Trematodes (flukes) are nonsegmented flattened helminths that are often leaflike in shape. Most have two suckers, one found around the mouth (oral sucker) and the other on the ventral surface. Most are hermaphroditic. The eggs, which are passed out of the host in sputum, urine, or feces, undergo several stages of maturation in other hosts before the larvae enter humans. The larvae are acquired either through ingestion of food (aquatic vegetation, fish, crayfish) or by direct penetration of the skin. After ingestion, most trematodes mature in the intestinal tract (intestinal flukes); others migrate and mature in the liver and bile duct (liver flukes), whereas still others penetrate the intestinal wall and migrate through the abdominal cavity to the lung (lung flukes). Diarrhea, abdominal pain, and anorexia are common symptoms associated with trematode infestation. Liver flukes may cause bile duct blockage, liver enlargement, upper right quadrant pain, and diarrhea. Liver function tests are usually altered. Lung flukes produce pulmonary symptoms such as cough, hemoptysis, and chest pain.
The schistosomes (blood flukes) are a distinct group of trematodes. These helminths are cylindrical at the anterior end and flattened at the posterior end. The sexes are separate. The larvae penetrate skin that is in contact with contaminated water and then migrate through the lymphatics and blood vessels to the liver. After maturing, schistosomes migrate into the mesenteric or vesicular vein, where the adults mate and release eggs. The eggs secrete enzymes that enable them to pass through the wall of the intestine (Schistosoma mansoni and Schistosoma japonicum) or bladder (Schistosoma haematobium). In addition, some eggs may be carried to the liver or the lung by the circulation. Penetration of the skin is associated with petechial hemorrhage, some edema, and pruritus that disappears after about 4 days. Approximately 3 weeks after trematode penetration, patients complain of malaise, fever, and vague intestinal symptoms. With the laying of eggs, acute symptoms of general malaise, fever, urticaria, abdominal pain, and liver tenderness are reported. Diarrhea or dysentery is associated with infestations by S. mansoni and S. japonicum, whereas hematuria and dysuria are commonly caused by S. haematobium. In the chronic form of the disease, fibrosis and hyperplasia can occur in the tissues the eggs inhabit.
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