Intestinal schistosomiasis (caused by the parasite Schistosoma mansoni) can be detected easily and accurately by measuring levels of an excreted parasite antigen called circulating cathodic antigen, or CCA, in an individual’s urine, according to new research published in PLoS Neglected Tropical Diseases. This method has been found to be at least as effective as the existing ‘gold standard’ diagnostic test the Kato-Katz assay—literally counting the number of parasite eggs per gram of poo—and also to blood tests that test for parasite-specific antibodies. Moreover, switching to these urine-based tests for schistomiasis diagnosis is, quite frankly, preferable to sifting through someone’s faeces (messy, risky, time consuming, and actually not that useful), and could also make it easier to assess the prevalence and resolution of S. mansoni infection, which could be beneficial for disease control programmes.
Taking advantage of a large study to assess schistosomiasis prevalence in young children, the researchers tested the sensitivity and specificity of two different types of CCA urine tests—a strip test designed for use in the laboratory (unfortunately no longer under production) and a cassette test suited for field use—and compared them to both the Kato-Katz technique for assessing S. mansoni egg burden and ELISAs to detect anti-schistosome antibodies. Stool, urine and blood samples were collected from 484 children (aged 1–15 years old) who lived in the village, Usoma, in Western Kenya, which is near Lake Victoria, an area known to have high S. mansoni infection rates. Even when taking into account the limitations of the Kato-Katz assay in their analysis, the urine-based diagnostics tests were still sensitive and specific with CCA test levels reflecting the stool egg burden, and thus the intensity of the infection (a finding that confirmed other study results). Moreover, the tests worked even if the children were also infected with other parasites (Ascaris lumbricoides, hookworm or Trichuris trichuria), which can affect the performance of some diagnostic tests.
The CCA diagnostic test for schistomiasis has now been shown to be effective in areas with a high burden of the disease, but more work is needed to see whether it is sensitive enough to detect disease in areas with low S. mansoni infection rates. Whether these urine tests will eventually replace the tried and tested Kato-Katz technique remains to be seen; one advantage of examining stool samples is that other parasites may be found and identified at the same time, which is not possible with the CCA test as it is specific for schistosomiasis.
Shane, H., Verani, J., Abudho, B., Montgomery, S., Blackstock, A., Mwinzi, P., Butler, S., Karanja, D., & Secor, W. (2011). Evaluation of Urine CCA Assays for Detection of Schistosoma mansoni Infection in Western Kenya PLoS Neglected Tropical Diseases, 5 (1) DOI: 10.1371/journal.pntd.0000951
geek! 