Lung Flukes Pamatmat

Paragonimiasis is a food-borne parasitic infection caused by the lung fluke which can cause a sub-acute to chronic inflammatory disease of the lung. These flukes are larger than liver flukes and infect meat- or fish-eating animals as well as humans. Their life cycle is similar to that of liver flukes except that their encysted larvae infect crabs and crayfish rather than plants or fish. Humans can ingest the encysted metacercariae from drinking contaminated water or eating raw or undercooked crabs and crayfish.

Acute Symptoms: -from period of invasion and migration of flukes -abdominal pain, diarrhea and urticaria, followed roughly 1 to 2 weeks later by fever, pleuritic chest pain, cough and/or dyspnea. Chronic Symptoms: -pulmonary manifestations include cough, expectoration of discolored sputum, hemoptysis, and chest radiographic abnormalities. -Chronic pulmonary paragonimiasis, the most common clinical pattern, is frequently mild, with chronic cough, brown-tinged sputum (the color being caused by expectorated clusters of reddish brown eggs rather than by blood) and true hemoptysis.

Causative Agent Kingdom: Animalia Phylum: Platyhelminthes Class: Trematoda Family: Paragonimidae Genus: Paragonimus Species: P. westermani More than 30 species of trematodes (flukes) of the genus Paragonimus have been reported to infect animals and humans. Among them, more than 10 species reported to infect humans, the most common is P. westermani, the oriental lung fluke.

Lung fluke is most commonly encountered in parts of Asia, Africa and South America. Southeast Asia is more predominately more infected because of lifestyles. Raw seafood is popular in these countries.

Paragonimus westermani eggs range from 80 to 120 µm long by 45 to 70 µm wide. They are yellow-brown, ovoid or elongate, with a thick shell, and often asymmetrical with one end slightly flattened. At the large end, the operculum is clearly visible. The opposite (abopercular) end is thickened. The eggs are unembryonated when passed in sputum or feces.

Cercariae are often indistinguishable between species. There is a large posterior sucker, and the exterior is spined. Metacercaria: Metacercariae are usually encysted in tissue. The exterior is spined and has two suckers

Adult flukes are typically reddish brown and ovoid, measuring 7 to 16 mm by 4 to 8 mm, similar in size and appearance to a coffee bean.They are hermaphroditic, with a lobed ovary located anterior to two branching testes. Like all members of the Trematoda, they possess oral and ventral suckers.

1. The eggs are excreted unembryonated in the sputum, or alternately they are swallowed and passed with stool. 2. In the external environment, the eggs become embryonated, 3. Miracidia hatch and seek the first intermediate host, a snail, and penetrate its soft tissues. 4. Miracidia go through several developmental stages inside the snail : sporocysts , rediae , with the latter giving rise to many cercariae , which emerge from the snail.

5. The cercariae invade the second intermediate host, a crustacean such as a crab or crayfish, where they encyst and become metacercariae. This is the infective stage for the mammalian host. 6. Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that harbor metacercariae of the parasite. 7. 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 8. Develop into adults[8] (7.5 to 12 mm by 4 to 6 mm). 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.

Transmission of the parasite P. westermani to humans and mammals primarily occurs through the consumption of raw or undercooked seafood. In Asia, an estimated 80% of freshwater crabs carry P. westermani. In preparation, live crabs are crushed and metacercariae may contaminate the fingers/utensils of the person preparing the meal. Accidental transfer of infective cysts can occur via food preparers who handle raw seafood and subsequently contaminate cooking utensils and other foods. Consumption of animals which feed on crustaceans can also transmit the parasite Food preparation techniques such as pickling and salting do not exterminate the causative agent. For example, in Chinese study eating “drunken crabs” was shown to be particularly risky because the infection rate was 100% when crabs are immersed in wine for 3–5 minutes and fed to cats/dog.

Animals such as pigs, dogs, and a variety of feline species can also harbor P. westermani. Humans only become infeted after eating raw fresh water crabs that have been encysted with the metacerciaria.

Time from infection to oviposition (laying eggs) is 65 to 90 days. Infections may persist for 20 years in humans.

Once in the lung or ectopic site, the worm stimulates an inflammatory response that allows it to cover its self in granulation tissue forming a capsule. These capsules can ulcerate and heal over time. The eggs in the surrounding tissue become pseudotubercles. If the worm becomes disseminated and gets into the spinal chord, it can cause paralysis; capsules in the heart can cause death. The symptoms are localized in the pulmonary system, which include a bad cough, bronchitis, and blood in sputum (hemoptysis).

Praziquantel is the drug of choice to treat paragonimiasis. It works by paralyzing the flukes' suckers, forcing them to drop away from the walls of the host's blood vessels. Bithionol is an alternative drug for treatment of this disease but is associated with skin rashes and urticaria. Depending on the species of fluke and the severity of infection, the course of treatment can vary from several days to several weeks. Cure rates vary from 50-95%. Most patients experience mild temporary side effects from these drugs, including diarrhea, dizziness, or headache.

No vaccines have been developed that are effective against lung or liver fluke infections. Prevention of these infections includes the following measures: boiling or purifying drinking water avoiding raw or undercooked fish or salads made from fresh aquatic plants; all food eaten in areas with fluke infestations should be cooked thoroughly; pickling or smoking will not kill fluke cysts in fish or shellfish control or eradication of the snails that serve as the flukes' intermediate hosts