This study was designed to evaluate the diagnostic potential of a RDT (SmCTF-RDT) which is capable of detecting antibodies with specificity for the two main Schistosoma species occurring in Africa, S. mansoni and S. haematobium. Here we have determined – for the first time – the performance of the SmCTF-RDT among preschool-aged children who are known to have mainly low intensity schistosome infections [30–32], which may be analagous to populations subjected to repeated rounds of preventive chemotherapy such as in areas targeted for schistosomiasis elimination .
We found that of the 118 preschool-aged children with complete data records, prevalence of S. mansoni and S. haematobium infections, according to egg-detection methods, was 20.3% and 5.1%, respectively. Around 70% of the children infected harboured light infections. This is in agreement with recent studies that have shown low prevalence and mainly light intensity infections in preschool-aged children [30–32]. However, egg-detection methods are known to lack sensitivity, particularly when infection intensities are light [1–5, 34]. The SmCTF-RDT employed here estimated that the prevalence of S. mansoni and S. haematobium infections was 66.9%, more than three times higher than that estimated by egg-detection methods. Single and duplicate POC-CCA tests estimated prevalence of S. mansoni infections to be 51.7% and 63.6%, respectively.
This study showed that a single SmCTF-RDT is more sensitive than a single Kato-Katz thick smear and at least as sensitive as duplicate Kato-Katz thick smears for the diagnosis of S. mansoni infection. It is also as sensitive as one urine filtration for the diagnosis of S. haematobium infection.
Specificity of the SmCTF-RDT was low compared to egg-detection methods, but this may be a direct consequence of their relative insensitivity. The specificity of the POC-CCA test was also low when using microscopy as the reference standard. There was little agreement between the ‘false-positive’ results seen with each assay and so this raises the question: which of the ‘false-positive’ results, by either the SmCTF-RDT or the POC-CCA, are in fact ‘true-positives’? Comparison of these two immunoassays with a potentially more accurate reference test (e.g. polymerase chain reaction, PCR) may provide an answer [35, 36].
Whilst the traditional parasitological methods employed here have their advantages, such as high specificity, provision of a quantitative measure of infection, and the Kato-Katz has the ability to concurrently diagnose S. mansoni and soil-transmitted helminths [4, 5, 37], they are insensitive, laborious and relatively expensive to perform . For large-scale schistosomiasis control programmes, a RDT is much more likely to be useful, particularly one that meets the ASSURED criteria for diagnostic tests . Both the SmCTF-RDT and POC-CCA are user-friendly, rapid, equipment-free and deliverable. They can be read by non-specialized individuals with limited training, they do not require use of a laboratory or even electricity, and they also give results within 20 min allowing diagnosis at the point-of-care.
An advantage of the commercially-available POC-CCA test over the SmCTF-RDT is that it works with urine rather than blood and so is less invasive. Since it detects circulating antigen rather than antibody, it is also able to discriminate between active and past infections. However, whilst the POC-CCA test is estimated to be at least as sensitive as one Kato-Katz thick smear for the diagnosis of S. mansoni infection, it unfortunately does not work well for the diagnosis of S. haematobium infections. The SmCTF-RDT, whilst unable to distinguish between them, has the ability to detect both S. mansoni and S. haematobium infections. Many areas of sub-Saharan Africa are co-endemic for both schistosome species, and so a test able to detect both infections will be particularly useful. Another downfall of the POC-CCA cassette is its cost, currently around US$ 1.75 per test. The SmCTF-RDT employed here is not yet commercially available, but it is estimated that it will cost less than US$ 1 per test.