Echinococcus granulosus
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p689-691
2025-11-03
43
GENERAL CHARACTERISTICS
Echinococcus is the smallest of all tapeworms (3 to 9 mm long) with three to five proglottids. It contains a scolex with four suckers and a rostellum with hooks to attach to the intestinal wall. E. granulosus is a tapeworm found in the small intestine of the definitive host, the canine. Eggs are ingested by the intermediate hosts and include a variety of mammals including sheep, cattle, moose, and humans. There are several strains of Echinocococcus granulosus that have been identified, with the dog-sheep strain being the most common. Humans are typically accidental hosts and are considered a dead-end since the life cycle of the organism is unable to continue in a human host. Oncospheres hatch in the intestine of the intermediate host and invade the circulatory system, where they develop into hydatid cysts. Disease symptoms vary with the site and size of the cyst. Echinococcosis (hydatid disease) results from the presence of one or more cysts (hydatids), which can develop in any tissue.
EPIDEMIOLOGY
E. granulosus is most common in cool, damp areas where sheep herds are prevalent, such as southern South America, Russia, East Africa, and the western United States. The eggs in the definitive host are passed through the feces and contaminate soil, water, or food. The eggs are able to survive freezing conditions and can remain viable within the environment for several years. Adult worms are found only in the intermediate hosts (Figure 1).
Fig1. Life cycle of Echinococcus granulosus (hydatid disease).
PATHOGENESIS AND SPECTRUM OF DISEASE
Hydatid disease in humans is potentially dangerous depending on the size and location of the cyst. Some cysts may remain undetected for many years until they grow large enough to affect other organs. Many humans live day-to-day without ever knowing they are infected. The cyst is very slow growing in humans. It is usually fluid-filled and has a germinal layer from which many thousands of scolices are budded. These are known as daughter cysts (brood capsules), which attach to the germinal layer or free-float in the cyst. The scolices in the hydatid fluid resemble grains of sand and are called hydatid sand (Figures 2, A and 3). The result is a unilocular cyst containing future adult worms. The cyst may resemble a slow-growing tumor. Infections in the liver or lungs may be asymptomatic for many years, but the pressure eventually causes noticeable symptoms. The majority of the hydatid cysts occur within the liver. Cysts within the liver cause chronic abdominal pain and allergic reactions and may result in cholangitis (infection of the common bile duct) and cholestasis (interference with flow of bile from the liver). Cysts that develop in the lungs may cause infections and abscesses and result in chronic cough, shortness of breath, and chest pain. During the life cycle of the cyst, there may be occasional seepage of fluid into the host tissue and circulation causing sensitization or activation of the immune response from the presence of the parasite. The rupture and release of the fluid of a hydatid cyst may cause ana phylactic shock as a result of the primary sensitization in a previously asymptomatic individual. If a cyst bursts within the human body, many new cysts may be released that are typically eliminated via the host’s cellular immune response. Leaking fluid from a cyst may cause notable eosinophilia.
Fig2. A, Echinococcus granules. B, Ovum. C, Scolex. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
Fig3. Echinococcus granulosus, hydatid sand (300×). (Inset) Two individual hooklets (1000×).
LABORATORY DIAGNOSIS
Clinical symptoms of a slow-growing abdominal tumor with or without eosinophilia are suggestive of infection. Human infection ranges from asymptomatic to severe, including death. Diagnosis is made through the identification of cysts in the infected organ, accompanied with positive serologic tests. A variety of serologic tests are available including ELISA, indirect hemagglutination, and latex agglutination. Both false positives and false negatives may occur; therefore clinical history is extremely important for diagnosis. Ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT) have improved the diagnosis and may provide visualization of the fluid-filled cysts. Calcified cysts can be visualized using conventional x-ray. Microscopic examination of the cyst fluid for the identification of the scolices can be useful in diagnosis. A 1% eosin stain may be added to the fluid to assist in the visualization and determination as to whether or not the cyst is viable. Nonviable scolices will stain with the eosin whereas viable scolices will not.
THERAPY
Surgery is the most common form of treatment. The procedure involves surgical removal of cysts or inactivation of hydatid sand by injecting the cyst with 10% formalin and then removing it. Extreme care must be taken to avoid spillage. Albendazole is the drug of choice to kill the scolices within the cyst, reduce the size of the cyst, and prevent recurrence. Mebendazole and praziquantel have also been shown to be effective.
PREVENTION
Preventive measures include avoiding contact with infected dogs and deworming animals regularly. Effective control includes educating the population concerning the danger and means of transmission of hydatid disease as well as maintaining good hygiene and practicing safe disposal of dog feces. Slaughtered animals must be disposed of properly, to prevent dogs from exposure to contaminated materials and interrupt the Echinococcus life cycle.
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