The problem of resistance of Oncomelania snail to sodium pentachlorophenate.

Sodium pentachlorophenate (NaPCP) is still one of the most commonly used molluscicides in Japan ; since 1953, the major endemic areas of schistosomiasis have been treated with this compound twice a year, in spring and autumn when irrigation is stopped. It is therefore of interest to know whether resistance of the snail to the molluscicide appears in the areas. Some authors (Okabe, Okahara and Ono, 1956; Ota and Sato, 1956) reported that the snails developed resistance to this compound. Some results obtained by later workers (Walton, Winn and Williams, 1958; Gancarz, 1958; Komiya, Yasuraoka and Hosaka, 19.61a,b) are not in full agreement with the earlier reports. The present paper describes the results of surveillance undertaken since 1958 to compare the LC50 values of NaPCP for snails. The Oncomelania nosophora snails used in the tests were collected at intervals of one to six months from villages in Yamanashi prefecture which have been treated with NaPCP twice a year since 1953. The same batch of NaPCP containing at least 93 % of the pure compound was used throughout this study. The chemical was dissolved in deionized water; a series of appropriate two-fold dilutions was prepared. The test procedure was essentially the same as that described by Komiya, Hosaka and Yasuraoka (1962) ; 10 freshly collected adult snails were used per dish in triplicate for each concentration. In all tests, snails were exposed to NaPCP at 25 C for 48 hr. Mortality count was made after a 48 hr recovery period in deionized water and LC50 was computed either by the method of Reed and Muench (1938) or of Litchfield and Wilcoxon (1949). Annual average LC50 values are listed in Table 1. The LC50 value for snails collected from areas untreated with NaPCP (Komiya et al., 1961a,b) is given in the table for comparison. There was no significant difference in the LC50 values, which indicates that no resistance to NaPCP develops in snails from areas treated with this molluscicide for 18 years. In the present study, LC50 for snails from the area treated with NaPCP was examined for 13 years, which corresponds to at least 20 to 25 generations. In the Yamanashi area the regular use of NaPCP at a dosage of 5 g/m2 has produced since 1958 a satisfactorily high snail mortality of 70-90 % (Yamanashi Hygiene Laboratory, 1970). From these results, there is no evidence for development of resistance to NaPCP by Oncomelania snails. Further studies on this problem should include such aquatic vector snails as Biomphalaria and Bulinus as well as Oncomelania.