Paragonimus westermani
Paragonimus westermani is a lung fluke and is most prominent in Asia and South America. 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. It’s one of the more recognized lung flukes with the widest geographical range.
Morphology
Size, shape, and color resemble a coffee bean when alive. Adult worms are 7.5 mm to 12 mm long and 4 mm to 6 mm wide. The thickness ranges from 3.5 mm to 5 mm. The skin of the worm (tegument) is heavily covered with scale like spines. The oral and ventral suckers are similar in size, with the later placed slightly pre-equatorially. The excretory bladder extends from the posterior end to the pharynx. The lobed testes are adjacent from each other located at the posterior end, and the lobed ovaries are off-centered near the center of the worm (slightly postacetabular). The uterus is located in a tight coil to the right of the acetabulum, which is connected, to the vas deferens. The vitelline glands, which produce the yolk for the eggs, are widespread in the lateral field from the pharynx to the posterior end. By viewing the tegumental spines and shape of the metacercariae, one could distinguish between the ~30 species of Paragonumus spp.
-Eggs: 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 .
-Cercaria : 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
-Adults: 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.
Life cycle
Unembryonated eggs are passed in the sputum of a human or feline. Two weeks later, miracidum develop in the egg and hatches. The miracidum 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.
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. Miracida 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 .
Pathology
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 cord, 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).
Human infection with Paragonimus may cause acute or chronic symptoms, and manifestations may be either pulmonary or extrapulmonary.
-Acute Symptoms: The acute phase (invasion and migration) may be marked by diarrhea, abdominal pain, fever, cough, urticaria, hepatosplenomegaly, pulmonary abnormalities, and eosinophilia. The acute stage corresponds to the period of invasion and migration of flukes and consists of abdominal pain, diarrhea and urticaria, followed roughly 1 to 2 weeks later by fever, pleuritic chest pain, cough and/or dyspnea .
-Chronic Symptoms: During the chronic phase, 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 .
Confusion with TB Practitioners should think about TB in chronic patient ases with fevers, cough, weight loss. However if in endemic areas, think about paragonamiasis. Flukes occasionally invade and reside in the pleural space without parenchymal lung involvement.
“In contrast to tuberculosis, pulmonary paragonimiasis is only rarely accompanied by rales or other adventitious breath sounds. Many patients are asymptomatic, and symptomatic patients frequently look well despite a prolonged course.”
In pleural paragonimiasis symptoms may be minimal and diagnosiss complicated, since ova are not coughed/spit out or swallowed and there is frequently no cough. Such patients may develop pleural effusions and, because of the coendemicity with Mycobacterium tuberculosis (and co-infection in some patients), such effusions are often misdiagnosed as tuberculous
Extra-pulmonary locations of the adult worms result in more severe manifestations, especially when the brain is involved. Extra-pulmonary paragonimiasis is rarely seen in humans for the worms migrate to the lungs but cysts can develop in the brain and abdominal adhesions resulting from infection have been reported. Cysts may contain living or dead worms; a yellow-brownish thick fluid (occasionally hemmorgahic). When the worm dies or escapes, the cysts gradually shrink, leaving nodules of fibrous tissues and eggs which can calcify .
Diagnosis and treatment
The way to properly diagnose this parasite infection is by looking at the sputum and finding the eggs. Sometimes eggs are shed in the feces. Radiological methods can be used to X-ray the chest cavity and look for worms. This method is easily misdiagnosed, because pulmonary infections look like tuberculosis, pneumonia, or spirochaetosis. A lung biopsy can also be used to diagnose this parasite. An assay that detects worm antigens using monoclonal antibody can also be used for diagnosis. The drug of choice is praziquantel. The recommended dosage of 75 mg/kg per day, divided into 3 doses over 2 days has proven to eliminate P. westermani. Bithionol is an alternative drug for treatment of this disease but is associated with skin rashes and urticaria.