A 23-year-old patient has a history of travel to China and the consumption of pickled crab. He now complains of rusty sputum and cough. What is the most likely causative organisms?

Correct Answer: D. Paragonimus westermani

Paragonimus westermani is a lung fluke endemic to East Asia (China, Korea, Vietnam, Thailand) that causes paragonimiasis. The clinical triad of rusty/blood-tinged sputum (hemoptysis), chronic cough, and chest pain is pathognomonic for pulmonary paragonimiasis. Transmission occurs through consumption of raw or undercooked freshwater crustaceans (crabs, crayfish) containing metacercariae. The adult flukes lodge in the bronchioles and alveoli, causing chronic inflammation, tissue necrosis, and hemorrhage—explaining the characteristic rusty sputum. The patient's travel history to China and specific consumption of pickled crab (a traditional preparation that may not achieve adequate cooking temperatures) is the epidemiological clue. Unlike Fasciola species (which cause hepatic disease) or Schistosoma japonicum (which causes intestinal/hepatosplenic disease), Paragonimus westermani uniquely targets the lungs. The rusty sputum—a hallmark of pulmonary tuberculosis—is also seen in paragonimiasis and can lead to diagnostic confusion in endemic areas. Diagnosis is confirmed by finding operculated eggs in sputum or stool samples.

Why the other options are wrong

A. Fasciola buski — Fasciola buski is an intestinal fluke acquired from water plants (bamboo shoots, water chestnuts) in Southeast Asia. It causes intestinal obstruction, malabsorption, and protein-losing enteropathy—not pulmonary disease. Patients present with abdominal pain and diarrhea, not rusty sputum. This is a classic NBE trap pairing an East Asian parasite with pulmonary symptoms to test whether students confuse intestinal and pulmonary flukes. B. Fasciola hepatica — Fasciola hepatica is a hepatic fluke acquired from watercress and aquatic plants worldwide (including India). It causes fascioliasis with hepatomegaly, jaundice, and cholangitis—not respiratory symptoms. The acute phase presents with fever and eosinophilia; chronic phase with biliary obstruction. Rusty sputum is not a feature. Students may select this if they recall only that Fasciola is a fluke without distinguishing organ tropism. C. Schistosoma japonicum — Schistosoma japonicum is endemic to China and the Philippines but causes intestinal and hepatosplenic schistosomiasis, not pulmonary disease. Transmission is through contaminated freshwater (not food), and eggs lodge in the intestinal wall and liver, causing granulomatous inflammation, portal hypertension, and splenomegaly. Rusty sputum is not a clinical feature. The geographic overlap with China may mislead students unfamiliar with the specific organ tropism of each parasite.

High-Yield Facts

Mnemonics

Lung Fluke = Paragonimus Paragonimus = Pulmonary (rusty sputum, hemoptysis, cough). Remember: Paragonimus is the ONLY trematode that causes primary LUNG disease. All other flukes target liver (Fasciola) or intestine/liver (Schistosoma). Crustacean Craving = Paragonimus Raw crabs or crayfish → Paragonimus westermani. Fasciola comes from aquatic plants; Schistosoma from contaminated water. Food source is the key discriminator.

NBE Trap

NBE pairs an East Asian parasite (Schistosoma japonicum, Fasciola buski) with pulmonary symptoms to test whether students confuse organ tropism. The rusty sputum—a classic sign of TB—may also distract students into thinking of bacterial causes rather than parasitic lung involvement.

Clinical Pearl

In Indian clinical practice, paragonimiasis is rare but must be considered in patients with chronic hemoptysis and a travel history to East Asia—especially those who consume traditional pickled or undercooked crab dishes. Misdiagnosis as pulmonary TB is common in endemic regions; sputum microscopy for operculated eggs is the key differentiator.

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 48 (Trematodes); Robbins & Cotran Pathologic Basis of Disease Ch. 8 (Parasitic Infections)_