Mem Inst Oswaldo Cruz, Rio de Janeiro, 96(1) January 2001
The Molluscicidal Activity of the Latex of Euphorbia splendens var. hislopii on Melanoides tuberculata(Thiaridae), a Snail Associated with Habitats of Biomphalaria glabrata (Pla
Departamento de Biologia, Instituto Oswaldo Cruz, Av. Brasil 4365, 21045-900 Rio de Janeiro, RJ, Brasil
*Escola Politécnica de Saúde Joaquim Venâncio-Fiocruz, Rio de Janeiro, RJ, Brasil
The use of the latex ofu00a0Euphorbia splendensu00a0var.u00a0hislopiiu00a0was considered as an effective control method foru00a0Biomphalaria glabratau00a0in Sumidouro, Rio de Janeiro. However, the appearance and expansion of the snailu00a0Melanoides tuberculatasince August 1997, with the concomitant reduction of the population ofu00a0B. glabratau00a0suggest that competitive exclusion might be taking place. Depending on the susceptibility of the thiarid to theu00a0E. splendensu00a0toxin, the natural control that is occurring could be interrupted by the employment of the latex if the planorbid were less susceptible to the toxin. The aim of this study is to investigate the molluscicidal activity of the latex onu00a0M. tuberculata.u00a0We used 420u00a0M. tuberculata, from Sumidouro. Fourteen different latex concentrations were tested using World Health Organization general methodology. Probit analysis was used for LD90u00a0and LD50u00a0determination. The LD50u00a0was 3.57 mg/l and LD90u00a0was 6.22 mg/l. At the highest concentration (10 mg/l) there was no survival. No significant differences among replicas (c2u00a0= 8.31; gl = 13; p > 0.05) were found. The LD90u00a0dose foru00a0M. tuberculatau00a0was 13.8 times greater than that foru00a0B. glabrata, so that the molluscicide in the presence of the thiarid may have a synergic effect on reduction ofu00a0Biomphalariau00a0populations.
Since 1991 our group has been making an eco-epidemiological study in Sumidouro, Rio de Janeiro, Brazil, to verify the abundance of Biomphalaria glabrata (Say, 1818) and its degree of infection with schistosomiasis. The use of latex fromEuphorbia splendens, which has good molluscicidal action against Biomphalaria (Vasconcellos & Schall 1986, Baptista et al. 1992, Mendes et al. 1992, 1997 Schall et al. 1998), was planned as a schistosomiasis control measure in this area. However, in August 1997 the presence of Melanoides tuberculata (Muller, 1774) was detected. This snail genus has medical and veterinary interest because some species act as intermediate hosts of pathogenic helminths to man [Paragonimus sp., Clonorchis sp. (Pessoa & Martins 1982, Kino et al. 1998) and Centrocestus formosanus (Pointier 1999)]. Nevertheless this species could be useful in the biological control of planorbids that are intermediate hosts ofSchistosoma mansoni (Prentice 1983 Pointier & Mccullough 1989, Pointier et al. 1989, Gomez et al. 1990, Gomez Perez et al. 1991, Pointier & Guyard 1992, Schlegel et al. 1997), the helminth responsible for schistosomiasis in Brazil.
Since the detection of the M. tuberculata population in Sumidouro there has been a severe decline in the B. glabrata population concomitant with the Melanoides species population growth and expansion indicating a process of competitive exclusion. The use of the toxin from E. splendens could therefore have a negative effect on control if it were more toxic towards on M. tuberculata than to B. glabrata.
The aim of this study was to verify the effects of the latex of E. splendens on M. tuberculata collected in B. glabrata habitats at Porteira Verde valley in Sumidouro.
MATERIALS AND METHODS
The latex from E. splendens var. hislopii NEB [Sin. Euphorbia milii Des Moul. Var. splendens (Hook.) Ursh & Leandri (Carter 1994)] was extracted from cuts made on the trunk. The plants used were grown in experimental plots located in the Fiocruz campus. One ml of latex was diluted in 9 ml of distilled water. Fourteen different concentrations were prepared following the World Health Organization standard method of 1983 (0; 0.2; 0.4; 0.6; 0.8; 1; 2; 2.5; 3; 3.5; 4; 4.5; 5; 10 mg/l).
Four hundred twenty M. tuberculata snails from Sumidouro were used. Shell sizes varied from 16 to 27 mm in length. For each concentration two beakers each with 15 snails (experiment and replicate) were used to complete 30 snails per concentration. Exposure was for 24 h after which the number of snails apparently dead was noted and the solution was replaced by distilled water. After further 24 h, the number of the dead snails was again counted. No food was supplied during the experiment.
Lethal doses for 50% and 90% of the population (LD50 and LD90) were obtained by Probit analysis, using the computer software of Finney (1971). Differences between replicas were verified using the chi-square test.
After exposure to the molluscicide, many snails had retracted into their shells, especially at the highest concentrations used, and their mobility was reduced. However, after the recovery period many snails returned to normal behaviour.
Concentrations of 2 mg/l or higher triggered intense mucus production but no escape behaviour was observed. Mortality started to occur at concentrations of 2.5 mg/l. The LD50 was 3.6 mg/l and the LD90 was 6.2 mg/l. At the highest concentration (10 mg/l) there were no survivors (Fig. 1).
No significant differences were observed between replicas (c2 = 8.31; gl = 13; p>0.05).
Mkoji et al. (1992) speculated that the use of molluscicide in areas where both B. pfeifferi and M. tuberculata occur would have negative effects on the control of schistosomiasis. The molluscicide would potentially affect the M. tuberculata population causing a diminution of the competitive pressure on the Biomphalaria sp. population. As a consequence, an increase in the Biomphalaria sp. population would be noted soon after the initial impact of the molluscicide.
However, the results obtained in this study demonstrate that this effect is not to be expected with the use of the latex ofE. splendens, at least with the studied populations. The LD90 of 0.45 mg/l for the B. glabrata control (Vasconcellos & Schall 1986) is 13.8 times lower than LD90 for M. tuberculata. Even the dosages used for the control of B. glabrata in the field are under the LD50 encountered for M. tuberculata in the present study. So the utilization of the latex of E. splendens in areas where both M. tuberculata and B. glabrata are present, should not reduce the Melanoides population and might even have a synergetic effect in reducing the B. glabrata population.
The introduction of M. tuberculata as a control measure for Biomphalaria should be considered with caution (WHO 1984) since M. tuberculata is an intermediate host for other parasites especially Paragonimus spp. that can affect the human population (Pessoa & Martins 1982). The transmission of paragonomiasis occurs through the ingestion of raw infected crustaceans (a second intermediate host), a habit that is spreading among fishermen and near-shore populations that inhabit the Paraná and Rio Grande rivers (State of São Paulo) in Brazil. The probability of autochthonous cases in these areas is high (Vaz et al. 1986). Besides, the effect of M. tuberculata populations on the bentonic macroinvertebrate fauna and on other organisms is very little known in the neotropics.
To complement the results obtained experiments in the field at different times of the year are necessary.
To Valdinei Valim for the technical assistance and to Marisa S Soares, from the Biology Department, for the suggestions and criticism. To Dr Eric S Loker, from The University of New Mexico, for the relevant comments and for revision of the English version.
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