Abstract
In recent years, the prevalence and infection intensity of Schistosoma japonicum in endemic areas of the Philippines have significantly decreased due to yearly population-based treatment strategies, yet transmission rates remain high and uninterrupted. An important indicator of active disease transmission is the presence of Schistosoma japonicum and its snail intermediate host Oncomelania hupensis quadrasi in freshwater habitats. In this study, we sought to apply a species-specific real-time PCR (qPCR) assay for the detection of S. japonicum and O. hupensis quadrasi in freshwater samples using environmental DNA approach that can complement the commonly utilized malacological survey in determining potential transmission foci in order to have a more effective snail surveillance strategy for schistosomiasis japonica in endemic areas. The newly developed assay was specific to S. japonicum and O. hupensis quadrasi with no amplification detected against non-target trematode Fasciola spp. and snails such as Lymnaea spp., Pomacea canaliculata, and Melanoides spp. that typically co-exist in the same environment. The assay effectiveness was determined using 19 environmental water samples collected from Northern Samar (N = 5 sites), Leyte (N = 11 sites) and Compostela Valley (N = 3 sites) and compared to malacological survey for determining O. hupensis quadrasi snail colonies and snail crushing to visualize S. japonicum cercariae. TaqMan qPCR targeting a short fragment of the cytochrome c oxidase subunit 1 (cox1) gene was positive for S. japonicum in 9 sites, for O. hupensis quadrasi in 9 sites, and for both S. japonicum and O. hupensis quadrasi in 5 sampling sites. Moreover, it was able to detect O. hupensis quadrasi in 3 out of 12 sites found negative and 6 out of 7 sites found positive through malacological survey, and in 4 of the 5 snail sites positive for snails with cercariae. Overall, this method can complement malacological surveys for monitoring of schistosomes in endemic areas of the Philippines, especially those with high risk of human infection.
Highlights
Schistosomiasis japonica remains to be a significant public health threat in the Philippines, causing morbidities in the majority of endemic rural communities and usually affecting school-children, farmers and fishermen [1,2,3]
Based on qPCR results, all aquaria except the negative set-ups were positive to O. hupensis quadrasi DNA, while 7 out of the 9 aquaria had detectable S. japonicum DNA (Table 2)
Regarding the field water samples, several sites tested positive for DNA of S. japonicum (9/ 19), O. hupensis quadrasi (9/19) or both (5/19) (Table 3)
Summary
Schistosomiasis japonica remains to be a significant public health threat in the Philippines, causing morbidities in the majority of endemic rural communities and usually affecting school-children, farmers and fishermen [1,2,3]. Despite the national schistosomiasis prevalence in the Philippines is less than 1% the variation very high, with some areas in Northern Samar reporting 48% prevalence of the disease [2]. This parasitic infection is considered both a water-borne and a snail-borne zoonosis [2,3,4]. In the Philippines, the pomatiopsid snail Oncomelania hupensis quadrasi (Mollendorff, 1895) serves as the intermediate host of Schistosoma japonicum [5] These snails can be found in both natural and man-made waterlogged and shady areas characterized by lush vegetation [1,6]. Children are at high risk due to frequent playing and water contact which exposes them to cercariae [2,6]
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