Our objective is to evaluate the habitat preference of freshwater snails in relation to environmental factors and the presence of the competitor snail Melanoides tuberculatus. In the first phase, snails was collected at 12 sites. This sampling sites presented a degree of organic input. In the second phase 33 sampling sites were chosen, covering a variety of lotic and lentic environments. The snail species found at Guapimirim, state of Rio de Janeiro, displayed a marked habitat preference, specially in relation to the physical characteristics of each environment. Other limiting factors for snail distribution at the studied lotic environments were the water current velocity and the amount of organic matter, mainly to Physa marmorata, M. tuberculatus, and Biomphalaria tenagophila. The absence of interactions between M. tuberculatus and another snails could be associated to the distinct spatial distribution of those species and the instability of habitats. This later factor may favor the coexistence of M. tuberculatus with B. glabrata by reduction of population density. In areas of schistosomiasis transmission some habitat modification may add to the instability of the environment, which would make room for the coexistence of M. tuberculatus and Biomphalaria spp. In this way, some of the usual measures for the control of snail hosts would prevent the extinction of populations of Biomphalaria spp. by M. tuberculatus in particular habitats.

Interactions between two species that result in reduced growth rates for both and extinction of one of the species are generally considered cases of asymmetric interspecific competition. Exploitative or interference competition is the usual mechanism invoked. Here we describe another mechanism producing the same result, named apparent competition through facilitation (ACF), observed between Melanoides tuberculata and Biomphalaria glabrata populations. The superior competitor actually gives some benefit to the other species, whose population becomes unstable with progressively increasing oscillations, leading to extinction. A model of ACF using difference equations suggests initial dynamics distinct from traditional interspecific competition. The dynamics of two freshwater snails in the field and in laboratory experiments suggest ACF, and these relations should be considered in studies of schistosomiasis control. ACF could occur in natural populations, but might have gone undetected because the final result is similar to traditional interspecific competition.

The aim of this study was to determine the toxicity of niclosamide (Bayluscide ®) on Melanoides tuberculata and Biomphalaria glabrata under laboratory conditions. The latter species is the intermediate host of Schistosoma mansoni (Sambon 1917). M. tuberculata was successfully used as competitor of B. glabrata in biological control programs in French West Indies. Both molluscicide and biological control using M. tuberculata have proved to be successful in reducing the population density of B. glabrata. The associated use of molluscicide in this area would be an effective measure if M. tuberculata were less susceptibility to the molluscicide than B. glabrata. Three hundreds individuals each of B. glabrata and of M. tuberculata, collected in Sumidouro, State of Rio de Janeiro, were used in the experiment. The molluscs were exposed to 14 different concentrations of niclosamide as recommended by the World Health Organization. Probit analysis was used to determine the LC 50 and LC 90. The LC 50 and LC 90 values for B. glabrata were 0.077 mg/l and 0.175 mg/l, respectively and the LC 50 and LC 90 values for M. tuberculata were 0.082 mg/l and 0.221 mg/l respectively. As the lethal concentrations of niclosamide were approximately the same to both species, this could be a disadvantage when controlling B. glabrata with niclosamide in an area of M. tuberculata occurrence. It migth therefore be preferable to utilize the latex extracted from the Euphorbia splendens, which presented a much higher efficiency for B. glabrata than to M. tuberculata.

The biological control of Biomphalaria glabrata, intermediate host of Schistosoma mansoni, is one the accepted options to fight schistosomiasis. One of the most promising candidates to control B. glabrata is the snail Melanoides tuberculata, a potential competitor. However, the mechanisms of interaction between the two species are not clear. Our objective is to determine if M. tuberculata indeed compete with B. glabrata, using two laboratory experiments. In Experiment 1, we tested the effect of the presence of M. tuberculata on the fecundity and mortality rates of B. glabrata. In Experiment 2, we tested if there was a direct or indirect interaction between the two species. In Experiment 1, M. tuberculata was eliminated after the peak in reproductive activity of B. glabrata. In Experiment 2, B. glabrata produced more egg masses when raised with M. tuberculata. The conditions leading to this unexpected positive effect of M. tuberculata on the fecundity of B. glabrata need further clarification, but emphasize that detailed studies of the interaction between these species in the conditions of the local environment should be considered.

OBJETIVOS: Investigar a distribuição espacial, a abundância e os índices de infecção natural de Biomphalaria glabrata, hospedeiro intermediário do Schistosoma mansoni, em localidade do Estado do Rio de Janeiro, RJ, Brasil. MÉTODOS: Na localidade de Pamparrão, município de Sumidouro, RJ, as coletas de moluscos foram realizadas bimestralmente no período de junho de 1991 a novembro de 1995. Foram estabelecidos 23 pontos de coleta ao longo do córrego Pamparrão e três de seus afluentes. Os moluscos capturados foram levados ao laboratório para diagnóstico da infecção. Para a análise dos dados, foram usados o coeficiente de Spearman (nível de 0,5% de significância) e o teste de qui-quadrado. RESULTADOS: A abundância populacional de B. glabrata foi bastante variável ao longo do tempo e entre os ambientes amostrados. A maioria dos pontos de coleta apresentou correlação negativa com a pluviosidade. O afluente B destacou-se dos demais corpos d'água por apresentar taxas de infecção de B. glabrata elevadas (acima dos 25% em alguns pontos de coleta) e persistentes. Foram encontrados mais moluscos infectados na estação seca do que na chuvosa (chi²=20,08; p=0,001). CONCLUSÕES: A população de moluscos foi influenciada negativamente pelo regime de chuvas, principalmente no córrego Pamparrão. A época de estiagem também parece ter favorecido a ocorrência de infecção, provavelmente devido ao menor volume de água dos córregos, o que aumentaria as chances de encontro do parasita com seu hospedeiro intermediário.

OBJECTIVES: To investigate the spatial distribution, abundance and natural schistosomiasis infection levels in the snail Biomphalaria glabrata, the intermediate host of Schistosoma mansoni in an area of the State of Rio de Janeiro, Brazil. METHODS: In the Pamparrão area, Sumidouro county, RJ, Brazil, snail captures were carried out every other month from June 1991 to November 1995. There were 23 collecting sites along the Pamparrão stream and its three tributaries (A, B and C). Captured snails were examined in the laboratory for infection. Data analyses were performed using Spearman coefficient (0.5% significance level) and Qui-square test. RESULTS: The abundance of B. glabrata was variable in both time and space. Most of the collection sites showed a negative correlation with rainfall. The tributary B had the highest and most steady infection rates (more than 25% in some sites). There were found significantly more infected snail in the dry season (chi2 = 20.08; p=0.001). CONCLUSIONS: B. glabrata population in the Pamparrão valley was negatively influenced by rainfall, especially at the Pamparrão stream. The dry season seems to promote the infection probably due to the lower water volume, increasing the chance of interaction between the intermediate host and the parasite.

OBJETIVOS: Investigar a distribuição espacial, a abundância e os índices de infecção natural de Biomphalaria glabrata, hospedeiro intermediário do Schistosoma mansoni, em localidade do Estado do Rio de Janeiro, RJ, Brasil. MÉTODOS: Na localidade de Pamparrão, município de Sumidouro, RJ, as coletas de moluscos foram realizadas bimestralmente no período de junho de 1991 a novembro de 1995. Foram estabelecidos 23 pontos de coleta ao longo do córrego Pamparrão e três de seus afluentes. Os moluscos capturados foram levados ao laboratório para diagnóstico da infecção. Para a análise dos dados, foram usados o coeficiente de Spearman (nível de 0,5% de significância) e o teste de qui-quadrado. RESULTADOS: A abundância populacional de B. glabrata foi bastante variável ao longo do tempo e entre os ambientes amostrados. A maioria dos pontos de coleta apresentou correlação negativa com a pluviosidade. O afluente B destacou-se dos demais corpos d'água por apresentar taxas de infecção de B. glabrata elevadas (acima dos 25% em alguns pontos de coleta) e persistentes. Foram encontrados mais moluscos infectados na estação seca do que na chuvosa (chi²=20,08; p=0,001). CONCLUSÕES: A população de moluscos foi influenciada negativamente pelo regime de chuvas, principalmente no córrego Pamparrão. A época de estiagem também parece ter favorecido a ocorrência de infecção, provavelmente devido ao menor volume de água dos córregos, o que aumentaria as chances de encontro do parasita com seu hospedeiro intermediário.

OBJECTIVES: To investigate the spatial distribution, abundance and natural schistosomiasis infection levels in the snail Biomphalaria glabrata, the intermediate host of Schistosoma mansoni in an area of the State of Rio de Janeiro, Brazil. METHODS: In the Pamparrão area, Sumidouro county, RJ, Brazil, snail captures were carried out every other month from June 1991 to November 1995. There were 23 collecting sites along the Pamparrão stream and its three tributaries (A, B and C). Captured snails were examined in the laboratory for infection. Data analyses were performed using Spearman coefficient (0.5% significance level) and Qui-square test. RESULTS: The abundance of B. glabrata was variable in both time and space. Most of the collection sites showed a negative correlation with rainfall. The tributary B had the highest and most steady infection rates (more than 25% in some sites). There were found significantly more infected snail in the dry season (chi2 = 20.08; p=0.001). CONCLUSIONS: B. glabrata population in the Pamparrão valley was negatively influenced by rainfall, especially at the Pamparrão stream. The dry season seems to promote the infection probably due to the lower water volume, increasing the chance of interaction between the intermediate host and the parasite.

The use of the latex of Euphorbia splendens var. hislopii was considered as an effective control method for Biomphalaria glabrata in Sumidouro, Rio de Janeiro. However, the appearance and expansion of the snail Melanoides tuberculata since August 1997, with the concomitant reduction of the population of B. glabrata suggest that competitive exclusion might be taking place. Depending on the susceptibility of the thiarid to the E. splendens toxin, 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 on M. tuberculata. We used 420 M. tuberculata, from Sumidouro. Fourteen different latex concentrations were tested using World Health Organization general methodology. Probit analysis was used for LD90 and LD50 determination. The LD50 was 3.57 mg/l and LD90 was 6.22 mg/l. At the highest concentration (10 mg/l) there was no survival. No significant differences among replicas (chi2 = 8.31; gl = 13; p > 0.05) were found. The LD90 dose for M. tuberculata was 13.8 times greater than that for B. glabrata, so that the molluscicide in the presence of the thiarid may have a synergic effect on reduction of Biomphalaria populations.