ESSENTIALS OF DIAGNOSIS
History of exposure to dogs or wild canines in an endemic area.
Large cystic lesions, most commonly of the liver or lung.
Positive serologic tests.
Echinococcosis occurs when humans are intermediate hosts for canine tapeworms. Infection is acquired by ingesting food contaminated with canine feces containing parasite eggs. The principal species that infect humans are Echinococcus granulosus, which causes cystic hydatid disease, and Echinococcus multilocularis, which causes alveolar hydatid disease. E granulosus is transmitted by domestic dogs in areas with livestock (sheep, goats, camels, and horses) as intermediate hosts, including Africa, the Middle East, southern Europe, South America, Central Asia, Australia, New Zealand, and the southwestern United States. E multilocularis, which much less commonly causes human disease, is transmitted by wild canines and is endemic in northern forest areas of the Northern Hemisphere, including central Europe, Siberia, northern Japan, northwestern Canada, and western Alaska. An increase in the fox population in Europe has been associated with an increase in human cases. The disease range has also extended southward in Central Asia and China. Other species that cause limited disease in humans are endemic in South America and China.
After humans ingest parasite eggs, the eggs hatch in the intestines to form oncospheres, which penetrate the mucosa, enter the circulation, and encyst in specific organs as hydatid cysts (eFigure 35–26). E granulosus forms cysts most commonly in the liver (65%) and lungs (25%), but the cysts may develop in any organ, including the brain, bones, skeletal muscles, kidneys, and spleen. Cysts are most commonly single. The cysts can persist and slowly grow for many years.
Life cycle of Echinococcus. The adult Echinococcus granulosus (3–6 mm long)
resides in the small bowel of the definitive hosts, dogs, or other canids. Gravid proglottids release eggs
that are passed in the feces. After ingestion by a suitable intermediate host (under natural conditions: sheep, goat, swine, cattle, horses, camel), the egg hatches in the small bowel and releases an oncosphere
that penetrates the intestinal wall and migrates through the circulatory system into various organs, especially the liver and lungs. In these organs, the oncosphere develops into a cyst
that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior. The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices
evaginate, attach to the intestinal mucosa
, and develop into adult stages
in 32–80 days. The same life cycle occurs with Echinococcus multilocularis
(1.2–3.7 mm), with the following differences: the definitive hosts are foxes, and to a lesser extent dogs, cats, coyotes, and wolves; the intermediate host are small rodents; and larval growth (in the liver) remains indefinitely in the proliferative stage, resulting in invasion of the surrounding tissues. With Echinococcus vogeli
(up to 5.6 mm long), the definitive hosts are bush dogs and dogs; the intermediate hosts are rodents; and the larval stage (in the liver, lungs and other organs) develops both externally and internally, resulting in multiple vesicles. Echinococcus oligarthrus
(up to 2.9 mm long) has a life cycle that involves wild felids as definitive hosts and rodents as intermediate hosts. Humans become infected by ingesting eggs
, with resulting release of oncospheres
in the intestine and the development of cysts
in various organs. (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)
Infections are commonly asymptomatic and may be noted incidentally on imaging studies or present with symptoms caused by an enlarging or superinfected mass. Findings may include abdominal or chest pain, biliary obstruction, cholangitis, portal hypertension, cirrhosis, bronchial obstruction leading to segmental lung collapse, and abscesses. Cyst leakage or rupture may be accompanied by a severe allergic reaction, including fever and hypotension. Seeding of cysts after rupture may extend the infection to new areas.
E multilocularis generally causes a more aggressive disease than E granulosus, with initial infection of the liver, but then local and distant spread commonly suggesting a malignancy. Symptoms based on the areas of involvement gradually worsen over years, with the development of obstructive findings in the liver and elsewhere.
Serologic tests, including ELISA and immunoblot, offer sensitivity and specificity over 80% for E granulosus liver infections, but lower sensitivity for involvement of other organs. Serology is somewhat more reliable for E multilocularis infections. Serologic tests may also distinguish the two major echinococcal infections.
Diagnosis is usually based on imaging studies, including ultrasonography, CT, and MRI (eFigures 35–27, 35–28, 35–29, 35–30, and 35–31). In E granulosus infection, a large cyst containing multiple daughter cysts that fill the cyst interior is highly suggestive of the diagnosis. In E multilocularis infection, imaging shows an irregular mass, often with areas of calcification.
Chest film of 19-year-old man showing a recently ruptured hydatid cyst in the right lung. Note fluid level (arrows). (Reproduced, with permission, from Goldsmith R, Heyneman D [editors]. Tropical Medicine and Parasitology. Originally published by Appleton & Lange. Copyright © 1989 by The McGraw-Hill Companies, Inc.)
Abdominal CT scan of a patient with an Echinococcus multilocularis lesion in the right lobe of the liver. Note irregular low-density zones alternating with scattered areas of calcification. (Used, with permission, from JF Wilson.)
Hydatid cysts in the liver on CT scan. Hydatid cysts in the liver are caused by parasitic infection by the Echinococcus tapeworm. On CT, they appear as low density, fluid-filled masses, often with septations. The cyst walls may enhance with contrast or may be calcified (arrows). (Used, with permission, from Nicholas Fidelman, MD.)
Echinococcosis of the liver with hepatobronchial fistula. This CT scan of the upper portion of the liver demonstrates a large, well-circumscribed, sharply demarcated cystic lesion of the right lobe of the liver near the diaphragm—typical of echinococcal cysts. The additional finding here is a change in the right lung at the posterior sulcus so that instead of the lung around the cyst being aerated, there is increased density within this area. This finding proved to be the result of a fistula from the echinococcal cyst through the diaphragm and into the right lung base.
Abdominal CT scan showing a large primary hydatid cyst (Echinococcus granulosus) in the right lobe of the liver. Note characteristic multiple internal septations indicating secondary daughter cyst formation. (Used, with permission, from P Braithwaite.)
The treatment of cystic hydatid disease is with albendazole, often with cautious surgical resection of cysts. When used alone, as in cases where surgery is not possible, albendazole (10–15 mg/kg/day orally) has demonstrated efficacy, with courses of 3 months or longer duration; alternating cycles of treatment and rest may be needed. Mebendazole (40–50 mg/kg/day orally) is an alternative drug, and praziquantel may also be effective. In some cases, medical therapy is begun, with surgery performed if disease persists after some months of therapy. Another approach, in particular with inoperable cysts, is percutaneous aspiration, injection, and reaspiration (PAIR). In this approach (which should not be used if cysts communicate with the biliary tract), patients receive antihelminthic therapy, and the cyst is partially aspirated. After diagnostic confirmation by examination for parasite protoscolices, a scolicidal agent (95% ethanol, hypertonic saline, or 0.5% cetrimide) is injected, and the cyst is reaspirated after about 15 minutes. PAIR includes a small risk of anaphylaxis, which has been reported in about 2% of procedures, but death due to anaphylaxis has been rare. Treatment of alveolar cyst disease is challenging, generally relying on wide surgical resection of lesions. Therapy with albendazole before or during surgery may be beneficial and may also provide improvement or even cure in inoperable cases.