Human echinococcosis is a zoonotic disease caused by tapeworms of the genus Echinococcus.
Echinococcosis occurs in 2 main forms:
cystic echinococcosis (CE) caused by infection with E. granulosus sensu lato;
alveolar echinococcosis (AE), caused by infection with E. multilocularis.
CE is principally maintained in a dog–sheep–dog cycle, yet several other domestic animals may be involved, including goats, swine, horses, and cattle. The parasite is transmitted to dogs when they ingest the organs of other animals that contain cysts. The cysts develop into adult tapeworms in the dog. Infected dogs shed tapeworm eggs in their feces which contaminate the ground. Intermediate hosts ingest tapeworm eggs in the contaminated ground; once ingested, the eggs hatch and develop into cysts in the internal organs.
E.multilocularis is mainly transmitted in a wildlife cycle involving red foxes as final hosts and rodents as intermediate hosts, although domestic dogs, cats and other species are also sporadically infected.
Transmission of echinococcus eggs to humans occurs both through direct contact with infected dogs, particularly intimate contact between children and their pet dogs, as well as by ingestion of soil, water, and vegetables contaminated with infected dog feces.
Echinococcus eggs that have been deposited in soil can stay viable for up to a year and survive freezing conditions.
The symptoms of CE are caused by production of cysts occurring in the course of many years. The cysts are usually 1-7cm in size, although they can be as big as 30 cm. One or more cysts are located mainly in the liver or lungs, and less frequently in other organs.
Germinal layer of cysts generates daughter cysts and protoscoleces into a central cavity filled with a clear “hydatid” fluid; it is surrounded first by an acellular laminated layer, then by the host reactive cells. Daughter vesicles of variable size and single protoscoleces may be present inside the “mother” cyst.
Cysts may persist without changes for years whereas alveoles are characterized by progressive growth.
The asymptomatic incubation period of CE can last many years until cysts grow to an extent that triggers clinical signs. Clinical symptoms of CE usually occur when the cyst compresses or ruptures into neighboring structures. Abdominal pain, nausea and vomiting are commonly seen when cysts occur in the liver. Rupture of cyst usually results in acute or chronic allergic manifestations.
CE is most commonly found in people involved in raising sheep and swine.
E. granulosus has several strains with distinct morphological, epidemiological and clinical characteristics, regarded by many as distinct species. Sheep strain (G1) and pig strain (G7) are dominant in Central and Eastern Europe.
CE is globally distributed and found especially in pastoral communities of South America, the Mediterranean littoral, Eastern Europe, the Near and Middle East, East Africa, Central Asia, China and Russia.
Worldwide, there may be > 1 million people living with echinococcosis. Prevalence levels for CE as high as 5%–10% may occur in parts of Argentina, Peru, East Africa, Central Asia and China.
A vigorous inflammatory and fibrous tissue reaction usually surrounds the larval mass.
A majority of AE/CE patients respond with a marked synthesis of parasitespecific antibodies, including all isotypes of immunoglobulins; only few patients fail to demonstrate a humoral immune response.
Generally, a decrease of parasite-specific serum antibody concentration can be observed after resection of larval mass. Presence of specific antibodies reflects the presence of a viable parasite. A restricted decrease of specific IgG antibody concentration was observed in most cases with favorable clinical courses 2 years after radical treatment, whereas specific IgA and IgE disappeared within 2 years in the same successfully treated patients.
The humoral immune response in humans can vary enormously, evidenced by the different patterns of parasite antigens recognized in the course of the immune response. Antibodies can be demonstrated against surface structures and soluble antigens derived from protoscoleces, germinal layer, laminated layer, and vesicular fluid.
Immunodiagnosis of human CE is currently best performed by use of native or recombinant antigen B (AGB) present in high concentrations in cyst fluid. AGB is a protease inhibitor that functions in suppression of immune response. High antigenicity of AGB is explained by its structure and ability to pass across the cyst wall and reach the host internal milieu. AGB is a 120-kDa protein composed of multimers of homologous but different 8-kDa subunits (AgB8/1, AgB8/2, and AgB8/3). AgB8/1 appears to be most useful for highly genusspecific serodiagnosis of human CE.
Most serological tests for immunodiagnosis of human AE employ heterologous E. granulosus antigen (E. granulosus hydatid fluid).
The diagnostic confirmation of AE/CE case according to WHO necessitates fulfillment of following criteria: 1)clinical and epidemiological history compatible with AE/CE; 2)imaging findings (MRI, ultrasound imaging); 3)serologic findings on 2 different tests; 4) demonstration of parasite viability (CE) or differentiation from liver carcinoma (AE).
Clinic symptoms of AE/CE are not specific enough, especially resembling symptoms of chronic hepatitis, liver cirrhosis, and liver carcinoma. Noninvasive imaging techniques such as computed tomography (CT) scans, magnetic resonance imaging (MRI), and ultrasound (US) imaging have proved valuable for diagnosis and preoperative evaluation by localizing and staging the lesion. CE/AE must be differentiated from benign cysts, cavitary tuberculosis, mycoses, abscesses, and benign or malignant neoplasms. Serologic tests, such as enzyme-linked immunosorbent assay (ELISA) and indirect hemagglutination test, are methods permitting such differentiation. Specific confirmation can be obtained by demonstrating antibodies to echinococcal antigens by immunodiffusion or immunoblot assays. Microscope examination of protoscoleces after cyst fluid aspiration gives evidence for viability of a cyst.
Besides its use for confirmation of suspected CE/AE case, serological testing could be also used in following situations:
1) For large-scale serological screening of human populations at risk or living in endemic areas. The level of parasite resectibility is heavily dependent upon the efficacy of screening programs for early diagnosis;
2)For monitoring therapy success. Complete surgical removal of the lesions results in a rapid decrease of specific antibodies, but in the case of undetected residual disease or relapse they persist.