The genus Paragonimus has undergone considerable taxonomic splitting over the past 40 years. Paragonimus westermani is the most common and widely distributed of the species. Members of the genus are all lung flukes. The egg capsules make their egress from the body through sputum or feces. If the eggs are deposited in freshwater, after 16 days to several weeks of embryonation, the miracidia emerge seeking a snail of the family Thieridae, such as Semisulcospira libertina. Once the miracidium has penetrated the snail, it sheds its ciliated epidermis and transforms into a sporocyst. The sporocyst in turn gives rise to rediae which produce numerous cercariae. The microcercous cercariae are characterized by their tail which is reduced to a small button at the posterior end. The cercariae "inch" over wet rocks where they encounter and actively penetrate 11 or more species of crabs. The cercariae encyst in the crustacean's flesh as metacercariae. A wide range of carnivores serve as definitive or final hosts. Humans may also be infected when they eat raw or poorly cooked crabs. In Southeast Asia, a delicacy is known as "drunken crab". Crabs are marinated overnight in wine which coagulates the protein and adds flavor. Still, the metacercariae are unharmed and lead to infections. When metacercariae are ingested, they excyst in the duodenum, eventually making their way through the wall of the intestine, diaphragm and lung pleural to the spaces of the lungs.
The major forms of disease are pulmonary (lung) and extrapulmonary (outside of the lung, such as in the abdomen or brain). Infections with this parasite have occurred in Orange County in persons who ate raw freshwater Japanese crabs, known as sawagani or regal crabs (shown at right).
Egg of Paragonimus westermani. The average egg size is 85 µm by 53 µm (range: 68 to 118 µm by 39 to 67 µm). They are yellow-brown, ovoidal or elongate, with a thick shell, and often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs of P. westermani are excreted unembryonated
Metacercaria of Paragonimus westermani. This is the infective stage for the definitive, vertebrate host and is found in a crab or crayfish. The young fluke inside is well along in its development. 400x
Paragonomiasis. Chest radiograph showing multiple rounded lesions in both lung fields, many of them calcified.
lLarge, noncalcified Paragonimus cyst in the right frontoparietal lobe of a Korean who also had pulmonary paragonimiasis. Eggs of the parasite were identified in the sputum. (A) A right internal carotid arteriogram shows slight displacement of the anterior cerebral artery across the midline on frontal projection. (B) A pneumoencephalogram shows distortion of the right lateral venticle which is shifted slightly across the midline by the large space-occupying mass in the right frontoparietal area. This case is presented as a historical record only. Both of these diagnostic studies were accepted methods of neurodiagnosis in the 1950s and 1960s, but the advent of CT in the 1970s and MRI in the 1980s completely replaced pneumoencephalography (thankfully, as any radiologist from that era will attest) and relegated cerebral arteriography to a less prominent role, used where CT and MRI are unavailable.
lCT scan of cerebral paragonimiasis in the early active stage in a 38-year-old Korean women with headache and left-sided hemiparesis. (A) Contrast-enhanced CT scan shows a conglomerate of ring-shaped lesions in the right parietal lobe with surrounding edema. (B) Follow-up contrast-enhanced CT scan obtained 4 months after praziquantel therpy shows that the enhancing ring-shaped lesions and surrounding edema have markedly decreased in size.
lMRI of cerebral paragonimiasis in its early active stage in a 24-year-old Korean woman. Contrast-enhanced TI-weighted images show multiple conglomerate, ring-enhancing lesions with massive surrounding edema in the right frontal lobe. Note the mass effect distorting and displacing the frontal horn and body of the right lateral ventricle. At surgery multiple abscess-like cystic lesions containing yellowish pus-like material and necrotic debris were removed. Paragonimus eggs were found in the histological specimen from the resected lesions.
lA : Preoperative axial T1-weighted magnetic resonace image shows ring-enhancing large cystic mass with small eccentric nodule
and wide peritumoral edema in the right cerebral convexity,
B : Active form of Paragonimus
westermani is identified.
Paragonimus westermani: Oriental lung fluke.
A. Epidemiology: Asia, especially Korea, Japan, China, Taiwan, Philippines, Indonesia. Also endemic foci in West Africa and Latin America.
B. Mode of transmission: Cercariae encyst in crayfish and fresh water crabs (the second intermediate hosts). Humans accidentally ingest the metacercaria on eating undercooked crustaceans or their juices. Dogs, cats and pigs are definitive hosts as well as humans.
C. Clinical manifestations: Most infections are asymptomatic. Cough, lung abscess and eosinophilia may be present with heavy infections.
D.Pathology: Metacercaria excyst in duodenum, penetrate the intestinal epithelium and migrate to the lungs. Adult flukes mature into 12mm x 5mm forms in the lungs, causing the characteristic host inflammatory response with fibrotic capsule formation. Adults deposit eggs that rupture into the bronchioles, are coughed up and swallowed. Abscess, chronic bronchitis and pleural effusion can result.
E.Laboratory diagnosis: Examination of sputum or stool to demonstrate eggs.
F. Treatment: Praziquantel.