis with TMP-SMX when CD4 <200 (standard of care under NACO ARTG)

Pulmonary Aspergillosis:
- Aspergillus fumigatus — septate hyphae, dichotomous branching at 45°
- Forms: (1) Aspergilloma — fungus ball in pre-existing cavity (e.g., TB cavity); (2) Allergic bronchopulmonary aspergillosis (ABPA) — asthma + central bronchiectasis; (3) Invasive aspergillosis — immunocompromised (neutropenia, transplant)
- Diagnosis: CT chest (halo sign in invasive), serum galactomannan ELISA (invasive), BAL culture, skin prick test (ABPA)

Cryptococcal pneumonia: Cryptococcus neoformans; thick capsule (India ink stain); primary infection via pigeon droppings; latent → reactivation in HIV (CD4 <100) → meningitis or pneumonia

Histoplasmosis (Histoplasma capsulatum): Dimorphic; not endemic in India but seen in travellers; small yeast inside macrophages on histology.

Parasitic LRTIs

Infographic comparing parasitic lower respiratory tract infections, showing hydatid cyst disease, pulmonary amebiasis, toxoplasmosis, and paragonimiasis with key diagnostic clues. — **Panel A:** Thoracic overview showing lungs, pleura, diaphragm, liver dome, hydatid cyst with daughter cysts, hepato-pulmonary spread, right pleural effusion/empyema, bilateral interstitial pneumonitis, and PCP differential note.. **Panel B:** Pulmonary hydatid disease showing Echinococcus granulosus dog-sheep-human zoonosis, lung hydatid cyst, daughter cysts on CT, Casoni test, and rupture causing anaphylaxis.. **Panel C:** Pulmonary amebiasis showing Entamoeba histolytica liver abscess, transdiaphragmatic hepato-pulmonary communication, right-sided pleural effusion, and empyema.. **Panel D:** Toxoplasmosis showing Toxoplasma gondii reactivation in an immunocompromised host, diffuse interstitial pneumonitis, and PCR diagnosis from BAL.. **Panel E:** Paragonimiasis showing Paragonimus westermani lung fluke, hemoptysis mimicking tuberculosis, operculated eggs in sputum, and endemic regions including NE India and Southeast Asia..

Pulmonary Hydatid Disease (Echinococcus granulosus):
- Cystic lesion (hydatid cyst) in lung; rupture → anaphylaxis; Casoni test (intradermal); CT shows daughter cysts
- Zoonosis from dogs/sheep; endemic in shepherding communities in India (Rajasthan, AP)

Pulmonary Ameobiasis (Entamoeba histolytica):
- Rare; right-sided pleural effusion/empyema due to hepato-pulmonary communication

Toxoplasmosis (Toxoplasma gondii):
- Reactivation in immunocompromised → interstitial pneumonitis; diagnosed by PCR of BAL

Paragonimiasis (Paragonimus westermani):
- Lung fluke; haemoptysis mimicking TB; eggs in sputum (operculated); endemic in NE India, SE Asia

SELF-CHECK

A 28-year-old HIV-positive patient on ART presents with 2-week progressive dyspnoea, dry cough and bilateral interstitial infiltrates. CD4 count is 120 cells/µL. BAL GMS stain shows helmet-shaped cysts. The MOST appropriate treatment is:

A. Liposomal amphotericin B

B. High-dose co-trimoxazole (TMP-SMX) + prednisolone

C. Oseltamivir

D. Isoniazid + rifampicin + pyrazinamide + ethambutol

Reveal Answer

Answer: B. High-dose co-trimoxazole (TMP-SMX) + prednisolone

GMS stain showing helmet-shaped cysts in BAL of an HIV patient with CD4 <200 is diagnostic of Pneumocystis jirovecii Pneumonia (PCP). First-line treatment is high-dose TMP-SMX. Corticosteroids (prednisolone) are added if PaO₂ <70 mmHg to reduce inflammation-mediated respiratory failure. Amphotericin B is for invasive fungal infections like Cryptococcus or Aspergillus, not PCP.

Sputum Processing and Microscopy (MI7.5 practical focus)

A four-panel microbiology diagram explains sputum collection, Murray-Washington quality criteria, lower respiratory tract specimen types, and key microscopy stain findings. — **Panel A:** Early morning sputum cup, collect, quality check, microscopy, interpretation, TB: 3 consecutive early morning samples. **Panel B:** Accepted sputum low-power field, rejected saliva field, squamous epithelial cells, PMNs, <10 squamous epithelial cells/LPF, >25 PMNs/LPF. **Panel C:** Sputum, bronchoalveolar lavage, tracheal aspirate, pleural fluid, LRTI indications, empyema testing: Gram stain, culture, ADA, cytology. **Panel D:** Gram-positive diplococci, Gram-negative rods, Ziehl-Neelsen acid-fast bacilli, auramine-rhodamine fluorescent bacilli, pneumococcus, Klebsiella, Pseudomonas, mycobacteria.

Sputum quality check: A good sputum sample has <10 squamous epithelial cells and >25 PMNs per low-power field (Murray-Washington criteria). Saliva samples are rejected.

Specimen types for LRTI:
- Sputum — most common; early morning; 3 consecutive samples for TB
- BAL (Bronchoalveolar Lavage) — immunocompromised patients, atypical infections
- Tracheal aspirate — intubated ICU patients (HAP evaluation)
- Pleural fluid — empyema; send for Gram stain + culture + ADA + cytology

Stains and interpretation:

Stain	Organism Targeted	Positive Finding
Gram stain	Bacteria	Gram+ve diplococci (pneumococcus); Gram-ve rods (Klebsiella, Pseudomonas)
ZN (Acid-fast) stain	Mycobacteria	Red/pink rods on blue background (AFB)
Auramine-rhodamine	Mycobacteria	Fluorescent yellow-green rods (more sensitive than ZN)
GMS (Gomori Methenamine Silver)	Fungi (Aspergillus, PCP)	Black fungal cell walls; PCP cysts
India ink	Cryptococcus	Clear halo (capsule) around dark organism
KOH + Calcofluor white	All fungi	Fluorescent fungal cell wall

Correlating Gram stain with clinical scenario:
- Gram+ve lancet-shaped diplococci + lobar consolidation + rusty sputum → S. pneumoniae
- Gram-ve thick capsulated rods + upper lobe + currant jelly sputum → K. pneumoniae
- No organisms + interstitial pattern + atypical features → Mycoplasma / viral / PCP
- AFB on ZN + cavitary upper lobe → MTB (confirm with GeneXpert)

Interactive practice: Quick Recall

Interactive practice: Multiple Choice

Interactive practice: True / False

Interactive practice: Multiple Choice

⚠ DRAFT — coordinates pending review

Interactive practice: Label the Diagram