Effect of sodium chloride solution on egg masses, juveniles and adults of Biomphalaria pfeifferi, host of schistosoma mansoni

Abstract

Human schistosomiasis is a parasitic disease caused by digenean trematode parasites in the genus Schistosoma, which live in the bloodstream of their mammalian hosts, and utilize freshwater gastropod snails as intermediate hosts. Schistosomiasis, a major public health problem in the sub-Saharan Africa, is among the neglected tropical diseases (NTDs) targeted for elimination by WHO by 2025.Although schistosomiasis control and elimination efforts, currently rely heavily on chemotherapy using the antischistosomal drug praziquantel, there is a growing consensus that chemotherapy alone will not help achieve the WHO elimination goal, and so, integrated approaches that include snail control among other strategies are being considered. However, snail control using synthetic molluscicides such as niclosamide is considered too costly for schistosomiasis endemic countries, and so, discovery of inexpensive but effective molluscicides should be explored. This study examined, under laboratory conditions, the potential of sodium chloride (NaCl) (common salt) as a molluscicide using the freshwater snail, Biomphalaria pfeifferi, as the test target.The molluscicidal activity of sodium chloride (NaCl) was tested against egg masses, neonates and the adults of B. pfeifferi snail. Field collected snails packed between layers of damp cotton gauze and transported to the lab in perforated plastic containers were used in these experiments. Prior to use the snails were screened for trematode infections and allowed to acclimatize for several days before use for experimental purposes. Only snails free of parasite infections were used directly as targets to evaluate the molluscicidal activity of sodium chloride (NaCl) solution or were allowed to lay egg masses for use in further experiments with egg masses and neonates.NaCl solution in concentrations of 3200 mg/L or less did not kill the adult snail (measuring 8mm or above in shell diameter) exposed for 24hr. However, at concentrations in the range 4400-6800mg/L, 10-100% of the adults were killed within 24 hr. of exposure. Concentrations of 1800mg/L or above killed 70-100% of the neonates snails, measuring 2mm or below in shell diameter. The concentration that killed 50% of the egg, neonates and adults was 495 mg/L, 1457 mg/L, and 5087 mg/L respectively. Interestingly, egg laying was completely inhibited in snails that had been exposed to non-lethal concentrations (5600 mg/L and above) of NaCl solution for 24hr, relative to unexposed controls. The molluscicidal effect of NaCl was significantly effective at all stages of B. pfeifferi eggs at 24hrs (d.f = 6, X2 = 42; P<0.05), neonates at 24 hrs (d.f = 6, χ2 = 30, (p<0.05) and adults at 24 hrs. (d.f = 4, χ2 = 15, (p<0.05). These results suggest that NaCl has potential as a molluscicide against freshwater snails of medical and veterinary importance, and given that it is more readily available and relatively inexpensive, further studies on it as a candidate molluscicide, and should be encouraged. In particular, further studies should be undertaken to determine the molluscicidal effects of NaCl on other snail species of medical or veterinary importance, and its effects on non-target organisms. Non-molluscicidal concentrations of NaCl will need to be tested for effects on snail fecundity, neonate growth and survival, and egg mass hatchability. Short-term or long-term effects of NaCl on the environment should also, be undertaken.