Among the more than 10 species reported to infect humans, the most common is Paragonimus westermani, the oriental lung fluke.

The vitelline glands, which produce the yolk for the eggs, are widespread in the lateral field from the pharynx to the posterior end.

P. westermani was discovered in the lungs of a human by Ringer in 1879[6] and eggs in the sputum were recognized independently by Manson and Erwin von Baelz in 1880.

Once eaten, the metacercaria excysts and penetrates the gut, diaphragm and lung where it becomes an adult worm in pairs.

In the next stage, the parasite miracidia hatch and invades the first intermediate host such as a species of freshwater snail.

Miracidia penetrate its soft tissues and go through several developmental stages inside the snail but mature into cercariae in 3 to 5 months.

Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite.

However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites.

[4] Transmission of the parasite P. westermani to humans and mammals primarily occurs through the consumption of raw or undercooked seafood.

[14] Consumption of animals that 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.

In Japan and Korea, the crab species Eriocheir is an important item of food as well as a notable second intermediate host of the parasite.

[4] Once in the lung or ectopic site, the worm stimulates an inflammatory response that allows it to cover itself in granulation tissue forming a capsule.

The symptoms are localized in the pulmonary system, which include a bad cough, bronchitis, and blood in sputum (hemoptysis).

[16] Diagnosis is based on microscopic demonstration of eggs in stool or sputum, but these are not present until 2 to 3 months after infection.

In the United States, detection of antibodies to Paragonimus westermani has helped physicians differentiate paragonimiasis from tuberculosis in Indochinese immigrants.

Case study:[17] 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 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].

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.

Flukes occasionally cause confusion when they invade the pleural space without entering the lung parenchyma.

[19][20][21] "In contrast to tuberculosis, pulmonary paragonimiasis is only rarely accompanied by rales or other adventitious breath sounds.

When the worm dies or escapes, the cysts gradually shrink, leaving nodules of fibrous tissues and eggs that can calcify.

[24] Other case studies: Prevention programs should promote more hygienic food preparation by encouraging safer cooking techniques and more sanitary handling of potentially contaminated seafood.

This recent study was conducted as a part of a broader effort to determine the status of Paragonimus species infection in Laos.

Consequently, several types of crabs were collected from markets and streams in a paragonimiasis endemic area for the inspection of metacercariae and were identified as the second intermediate host of the Paragonimus species.

In addition to this characteristic feature of the villagers' food culture, the denizens of this area drink fresh crab juice as a traditional cure for measles, and this was also thought to constitute a route for infection.