Common name. Lung fluke
Geographic distribution. The Genus Paragonimus composed of over 40 species. Human infections occur in restricted areas where local people consume improperly cooked freshwater crustaceans. Paragonimus westermani, a type species, causes inflammatory lung diseases in several Asian countries of Korea, Japan, China, Far-east of Russia, Taiwan, the Philippines, Malaysia, Indonesia, and India. P. miyazakii in Japan, P. skrjabini in China, P. heterotremus in Southeast Asia and in southern part of China, P. uterobilateralis and P. africanus in Africa, P. mexicanus in Latin America, and P. kellicotti in North America also invoke human infection. These species also invoke pleuropulmonary and subcutaneous infections.
Infection rate. The total number of persons infected worldwide is not determined. Annally about 200 cases and 500 cases are sporadically detected in Japan and Korea, respectively. The infection is also reported to be high in the Philippines and India.
Life cycle. Paragonimus is a typical digenean trematode of carnivorous mammals. Large immature eggs are spread throughout in stool, and mature to miracidia in freshwater. They infect snails in which they undergo three generations of asexual reproduction (redia, daughter redia, and cercaria). Microcercous cercariae are released into water where they infect freshwater crustaceans (crabs and crayfish). As encysted, they become metacercariae. The metacercariae are infective to definitive hosts. They also infect paratenic (transport) hosts. Carnivorous mammals are the most important definitive hosts. In case of P. westermani, rodent and boar are known to be transport hosts. The metacercariae excyst in the duodenum, penetrate the intestinal wall, and stay in the peritoneal cavity for a while. The parasites finally arrive to the lung in which they become adult worms approximately 8-10 weeks after metacercarial infection. They liberate eggs in sputum and stool. The worms are thought to thrive approximately 5 years in men. Main habitat of the parasite in the definitive hosts is the bronchiolar lumen and peribronchial tissues.
Morphology. Living adult worms are pinkish-brown in color and bean-shaped (7 to 15 mm in length, 3 to 8 mm in width, and 3 to 5 mm in thickness). It contains characteristic ovary in the middle part of the worm. The golden brown colored large immature eggs are approximately 45-60 x 80-100 µm. They contain operculum at one side and abopercular thickening at other side. The metacercariae in the 2nd intermediate host are spherical in shape measuring 220-450 µm.
Pathology and clinical symptoms. Usually no symptoms are observed when the parasites migrate in the peritoneal cavity. In rare cases of heavy infection, non-specific symptoms (such as easy fatiguability, myalgia, and mild fever) can be manifested. When parasites invade the lung, several symptoms including pleuritic and chest pain, cough, and rusty sputum may be present. Remarkable laboratory findings are peripheral eosinophilia, leukocytosis, and elevated serum levels of total IgE. Hydropneumothorax, parenchymal infiltration, and pleural effusion are the main pathologic changes. As time passes, worms are surrounded by thick fibrous granuloma. Generalized or localized fibrosis of the lungs and cystic dilatation of bronchi are the main histopathologic changes. The patients continue to complain about several symptoms. If not treated, the symptoms can last several years. Ectopic paragonimiasis results in granulomatous lesions in the organs other than the lung. The most commonly affected organs include liver, spleen, omentum, and ovary. Migrating subcutaneous lesions can also be developed. The most serious illness is cerebral paragonimiasis, which is associated with several neurological symptoms. Migrating worms frequently invoke subacute hematoma. Chronic cerebral paragonimiasis, which results from calcifying granuloma, is mainly manifested by partial seizure, headache, and homonymous hemianopia. Predilection sites of the cerebral involvement include temporal, occipital, and parietal lobes.
Diagnosis. Detection of eggs in feces is the diagnostic choice. Intradermal test is also available in large-scale based screening but not good for individual patient due to its low specificity. Enzyme-linked immunosorbent assay (ELISA) and immunoblot are reliable in detecting specific IgG antibodies. In case of cerebral paragonimiasis, CSF tests are highly recommended. Radiologic examination by high resolution chest CT provide definitive clue for paragonimiasis. Brain CT/MRI are also highly diagnostic either in active or in chronic calcified neuroparagonimiasis.
Treatment. Praziquantel is a drug of choice with more than 95% efficiency. Praziquantel is definitely effective in early stage of active cerebral paragonimiasis. Its adverse effects include headache, dizziness, gastrointestinal disturbance, and blurred vision, albeit mild and transient. Triclabenazole is also effective. Any significant side effect has not been reported. In the cases of chronic calcified cerebral paragonimiasis, surgical removal is recommended.
Prevention. Human infection is closely related with eating habits of foods. In many endemic regions, people acquire the infection by eating raw/undercooked freshwater crustaceans. Raw boar meat harboring excysted metacercariae are also infective.
Comments. Differential diagnosis from Fasciola eggs is necessary. Serological cross-reactions with other helminthiases such as clonorchiasis and fascioliasis should be accounted.
Egg of Paragominus westermani. 97 um x 55 um. x400.
Egg of Paragonimus westermani from a dog which was experimentally infected.
Metacercariae of Paragonimus westermani isolated from a crayfish.
Metacercaria of Paragonimus westermani isolated from a crayfish.
Adult worm of Paragonimus westermani.
Tai Soon Yong
Paragonimus westermani, adult worm. x4.
The immature worm of Paragonimus westermani from a naturally infected cat.
Semisulcospira libertina : first intermediate host of Paragonimus westermani
PR Chung/YK Park
Crayfish, Cambaroides similis, the second intermediate host of Paragonimus westermani.
Crayfish, Cambaroides similis.
Cambaroides similis, the intermediate host of Paragonimus westermani. Woon-Mok Sohn
An egg of Paragonimus westermani in a wet smear prepared from resected brain mass of a chronic paragonimiasis patient. 82 um long and 53 um wide. (Kang et al. 2000. Korean J Parasitol 38:167-71).
Dog lung infected with Paragonimus westermani shows worm nodules and an opening from which an adult worm escaped.
Dog lung infected with Paragonimus westermani shows multiple worm nodules.
Paragonimus westermani, paired in a worm capsule. x100.
Lung of a dog, which was experimentally infected with the metacercariae of P. westermani.
Magnetic resonance image shows a conglomerate of multiple ring-shaped cluster in the right lower temporo-occipital lobe of a chronic cerebral paragonimiasis patient. (Kang et al. 2000. Korean J Parasitol 38:167-71).
A computerized tomography shows multiple calcified density in the right frontal and temporal areas of chronic cerebral paragonimiasis patient. (Kang et al. 2000. Korean J Parasitol 38:167-71).