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.
Notwithstanding the great number of tests applicable to the schistosomiasis diagnosis, few have resisted to scrutiny of effectiveness, reproducibility, cross reactivity and predictive values. The microscopic demonstration of the parasite's eggs in feces or urine remains the more widespread tool, mainly due to its low operational cost and its feasibility in situations of precarious laboratorial structure. A significant increase in the sensitivity of the parasitological examination may be reached by the exam of a larger number of samples, indicated especially after treatment and in low prevalence settings where expected number of eggs is usually minimal.Up to the moment antibody detection using crude antigens offers low specificity and consequent low positive predictive value making these methods unsuitable for determination of prevalence and evaluation of control measures.Detecting eggs or adult worm antigens in urine and sera of Schistosoma sp. infected individuals differentiates past and current infections. The specificity close to 100% is the main advantage that the detection of circulating antigens offers over the search for antibodies, but the disadvantages include low sensitivity in light infections, high cost, difficult approach and dependence on monoclonal antibodies.Recently, our group has developed a novel method for the detection of S. mansoni in human samples (Pontes (Steiner et al. 1995), a simple and inexpensive DNA extraction technique and a rapid two step polymerase chain reaction (PCR) it was possible to amplify S. mansoni DNA in human fecal and serum samples. Primers were designed to amplify the 121 bp tandem repeat DNA sequence of S. mansoni described by Hamburger et al. (1991).The high sensitivity of the approach enabled the detection of the parasite DNA in fecal samples and was confirmed using artificial mixtures of eggs and feces: four positive fecal samples were prepared starting from a sample containing 216 eggs/g (determined by Kato-Katz stool examination) and mixed to negative feces on ten fold dilutions resulting on samples with estimated concentrations of approximately 21.6, 2.16 and 0.216 eggs/g of feces. DNA was still detected in the sample containing 2.16 eggs/g of feces, which makes this technique ten times more sensitive than the Kato-Katz examination. A detection limit of 1 fg of Schistosoma sp. DNA was determined when pure DNA was used as PCR template.The amplification reaction showed to be specific without cross-reaction with DNA from four related helminthic parasites: Ascaris lumbricoides, Ancylostoma duodenales, Taenia solium, Trichiuris trichiuria.On a first step evaluation of the efficacy of PCR for a community-based diagnosis (manuscript in preparation), 194 inhabitants from Comercinho, State of Minas Gerais, Brazil, provided three stool samples. Prevalence of infection was 25.3%, 29.4% and 30.9% examining one, two and three fecal samples by the Kato-Katz method, respectively and 38.1% examining one fecal sample by the PCR. No eggs were found in the feces of 16 individual...
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