Avaliação de métodos diagnósticos visando a interrupção de transmissão da esquistossomose

Abstract

Schistosomiasis is an infection that affects around 240 million people worldwide, and it is estimated that more than 700 million people live at risk of infection in endemic areas. The direct visualization of parasite eggs in the feces of the infected individual is the confirmatory diagnosis, however, negative results do not rule out the occurrence of infection, especially in areas of low endemicity, where individuals have low parasitic loads, consequently, a reduced number of eggs in the feces. feces, which may lead to underestimated prevalence rates or false negative results when low-sensitivity diagnostic methods are applied. One of the priorities of the World Health Organization is the development of studies in order to establish criteria to certify the interruption of transmission in the future, however, new diagnostic screening tools are necessary to overcome the less than adequate sensitivity of Kato-Katz, currently used as thick smear method. Although development of such tools is ongoing, extensive rigorous performance evaluation is required before they are introduced as resources for control activities in endemic areas. From this perspective, the main objective of this work was to evaluate diagnostic methods for schistosomiasis mansoni with cooperating to interrupting transmission. The methods used in the comparative evaluation were: For the parasitological examination of feces, a sowing experiment was carried out using two techniques, the TF-Test® and Lutz methods, in which different loads of S. mansoni eggs were extracted from mouse liver experimentally infected individuals were sown (50, 100, 200 and 450) in eight sets containing approximately 1g of feces for each parasite load sown. For serological screening, we employed the IgG/IgM ELISA method, using soluble egg antigen (SEA) and adult worm antigen (AWA) respectively, and an immunochromatographic test using adult worm antigen (AWA). The Helmintex (HTX) method was used to define truly positive samples. Serum samples were collected in the town of Candeal, Municipality of Estância, State of Sergipe, Northeast Brazil. Concomitant fecal samples were examined by the HTX method for egg detection, as a reference method, describing the following egg load categories: ≤1 e >1 OPG. The egg recovery rate varied between 32% (450 eggs) and 7% (50 eggs) for TF-Test and between 6% (200) and 5% (450, 100 and 50 eggs) for Lutz. Examining only 1 slide, sensitivity (S) was 100%, 38%, 38% and 25% for TF-Test and 100%, 100%, 38% and 50% for Lutz, respectively for 450, 200, 100 and 50 eggs. The results allow a more detailed evaluation of the performance of the TF-Test and Lutz, where a progressive reduction in sensitivity and probable better performance of the Lutz method are observed from loads lower than 200 eggs per gram. TF-Test has a great operational advantage in sample handling and preparation, however, in smaller parasitic loads, it has lower sensitivity. Serological analyzes revealed that sensitivity was greater than 90% for both IgG-ELISA and ICT, but specificity was 40% and 18%, respectively. For IgM-ELISA, values were estimated at 55% and 47%, respectively. Low specificity and positive predictive values prevent these tests from being recommended for screening populations in low-intensity endemic areas. For any application, serological tests must be adequately standardized and evaluated.