Microscopy and Molecular Assay Compared for Schistosomiasis Diagnosis

The current reference test for the detection of Schistosoma mansoni in endemic areas is stool microscopy based on one or more Kato-Katz stool smears, but this technique is highly observer-dependent and has suboptimal sensitivity.

Polymerase chain reaction (PCR), in a multiplex format, has the some advantages over microscopy, with greater flexibility as a multiplex PCR can detect all Schistosoma and other helminth species at the same time, and at any moment after the stool has been collected.

Scientists at Leiden University Medical Center (the Netherlands) led an international team to compare the performance of stool microscopy with the highly specific real-time polymerase chain reaction (RT-PCR) for the detection and quantification of parasite specific DNA. Microscopy and PCR were performed in a Senegalese community of 197 in an area with high S. mansoni transmission and co-occurrence of S. haematobium, and in 760 Kenyan schoolchildren from an area with comparatively low S. mansoni transmission.

In Senegal, two stool and two urine samples were collected from each participant on consecutive days. From each stool sample, a duplicate 25 mg Kato-Katz slide was prepared for quantitative detection of Schistosoma spp. eggs and qualitative diagnosis of soil transmitted helminths (STHs) Ascaris lumbricoides and Trichuris trichiura by microscopy. A Schistosoma multiplex real-time PCR (Schisto-PCR) was performed on DNA isolated from feces samples. This PCR targets the Schistosoma-specific internal transcriber-spacer-2 (ITS2) sequence of S. mansoni, S. haematobium, and S. intercalatum. Amplification, detection and data analysis were performed with the CFX96 Real-Time System version 1.1 (Bio-Rad; Hercules, CA, USA).

Despite the differences in Schistosoma endemicity the PCR performed very similarly in both areas; 13%–15% more infections were detected by PCR when comparing to microscopy of a single stool sample. Even when two to three stool samples were used for microscopy, PCR on one stool sample detected more infections, especially in people with light-intensity infections and in children from low-risk schools. The low prevalence of soil-transmitted helminthiasis in both populations was confirmed by an additional multiplex PCR.

The authors concluded that the ITS2-based PCR was more sensitive than standard microscopy in detecting Schistosoma spp. This would be particularly useful for S. mansoni detection in low transmission areas, and post-control settings, and as such improve schistosomiasis control programs, epidemiological research, and quality control of microscopy. Moreover, it can be complemented with other multiplex real-time PCRs to detect a wider range of helminths and thus enhance effectiveness of current integrated control and elimination strategies for neglected tropical diseases. The study was published on July 28, 2015, in the journal Public Library of Science Neglected Tropical Diseases.

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