Factors conditioning the habitat of bilharziasis intermediate hosts of the family Planorbidae.

In this article, the author examines certain physical, chemical and biological characteristics of water-bodies which make them suitable or unsuitable as habitats for planorbid snails acting as vectors of bilharziasis. The principal conditioning factors appear to be: amount of food available; extent of the growth of aquatic weeds; oxygen content of the water; amount of sunlight able to penetrate the water; strength of the current; nature of the substratum; ionic composition of the water; and presence or absence of parasites and predators. Several of these factors are interdependent. Although there are differences between the various species in their habitat requirements, their ranges of tolerance were found to overlap greatly. The optimum conditions are similar for all species, but extremes are tolerated better by some species than by others. Theoretically, extremes of certain factors should be capable of eliminating snails from a body of water; in practice such extremes rarely occur, and the absence of vectors must be attributed to the combined effect of several factors. Although certain parasites and predators exterminate vectors in the laboratory, the author considers it unlikely that they would do so in nature, as under laboratory conditions the biological balance is disturbed to the disadvantage of the snail. The data available are still too scanty for an exact assessment to be made of the importance of individual environmental factors in controlling the size of vector populations; but this review of present knowledge indicates the lines along which further investigation can be most profitably pursued.