Paragonimiasis is a zoonois which is caused by a trematode called Paragonimus westermani. This parasite lives in the human lung, though some ectopic parasites are found in other locations, suggesting that this parasite is not well adapted to living in man. P. westermani is also found in the lungs of many other mammals. The disease is endemic in Korea, China, Japan, Burma, Vietnam, Cambodia and Laos. It also exists in South and Central America, West and Central Africa. Its reservoir hosts include dogs, cats, lions, leopards, tigers and pigs.
The adult fluke is robust reddish–brown and is covered with minute cuticular spines. It measures 7.5 to 12 mm long by 5 mm wide and 4 mm thick.
Adult trematodes lay golden brown, operculate eggs in the lungs. The eggs are passed out in the air passages of the lungs and are coughed up and spat out or swallowed. Those eggs that are swallowed are voided with the host’s faeces. The eggs take 17 to 21 days at 30 oC to hatch into motile ciliated miracidia. At lower temperatures, this period can extend up to 6 weeks. The intermediate hosts of P. westermani are snails of the genera Brotia, Thiara, Melania, and Semisulcospira. Once the miracidia find the right snail, they penetrate it and change into tube-like sporocysts. The sporocysts develop into first generation rediae. These first generation rediae or mother rediae, as they are also called, give rise to daughter rediae or second-generation rediae. Finally, thousands of cercariae emerge from the daughter rediae.
The cercariae that can live for up to 48 hrs do not swim but either crawl or are carried by currents in the water until they find a second intermediate host, usually a crab or a crayfish, which they actively penetrate and then migrate to the muscles or heart and encyst as metacercariae . About six weeks later, the metacercariae become infective.
Human infection is caused by eating raw crayfish or raw crabs. After ingestion, the metacercariae excyst within an hour of ingestion and the young worms penetrate the intestinal wall and make their way via the diaphragm into the pleural cavity. The worms may wander about and invade many other body organs, such as the brain, intestine, reproductive organs, spinal cord, scrotum, etc. Within 2 to 4 weeks post-infection, the worms arrive in the lungs, where they will undergo growth to maturity before shedding eggs. Not all the flukes reach the lungs; some end up in the brain, spleen, liver, and kidneys where they cause inflammation and abscesses.
Larvae migrating to the lungs from the intestine cause lesions along their path. Sometimes, they may stray into aberrant locations including the abdomen causing diarrhoea and abdominal pain, or the spinal cord and brain causing cerebral paragonimiasis. Cerebral paragonimiasis is a serious condition that is characterized by epilepsy, meningitis and other neurological complications.
The usual symptoms are chronic cough, severe stabbing pains in the chest and haemorrhage. The sputum is tinged with blood and its examination reveals the presence of operculate eggs. There may also be other symptoms such as fever, chills and sweating and loss of weight. In addition, there may be chest pains and dyspnoea, bronchiectasis and fibrosis of the lung tissue.
Humans acquire the infection by eating pickled or marinated infected crabs or crayfish. In endemic countries like China, crabs are sprinkled with salt and spices and eaten raw. The salt can kill some of the metacercariae but not all. Another common habit is to soak the crabs in wine overnight and eat them raw. Infection may also be acquired by eating salads contaminated with infected crabmeat. Some people in Japan and Korea use the juice prepared from the crayfish for medicinal purposes, while some communities in West Africa regard eating raw crabs as treatment for infertility.
Paragonimus exists as a zoonois in cats, dogs, pigs and a large number of many other animals. These animals are capable of transmitting infection to humans.
Diagnosis is done by recovery of eggs of P. westermani in sputum, faeces, and pleural aspirate or from abscesses.
Treatment with bithionol is effective and is the drug of choice. Chloroquine and praziquantel have also been used successfully. Infection may be prevented by avoiding eating raw crabs or crayfish as well as maintenance of sanitary conditions.