Paragonimus Westermani - Life Cycle

Life Cycle

Unembryonated eggs are passed in the sputum of a human or feline. Two weeks later, miracidia develop in the egg and hatches. The miracidia penetrate its first intermediate host (snail). Within the snail mother sporocyst form and produce many mother rediae, which subsequently produce many daughter rediae which shed crawling cercariae into fresh water. The crawling cercariae penetrate fresh water crabs and encyst in its muscles becoming metacercaria. Humans or felines then eat the infected crabs raw. Once eaten, the metacerciaria excysts and penetrates the gut, diaphragm and lung where it becomes an adult worm in pairs.

The first intermediate hosts of the Paragonimus westermani are freshwater snails:

• Semisulcospira amurensis
• Semisulcospira calculus
• Semisulcospira cancellata
• Semisulcospira extensa
• Semisulcospira gottschei
• Semisulcospira libertina - synonym: Semisulcospira toucheana
• Semisulcospira mandarina - synonym: Semisulcospira wegckiangensis
• Semisulcospira multicincta
• Semisulcospira nodiperda
• Semisulcospira nodiperda quinaria
• Semisulcospira paucincta
• Semisulcospira peregrinomum

For many years Tarebia granifera was believed to be an intermediate host for the Paragonimus westermani, but Michelson showed in 1992 that this was erroneous.

Paragonimus has a quite complex life-cycle that involves two intermediate hosts as well as humans. Eggs first develop in water after being expelled by coughing (unembryonated) or being passed in human feces. In the external environment, the eggs become embryonated. 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. Cercariae next invade the second intermediate host such as crabs or crayfish and encyst to develop into metacercariae within 2 months. Infection of humans or other mammals (definitive hosts) occurs via consumption of raw or undercooked crustaceans. Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite. The metacercariae excyst in the duodenum, penetrate through the intestinal wall into the peritoneal cavity, then through the abdominal wall and diaphragm into the lungs, where they become encapsulated and develop into adults. The worms can also reach other organs and tissues, such as the brain and striated muscles, respectively. However, when this takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites.