Paragonimus westermani

**Classification:** Trematode (fluke), a hermaphroditic flatworm. Endemic in East and Southeast Asia (China, Korea, Japan, Philippines), with related species in West Africa and the Americas (P. kellicotti in North America, acquired from crayfish).

**Life cycle:** 1. Eggs are coughed up in sputum or swallowed and passed in stool. They reach fresh water. 2. A miracidium hatches and infects the first intermediate host, a freshwater snail (Semisulcospira). 3. Cercariae are released and encyst as metacercariae in the second intermediate host, a freshwater crab or crayfish. 4. A human eats raw, undercooked, or pickled/"drunken" crab or crayfish. Metacercariae excyst in the duodenum. 5. Juveniles penetrate the intestinal wall, cross the peritoneum and diaphragm, and enter the lung parenchyma. There they form fibrotic cysts, usually in pairs, and mature into adults. 6. Adults produce eggs, which are released into the bronchioles.

**Pathophysiology:** Adult worms in the lung provoke an eosinophilic and granulomatous inflammatory response. This produces fibrous cysts that communicate with bronchioles.

**Clinical features:** - Chronic productive cough. - Hemoptysis, with rusty or chocolate-brown sputum from eggs and blood. This is classic. - Chest pain and dyspnea. - Peripheral eosinophilia, especially early. - Imaging shows nodules, cavities, ring shadows, or pleural effusion. It is often misdiagnosed as tuberculosis, so consider it in TB-negative patients with an appropriate exposure history. - Ectopic disease: the brain is the most important site (seizures, meningitis, a mass lesion), and the skin and abdomen can also be involved.

**Diagnosis:** - Operculated, golden-brown eggs in sputum, stool, or bronchoalveolar lavage fluid. - Serology (ELISA, immunoblot) when eggs are not found. - Eosinophilia and elevated IgE.

**Treatment:** Praziquantel (first line). Triclabendazole is an alternative. Bithionol is an older option.

**Prevention:** Cook crustaceans thoroughly and avoid eating them raw or pickled.

**High-yield board associations:** - Source: raw crab or crayfish → lung disease with hemoptysis that mimics TB. - Compare with other flukes: - *Clonorchis sinensis* (raw freshwater fish) → biliary disease and cholangiocarcinoma. - *Fasciola hepatica* (watercress) → liver disease. - *Schistosoma* (skin penetration by cercariae in fresh water) → hepatosplenic disease (S. mansoni, S. japonicum), hematuria and squamous cell bladder cancer (S. haematobium). - Praziquantel works by increasing calcium permeability in the parasite, causing tetanic paralysis.