A novel method for the detection of Schistosoma mansoni in human samples that is based on the amplification of a highly repeated DNA sequence has been developed. By use of simple DNA extraction techniques and a rapid 2-step polymerase chain reaction (PCR), it was possible to amplify S. mansoni DNA in human fecal and serum samples. The high sensitivity of the approach enabled the detection of the parasite DNA in fecal samples containing as few as 2.4 eggs per gram of feces, which makes it 10 times more sensitive than the Kato-Katz examination. A detection limit of I fg of Schistosoma sp. DNA was determined when pure DNA was used as PCR template. The amplification reaction showed to be specific giving no cross-reaction with DNA from other helminths. The PCR assay developed in this study may constitute a valuable alternative for the diagnosis of the Schistosoma sp. infection.

Detection by polymerase chain reaction of Schistosoma mansoni DNA in human serum and feces.

Introduction
Schistosomiasis (bilharzia), one of the most relevant parasitoses of humans, is confirmed by microscopic detection of eggs in stool, urine, or organ biopsies. The sensitivity of these procedures is variable due to fluctuation of egg shedding. Serological tests on the other hand do not distinguish between active and past disease. In patients with acute disease (Katayama syndrome), both serology and direct detection may produce false negative results. To overcome these obstacles, we developed a novel diagnostic strategy, following the rationale that Schistosoma DNA may be liberated as a result of parasite turnover and reach the blood. Cell-free parasite DNA (CFPD) was detected in plasma by PCR.

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Diagnosing Schistosomiasis by Detection of Cell-Free Parasite DNA in Human Plasma

DNA probe and PCR-based assays to identify and detect parasites are technically complex; however, they have high sensitivity, directly detect parasites independent of the immunocompetence or previous clinical history of the patient, and can distinguish between organisms that are morphologically similar. Diagnosis of parasites is often based on direct detection by microscopy, which is insensitive and laborious and can lack specificity. Most PCR-based assays were more sensitive than DNA probe assays. The development of PCR-based diagnostic assays requires multiple steps following the initial selection of oligonucleotide primers and reporter probe. Generally, the ability to detect the DNA of one parasite was attained by PCR; however, advances in the preparation of samples for PCR (extraction of DNA while removing PCR inhibitors) will be required to achieve that sensitivity with human specimens. Preliminary PCR systems have been developed for many different parasites, yet few have been evaluated with a large number of clinical specimens and/or under field conditions. Those evaluations are essential for determination of clinical and field utility and performance and of the most appropriate application of the assay. Several situations in which PCR-based diagnosis will result in epidemiologic, medical, or public health advances have been identified.

DNA probes and PCR for diagnosis of parasitic infections

Fecal samples from 194 individuals living in an area of Brazil endemic for Schistosoma mansoni were analyzed by a polymerase chain reaction (PCR) and the Kato-Katz parasitologic examination. Statistical analysis of the results showed a kappa index of 0.8 between the two methods. The prevalence of infection was 30.9% in three fecal samples examined by the Kato-Katz method, but 38.1% in one fecal sample examined by the PCR technique. Repeated survey of discordant results showed that five (41.6%) of 12 parasitologically negative cases for which PCR gave positive results were misdiagnosed by Kato-Katz examinations. The PCR technique showed a sensitivity of 96.7% and a specificity of 88% when the parasitologic examination was used as the reference test. The efficacy of cure with praziquantel was 87.8% in three parasitologic stool examinations and 75.6% in one PCR survey. These results demonstrate that the PCR assay might be a valuable alternative for diagnosing Schistosoma infections.

Comparison of a polymerase chain reaction and the Kato-Katz technique for diagnosing infection with Schistosoma mansoni.

http://www.bio.umass.edu/micro/klingbeil/590s/Reading/Schistotrans2006.pdf

Tikva Turetski

Papers

Highly repeated short DNA sequences in the genome of Schistosoma mansoni recognized by a species-specific probe.