Biologie et compatibilité à schistosoma mansoni sambon, 1909 de biomphalaria pfeifferi (krauss, 1848, planorbis) et biomphalaria camerunensis (Boettger, 1941, Australorbis) au Cameroun

Abstract

Schistosomiasis is largely distributed in Cameroon and schistosome’s transmission is still heterogenous and highly dynamic despite the reduction of the disease’s mortality. The integration and application of snail control in schistosomiasis control programs could be the most efficient way to interrupt schistosome transmission. Preliminary studies are needed for the correct implementation of this measure. Few data are available until now about the biology and the host-parasite compatibility of intermediate hosts belonging to the genus Biomphalaria Preston, 1910 in Cameroon. This thesis was then designed with the aim to highlight the intrinsec characteristic of Biomphalaria pfeifferi and Biomphalaria camerunensis, which are involved in the heterogeneity and the persistence of Schistosoma mansoni transmission in Cameroon. Five specific objectives were assigned to achieve this goal : i) to identify Biomphalaria species based on shell morphology and a DNA taxonomy methodology; ii) to analyse live history traits such as age at first maturity, fecondity, haching and survival rate, mating behaviour ; iii) to analyse the mating system in the two species ; iv) to evaluate the susceptibility rate of B. pfeifferi and B. camerunensis populations towards S. mansoni; v) to evaluate cercarial production of susceptible snails. The snails collected were raised in a snail rearing room situated at the Faculty of Science of Yaoundé I University. Snails were fed ad libitum with dried lettuce. The mating system analysis was performed following a three-step experimental design as follows: (i) for each species, a sample of young snails (G1), virgin and sexually mature was constituted and divided into two groups; (ii) in the first group, individuals were maintained isolated for the evaluation of the impact of self-fertilization on life history traits while in the second group, individuals were paired for few hours for the evaluation of cross fertilization impact; (iii) in each group, fitness parameters (fecundity of G1 snails and survival of G2 offspring) were monitored during one month. Mating behaviour was observed in three populations, one of B. pfeifferi and two of B.camerunensis. The courtship behaviour, the number of copulations, as well as the sexual role assumed by individuals were recorded. The susceptibility of three B. pfeifferi and f (G1) of each population were infected with S. mansoni miracidia, and prepatent period, infection and survival rates of infected snails, as well as cercarial production were recorded and compared between snail species and populations. The effect of parasitism on growth and fecundity of infected B. pfeifferi individuals was analysed. The sexual maturity (age at first egg laying) was reached, on average, at 63.9 (SD: 3.0) and 103.7 (SD: 36.6) days for B. pfeifferi and B. camerunensis, respectively. In B. pfeifferi, the fecundity (number of egg capsules and eggs) of young G1 individuals and survival of G2 offspring on D0 and D8 were similar between selfing and outcrossing individuals, and a very low inbreeding depression (0.063) was observed. In B. camerunensis, the fecundity of outcrossed individuals was significantly higher than that of selfed individuals. The hatching rate was significantly higher and the incubation time significantly shorter for cross-fertilized eggs as compared with self-fertilized eggs, and a high inbreeding depression (0.71) was observed. Snails from both species easily copulated and individuals displayed elaborate stereotyped courtship behaviour. Compatibility tests were performed on 827 snails: 344 B. pfeifferi and 483 B. camerunensis. Infection rates were quite heterogeneous, higher in B. pfeifferi (61.5%) as compared to B. camerunensis (7.8%) (Chi-square test: χ2 = 258.88, df = 1, P < 0.0001). All the three B. pfeifferi-infected populations were susceptible to S. mansoni. However, among the five B. camerunensis populations tested, four were susceptible to S. mansoni, with 21.9% as the highest infection rate. The prepatent period was, on average, shorter in B. pfeifferi than in B. camerunensis (P < 0.0001), but the cercarial production was significantly higher in B. camerunensis as compared to B. pfeifferi (P < 0.001).The growth was significantly higher in healthy individuals compared to infected B. pfeifferi individuals (P= 0, 01). It was also observed a negative effect of parasitism on the fecundity of infected individuals. The biology diversity and high susceptibility polymorphism shown by these Biomphalaria could contribute to the heterogeneity of S. mansoni transmission in Cameroon. The susceptibility of some B. camerunensis populations to S. mansoni should be taken into account when analyzing strategies of intestinal schistosomiasis elimination in Cameroon.ive B. camerunensis populations to a strain of S. mansoni was assessed. Juvenile snails