Zoonotic Diseases of the Lung — MD Exam Answer (15 Marks)

Introduction

Classification of Pulmonary Zoonoses

Detailed Description of Major Diseases

1. Q Fever (Coxiella burnetii)

• Source: Inhalation of aerosols from placentas, amniotic fluid, urine, feces of infected ruminants (cattle, sheep, goats). Farmers, veterinarians, abattoir workers at risk.
• Pathology: Obligate intracellular organism; inhaled organisms reach alveolar macrophages.
• Clinical Features:
  • Acute: High fever, severe headache, myalgia, atypical pneumonia (dry cough, minimal auscultatory findings disproportionate to radiological changes), hepatitis.
  • CXR: Rounded opacities ("doughnut lesions"), segmental consolidation, or patchy infiltrates.
• Diagnosis: Serology — Phase II IgG ≥1:200 (acute); Phase I IgG ≥1:800 (chronic/endocarditis). PCR on blood.
• Treatment: Doxycycline 100 mg BD × 14–21 days (drug of choice). Chronic Q fever (endocarditis): Doxycycline + hydroxychloroquine × ≥18 months.

2. Psittacosis (Chlamydophila psittaci)

• Source: Inhalation of dried secretions/feces of infected birds (parrots, cockatiels, pigeons, poultry). Pet shop workers, bird handlers at risk.
• Clinical Features:
  • Atypical pneumonia: Fever, relative bradycardia (Faget sign-like), dry cough, myalgia.
  • Extrapulmonary: Horder's spots (pink macular rash), endocarditis, encephalitis, hepatosplenomegaly.
  • CXR: Lobar or patchy consolidation, may be bilateral.
• Diagnosis: Serology (complement fixation, microimmunofluorescence), PCR.
• Treatment: Doxycycline 100 mg BD × 10–21 days (first line). Alternatives: azithromycin, erythromycin (in pregnancy/children).

3. Inhalational Anthrax (Bacillus anthracis)

• Source: Inhalation of B. anthracis spores from contaminated animal products (wool, hides, bone meal), or bioterrorism.
• Pathogenesis: Spores phagocytosed by alveolar macrophages → germinate → produce exotoxins (lethal toxin, edema toxin).
• Clinical Features (Biphasic):
  • Phase 1 (1–5 days): Mild fever, malaise, nonproductive cough (flu-like).
  • Phase 2 (sudden onset): High fever, drenching sweats, severe dyspnea, cyanosis, mediastinal widening, stridor, shock.
• CXR/CT: Widened mediastinum (hemorrhagic mediastinitis), pleural effusions, pulmonary infiltrates. (Pathognomonic sign)
• Diagnosis: Blood culture (gram-positive bacilli), PCR, ELISA for anthrax toxin.
• Treatment: Ciprofloxacin 400 mg IV BD + clindamycin or linezolid (for antitoxin effect) ± antitoxin (raxibacumab). Duration: 60 days (to eliminate persisting spores).
• Mortality: >80% if untreated; ~50% even with treatment.

4. Pneumonic Plague (Yersinia pestis)

• Source: Primary plague — inhalation of respiratory droplets from infected humans/animals. Secondary plague — hematogenous seeding of lungs from bubonic/septicemic plague.
• Clinical Features: Rapid onset fever, cough with bloody/frothy sputum, dyspnea, cyanosis, respiratory failure within 24–48 hours.
• CXR: Bilateral bronchopneumonic infiltrates.
• Diagnosis: Blood/sputum culture, Gram stain (bipolar "safety pin" staining with Wayson/Giemsa), PCR, serology.
• Treatment: Streptomycin 1 g IM BD (drug of choice) or gentamicin. Alternatives: doxycycline, chloramphenicol (meningitis). Duration: 10 days. Respiratory isolation mandatory.
• Prognosis: Near 100% fatal if untreated.

5. Tularemia — Pneumonic Form (Francisella tularensis)

• Source: Contact with infected rabbits/rodents, tick bites, inhalation of contaminated dust or aerosols (typhoidal/pneumonic forms). Laboratory workers at risk.
• Clinical Features: Fever, headache, dry cough, pleuritic chest pain; may progress to ARDS.
• CXR: Oval opacities, hilar adenopathy, pleural effusion.
• Diagnosis: Culture (BSL-3 pathogen — lab must be warned), serology (agglutination titers), PCR.
• Treatment: Streptomycin 1 g IM BD × 10 days (drug of choice) or gentamicin. Doxycycline/ciprofloxacin for mild cases.

6. Hantavirus Pulmonary Syndrome (HPS)

• Source: Inhalation of dried urine, feces, saliva of infected deer mice (Peromyscus maniculatus) in the Americas. Sin Nombre virus (USA); Andes virus (South America — can spread person-to-person).
• Pathogenesis: Virus infects pulmonary endothelium → massive capillary leak → non-cardiogenic pulmonary edema.
• Clinical Features (Three Phases):
  1. Prodrome (3–5 days): Fever, myalgia, headache, GI symptoms.
  2. Cardiopulmonary phase: Abrupt onset dyspnea, bilateral pulmonary edema, hypotension, cardiogenic shock (myocardial depression). No prominent cough/sputum.
  3. Diuretic phase: Recovery (in survivors).
• Lab: Thrombocytopenia (pathognomonic), hemoconcentration, circulating immunoblasts, elevated LDH.
• CXR: Bilateral interstitial infiltrates ("batwing" pulmonary edema) — rapidly progressive.
• Diagnosis: Serology (IgM ELISA), PCR, immunohistochemistry.
• Treatment: Supportive — mechanical ventilation, ECMO for refractory cases. Ribavirin (limited benefit in HPS; more useful in hemorrhagic fever with renal syndrome). No proven antiviral.
• Mortality: 30–40%.

7. Avian Influenza (H5N1 / H7N9)

• Source: Direct contact with infected poultry or contaminated environments. Poultry farmers, live bird market workers.
• Clinical Features: Fever, cough, rapidly progressive pneumonia → ARDS. High case fatality rate (H5N1: ~60%; H7N9: ~30%).
• CXR/CT: Bilateral ground-glass opacities, consolidation progressing to "white-out."
• Diagnosis: RT-PCR from respiratory specimens (nasopharyngeal/tracheal aspirates).
• Treatment: Oseltamivir 75 mg BD × 5 days (higher doses and longer duration debated). Avoid amantadines (resistant). Supportive care, ICU.

8. Pulmonary Echinococcosis (Hydatid Disease of Lung)

• Organism: Echinococcus granulosus (cystic); E. multilocularis (alveolar — more aggressive).
• Source: Ingestion of tapeworm eggs from dogs (definitive host); sheep, cattle (intermediate hosts). Shepherds, farmers at risk.
• Pathology: Oncospheres reach lung via portal circulation/IVC → form hydatid cysts (3-layered: pericyst, ectocyst, endocyst/germinal layer).
• Clinical Features:
  • Small cysts: Asymptomatic.
  • Large/ruptured cysts: Cough, hemoptysis, chest pain, vomiting of hydatid material (hydatoptysis), urticaria, anaphylaxis on rupture.
• CXR/CT:
  • Intact cyst: Smooth, round, homogeneous opacity.
  • Air enters (crescent sign / Meniscus sign): Crescent of air above cyst.
  • Detached endocyst floating in fluid: "Water lily sign" (Camelotte sign) — pathognomonic.
  • Collapsed membranes: "Serpent sign".
• Diagnosis: CT scan, serology (ELISA, indirect hemagglutination), Casoni test (obsolete). Aspiration/biopsy contraindicated (risk of anaphylaxis and dissemination).
• Treatment:
  • Surgery (PAIR — Puncture, Aspiration, Injection of scolicidal agent, Re-aspiration): For accessible cysts.
  • Albendazole 400 mg BD × 1–6 months (pre/perioperative; or for inoperable cases).
• Complications: Rupture (anaphylaxis, dissemination, secondary infection).

9. Paragonimiasis (Paragonimus westermani)

• Source: Ingestion of raw/undercooked freshwater crabs and crayfish. Endemic in East/Southeast Asia, West Africa, Latin America.
• Life Cycle: Metacercariae penetrate gut wall → cross diaphragm → reach lungs → form cysts.
• Clinical Features: Chronic cough, rusty-brown (iron-colored) sputum, hemoptysis, pleuritic chest pain. Mimics pulmonary TB closely.
• CXR: Nodular shadows, ring shadows (worm cysts), pleural effusion, pleural thickening.
• Diagnosis: Sputum microscopy (operculated eggs), stool microscopy, serology (ELISA), CT.
• Treatment: Praziquantel 25 mg/kg TDS × 2 days (drug of choice). Alternative: triclabendazole.

10. Pulmonary Dirofilariasis (Dirofilaria immitis)

• Source: Canine heartworm transmitted to humans via mosquito bite. Humans are accidental/dead-end hosts.
• Pathology: Worm dies in pulmonary artery → thrombosis → infarction → granuloma formation.
• Clinical Features: Usually asymptomatic, presenting as a solitary pulmonary nodule (coin lesion) on CXR/CT. Occasionally: chest pain, hemoptysis, dry cough.
• CXR: Well-defined round opacity, typically <3 cm — mimics malignancy.
• Diagnosis: Excisional biopsy (both diagnostic and curative). Eosinophilia is not typically elevated.
• Treatment: Surgical excision of the nodule. Does not respond to antifilarial drugs.

11. Histoplasmosis (Histoplasma capsulatum)

• Source: Inhalation of microconidia from soil contaminated with bat or bird droppings (caves, old buildings, river valleys). Ohio-Mississippi river valley endemic.
• Clinical Forms:
  • Acute pulmonary: Flu-like illness, self-limited in immunocompetent.
  • Chronic pulmonary: Upper lobe cavities — mimics TB closely.
  • Disseminated: Immunocompromised (HIV, transplant patients).
• CXR: Diffuse miliary pattern (acute heavy exposure), mediastinal adenopathy, calcified granulomas ("buckshot calcifications").
• Diagnosis: Urine/serum Histoplasma antigen (most sensitive for disseminated), culture, serology, biopsy.
• Treatment: Mild — self-limited. Moderate-severe: Itraconazole 200 mg TDS × 3 days, then BD × 12 weeks. Severe/disseminated: Amphotericin B, then step down to itraconazole.

Summary Comparison Table — Key Exam Points

Exam Mnemonics

• "QPBATH-PDP" for bacterial/viral/fungal/parasitic list:
  • Q = Q fever, P = Psittacosis, B = Brucellosis, A = Anthrax, T = Tularemia, H = Hantavirus / Histoplasmosis, P = Plague, D = Dirofilariasis, P = Paragonimiasis
• Water lily sign = Hydatid (Echinococcosis)
• Widened mediastinum = Anthrax (most feared sign)
• Rusty sputum + operculated eggs = Paragonimiasis
• Solitary pulmonary nodule + dog exposure = Dirofilariasis
• Thrombocytopenia + ARDS + deer mouse = HPS

Important Clinical Pearls for MD Exam

1. Occupational/animal exposure history is the single most important clue in diagnosing pulmonary zoonoses.
2. Many zoonoses (Q fever, psittacosis, tularemia, plague) are Category A/B bioterrorism agents — important in differential of unexplained pneumonia clusters.
3. Aspiration of hydatid cyst is absolutely contraindicated due to risk of anaphylaxis and peritoneal seeding.
4. In HPS, thrombocytopenia is an early and consistent finding; it helps distinguish HPS from other atypical pneumonias.
5. Paragonimiasis is a classic mimic of pulmonary tuberculosis — always consider in endemic area patients with apparent TB not responding to ATT.
6. Dirofilariasis presents as a coin lesion on CXR and is often resected as a suspected carcinoma — diagnosis is histological.
7. Doxycycline is the backbone treatment for many intracellular bacterial zoonoses (Q fever, psittacosis, brucellosis, tularemia — mild).