Parasites In Humans
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Paragonimus Westermani - Lung Fluke

Human lung fluke, Paragonimus westermani, infects 22 million people in Africa, Asia and South and Central America. Southeast Asia in particular is affected because raw seafood is very popular there. Humans get infected with the disease, paragonimiasis, by eating raw crabs or fish that are carrying the parasite. Even properly cooked sushi can cause infection, if the cook or waiter is careless when preparing the food. In Asia about 80 % of freshwater crabs are infected with the lung fluke.

Life cycle of a lung fluke begins, when the female lays eggs that are carried out from the human lungs in the sputum by the motion of microvilli. Then the eggs are taken through the gastrointestinal tract and out of the body. If the feces get in contact with water, then after two weeks larvae called miracidia hatch and start to grow. A miracidium finds a snail and penetrates its skin. In 3–5 months miracidium develops further and produces another larval form called cercaria. The cercaria crawls out of the snail to find fresh water crayfish (a lobster-like creature) or crabs. It finds its way to the muscles of the crab and starts forming a cyst. Within two months it transforms into metacercaria which is the resting form of cercaria. If a human eats this infected crab raw, the metacercaria cyst gets into the stomach. Once inside the beginning of the small intestine, duodenum, the metacercaria excysts and penetrates the intestinal wall. It continues through abdominal wall and diaphragm into the lungs where it forms a capsule and develops into an adult. Male and female lung worms reproduce and the cycle starts again.

Sometimes lung fluke larvae accidentally travel to the brain or other organs and reproduce there. But because the secretion of the eggs from the brain is blocked the life cycle will not happen. If the worm goes to the spinal cord instead of the lungs, the host might become paralyzed. If it infects the heart, the host could die.

Lung flukes cause pain and severe coughing (there might be some blood, too). Paragonimiasis diagnosis is done by looking at sputum (slime from the lungs), to see if there are any lung fluke eggs. Feces can be examined, too. Alternatively X-rays and biopsies can be taken. Paragonimiasis is usually treated with a drug called praziquantel.

Salting food does not kill the parasite, cooking and freezing will. After ingestion it takes about three months for the lung fluke to start laying eggs. The host might stay infected up to 20 years.

Paragonimus westermani life cycle

Paragonimus westermani body parts adult lung flukeParagonimus westermani (adult) Paragonimus westermani egg (egg)

Adult lung flukes are 4–6 mm wide, 3–5 mm thick and 7–12 mm long. They are red-brown looking almost like a coffee bean. They hold on to tissue with two suckers. The oral sucker is in the front and just before the center of its lower body is the ventral sucker.

In addition to humans, Paragonimus westermani infects other carnivores such as felids (cats etc.), canids (dogs etc.), rodents (rats etc.), weasels and pigs.