Biological control for freshwater snails

Parasitology Today, vol. 7, no. 5, 1991 124 Biological Control for Freshwater Snails i||1111 With respect to Madsen's recent review ~on the potential of biological methods for the control of freshwater snails,we feel that some additional information should be provided. Although it is indisputable that, as Madsen concludes, "the most promising results have been obtained from experimental introductions of competitive snail species", the author drew exclusively on examples of competition between Biomphalaria and other genera of aquatic snails such as Thiara, Marisa, Helisoma and Melanoides. As was pointed out, this approach to biological control is prone to a series of drawbacks, especially when the introduced snail also attacks cultivated crops such as in the case ofMarisa sp. and rice. Surprisingly, nothing is said in the review about some alternative biological control experiments also reported in the literature. Such experiments are based upon competition between intrageneric taxa, particularly between B. glabrata and B. straminea. Under natural conditions, the first report of competitive displacement ofB. glabrata by another species of Biomphalaria was published by Barbosa2, based upon fieldwork conducted in northeastern Brazil. In this study, Barbosa noted the gradual substitution ofB. glabrata by B. straminea in major breeding places, a process that took three years. Later, this author confirmed his early observations in other sites of the same region3. A malacological survey conducted in the southeastern Brazilian state of S~o Paulo also revealed that the introduction of B. straminea into B. glabrata breeding places led to the exclusion of the latter species4. Also in this state, other researchers observed the substitution ofB. glabrata by B. tenagophila s. Outside Brazil, studies conducted in the island of Martinique concluded that B. glabrata is becoming a rare species while B. straminea is rapidly spreading throughout the major breeding sites in the region6-8 (A. Guyard andJ.P. Pointier, pers. commun.). Laboratory experiments further confirmed previous field observations on the competitive displacement ofB. glabrata by B. straminea 9,~o. In one particular study ~°,the author simulated semi-natural conditions and obtained impressive results, with B. straminea managing to totally exclude B. glabrata in a period of 100 weeks. More recently, the results from the largest controlled and longterm field experiment so far conducted on congeneric competition in Biomphalaria (C.C.S. Barbosa-de-Figueiredo, MSc thesis, Universidade Federal da Paraiba, 1989)were made available. In this study, the author introduced schistosome-resistant B. straminea (R-3 strain)in nine natural breeding places in northeast Brazil, where B. glabrata was the sole species represented in this genus. In seven of these sites, B. glabrata was completely displaced by the introduced B. straminea strain, a phenomenon that did not revert after seven years of continuous fieldwork. The examples from Brazil and Martinique constitute innovative approaches to the issue of schistosomiasis control. In Martinique, the schistosome-resistant strain ofB. straminea is very efficient in displacing the autochthonous B. glabrata. According to recent reports, the elimination of the latter species from most breeding places is probably the single major factor in the curtailment of schistosomiasis transmission on the island7 (A. Guyard and J.P. Pointier, pers. commun.). The Brazilian experience also provides promising results in controlling B. glabrata and consequently the transmission of schistosomiasis in highly endemic areas where this species is the major intermediate host2'3,10(C.C.S. Barbosa-deFigueiredo, MSc thesis, Universidade Federal da Paraiba, 1989). In both countries, biological methods based on the principle of competitive displacement between close species hold important potential in the control of schistosomiasis. References I Madsen,H. (1990) Parasitology Today 6, 237 24 I 2 Barbosa,F.S.(1973) Malacologia 14,401-408 3 Barbosa,F.S.( 1981) Mere. Inst. Oswaldo Cruz 76, 361-366 4 Teles,H.M.S.(1988) Ci6n. Cult. 40, 374-379 5 Kawazoe,U. et al. (1980) Rev.SaddeP(~bL 14, 65-87 6 Pointier, J.P.(1982)Malacologia22, 395-398 7 Pointier,J.P.(1983) Symbiosis I 5, 85-9 I 8 Guyard, A. and Pointier,J.P.(1979)Ann. ParasitoL Hum. Comp. 54, 193-205 9 Michelson,E.H.and Dubois, L. (1979) Rev. Inst. Meal. Trop. SSoPaulo 2 I, 277-286 10 Barbosa,F.S.and Arruda-da-Silva,F.T.(I 984) Mere. Inst. Oswaldo Cruz 79, 163-167 Carlos E.A. Coimbra, Jr Departmento de Epidemiologia EscolaNacionalde SaudeP~blica RuaLeopoldo Bufh6es 1480 21041 - Rio deJaneiro,Brazil Reply I agree that there is convincing evidence that Biomphalaria straminea is replacing B. glabrata in some areas and therefore should have been mentioned in the review. However, my main concern about B. straminea is that a potential intermediate host is introduced and that it may achieve very high population densities and thereby make up for its relatively low susceptibility to Schistosoma mansoni infection. Even if insusceptible strains are used, there is no guarantee that they stay resistant to new strains ofS. mansoni that might be introduced into the area. Although B. straminea is relatively poorly susceptible or insusceptible to S. mansoni, it is nevertheless the only or even an important intermediate host in some areas of Brazil ~'2. Malek and Rouquayrol2 give a review of reported natural infection rates with S. mansoni in B. straminea. Generally, these infection rates were low, but one was reported of up to 27.2%. Experimental infection of field-collected B. straminea and B. tenagophila showed that 70% of the populations were susceptible to S. mansoni with infection rates of up to 21% in B. straminea and up to 28% in B. tenagophila 3. Also, B. tenagophila has been found naturally infected4 and may under certain circumstances be an effective host5. Also, the W H O Special Programme for Research and Training in Tropical Diseases (TDR) informal consultation on the biological control of snail intermediate hosts 6 did not recommend that research on this approach be supported, for both practical and theoretical reasons, except in very special situations. The latter presumably applies to Martinique, where B. straminea has not been incriminated as an intermediate host of S. mansoni. As for B. tenagophila, little evidence of its effectiveness as a competitor of B. glabrata exists7. Therefore, if this species was to be included, I should also have mentioned most of the other species that have been mentioned at one stage or another as potential biocontrol agents (eg. Potarnopyrgus jenkinsi, Pomacea haustrum, Saulea vitrea and Melanopsis pr aemorsa ). References I Paraense,W.L. ( 1983)Mere.Inst. Oswaldo Cruz 78, 343-36 I 2 Malek,E.A.and Rouquayrol,M.Z. (I 986)Rev. Inst. Med. Trop. S~oPaulo 28, 160-165 3 de Souza,C.P.(1986) Rev.Inst. Meal Trap. $8o Paulo 28,287-292 4 de Melo,J.L.and Pereira,L.H.(I 985)Rev.Inst. Med. Trop. S~oPaulo 27, 99-101 5 de Souza,C.P.et al. (I 987)Mem Inst. Oswaldo Cruz, 82, 67-70 6 World HealthOrganization(1984) TDR/BCVSCH/SIH/84.3 7 Kawazoe,U. et al. (1980) Rev.SaddeP(~bl.$8o Paulo 14,65-87 Henry Madsen DanishBilharziasisLaboratory JaegersborgAlle I D DK-2920 Chadottenlund Denmark Letters to the Editor ParasitologyTodaywelcomes letters to the editor. Please address letters to: Dr Caroline Ash ParasitologyToday Elsevier Trends Journals 68 Hills Road Cambridge CB2 I LA, UK and mark clearly whether they are intended for publication.