Resistance to the drought of Bulinus molluscs, human and animal trematodosis vectors in Senegal. II. Study under natural conditions in North-Sudanian area. Ecology and resistance to the drought of Bulinus umbilicatus and B. sene .

Abstract

The authors report on the results of a 2 years study on the ecology and resistance to drought of B. umbilcatus and B. senegalensis on 3 temporary ponds in the North-Sudanian area (region of Tambacounda, Senegal). The variations of some abiotic factors like the temperature and the pH of water do not seem to have a strong influence on the ecology while the rainfall has a great importance on the distribution and the density of molluscs. As a fact, the quantity of water and the drying out period of these ponds depend upon rain factors which rule the existence and the survival on the malacological faun. In the second part of the rain season, the population reaches its maximum. i.e. during the reproduction period. Monthly observations show that the relative abundance of B. umbilicatus is higher than that of B. senegalensis. These ponds are dry during 6 to 8 months per year. However the populations of molluscs regenerate regularly, a fact which presupposes a certain ability to resist drought. Sorne corne through this period successfully, but the middle sized ones (7 to 9.9 mm) resist better than other (70 to 80 per cent of the population). Immediately after the first rains they resume their activity and lay intensively in order to reconstitute the population. B. umbilicatus and B. senegalensis are potential interme­diate hosts for human and animal trematodosis, but in the studied region only B. umbilicatus intervenes in the transmission of S. haematobium and S.curassoni which occurs between September and November. In the natural Sahel conditions the epidemiological cycle is short and everything happens within 4 to 6 months with the regeneration and the growth of the population of molluscs, its infestation and the transmis­ sion of the trematodosises.
The ecological behaviour of these molluscs in the North-Sudanian region is very important in the epidemiology of human and animal trematodosis and requires a new controlling strategy. The destruction of molluscs is more effective and more economic at the end of the rain season, which is the beginning of drying of ponds, a period in which they concentrate in the residual water pools.