Some effects of the molluscicide bis (tri-N-butyltin) oxide (TBTO) on the snail Oncomelania quadrasi and the larval stages of its trematode parasite Schistosoma japonicum

The effects of sublethal concentrations (6 $\times$ 10$\sp{-5}$, 3 $\times$ 10$\sp{-4}$ and 1.2 $\times$ 10$\sp{-3}$ ppm) of the organotin molluscicide, bis (tri-n-butyltin) oxide (TBTO), on the Philippine strain of the snail Oncomelania quadrasi and the larval stages of its trematode parasite, Schistosoma japonicum, were investigated. This study included the effects of TBTO alone and the effects of TBTO jointly with schistosoma infection on O. quadrasi. TBTO in a concentration of 1.2 $\times$ 10$\sp{-3}$ ppm reduced growth of Oncomelania quadrasi. TBTO concentrations of 6 $\times$ 10$\sp{-5}$ to 1.2 $\times$ 10$\sp{-3}$ ppm reduced the reproduction/fecundity of the snails. The number of offspring of O. quadrasi was inversely related to TBTO concentration and was reduced 41-55% from that of the control. Schistosoma japonicum infection increased the effect of TBTO on fecundity of the snails. Snails infected with five miracidia and treated with 6 $\times$ 10$\sp{-5}$ to 1.2 $\times$ 10$\sp{-3}$ ppm TBTO produced fewer offspring than uninfected treated snails, i.e., 41-90% reduction. TBTO or S. japonicum alone had little effect on the metabolic rate of O. quadrasi. However, metabolic rates did increase for infected snails treated with 3 $\times$ 10$\sp{-4}$ to 1.2 $\times$ 10$\sp{-3}$ ppm TBTO. Concentrations of 1.2 $\times$ 10$\sp{-3}$ ppm TBTO retarded the development of Schistosoma japonicum in the snail hosts, as shown by the reduced number of cercariae emerging from snails that were treated with TBTO the day following infection. TBTO at the lower concentrations (6 $\times$ 10$\sp{-5}$ and 3 $\times$ 10$\sp{-4}$ ppm) showed the greatest effect was on the late prepatent group (i.e., during the developing period of the cercariae in the snails). TBTO had some effect on the early prepatent groups (i.e., the developing period of the sporocysts in the snails), but had no effect on the patent groups (i.e., after completion of development of the cercariae in the snails). Thus, TBTO had a greater effect on the developing cercariae than on the developing sporocysts. Also, 6 $\times$ 10$\sp{-5}$ to 1.2 $\times$ 10$\sp{-3}$ ppm TBTO retarded development of S. japonicum in the snail hosts treated 15 days before infection. Therefore, TBTO at the lower, sublethal concentrations should be of value in controlling schistosomiasis by reducing both the number of target snails in specific localities and their schistosome parasite larvae.