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Human paragonimiasis is acquired through ingestion of raw or undercooked crabs or crayfish, and is usually a lung infection. After ingestion, metacercariae excyst in the small intestine and release larvae that penetrate the duodenal wall and enter the peritoneal cavity. The larvae migrate for approximately 1 week, then penetrate the diaphragm, enter the pleural cavity, and migrate directly through lung tissue to reach the bronchi. There they form cystic cavities and develop into adult worms in 5-6 weeks. The adult parasites are reddish brown and ovoid, measuring 7.5-12 mm by 4-6 mm. Adult worms induce an inflammatory response in the lungs, generating a fibrous cyst that contains a purulent, bloody effusion and eggs released by the flukes which are passed into the environment via expectoration, or may be swallowed and passed with feces. When deposited in fresh water, eggs hatch to release miracidiae, which then invade specific snail hosts. Thousands of cercariae are later released from the infected snail, which encyst (as metacercariae) in the gills, muscles, legs, and viscera of freshwater crustaceans (crabs or crayfish).
The clinical picture of chronic paragonimiasis resembles chronic bronchitis or tuberculosis. Persons may cough up coffee-colored or blood-tinged sputum, often accompanied by chest pain and/or shortness of breath. The sputum may be peppered consisting of clumps of eggs produced by the adult fluke living in the lung.
Peripheral eosinophilia is common and can be intense, especially during the early larval migration stages. Many patients have a spectrum of abnormalities on chest radiographs: lobar infiltrates, coin lesions, cavities, calcified nodules, hilar enlargement, pleural thickening and effusions. Ring-shaped opacities of contiguous cavities giving the characteristic appearance of a bunch of grapes are highly suggestive of pulmonary paragonimiasis. Central nervous system disease may provide similar “grapebunch” findings, characteristically seen in the temporal and occipital lobes on computed tomography of the brain. CNS involvement occurs in up to 25% of hospitalized patients and may be associated with Paragonimus-induced meningitis. CNS symptoms may include headaches, seizures, and visual disturbances. Paragonimus flukes may also invade the liver, spleen, intestinal wall, peritoneum, and abdominal lymph nodes.
Sputum examined microscopically may reveal Paragonimus eggs released by the flukes in the lungs. Keep in mind that the acid-fast stain that is used for TB testing of sputum destroys eggs. The eggs may also be found by multiple stool exams on different days as a result of coughed-up eggs that are swallowed. The microscopic eggs are yellowish brown, 80-120 µm long by 45-70 µm wide, thick-shelled, and with an obvious operculum. Serologic tests can be especially useful for early infections (prior to maturation of flukes) or for ectopic infections where eggs are not passed in stool.
Ectopic lesions from aberrant migration of flukes can involve any organ, including abdominal viscera, the heart, and the mediastinum. The infection can also affect the liver, spleen, abdomen, and skin. The most clinically recognizable ectopic lesions arise from cerebral paragonimiasis, which, in highly endemic countries, more commonly affects children. These children present with eosinophilic meningoencephalitis, seizures, or signs of space-occupying lesions. Many patients with central nervous system disease also have pulmonary infections. P. skrjabini often produces skin nodules, subcutaneous abscesses, or a type of creeping eruption known as “trematode larva migrans.”
Praziquantel is the drug of choice: adult or pediatric dosage, 25 mg/kg given orally three times per day for 2 consecutive days. For cerebral disease, a short course of corticosteroids may be given with the praziquantel to help reduce the inflammatory response around dying flukes.
Alternative: Triclabendazole, 2 doses of 10 mg/kg given 12 hours apart in patients 6 years of age and older.
Praziquantel is pregnancy category B. There are no adequate and well-controlled studies in pregnant women. However, the available evidence suggests no difference in adverse birth outcomes in the children of women who were accidentally treated with praziquantel during mass prevention campaigns compared with those who were not. In mass prevention campaigns for which the World Health Organization (WHO) has determined that the benefit of treatment outweighs the risk, WHO encourages the use of praziquantel in any stage of pregnancy. For individual patients in clinical settings, the risk of treatment in pregnant women who are known to have an infection needs to be balanced with the risk of disease progression in the absence of treatment.
Pregnancy Category B: Either animal-reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).
Praziquantel is excreted in low concentrations in human milk. According to WHO guidelines for mass prevention campaigns, the use of praziquantel during lactation is encouraged. For individual patients in clinical settings, praziquantel should be used in breast-feeding women only when the risk to the infant is outweighed by the risk of disease progress in the mother in the absence of treatment.
The safety of praziquantel in children aged less than 4 years has not been established. Many children younger than 4 years old have been treated without reported adverse effects in mass prevention campaigns and in studies of schistosomiasis. For individual patients in clinical settings, the risk of treatment of children younger than 4 years old who are known to have an infection needs to be balanced with the risk of disease progression in the absence of treatment.
There are no available data on the use of triclabendazole in pregnant women to inform a drug-associated risk for major birth defects, miscarriage, or adverse maternal or fetal outcomes. See additional perspective in the product labelpdf iconexternal icon.
According to the product labelpdf iconexternal icon, there are no data on the presence of triclabendazole in human milk, the effects on the breastfed infant, or the effects on milk production. Published animal data indicate that triclabendazole is detected in goat milk when administered as a single dose to one lactating animal. When a drug is present in animal milk, it is likely that the drug will be present in human milk. The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for triclabendazole and any potential adverse effects on the breastfed infant from the medication or from the underlying maternal condition.
According to the product labelpdf iconexternal icon, which addresses treatment of fascioliasis, the safety and effectiveness of triclabendazole have been established for pediatric patients aged 6 years and older (the age group for which the drug has been approved by FDA for treatment of fascioliasis) but have not been established for younger patients.