Héctor H. García, Saul Santivañez, and Robert H. Gilman
Five species of Echinococcus have been recognized to date. Echinococcus granulosus, E multilocularis, and more rarely E vogeli and E oligarthrus infect humans with their larval stages. The definitive hosts are canids, except for E oligarthrus, which includes wild felids as intermediate hosts. Humans become accidental intermediate hosts when the eggs from the feces of dogs, wolves, or other canids are ingested. A fifth species, E. shiquicus, has been recently recognized, but it is not known whether it may infect humans.1,2
The adult worm of E granulosus is found in the intestine of dogs, wolves, and other canids. The worm measures only about 0.5 cm in length. It has a scolex with hooks; a neck region; one immature, one mature, and one gravid proglottid. The dog usually harbors hundreds or thousands of adult tapeworms.1,3 The eggs, which are morphologically similar to those of Taenia species, are excreted in the feces. When an intermediate host, such as sheep, ingests the eggs, the embryo hatches from the egg, penetrates the intestinal mucosa, and enters lymphatics or blood vessels. The host defense mechanisms destroy many embryos, but those surviving develop into expanding cystic structures called hydatid cysts (Fig. 338-1). The rapidity of cyst growth is quite variable and partially dependent upon the tissue localization, but an increase in diameter of 1 cm or more per year is not uncommon, with faster growth in children. Spherical brood capsules arise from the inner germinal membrane of the cyst wall. Protoscolices, the precursors to the scolices of the adult worms, develop from germinal membrane and the inner surface of the brood capsules and accumulate within the cyst as “hydatid sand” (Fig. 338-2). If the cyst, or a portion of it, is eaten by a suitable definitive host, adult tapeworms develop in the small intestine. Hydatid cysts are capable of developing in nearly any tissue, including the central nervous system and bone; however, 90% of them develop in either the liver or the lung, most frequently in the liver.3,4
FIGURE 338-1. A hydatid cyst capsule removed surgically from the lung. (Source: http://en.wikipedia.org/wiki/File:Hydatid_cyst_membrane.jpg.)
Human infection with hydatid cysts is most common in sheep- and cattle-raising areas such as the countries bordering the Mediterranean, Australia, New Zealand, and the Andean region in South America, particularly Peru and Argentina.1,5 In the United States, most infections are found among immigrants from endemic areas. However, there have been foci of infection among Basque shepherds in California, Mormon ranchers in Utah, and Native Americans in Arizona and New Mexico. Uganda and Kenya have very high prevalence rates of human infection.
Adult worms of Echinococcus multilocularis are smaller but morphologically similar to E granulosus. The larval stage of E multilocularis in the intermediate host grows by external budding, resembling a malignant tumor, and does not produce large cystic structures (alveolar hydatid disease). Liver tissue is progressively destroyed, contiguous structures are invaded, and more rarely, metastatic lesions may develop in distant sites. Foxes are usually the definitive hosts, and rodents are intermediate hosts. Hunters and fur traders exposed to foxes and fox fur are at risk. This infection occurs only in the Northern Hemisphere. It has a wide distribution in the northern midwestern states of the United States, in Canada, the former Soviet Union, Switzerland and adjacent countries, and in northern Japan. Sled dogs in Arctic villages may be sources of human infection.6
Human polycystic echinococcosis, caused by E vogeli or E oligarthrus, occurs much less frequently, always in Central and South America. Polycystic disease consists of multiple fluid-filled cysts, up to 3 cm in diameter, often interconnected and multichambered.
The majority (70%) of patients with hydatid disease caused by E granulosus have single cysts. When multiple cysts are present, they are most commonly in the same organ, but they can develop in multiple sites. Some investigators feel that pulmonary echinococcosis is more common in children and requires special attention and unique surgical approaches.7 About 20% of children with pulmonary hydatid cysts will also have hepatic cysts. Both hepatic and pulmonary cysts may attain considerable dimensions and lead to parenchymal damage,7 or conversely, some may resolve by natural evolution.5 It is not uncommon to find asymptomatic calcified cysts in the liver or spleen of infected adults as an incidental finding on radiologic studies or at autopsy. It takes many years for cysts to die and calcify; therefore calcifications are rarely seen in children.1,3
Large hepatic cysts may cause pain and tenderness in the right upper quadrant. In some instances, a mass may be palpable. Biliary tract obstruction may develop, depending upon the size and location of the hepatic cyst. Between 5% and 15% of hepatic cysts rupture into the biliary tract, causing fever, pain, and jaundice. The release of antigenic cyst fluid may cause severe allergic reactions including anaphylaxis. Patients who survive intraperitoneal cyst rupture are in danger of multiple secondary cysts developing as a secondary infection within the abdomen.
Although pulmonary cysts often are asymptomatic, about one third of them rupture into a bronchus or into the pleural space. Secondarily infected lung cysts appear as lung abscesses. Cysts that rupture into a bronchus may be coughed up. The patient may describe the membranes in the sputum. Complete evacuation of a pulmonary cyst results in a cure. Partial evacuation of the cyst sets the scene for bacterial growth and the production of a lung abscess.
Bone involvement may present as a bone deformity or as a pathologic fracture. The hydatid begins growth within the marrow cavity. The typical laminated membrane does not develop. Bone destruction by the parasite resembles that caused by tumor or infection. Involvement of the vertebral body causes pain and tenderness to palpation and may produce spinal cord or nerve root compression with neurologic signs and symptoms. Intracranial hydatid cysts are rare and occur most frequently in children. The symptoms are those of an expanding mass, usually causing intracranial hypertension with headache, nausea, and vomiting. Seizures may also develop.
FIGURE 338-2. Hydatid sand. A. Scolices invaginated into cyst membrane (140×). B. Evaginated scolex with hooklets; stalk is present, by which the scolex is continuous with the germinal epithelium (140×).
Although alveolar hydatid disease caused by E multilocularis has been described in a 5-year-old child, it is usually a disease of adults. The progressive destruction of the liver takes many years. Tender hepatomegaly, abdominal masses arising from the liver, and jaundice are common presenting findings. Extension of the parasite into large vessels may result in metastatic lesions in the lungs or brain.3,6
A history of exposure to dogs or other canids in an area endemic for echinococcosis is very helpful. Imaging techniques such as ultrasound are effective in delineating the contents of cystic structures. The presence of daughter cysts can be diagnostic. Features noted from CT scanning or MRI may be highly characteristic septate densities. Intact pulmonary cysts appear as sharply demarcated smooth, spherical, or ovoid radiopaque “cannonball” lesions. If the cyst has ruptured, an air-fluid level may be present. A collapsed membrane on the surface of the fluid may produce the classic “water lily” sign. The WHO classification provides a very useful tool for ultrasound staging and follow-up of liver cysts.8
Serologic testing using ELISA, immunoblots, or indirect hemagglutination tests is available through a few reference laboratories and the CDC. False-negative serologic tests have been problematic, particularly in lung cysts and other sites outside of the liver. Cross-reactivity has been noted with cysticercosis, hymenolepiasis, and other cestode infections.
Examination of the cyst and its contents at surgery proves the diagnosis by the presence of protoscolices and hooklets (hydatid sand). Percutaneous needle aspiration for diagnostic purposes is usually not needed and should be done only after other techniques have failed.
Alveolar hydatid disease is usually suspected when plain films of the liver show amorphous calcification surrounding 2-mm to 4-mm radiolucent areas. The recent development of a highly sensitive and specific ELISA using an epitope, Em 18, which is not shared with E granulosus, appears very promising for diagnosis and for following the response to treatment.
The advent of chemotherapy with benzimidazole compounds (initially mebendazole and later albendazole), and the success of percutaneous aspiration and injection techniques, make it necessary to carefully reassess the dominant role of surgery in the treatment of cystic hydatid disease caused by E granulosus.9,10
Surgery is still considered appropriate therapy for large liver cysts with multiple daughter cysts, superficial cysts that are subject to spontaneous or traumatic rupture, cysts communicating with the biliary tract, and infected cysts. Surgery should be preceded by antiparasitic treatment to decrease the risk of cyst rupture at surgery, by decreasing pressure inside the cyst and the viability of the protoscolices if rupture occurs. The combination of albendazole plus praziquantel seems more effective than albendazole alone for prevention of secondary infection.11 Some authors suggest continuing antiparasitic therapy for 1 month or more following surgery to prevent recurrence and/or the development of secondary cysts. It is common practice to cover the operatory field with pads soaked in hypertonic saline and to introduce a scolicidal substance such as hypertonic saline, alcohol, or cetrimide into the cyst during the surgical procedure. All of the commonly used scolicides have the potential to cause sclerosing cholangitis if there is communication between the cyst contents and the biliary tract.
Percutaneous aspiration, injection of protoscolicide, and reaspiration (the PAIR technique) have replaced surgery in select cases of liver cystic hydatid disease. The types of cyst that would mostly benefit from PAIR are those in stages I and III of the WHO ultrasound classification of liver cystic hydatid lesions.7 When feasible, PAIR eliminates the need for open surgery, decreases the hospital stay, and significantly reduces costs.
Chemotherapy alone may be effective. Albendazole has replaced mebendazole as the drug of choice. Recent studies show that a combination of albendazole and praziquantel is more successful than albendazole alone for the treatment of this disease. Smaller, uncomplicated cysts appear to respond to albendazole more readily than do larger cysts. There are reports of central nervous system lesions and muscle, vertebral, and other bone lesions resolving completely with medical therapy. Because of the evolving therapy of cystic echinococcosis, consultation with the CDC is advised to obtain current information (contact the United States CDC Parasitic Disease Service at 770-488-7775).
Alveolar hydatid disease is treated with aggressive surgery, including partial hepatectomy or lobectomy. Unfortunately, less than 30% of patients have resectable lesions at the time of diagnosis. Long-term albendazole therapy may benefit a significant number of patients with inoperable lesions. Liver transplantation has been used in select patients. However, there is the risk of regrowth and metastatic spread associated with the immunosuppression that is necessary to preserve the transplant.6 Albendazole also appears to be beneficial in the treatment of polycystic hydatid disease caused by E vogeli.
Hydatid disease occurs in communities where the dog-livestock cycle is maintained, and usually dogs are given infested offal following non-supervised slaughter. The disease has been eliminated in several countries after very long periods of sustained control, including dog treatment and purge and health education. New tools, including an effective livestock and/or dog vaccine for the tapeworm stage may reduce the time required for successful disease control to be achieved.1