[Studies on standardization of methods for screening molluscicides in laboratory IV sensitivity of Oncomelania snails from different months to niclosamide].

Objective: To understand the sensitivity of Oncomelania snails collected from different months to niclosamide, so as to provide the scientific evidence for the standardization of methods for molluscicide screening and efficacy evaluation in laboratory.

Methods: The snails collected from the marshland of Zhenjiang City, Jiangsu Province month by month from June 2010 to May 2011. After being raised in laboratory for 24 h, the snails were randomly grouped, and then immersed in different concentrations of 50% wettable powder formulation of niclosamide ethanolamine salt at (25 +/- 1) degrees C with a humidity of 60%. The dead snails were identified and counted, and the mortality rate of snails and median lethal concentration (LC50) were calculated.

Results: When the snails were immersed in the solutions of niclosamide at concentrations of more than 0.5 mg/L for 24 h, all the snails collected from different months were dead, while 60%-100% of the snail mortality was achieved for 0.250 mg/L niclosamide, 3%-27% for 0.125 mg/L niclosamide, and 3%-20% for 0.062 5 mg/L niclosamide. When the concentration was lower than 0.032 mg/L, the niclosamide was not toxic to the snails within 24 h. The LC50 value was 0.140-0.209 mg/L for 24 h. When the snails were immersed in the solutions of niclosamide at concentrations of more than 0.5 mg/L for 48 h, all the snails collected from different months were dead, while 90%-100% of the snail mortality was achieved for 0.250 mg/L niclosamide, 3%-57% for 0.125 mg/L niclosamide, 3%-13% for 0.062 5 mg/L niclosamide, and 0-10% for 0.032 mg/L niclosamide. When the concentration was lower than 0.016 mg/L, the niclosamide was not toxic to the snails within 48 h. The LC50 value was 0.112-0.170 mg/L for 48 h. There were no significant differences in the mortality of snails caused by niclosamide treatment observed at 24 and 48 h (P values = 0.374 and 0.267, respectively).

Conclusions: There are little changes in the sensitivity of snails collected from different months to niclosamide, with minor fluctuations in LC50 values. Such a finding indicates that the snails collected from different months have few effects on screening of molluscicides and efficacy evaluation.