• The trematodes, or flatworms, may be classified according to the tissues invaded by the adult stage of the fluke: blood, biliary tree, intestines, or lungs.
• The life cycle involves a definitive mammalian host (e.g., humans), in whom adult worms produce eggs through sexual reproduction, and an intermediate host (e.g., snails), in which miracidial forms undergo asexual reproduction to form cercariae. Worms do not multiply within the definitive host.
• Human infection results from either direct penetration of intact skin or ingestion.
• Infections with trematodes that migrate through or reside in host tissues are associated with a moderate to high degree of peripheral-blood eosinophilia.

Schistosomiasis

Microbiology and Epidemiology Five species cause human schistosomiasis: Schistosoma mansoni, S. japonicum, S. mekongi, and S. intercalatum cause intestinal and hepatic schistosomiasis, and S. haematobium causes urogenital schistosomiasis.

• After infective cercariae penetrate intact skin, they mature into schistosomula and migrate through venous or lymphatic vessels to the lungs and ultimately the liver parenchyma. Sexually mature worms migrate to the veins of the bladder and pelvis (S. haematobium) or the mesentery (S. mansoni, S. japonicum, S. mekongi, S. intercalatum) and deposit eggs.
  • - Some mature ova are extruded into the intestinal or urinary lumina, from which they may be voided and ultimately may reach water and perpetuate the life cycle.
  • - The persistence of ova in tissues leads to a granulomatous host response and fibrosis.
• These blood flukes infect 200-300 million persons (mostly children and young adults) in South America, the Caribbean, Africa, the Middle East, and Southeast Asia.

Clinical Manifestations

Schistosomiasis occurs in three stages that vary by species, intensity of infection, and host factors (e.g., age, genetics).

• Cercarial invasion, most often with S. mansoni and S. japonicum infections, causes a pruritic maculopapular rash (“swimmers' itch”) 2-3 days later.
• Acute schistosomiasis (Katayama fever) presents 4-8 weeks after skin invasion as a serum sickness-like illness characterized by fever, generalized lymphadenopathy, hepatosplenomegaly, and significant eosinophilia.
• Chronic schistosomiasis causes manifestations that depend primarily on the schistosome species.
  • - Intestinal species cause colicky abdominal pain, bloody diarrhea, anemia, hepatosplenomegaly, portal hypertension, and esophageal varices with bleeding.
  • - Urinary species cause dysuria, frequency, hematuria (sometimes only at the end of voiding), obstruction with hydroureter and hydronephrosis, fibrosis of bladder granulomas, and late development of squamous cell carcinoma of the bladder.
  • - Pulmonary disease (e.g., endarteritis obliterans, pulmonary hypertension, cor pulmonale) and CNS disease (e.g., Jacksonian epilepsy, transverse myelitis) can occur and are due to granulomas and fibrosis.

Diagnosis Diagnosis is based on geographic history, clinical presentation, and presence of schistosome ova in excreta.

• Serologic assays for schistosomal antibodies (available through the CDC in the United States) may yield positive results before eggs are seen in excreta.
• Infection may also be diagnosed by examination of tissue samples, typically from rectal biopsies.

Prevention Travelers to endemic regions should avoid contact with all freshwater bodies.

Liver (Biliary) Flukes

• Clonorchiasis (due to Clonorchis sinensis) and opisthorchiasis (due to Opisthorchis viverrini and O. felineus) occur in Southeast Asia and Eastern Europe.
  • - Infection is acquired by ingestion of contaminated raw freshwater fish; larvae travel through the ampulla of Vater and mature in biliary canaliculi.
  • - Most infected individuals are minimally symptomatic; chronic or repeated infection causes cholangitis, cholangiohepatitis, and biliary obstruction and is associated with cholangiocarcinoma.
  • - Therapy for acute infection consists of praziquantel administration (25 mg/kg tid for 2 or 3 days).
• Fascioliasis (due to Fasciola hepatica and F. gigantica) is endemic in sheep-raising countries and has a worldwide prevalence of 17 million cases.
  • - Infection is acquired by ingestion of contaminated aquatic plants (e.g., watercress, water caltrop, water chestnuts).
  • - Acute disease develops 1-2 weeks after infection and causes fever, RUQ pain, hepatomegaly, and eosinophilia. Chronic infection is infrequently associated with bile duct obstruction and biliary cirrhosis.
  • - For treatment, triclabendazole is given as a single dose of 10 mg/kg.
• Stool ova and parasite (O & P) examination diagnoses infection with liver flukes. Serologic testing is helpful, particularly in lightly infected pts.

Lung Flukes

Infection with Paragonimus spp. is acquired by ingestion of contaminated crayfish and freshwater crabs.

• Acute infection causes lung hemorrhage, necrosis with cyst formation, and parenchymal eosinophilic infiltrates. A productive cough with brownish or bloody sputum, in association with peripheral-blood eosinophilia, is the usual presentation in pts with heavy infection.
  • - In chronic cases, bronchitis or bronchiectasis may predominate.
  • - CNS disease can also occur and can result in seizures.
• The diagnosis is made by O & P examination of sputum or stool; serology can be helpful.
• Praziquantel (25 mg/kg tid for 2 days) is the recommended therapy.