Coproantigens in taeniasis and echinococcosis

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Abstract

The application of modern immunodiagnostic or molecular diagnostic techniques has improved the diagnosis of the taeniid cestode infections, echinococcosis and taeniasis. One particularly promising approach is the detection of parasite-specific antigens in faeces (coproantigens). This approach has been applied to both Echinoccocus and Taenia species and it has gained increasingly widespread use. Taeniid coproantigen tests are based on either monoclonal or polyclonal antibodies raised against adult tapeworm antigens. These tests have the following common characteristics; they are largely genus-specific, specificity is high (> 95%), parasite antigen can be detected in faeces weeks prior to patency, levels of coproantigen are independent of egg output, coproantigen is stable for days at a range of temperatures (− 80 °C to 35 °C), for several months in formalin-fixed faecal samples, and coproantigen levels drop rapidly (1–5 days) following successful treatment. In the genus Taenia, most work has been done on Taenia solium and coproantigen tests have reliably detected many more tapeworm carriers than microscopy. For Echinococcus species, there is a broad positive correlation between test sensitivity and worm burden with a reliable threshold level for the test of > 50 worms. Characterisation of taeniid coproantigens in order to further improve the tests is ongoing. Studies indicate taeniid coproantigens to include high molecular weight (> 150 kDa), heavily glycosylated molecules with carbohydrate moieties contributing substantially to the levels of antigen detected in faeces. Application of the existing coproantigen tests in epidemiological and control programmes for Echinococcus and Taenia species infection has begun to contribute to an improved understanding of transmission and of surveillance of these important zoonotic cestodes.

Section snippets

Parasitological diagnosis of taeniid cestodes

Broadly speaking the definitive host range for the zoonotic taeniid species is more restricted than the intermediate host range of the same parasite. For example, humans represent the sole natural definitive host for both Taenia solium and Taenia saginata. This therefore leaves these two species theoretically susceptible to interventions designed at reducing the rate of humans carrying the intestinal stage. Thus, from a control perspective, the diagnosis of the adult stages of these parasites

Alternatives to parasitological diagnosis

The application of alternative means to the diagnosis of these cestodes is not new. Immunodiagnosis of intestinal Taenia was first attempted in the early years of the 20th century and a number of immunological and latterly molecular approaches have been applied to the diagnosis of intestinal stages of both Echinococcus and Taenia in the intervening years [4], [5]. One of these approaches will be reviewed here.

Coproantigen-based diagnosis

The diagnosis of intestinal infections by detection of pathogen-/parasite-specific antigens in faeces (coproantigens) is an approach now widely applied to a broad range of infectious organisms. If these organisms release products of metabolism into the intestine, these should be amenable to immunological detection. Theoretically, if these antigens are not directly associated with parasite reproduction, they should be present when reproductive material (like taeniid eggs) is absent from the

Time course infection

Babos and Nemeth [6] first showed antigen to be present in the faeces of E. granulosus-infected dogs prior to the onset of egg production. Follow-up on work with taeniid cestodes in time course infections demonstrated that antigen is present in the faeces of Taenia hydatigena-, Taenia pisiformis- and E. granulosus-infected dogs before patency, apparently reaching a plateau several weeks before onset of egg production and is independent of egg output thereafter [10], [13], [19], [28]. Where

Sensitivity/Worm burden determination

Of the Taenia species infecting humans, most coproantigen application has been carried out on T. solium. These studies have utilised either a microplate format ELISA test [9] or a dipstick format test [11]. Several studies involving follow-up on coproantigen positive results to determine firstly whether the diagnostic result could be parasitologically confirmed, and secondly the species of tapeworm involved have indicated that coproantigen testing by microplate ELISA detects over twice as many

Specificity

Despite most of the described tests being based on polyclonal antibodies raised to relatively crude adult worm derived antigens, coproantigen tests for taeniid species have proved remarkably specific. Tests raised using antibodies to Taenia species provide genus-specific results: polyclonal rabbit sera raised against T. saginata, T. solium, or T. hydatigena have, for example, proved capable of detecting antigens in faeces of hosts harbouring Taenia genus infections but are negative when tested

Coproantigen stability

Taenia or Echinococcus coproantigens have been shown to be very stable, being capable of detection in faecal samples stored at room temperature (20 °C) over several days [13]. Samples can be also be reliably stored in 5% or 10% formalin solution for several months [10], [15]. This considerably simplifies sample collection and storage, particularly under field conditions. It should be noted however that coproantigens are not detectable in faeces treated with organic solvents, and are not

Field applicability/epidemiological studies

Since first being described, taeniid coproantigen tests have generated useful data through their application in epidemiological studies on the important zoonotic species. Some of the aspects discussed above should be taken into account when applying such tests. Nonetheless, our understanding of the epidemiology of the intestinal stages of T. solium, E. granulosus and E. multilocularis has benefited from the application of coproantigen tests. Some specific examples are provided here:

Antigen characterisation

The existing coproantigens tests, whilst useful, are still not optimized in all respects. Issues remain with both sensitivity and specificity, and a reasonable level of laboratory infrastructure is needed to use most of the test formats developed to date. Ideally, improved tests could be capable for example of differentiating the human Taenia species, providing reliable estimate of intestinal Echinococcus burden and/or being easier to apply with minimal laboratory infrastructure.

In this regard,

Acknowledgements

The Cestode Zoonoses Research Group is grateful for funding support from the Wellcome Trust, European Commission, NIH/NSF and British Council.

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