A Blood Based Real-time PCR for Diagnosis of Schistosomiasis

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A Blood Based Real-time PCR for Diagnosis of Schistosomiasis

Discussion


To our knowledge this is the first multicenter study on acute schistosomiasis in travellers residing in non-endemic countries. Moreover, it constitutes the largest prospective study on a blood based real-time PCR for the diagnosis of the disease. The high rate of positive PCR results within the study population may be in part attributed to the fact that the participating centres are well familiar with this otherwise rare disease of acute schistosomiasis as all centres are specialized in travel and tropical medicine.

The results presented here support previous findings that PCR for the detection of Schistosoma DNA in serum outperforms other diagnostic test such as serology or microscopy in the early phase of the infection, particular in the prepatent stage of human schistosomiasis, as demonstrated by the twelve patients presenting within 42 days after the suspected exposure. By PCR, a correct diagnosis was made in 95% of patients, despite the heterogeneous characteristics of the study population (17 endemic countries - cluster and single patients), which underlines the robustness of blood base real-time PCR testing for the diagnosis of acute schistosomiasis. In addition, PCR was able to exclude active Schistosoma infections in six patients, of whom 50% were suspected due to positive serology (1 positive, 2 inconclusive). Thus, the results support previous observations that the PCR test system used in this study is highly sensitive and specific. Accordingly, this test system has substantial added value for an early and rapid decision making to circumvent cost-intensive and time-consuming diagnostic procedures and to prevent unnecessary therapies. Cost for one PCR test (materials and reagents) are roughly 2 US$, thus the test is a useful addition in the differential diagnosis in patients with fever and eosinophilia after travelling to schistosomiasis endemic countries but is not generally recommended as a screening assay for returning travelers.

A considerable disadvantage of classical Schistosoma diagnostics is the persistence of serologic markers and egg shedding after successful treatment. Previous studies in experimentally infected mice suggested that blood based real-time PCR testing might be suitable not only for initial diagnosis but also for follow up monitoring. This is further supported by our findings in 8 patients, for whom follow up data were available. These results suggest a significant decline in Ct-values in response to treatment. Clearly negative PCRs are expected after a longer period of time due to the high sensitivity of PCR and the carry over of DNA from slowly degenerating eggs in the tissue. However, for a definite conclusion on the usefulness of PCR for monitoring of schistosomiasis treatment the follow-up of a larger number of patients is required.

The highly repetitive 121 bp DNA fragment used as target sequence in the real-time PCR assay constitutes about 12% of the S. mansoni genome. Although this sequence is detected in S. haematobium and S. japonicum as well sensitivity of PCR for the latter two species might be reduced. As far as species could be determined by microscopic egg morphology, the vast majority of cases in this study were due to infections by S. mansoni. Thus, our results may not allow generalization to infections with Schistosoma species other than S. mansoni. Evaluation of blood based PCR for the diagnosis of acute schistosomiasis due to other Schistosoma species such as S. haematobium or S. japonicum requires further studies. Previous results with the assay used in this study indicated that the performance of the PCR using the 121 bp tandem target sequence on 2 ml serum samples works very well in S. mansoni infection, but not in S. haematobium and in S. mekongi infection (Jan Clerinx, personal communication). Likewise, additional studies are required to evaluate the usefulness of blood-based PCR for the diagnosis of chronic schistosomiasis, in particular, in suspected cases from endemic countries, with a positive serology but negative parasite microscopy. Moreover, recent studies on, PCR performed with urine or fecal samples to detect S. haematobium or S. mansoni infection have shown rather promising results. However, further studies are required to determine, whether urine and/or feces PCR is sensitive to detect early schistosoma infection as it is the case in patients with acute schistosomiasis.

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