Echinococus granulosus

BACKGROUND

Echinococcosis is the general term for 3 diseases caused by the larval stage of Echinococcus tapeworms, the smallest tapeworms in the Taeniidae family. Echinococcus granulosus causes cystic echinococcosis. This species occurs worldwide, typically in rural areas of Africa, the Middle East, southern Europe, Russia, China, Australia, and South America (especially Argentina and Uruguay). Echinococcus multilocularis causes alveolar hydatid echinococcosis and occurs only in the northern hemisphere. Echinococcus vogeli causes polycystic echinococcosis and occurs in Central and South America.

Pathophysiology;

The tapeworm's life cycle (see Image 1) involves a definitive host, usually a dog (although foxes and coyotes have also been implicated), and intermediate hosts such as sheep, goats, and swine. The 3- to 6-mm adult tapeworm is found in the definitive host's intestines. Eggs are excreted via the feces into the environment. Dogs typically become infected by eating the remains of infected sheep and other livestock.

Humans are accidental hosts and are not typically involved in the life cycle of the organism. Humans usually become infected through exposure to canine feces. Humans become infected by eating food contaminated with tapeworm eggs, which hatch in human small intestine under the influence of gastric and intestinal secretions. Larvae hatched from these eggs can penetrate the small intestine and become widely distributed in the human body. These larvae develop into a hydatid cyst. In alveolar echinococcosis, the liver is almost exclusively involved. In cystic echinococcosis, the liver is involved in two thirds of cases; in the remaining cases, other sites of involvement, in descending order of frequency, are the lung, spleen, skin, muscle, kidney, retroperitoneum, bone, heart, and brain.

Hydatid cysts may contain many protoscolices, each with the ability to develop into an adult tapeworm in a definitive host. The definitive host usually becomes infected by ingesting the protoscolices in hydatid cysts from the intermediate host; adult tapeworms only develop in the intestines of the definitive host.

Human disease is usually caused by the mass effect of slow-growing hydatid cysts, although bacterial superinfection or allergic symptoms from cyst rupture has also been reported. Because of the hydatid cyst's unhurried growth, infected humans may remain asymptomatic for 20 years.
Frequency
United States

Echinococcosis is rare. Indigenous cases have been reported in Minnesota and among Native Americans in western Alaska. Imported cases are uncommon.
International

Human infection rates are generally highest in areas with poor sanitation practices for sheep-raising, particularly in areas with dogs. Dogs in these areas may become infected by eating the entrails of infected butchered sheep or other herded animals. Humans then become infected when they ingest Echinococcus eggs from dog feces. Human infection rates are also high in areas where intestines are part of the diet.

Echinococcosis has been reported in Manitoba, Canada. Outside North America, incidence rates vary considerably, from less than 1 case per 100,000 people in many parts of the world, to 13 cases per 100,000 people in Greece, 143 cases per 100,000 people in Argentina, 197 cases per 100,000 people in the Xinjiang province of China, and 220 cases per 100,000 people in Kenya's Turkana district.

The Turkana district's particularly high incidence rate is attributed to 2 cultural practices. Some tribes eat canine intestine that has been roasted on a stick over a campfire; disease transmission likely occurs when infected intestine is not thoroughly cooked. Some tribes also do not bury their dead; carnivores may become the direct intermediate host after eating corpses.
Mortality/Morbidity

Most human infection is probably asymptomatic. Death may occur from hydatid rupture, which may lead to anaphylactic shock, or from the extreme progression of cysts in vital organs.

Sex;

Males and females are equally affected, according to a Bavarian survey.

Age;

A European study reported most patients infected by E granulosus were aged 21-50 years. Patients infected by E multilocularis were typically older; most were aged 31-80 years.

History;

· Patients with echinococcosis often remain asymptomatic for many years.

· Routine imaging may reveal incidental evidence of echinococcosis.


Physical;

· Echinococcosis symptoms may be secondary to compression of adjacent structures.

o Abdominal pain, a palpable mass, or biliary obstruction is possible.

o Patients with pulmonary involvement may occasionally have respiratory symptoms.

o The location of lesions determines other symptoms.

§ Clinical manifestations occur very early in nervous system infection.

§ In bone infections, necrosis may occur and cause thin and fragile bone, usually a cause of spontaneous fracture.

· Bacterial superinfection may occur with pyogenic abscess formation within the cyst. Cyst leakage or rupture may lead to allergic symptoms, including the following:

o Anaphylaxis

o Fever

o Asthma

o Urticaria

· Loss of appetite is a frequent symptom, and weight loss and weakness may occur. Despite popular belief, hunger pains are uncommon.

· Alveolar hydatid disease may resemble a hepatic neoplasm, with local destruction, biliary obstruction, and occasional metastasis to the brain and lung.


Causes;

Echinococcosis is caused by the larval stage of the Echinococcus tapeworm.

Lab Studies;

· Chest radiography, CT scanning, ultrasonography, or MRI findings usually suggest the diagnosis of echinococcosis, especially in patients with epidemiological risk factors. Serologic tests are then typically used to help confirm the diagnosis. If radiologic studies and epidemiology suggest echinococcosis, consider further evaluation when serology test results are negative.

· Serologic testing

o The Centers for Disease Control and Prevention in Atlanta, Ga provides the following 3 types of serologic tests:

§ Indirect hemagglutination

§ Indirect fluorescent antibody

§ Enzyme immunoassay/enzyme-linked immunosorbent assay

o Sensitivities range from 60-95%. Liver cysts are more likely to yield positive results than pulmonary cysts. Positive test results are less likely with calcified or dead cysts and more likely with ruptured cysts.

o False-positive serologic test results may occur in patients with other parasitic infections (especially cysticercosis) and in patients with cancer or an immune dysfunction.

· Antigen testing may be helpful if antibody test results are negative, although this modality is not available in the United States.

· Stool evaluation is generally not useful for diagnosis.

Imaging Studies;

· Chest radiography may reveal cysts of 1-20 cm in any location. These cysts do not calcify, nor do they have daughter cysts. Approximately one third of patients with a positive finding on chest radiography have definable cysts in the liver.

· Liver cysts may be diagnosed based on findings of CT scanning, ultrasonography, or MRI. Most liver cysts are in the right lobe and may sometimes be difficult to differentiate from abscesses or neoplasms. Liver cysts may be single or multiple.

· Ultrasonography may reveal irregular, heterogeneous, hypoechoic lesions, some with calcifications. Ultrasonography is a simple and noninvasive method to ascertain cyst size.

Medical Care;

· Preoperative and postoperative albendazole therapy may be beneficial. As an alternative to surgical management, a randomized controlled study of albendazole in uncomplicated hepatic hydatid disease in adults concluded that 10 mg/kg/d for 3 months without rest periods was effective and could be tried before surgical intervention. In children, the dose is 15 mg/kg/d for 28 days and is repeated, as necessary.

· The World Health Organization (WHO) recommends postoperative chemotherapy for 2.5 years after radical surgery for alveolar hydatid disease.

· Inoperable cases can be treated with albendazole or mebendazole. An overall response rate of 55-79% has been documented, with cure in 29% of cases. Albendazole is the drug of choice because of its greater absorption from the gastrointestinal tract with higher plasma levels. Mebendazole is poorly absorbed and must be taken at higher doses for several months for a therapeutic effect. Albendazole is administered in 3 or more cycles of 400-800 mg twice daily for 4 weeks (adult dose), followed by a 2 week period of rest. If response to 3 cycles is not evident, subsequent courses are unlikely to be beneficial. Response to therapy is best monitored by serial imaging studies.

Surgical Care;

Surgery is the treatment of choice for most cases of cystic echinococcosis and is usually successful. Alveolar echinococcosis is less amenable to surgery, and medical therapy may be useful.

· Liver surgery options include the following:

o Partial hepatic resection

o Pericystectomy

o Cystectomy

· In 20% of cases, disease recurs despite surgery.

· Some clinicians have inactivated the fertile cyst with formalin or 30% saline in combination with resection. Sclerosing cholangitis may occur as a complication of this procedure.

· An alternative to surgery is the PAIR method (ie, puncture the cyst, aspirate fluid, introduce a protoscolicidal agent, then reaspirate), which requires ultrasonographic guidance. Extreme care is essential to prevent spilling hydatid fluid into a body cavity because this may lead to anaphylactic shock. Albendazole therapy may be combined with PAIR from 10 days before to 30 days after the procedure. PAIR is a promising technique, although large-scale clinical trails have not yet been conducted.

· Some severe cases of alveolar hydatid disease have led to liver transplantation.

Consultations;

Recommended consultations for echinococcosis include surgeons and infectious diseases specialists with experience in diagnosis and treatment of the disease.

Diet

To prevent repeat infections, instruct patients to wash fruits and vegetables thoroughly and to consume only well-cooked meats.


Albendazole is considered the drug of choice. Praziquantel should not be used because the drug may aid alveolar hydatid growth.

Drug Category: Anthelmintics

Parasite biochemical pathways are sufficiently different from the human host to allow selective interference by chemotherapeutic agents administered in relatively small doses.

Drug Name;


Albendazole (Albenza)

Description


Decreases ATP production in tapeworms, causing energy depletion, immobilization, and death. To avoid inflammatory response in CNS, patient must be started on anticonvulsants and high-dose glucocorticoids. Administer with food to increase absorption.

Adult Dose;


<60 kg: 15 mg/kg/d PO divided q12h for 1-3 mo; not to exceed 800 mg/d
>60 kg: 400 mg PO bid for 1-3 mo

Pediatric Dose;


15 mg/kg/d PO divided q12h for 28 d; not to exceed 800 mg/d; may repeat dosage cycle as needed for 1-3 mo

Contraindications;


Documented hypersensitivity; hepatic disease

Interactions


Coadministration with carbamazepine may decrease efficacy; dexamethasone, cimetidine, ritonavir, and praziquantel may increase toxicity

Pregnancy


C - Safety for use during pregnancy has not been established.

Precautions


Discontinue use if liver function test results increase significantly (resume when levels decrease to pretest values); treat in liver damage with reduced doses, if at all; use only under constant medical supervision and regular monitoring of serum transaminase concentrations and of leucocyte, RBC, and platelet counts (rare cases of bone marrow damage have been reported)

Further Inpatient Care

· Patients with homogeneously calcified cysts do not usually require surgery and may undergo observation.

Deterrence/Prevention

· Preventive measures include the following:

o Wash all fruits and vegetables.

o Wash hands after handling pets or other canines.

o Use fences to keep animals away from gardens.

o Use gloves when handling foxes, coyotes, or other wild canines.

o Thoroughly cook meat, especially intestines prepared for human consumption.

Patient Education

· Public education about proper sanitation methods, especially sanitary sheep butchering, can dramatically reduce disease transmission.

· Avoid feeding entrails to canines.

Special Concerns

· Illegal importation of wild foxes from Alaska to southeastern states in the United States may increase the risk of echinococcosis transmission within these areas.