Certain Paragonimus species of flukes cause infections of the lungs.
People are infected when they swallow cysts containing fluke larvae in raw, undercooked, or pickled freshwater crabs or crayfish.
Most infected people do not have symptoms, but those who do may have diarrhea, abdominal pain, fever, cough, itching, and later, symptoms due to damage to the lungs and other organs.
Doctors diagnose the infection when they see eggs in a person's sputum or eggs from swallowed sputum in stool.
Flukes are parasitic flatworms. There are many species of flukes. Different species tend to infect different parts of the body. There are more than 30 species of Paragonimus, and 10 species can infect people, causing paragonimiasis. However, most cases of paragonimiasis are caused by
Paragonimus westermani
People get lung fluke infections when they swallow cysts containing immature flukes (larvae) in raw, undercooked, or pickled freshwater crabs or crayfish. These infections occur most often in Asia. (See also Overview of Parasitic Infections.) Other Paragonimus species cause paragonimiasis in Africa, Central and South America, and rarely in North America.
After the cysts are swallowed, the larvae leave the cyst, penetrate the wall of the intestine and enter the abdominal cavity. Then they pass through the diaphragm, and invade the lungs. There, they develop into adults and produce eggs. Adults can live 20 years if not treated.
The flukes may also go to the brain, liver, lymph nodes, skin, or spinal cord where they form cysts and produce eggs. However, the life cycle cannot be completed in these organs because the eggs have no way to exit the body.
From the lungs, eggs are passed in the sputum that is coughed up and spit out or swallowed and passed in stool. If the eggs enter fresh water, they hatch into larvae that are ingested by snails. Inside the snail, the larvae develop into a form (called cercariae) that can swim. Cercariae released from infected snails then infect crabs or crayfish and form cysts (called metacercariae).
Image from the Centers for Disease Control and Prevention Image Library.
See also the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) information on paragonimiasis.
Symptoms of Fluke Lung Infections
Most people with fluke lung infections do not develop symptoms. However, soon after getting a lung fluke infection, people may have diarrhea, abdominal pain, fever, cough, and itching.
Later, the infection damages the lungs the most but may affect other organs, including the skin. People slowly develop symptoms such as a chronic cough, chest pain, and difficulty breathing. They may cough up blood. Bumps may form on the skin.
If the brain is affected, people may have seizures, difficulty using or understanding language, or problems seeing. They may be paralyzed.
Diagnosis of Fluke Lung Infections
Examination of a sputum or stool sample
Sometimes blood tests to detect antibodies to the parasite
Imaging tests of the lungs
Doctors diagnose lung fluke infections when they see eggs in a person's sputum or stool. Sometimes a sample of fluid is removed from the lungs and checked for eggs. Eggs may be difficult to find because only a few are released at time and they are not released regularly. Laboratories may use special concentration techniques to help them identify the eggs.
Blood tests to detect antibodies to the parasite may be useful. (Antibodies are proteins produced by the immune system to help defend the body against attack, including that by parasites.)
Other tests, such as an x-ray or computed tomography (CT) of the chest, may be done to check for damage to the lungs or other organs.
Treatment of Fluke Lung Infections
A medication to eliminate the flukes from the body
Sometimes surgery
If the brain is infected, corticosteroids may also be given. They help control the inflammation that develops when the medication kills the flukes. Antiseizure drugs are used to control seizures.
Sometimes surgery is needed to remove bumps on the skin or rarely cysts in the brain.
Prevention of Fluke Lung Infections
For travelers to areas where lung fluke infections occur, prevention involves not eating raw or undercooked freshwater crabs and crayfish.
