These larvae released into water and penetrate crabs, crayfish and other crustaceans (second intermediate host). The cercariae situate themselves into the gills, liver and muscles where they further develop into metacercariae. When the parasite-filled crustacean is eaten, the metacercariae hatch in the intestine. These young worms penetrate intestinal wall, peritoneum, the diaphragm and the pleura where they finally reach the lungs. Here they live in pairs, lay eggs that are coughed up in sputum to restart the cycle. Geographic distribution
There are more than 30 known species of Paragonimus. Species of Paragonimus are widely distributed in Asia, Africa, and North and South America. P. westermani is found in southeast Asia and Japan, while P. kellicotti is endemic to North America. P. africanus is found in Africa and P. mexicanus is found in central and South America. Just as the species names imply, paragonimiasis  is more prominent in Asians, Africans and Hispanics because of their habitats and cultures. Prominence increases with age from older children to young adults then decreases with age. It is also higher among the female populations. This is a very common parasite of crustacean-eating mammals. Symptoms and diagnosis
Paragonimiasis causes pneumonia with characteristic symptoms including prolonged cough, chest pain, shortness of breath, and hemoptysis. Owing to the diverse symptoms it presents, the disease is variously known as endemic haemoptysis, oriental lung fluke infection, pulmonary distomiasis, parasitical haemoptysis, and parasitare haemopte. Pulmonary paragonimiasis is the most common clinical manifestation, accounting for 76 to 90% of all infections. It has the classic symptoms of pneumonia.
The flukes can also reach other organs and tissues, such as the brain and skeletal muscles. However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites. Time from infection to laying of eggs is 65 to 90 days. Infections may persist for 20 years in humans. Animals such as pigs, dogs, and a variety of feline species can also harbor P. westermani. For other species, rodents and deer are also additional (paratenic) hosts. By consuming infected animals of these reservoir species, even animals and humans that do not eat crustaceans directly can become infected. Background
The first human case was seen in 1879 in Taiwan. An autopsy was done and adult trematodes were found in the lungs. The adult flukes have a reddish-brown in color with an ovoid shape. They have two muscular suckers, the first an oral sucker located anteriorly and the second a ventral sucker located mid-body. The adult flukes can live up to 20 years. The eggs are golden brown in color and are asymmetrically ovoid. They have a very thick shell. As seen above, these trematodes have a very complex life cycle with seven distinct phases involving intermediate hosts and humans. These seven phases are outlined as follows: eggs reach fresh water where they develop into miracidia. These penetrate many species of aquatic snails (first intermediate host) where they go through three distinct stages: first sporocysts, then rediae, and finally cercariae, also referred to as the larvae.