Paragonimus westermani is the major species of lung fluke that infects humans, causing paragonimiasis. The species sometimes is called the Japanese lung fluke or oriental lung fluke. Human infections are most common in eastern Asia and in South America. Paragonimus westermani was discovered when two Bengal tigers died of paragonimiasis in zoos in Europe in 1878. Several years later, infections in humans were recognised in Formosa.
More than 30 species of trematodes (flukes) of the genus Paragonimus have been reported to infect animals and humans. Among the more than 10 species reported to infect humans, the most common is Paragonimus westermani, the oriental lung fluke.
Transmission of the parasite P. westermani to humans and mammals primarily occurs through the consumption of raw or undercooked seafood. In Asia, an estimated 80% of freshwater crabs carry P. westermani. In preparation, live crabs are crushed and metacercariae may contaminate the fingers/utensils of the person preparing the meal. Accidental transfer of infective cysts can occur via food preparers who handle raw seafood and subsequently contaminate cooking utensils and other foods. Consumption of animals which feed on crustaceans can also transmit the parasite, for cases have been cited in Japan where raw boar meat was the source of human infection. Food preparation techniques such as pickling and salting do not exterminate the causative agent. For example, in a Chinese study eating "drunken crabs" was shown to be particularly risky because the infection rate was 100% when crabs are immersed in wine for 3–5 minutes and fed to cats/dog.
Animals such as pigs, dogs, and a variety of feline species can also harbor P. westermani.
Time from infection to oviposition (laying eggs) is 65 to 90 days. Infections may persist for 20 years in humans.
An 11½-year-old Hmong Laotian boy was brought into the emergency room by his parents with a 2- to 3-month history of decreasing stamina and increasing dyspnea [shortness of breath] on exertion. He described an intermittent nonproductive cough and decreased appetite and was thought to have lost weight. He denied fever, chills, night sweats, headache, palpitations, hemoptysis [coughing up blood], chest pain, vomiting, diarrhea or urticaria [skin rash notable for dark red, raised, itchy bumps]. There were no pets at home. At the time of immigration to the United States 16 months earlier, all family members had negative purified protein derivative intradermal tests except one brother, who was positive but had a normal chest radiograph and subsequently received isoniazid for 12 months... a left lateral thoracotomy was performed during which 1800 ml of an odorless, cloudy, pea soup-like fluid containing a pale yellow, cottage cheese-like, proteinaceous material was removed, along with a solitary, 6-mm-long, reddish brown fluke subsequently identified as Paragonimus westermani
Human infection with Paragonimus may cause acute or chronic symptoms, and manifestations may be either pulmonary or extrapulmonary.
Acute symptoms: The acute phase (invasion and migration) may be marked by diarrhea, abdominal pain, fever, cough, urticaria, hepatosplenomegaly, pulmonary abnormalities, and eosinophilia. The acute stage corresponds to the period of invasion and migration of flukes and consists of abdominal pain, diarrhea and urticaria, followed roughly 1 to 2 weeks later by fever, pleuritic chest pain, cough and/or dyspnea. Chronic Symptoms: During the chronic phase, pulmonary manifestations include cough, expectoration of discolored sputum, hemoptysis, and chest radiographic abnormalities. Chronic pulmonary paragonimiasis, the most common clinical pattern, is frequently mild, with chronic cough, brown-tinged sputum (the color being caused by expectorated clusters of reddish brown eggs rather than by blood) and true hemoptysis.