Fasciola hepatica, a parasite causing fasciolosis, is described in Africa, Central Europe, and South America. People are infected by ingestion of contaminated plants and water. The clinical presentation may simulate hepatic or cholangic tumor due to hepatomegaly. Computer tomography of the liver shows several low-density nodules or irregular structures in the biliary tract in the liver suggesting abscesses or tumors. The diagnosis of fasciolosis may be suggested by marked eosinophilia and serology. Biopsies contain eggs measuring 130 × 70 μm with polar opercula, surrounded by eosinophils and Charcot-Leyden crystals (Fig. 2.20).

Clonorchis sinensis is described in Taiwan, Japan, Korea, Vietnam, Russia, and Turkey, while Opistorchis Viverrini is endemic in Laos, Thailand, and generally around the Mekong River. These parasites are hosted by the biliary tract and induce asymptomatic infections and chronic cholangitis, sometimes mimicking cholangiocarcinoma. Biopsies show inflammatory changes with dilated bile ducts, adenomatous proliferation of bile ducts, parasite eggs (15 x 30 μm) combined with Charcot-Leyden crystals and eosinophils in areas of necrosis. Chronic infection leads to cholangiocarcinoma.

Several trematodes in the genus Paragonimus induce human infection in Eastern Asia and Western Africa. These parasites are transmitted by contaminated water from crabs and crayfish, containing larval stages that cross the intestinal wall and the diaphragm to reach the lung. The main clinical symptoms include productive chronic cough, and hemoptysis. Chest radiograph shows a cavitating lesion or an irregular mass in the upper lung frequently accompanied by pleural effusion that may easily be confused with tuberculosis or a neoplasm. These infiltrates are transient, migratory, and combined with eosinophils. The diagnosis is made by the detection of characteristic ova in the sputum or stools measuring 120 μm with a polar opercules. The wall of these ova are able to deviate the polarized light as all the ova of parasites causing distomatosis. Other localizations have also been reported, such as the brain and gastrointestinal tract.

Schistosoma mansoni, S. japonicum, and S. mekonji cause pathologic changes in the colon and rectum. S. hematobium colonizes the colon in about 60% of cases, while the main infection is seen in the urinary system. In the digestive system, only 20% of patients infected with Schistosoma have gross abnormalities in the form of plaques and polyps in the colon and rectum. Polyps are covered with thin mucosa and contain several granulomas with epithelioid cells, eosinophils, and a variable amount of eggs with characteristic morphology. Eggs of S. mansoni measure 140 × 70 μm with a lateral spine which is Ziehl-Neelsen positive (Fig. 2.21). S. japonicum (120 μm) and S. mekongi (80–100 μm) are smaller and without a spine. Old lesions are characterized by extensive fibrosis of the colon/rectum wall, peritoneum, and the liver with portal hypertension. The morphology of eggs in fibrosis may be variable with deformation and numerous atypias. Diagnosis is made by the demonstration of eggs in biopsies, but serology is helpful.

Schistosoma may target also the skin, lung, central nervous system (frequent with S. Japonicum), spinal cord (S. haematobium and mansoni), genital system, and peritoneum.

Hydatidosis is a parasitic infection caused by larval tapeworms of Echinococcus. Two main species are endemic: The most common, Echinococcus granulosus, is transmitted by dogs, while the less common Echinococcus multilocularis is transmitted also by foxes and rodents. The former is endemic in Southern Europe, the Mediterranean basin, most countries of Africa, Madagascar, and South America (Chile and Peru). The latter is observed in the eastern part of France, Germany, Italy, Austria, and Western Europe. Both types are endemic in Canada, Australia, New Zealand, and in the western parts of the United States.

Parasites form cystic lesions that are found mainly in the liver, less commonly in the lungs, and rarely in other organs. Hydatid cysts grow slowly and are therefore seldom observed in small children. Symptoms are related to localization of the cyst. In the liver, they may produce abdominal pain, acute or chronic jaundice, portal hypertension, and episodes of cholangitis. Patients with hydatid cysts in the lung may complain of chest pain, coughing, and hemoptysis. However, most patients are asymptomatic and cysts are an accidental finding. Hydatoid cysts are usually diagnosed by a combination of characteristic ultrasound image and positive serology for anti-Echinococcus IgG antibodies. Small-volume biopsy is not a method of choice for obtaining a diagnosis because there is a risk of inoculation, and some anaphylactic reactions have been reported following the procedures. However, ultrasound images may not be characteristic when cysts are complicated by hemorrhage or infection [11]. Furthermore, 10% of patients do not produce detectable antibodies, especially children between 3 and 15 years of age. In such cases FNA is indicated because lesions simulate tumors. The material obtained by FNA macroscopically resembles abscess. Background of smears contains granular necrotic precipitate with various amounts of parasite remnants depending on the duration of the lesion. Almost intact scolices, complete with the rostrum of hooklets, may be found, surrounded by many free hooklets (Fig. 2.22). Alternately, degenerated, unrecognizable scolices may be present, or only a few hooklets. Klopčič et al. described scolices and hooklets in 24 of 38 FNA samples from hydatid cysts; the remaining 14 specimens contained only free hooklets [12]. In tru-cut specimens of Echinococcus multilocularis a conglomerate of cysts and vesicles were described by Ciftçioğlu et al., containing PAS-positive mucoid material [13]. They also observed coagulative necrosis and occasionally foreign body giant cells. In the FNA specimens the same authors describe necrotic background, PAS-positive cuticular structures, and mucoid globules, as well as foreign body giant cell in some cases [13].

Taenia solium cysticercosis is the development of the larval stage of the tapeworm occurring in many organs. Most of the infections are asymptomatic. The main target is the brain mimicking brain tumor. Histologically there is a cyst with pseudoglandular pattern (Fig. 2.23).