Cystic echinococcosis (CE) is an important helminthic zoonotic disease caused by the
Cystic echinococcosis (CE) is a parasitic disease caused by the larval stage (metacestode) of
CE diagnosis and monitoring firstly rely on imaging techniques. Ultrasonography (US) standardized classification of stage-specific cystic images has been issued by the WHO Informal Working Group on Echinococcosis (WHO-IWGE) for the diagnosis and the clinical management of CE [
In this paper we summarize the current knowledge on the use of HF for human CE diagnosis. Additionally, results obtained from different purified fractions of parasite antigens, recombinant antigens, and synthetic peptides are also revised. A comprehensive review of the different available antigens and their performance in the diagnosis of CE was published by Carmena and colleagues [
HF is a complex mixture of parasite-derived proteins, mainly produced by the germinal layer of the cyst. Some of the HF components have been characterized as highly immunogenic, reaching the host environment and triggering antibody responses. The HF is the main antigenic component in the majority of commercially and in-house serological assays. This antigen mixture is used in several techniques such as the enzyme linked immunosorbent assay (ELISA), the indirect haemagglutination test (IHA), and the immunoblotting (IB). Both the ELISA and the IHA are usually the first line tests for CE patients, while the IB is used as confirmatory test. As mentioned, the use of HF for the detection of CE specific antibodies is limited by several drawbacks. First, a percentage of CE patients are serological negative against HF. Specifically, the use of HF for the detection of total IgG in ELISA test leads to variable results regarding Se and Sp. In Table
Performance of the hydatid cyst fluid in ELISA test for the detection of total IgG in CE patients (articles published from 2006).
Number of CE patients | Confirmatory test | Sensitivity (%) | Negative serology more frequent when | Reference |
---|---|---|---|---|
23 | Histopathology | 100 | Not specified |
[ |
5 | Imaging techniques plus serology | 80 | Not specified | |
13 | Serology | 100 | Not specified | |
|
||||
41 | Surgery | 95.1 | Not specified | [ |
|
||||
6 | Imaging techniques | 66.7 | Cyst location other than liver | [ |
|
||||
144 | Imaging techniques | 92.4 | CE1, CE4, and CE5 cyst stages | [ |
|
||||
123 | Imaging techniques | 64.8 |
CE4 and CE5 cyst stages, no pretreatment | [ |
|
||||
59 | Surgery | 95.1 | Not specified | [ |
|
||||
10 | Surgery | 100 | Not specified | [ |
|
||||
54 | Surgery | 81.5 | Single cyst, no pretreatment |
[ |
186 | Surgery | 83.3 | ||
|
||||
32 | Imaging techniques | 93.8 | CE4 and CE5 cyst stages | [ |
|
||||
155 | Surgery | 90.3 |
Not specified | [ |
|
||||
40 | Surgery | 92.5 | Not specified | [ |
|
||||
47 | Surgery | 95.71 | HF collected from cysts of different hosts or of different anatomical locations from the same host |
[ |
93.62 | ||||
91.43 | ||||
97.84 | ||||
93.65 | ||||
78.56 | ||||
72.27 | ||||
|
||||
63 | Surgery | 90.5 | Not specified |
[ |
82.4 |
||||
|
||||
68 | Imaging techniques | 92.6 | Lung cysts | [ |
The detection of antibodies other than IgG has shown some promising results in relation to cyst activity, relapses, and follow-up. It has been shown that both IgG2 and IgG4 could be related to cyst stages, disease evolution, and relapses [
In an attempt to overcome the problems related to the use of HF for the detection of antibodies in CE patients, many authors have described the production and the use in serological tests of partially purified native antigens, recombinant antigens, and synthetic peptides. These are mainly represented by the two most immunogenic antigens in HF: antigen B (EgAgB) and antigen 5 (EgAg5).
EgAgB is a polymeric protein of 120–160 kDa that dissociates under reducing conditions in 8, 16, and 20–24 kDa subunits. Its biological role includes the protease inhibitor activity, neutrophil chemotaxis inhibition, triggering of nonprotective Th2 responses, induction of apoptosis of immune cells, and sequestration of xenobiotics [
Use of antigen B and derivatives for the diagnosis of CE patients (articles published from 2006).
Antigen | Number of CE patients | Confirmatory test | Technique, antibody | Sensitivity (%) | Negative serology more frequent when | Reference |
---|---|---|---|---|---|---|
Purified | 21 | Surgery | Immunochromatography, IgG | 100 | Not specified |
[ |
Immunochromatography, IgG4 | 95 | |||||
23 | Surgery | ELISA, IgG | 100 | Not specified |
[ | |
5 | Imaging techniques and serology |
80 | ||||
13 | Serology |
0 | ||||
32 | Imaging techniques | ELISA, IgG | 93.8 | CE4 and CE5 cyst stages | [ | |
40 | Surgery | ELISA, IgG | 87.5 | Not specified |
[ | |
ELISA, IgG4 | 80 | |||||
108 | Surgery | DIGFA, IgG | 89.8 | Cyst location other than liver; CE1, CE4, and CE5 cyst stages; small cysts | [ | |
113 | Imaging techniques | DIGFA, IgG | 92.9 | Not specified | [ | |
35 | Imaging techniques | ELISA, IgG | 54 | CE1, CE4, and CE5 cyst stages | [ | |
56 | Surgery | ELISA, all (Protein G) | 96.91 | Not specified |
[ | |
82.12 | ||||||
|
||||||
Recombinant B1 | 31 | Surgery | ELISA, all (Protein G) | 71 | Parasite genotype other than G1 | [ |
124 | Surgery |
ELISA, IgG | 83 | Not specified | [ | |
246 | Imaging techniques | ELISA, all (Protein G) | 77.6 | Not specified | [ | |
113 | Imaging techniques | DIGFA, IgG | 77.9 | Not specified | [ | |
56 | Surgery | ELISA, all (Protein G) | 94.6 | Not specified | [ | |
123 | Imaging techniques | ELISA, IgG | 73.9 | CE4 and CE5 cyst stages, no pretreatment | [ | |
|
||||||
Recombinant B2 | 543 | Surgery | ELISA, IgG | 77.8 | Single cyst, no pretreatment |
[ |
1864 | 79 | |||||
124 | Surgery |
ELISA, IgG | 62.9 | Not specified | [ | |
|
||||||
Recombinant 2B2 | 543 | Surgery | ELISA, IgG | 92.6 | Single cyst, no pretreatment |
[ |
1864 | 87.6 | |||||
|
||||||
Recombinant B3 | 124 | Surgery |
ELISA, IgG | 29 | Not specified | [ |
|
||||||
Recombinant B4 | 124 | Surgery |
ELISA, IgG | 75.8 | Not specified | [ |
36 | Surgery | ELISA, IgG | 91.7 | Not specified | [ | |
|
||||||
Recombinant B5 | 124 | Surgery |
ELISA, IgG | 41.3 | Not specified | [ |
|
||||||
P176 peptide | 61 | Surgery | ELISA, IgG | 78.7 | Lung cysts, no complications, single cyst, and small cysts | [ |
63 | Surgery | ELISA, IgG | 23.8 | Not specified | [ | |
|
||||||
Long D8-9 peptide | 35 | Imaging techniques | ELISA IgG | 74.3 | Not specified | [ |
In its recombinant form (Rec), mainly two isoforms have been assayed: RecEgAgB1 and RecEgAgB2. RecEgAgB1 gives rise to variable sensitivities, ranging from 55% to 84% [
As mentioned, several synthetic peptides derived from EgAgB1 have been also tested in ELISA for the detection of specific antibodies. From those tests, the most promising results have been obtained with the p176 peptide ([
In the light of the results obtained by several authors using EgAgB, this antigenic family looks promising for the serodiagnosis of CE patients and potentially useful in the follow-up. Anyway, testing of this antigen family obtained in a standardized format, with a well-defined and wide panel of CE samples, including the clinical information influencing the interpretation of the test (the parasite genotype, location, number, size, and type of cysts, and presence of complications and coinfections), should be further performed.
Antigen 5 (EgAg5) is also an abundant highly immunogenic component of HF. EgAg5 is a thermolabile protein of around 400 kDa, composed of two subunits of 57 and 67 kDa. Studies on the N-terminal sequence of the 38 kDa subunit have shown that several isoforms exist similar to EgAgB, and thus EgAg5 could be also coded by a multigenic family [
Molecules very similar to EgAg5 (more than 90% identity) are also expressed by
The most frequently used crude extract of
Additionally, several recombinant antigens different from EgAgB and EgAg5 have been obtained by several authors and tested for the diagnosis of CE. These include, among others, the malate dehydrogenase (RecEgMDH), the calcium binding protein (RecEgCaBP), the actin filament fragmenting protein (RecEgAFFP), the RecEgEpC1, the thioredoxin peroxidase (RecEGTPx), and the RecEg19 [
The clinical management of CE should be based on recommendations done by experts and driven by cystic stages identification [
Some antigens could be mainly expressed by defined cyst stages, showing that serology could be potentially useful for the definition of cyst activity and thus for the clinical management of CE patients. As mentioned, the HF has been also evaluated for the follow-up of patients treated by surgery and/or chemotherapy, but its usefulness is hampered by the long persistence of antibodies in patients with nonactive cysts [
Similarly, other authors have detected the loss of defined bands in IB after cure. The identification and characterization of the above-mentioned antigens could support the clinical decision making actually based on imaging techniques. Similarly, the shift of defined subisotypes of IgG antibodies during cyst evolution could be used for the follow-up of CE patients. As mentioned, IgG4 levels could be correlated with cyst activity, and declining of the levels of isotypes other than IgG seems to be useful to define the success of treatment [
Diagnosis and follow-up of CE patients are mainly based on imaging techniques. These can be used for the identification of cystic stages, leading to a stage-specific approach in CE clinical management. Serological tools supporting imaging techniques would be desirable. However the available tests are based on antibodies against crude antigens and thus marred by poor Sp and Se, with low or no usefulness for the follow-up of patients during the treatment. Specific recombinant antigens have good potential as diagnostic and follow-up tools for CE, but progress in this field is hampered by lack of standardization. Thus, a challenge still exists to develop a reliable world standard based on serology for the diagnosis and monitoring of CE patients. In this respect, a multicentre study with a wide panel of sera from CE patients, including relevant clinical data to properly define the usefulness of the available recombinant antigens, should be performed. Along this path, a project focusing on CE has been recently funded by the European commission under the Seventh Framework Programme (HERACLES;
The funding sources had no involvement in the preparation, ideas, writing, interpretation, or the decision to submit this paper.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research that will lead to these results has received funding from the European Community’s Seventh Framework Programme under the Grant agreement 602051 (Project HERACLES: Human cystic Echinococcosis ReseArch in CentraL and Eastern Societies;