Paragonimus lung diseases for md exam answer 10 marks from fishman and Murray only

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Paragonimiasis (Lung Fluke Disease) - 10 Mark Answer

Sources: Fishman's Pulmonary Diseases and Disorders, 5e & Murray & Nadel's Textbook of Respiratory Medicine, 7e

Definition and Etiology

Paragonimiasis is a parasitic lung infection caused by trematodes (flukes) of the genus Paragonimus. The principal pathogen in humans is P. westermani (the Oriental lung fluke), responsible for >90% of cases worldwide, predominantly in Asia. In North America, P. kellicotti is the only indigenous species and causes most locally acquired infections through crayfish ingestion. Other species - P. africanus, P. uterobilateralis, P. heterotremus - cause disease in Africa and Southeast Asia. An estimated 25 million people are infected globally, with 300 million at risk.

(Fishman's, p. 1219; Murray & Nadel's, p. 1347)

Life Cycle and Pathogenesis

The life cycle is complex, requiring two intermediate hosts:

Definitive host: Humans, cats, dogs, wild boar, and other mammals
First intermediate host: Freshwater snails (cercariae develop here)
Second intermediate host: Crustaceans - crabs or crayfish (metacercariae encyst here)

Infection route: Humans are infected by eating raw, partially cooked, or pickled crustaceans containing encysted metacercariae. The metacercariae excyst in the duodenum, penetrate the duodenal/intestinal wall, cross the peritoneal cavity, migrate through the diaphragm, and enter the pleural space and lung parenchyma, where they mature to adult worms.

Adult worm behavior: Pairs of adult worms live in cystic cavities near bronchial passages and produce large, thick-shelled operculated eggs (~80 x 50 μm). Cystic cavities eventually rupture into bronchioles, allowing eggs to be expectorated or swallowed and passed in feces. Unembryonated eggs released into freshwater infect snails, completing the cycle.

Extrapulmonary migration: Paragonimus has a unique tropism for the lung, but aberrant migration can occur to the brain (most common extrapulmonary site), skin, and peritoneum.

(Murray & Nadel's, p. 1347-1348; Fishman's, p. 1219)

Clinical Features

The clinical syndrome reflects the parasite's migratory path and host immune interaction. Symptoms typically begin 2-16 weeks after ingesting infective larvae.

Acute Phase

Fever, abdominal pain, diarrhea, urticaria
Chest pain, cough
Peripheral eosinophilia and elevated serum IgE (present in 80% of infected patients)
Many acute infections are asymptomatic

Chronic/Late Phase (5-10 years after exposure)

Chronic cough productive of thick, rusty-colored or blood-tinged sputum containing Charcot-Leyden crystals
Hemoptysis - may be frank and resemble that seen in tuberculosis (this is the classic TB mimic)
Fever and eosinophilia are often absent at this stage
Pleuritic chest pain
Pneumonia, bronchiectasis, and vasculitis may be present

Pleural Disease

Pleural involvement is common - in a series of 71 patients, 61% had pleural disease:

Unilateral pleural effusion (most common)
Bilateral effusion
Unilateral or bilateral hydropneumothorax
Pleural thickening

Characteristic pleural fluid in paragonimiasis (unique and diagnostically important):

Exudative
Glucose < 10 mg/dL (very low - < 0.56 mmol/L)
LDH > 3 times upper limit of normal for serum
pH < 7.10
High percentage of eosinophils on cell differential
May contain cholesterol crystals or chyle

This combination of low pH + low glucose + eosinophilia is found in only TWO conditions: (1) Paragonimiasis, and (2) Eosinophilic granulomatosis with polyangiitis (EGPA). This is a classic exam fact.

(Murray & Nadel's, p. 1348-1349; block 25)

Diagnostic Investigations

Laboratory

Blood eosinophilia and raised IgE - present in ~80%
Definitive diagnosis: Detection of Paragonimus eggs in sputum, stool, BAL fluid, gastric aspirate, or pleural fluid - however, eggs are detected in only 11.7% of cases (Japan series)
Bloody sputum is most likely to yield positive results
Eggs are operculated, golden-brown, ~80 x 50 μm (see Fig. 58.4D in Murray & Nadel's)
ELISA and immunoblot assays: Most sensitive; used when egg detection fails
Complement fixation titer > 1:8 for P. westermani is strongly suggestive

Pathology

Lung biopsy: Fibrous cysts (1-3 mm) adjacent to or communicating with airways, containing adult worms
Granulomas with eggs at the center (see Fig. 58.4B)
Acute and chronic changes may coexist within the same lesion

Radiology

Chest X-Ray may show:

Minimal perihilar linear/reticular opacities (early)
Consolidation or focal parenchymal infiltrates
Cavitary lesions up to 4 cm in diameter
Small cysts; calcified or non-calcified nodules
Pleural effusion, pneumothorax, pleural thickening (minority)

CT Scan (more sensitive, Fishman's):

Pleural effusion - most common CT finding
Early: parenchymal consolidations (during lung penetration)
Late: peripheral nodules or worm-containing cysts
Serpentine worm migration tracks
Pneumothorax, lymphadenopathy

(Fishman's, p. 1219; Murray & Nadel's, p. 1347-1349)

Pathology Image (Murray & Nadel's, Figure 58.4)

Paragonimus westermani - CXR, granuloma with ova, cross-section of adult pair in lung, ovum in sputum

Figure 58.4 (Murray & Nadel's): (A) CXR showing perihilar linear and reticular opacities in a patient with hemoptysis; (B) Granuloma surrounding ova; (C) Cross-section of a pair of adult worms in the lung; (D) Operculated ovum in sputum.

Differential Diagnosis

The chief differential is pulmonary tuberculosis - paragonimiasis can be mistaken for TB due to:

Similar geographic distribution (Asia)
Chronic cough, hemoptysis, cavitary lesions on CXR
Night sweats and constitutional symptoms

Other differentials: lung abscess, bronchiectasis, lung cancer (when nodules/cavities are present).

Treatment

Drug of choice: Praziquantel - 75 mg/kg/day in 3 divided doses for 2 days
Treatment failure is rare
Untreated pulmonary paragonimiasis may resolve within 5-10 years (the lifespan of adult worms), but chronic infection may lead to extensive pulmonary fibrosis

(Fishman's, p. 1219; Murray & Nadel's, p. 1349)

Prevention

Avoid eating raw, undercooked, or pickled crustaceans (crabs, crayfish)
Adequate cooking destroys metacercariae

Summary Table

Feature	Details
Causative agent	Paragonimus westermani (Asia), P. kellicotti (North America)
Mode of infection	Eating raw/undercooked crabs or crayfish
Site of adult worm	Cystic cavities near bronchi in lung parenchyma
Egg size	~80 x 50 μm, operculated, thick-shelled
Classic presentation	Hemoptysis + Rust-colored sputum + Eosinophilia
TB mimic	Yes - chronic hemoptysis + cavitation
Pathognomonic pleural fluid	Low glucose (<10) + Low pH (<7.1) + Eosinophils
Diagnosis	Eggs in sputum/stool; ELISA; complement fixation >1:8
CT finding	Pleural effusion, nodules, worm cysts, serpentine tracks
Treatment	Praziquantel 75 mg/kg/day x 2 days
Extrapulmonary sites	Brain > skin > peritoneum

Sources:

Murray & Nadel's Textbook of Respiratory Medicine, 7e - Chapter 58 (Parasitic Infections), p. 1347-1349
Murray & Nadel's Textbook of Respiratory Medicine, 7e - Chapter (Pleural Disease), p. (block 25, pleural paragonimiasis section)
Fishman's Pulmonary Diseases and Disorders, 5e - p. 1219-1220

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