Abstract
Of the approximately 34 identified Biomphalaria species,Biomphalaria alexandrina represents the intermediate host of Schistosoma mansoni in Egypt. Using parasitological and SOD1 enzyme assay, this study aimed to elucidate the impact of the age of B. alexandrina snails on their genetic variability and internal defence against S. mansoni infection. Susceptible and resistant snails were reared individually for self-reproduction; four subgroups of their progeny were used in experiment. The young susceptible subgroup showed the highest infection rate, the shortest pre-patent period, the highest total cercarial production, the highest mortality rate and the lowest SOD1 activity. Among the young and adult susceptible subgroups, 8% and 26% were found to be resistant, indicating the inheritance of resistance alleles from parents. The adult resistant subgroup, however, contained only resistant snails and showed the highest enzyme activity. The complex interaction between snail age, genetic background and internal defence resulted in great variability in compatibility patterns, with the highest significant difference between young susceptible and adult resistant snails. The results demonstrate that resistance alleles function to a greater degree in adults, with higher SOD1 activity and provide potential implications for Biomphalaria control. The identification of the most susceptible snail age enables determination of the best timing for applying molluscicides. Moreover, adult resistant snails could be beneficial in biological snail control.
Highlights
Schistosomiasis is recognised as a major neglected tropical disease, second only to malaria in terms of its adverse socioeconomic and public health impact in tropical and subtropical third-world developing countries
Resistance of B. glabrata to S. mansoni infection was found to vary with age; it is controlled in juveniles by at least four genes, each with several alleles, whereas in adulthood, only a single dominant gene determines this trait (Richards 1977, Richards & Shade, Richards et al 1992, Spada et al 2002, Ittiprasert et al 2010)
Parasitological study - pre-patent period (PPP) - Snails of each subgroup were individually examined for cercarial shedding twice weekly, starting from the 28th day after exposure to infection until the 49th day (Table I)
Summary
Schistosomiasis is recognised as a major neglected tropical disease, second only to malaria in terms of its adverse socioeconomic and public health impact in tropical and subtropical third-world developing countries. The worldwide distribution of Schistosoma mansoni, which causes intestinal schistosomiasis, is promoted by the broad geographic range of susceptible species of its intermediate host: pulmonate freshwater snails of the genus Biomphalaria. These snails serve as obligatory hosts for the larval stage, which infects humans. Different snail-specific factors are critical in determining Biomphalaria susceptibility to S. mansoni (Negrão-Corrêa et al 2007, El Naga et al 2010). For B. alexandrina, little is known about the effect of age on genetic modulation and S. mansoni infection compatibility Another important factor determining the compatibility of Biomphalaria species to S. mansoni infection is the snail’s internal defence system (IDS). This is in contrast to O2-, which does not readily cross biological membranes, as its reaction with SOD1 occurs in the compartment where it is generated (Goodall et al 2004, Bender et al 2007)
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