The paper draws together a large and scattered body of empirical evidence concerning the prevalence of snail infection with schistosome parasites in field situations, the duration of the latent period of infection in snails (and its dependence on temperature), and the mortality rates of infected and uninfected snails in field and laboratory conditions. A review and synthesis of quantitative data on the population biology of schistosome infections within the molluscan host is attempted and observed patterns of infection are compared with predictions of a schistosomiasis model developed by May (1977) which incorporates differential snail mortality (between infected and uninfected snails) and latent periods of infection. It is suggested that the low levels of prevalence within snail populations in endemic areas of schistosomiasis are closely associated with high rates of infected snail mortality and the duration of the latent period of infection within the mollusc. In certain instances, the expected life-span of an infected snail may be less than the duration of the latent period of infection. Such patterns generate very low levels of parasite prevalence. A new age prevalence model for schistosome infections within snail populations is developed and its predictions compared with observed patterns.The implications of this study of observed and predicted patterns of snail infection within molluscan populations are discussed in relation to the overall transmission dynamics of schistosomiasis.

Prevalence of schistosome infections within molluscan populations: observed patterns and theoretical predictions

Publisher Summary Schistosomiasis is considered to be the second most important parasitic disease affecting man. The name of the species of mammalian schistosomes, their intermediate hosts, their usual definitive hosts, and their geographic distribution in Africa are tabulated in this chapter. The taxonomy and distribution of the African snails is also discussed. In the south and west of the African continent, Schistosomiasis mattheei (S. mattheei) replaces S. bovis but the morphological similarity of their spindle-shaped eggs causes considerable problems in identification. Further problems arise because a number of factors are known to cause variation in the shape of schistosome eggs and it can be difficult to identify the African species, other than S. mansoni and S. rodhaini, on the basis of egg shape alone. An important problem in studying naturally infected snails is that there is no reliable method of distinguishing the cercariae of the different mammalian schistosomes. Progress has been made in studying populations of cercariae by the indirect method of exposing laboratory animals in the natural habitats and using the resulting infection rates and worm burdens as an assessment of the transmission potential of the habitat at the time of exposure. This disposes off the problems caused by turbidity and of identifying the cercariae and, epidemiologically, has the advantage of detecting only those cercariae, which are infective.

Snail problems in African schistosomiasis.

Aestivation patterns in Bulinus rohlfsi and B. globosus were studied by digging transects across the floor of their dried habitats and by monitoring changes in snail population structure during the period when the habitats were flooded. Bulinus rohlfsi, which inhabited a small man-made lake, was found to aestivate towards the bottom of its habitat, aestivation occurring during the last six weeks before the lake dried. The data on B. globosus from a temporary pool are less complete, but they show that this species also aestivates at the bottom of its habitat and can be found buried at depths up to 3 cm. A wide size range of both species of snail were found to aestivate, but there was differential mortality of young B. rohlfsi soon after the lake re-fllled, and the optimally surviving size group of B. globosus was 9 mm. The beginning of aestivation in B. rohlfsi did not correlate with any of the physical parameters measured, but it coincided with the dying off of a bloom of unicellular algae. The importance...

Schistosomiasis in Kano State, Nigeria

Freshwater snails act as intermediate hosts (IH) for schistosomiasis, a tropical disease affecting over 200 million people worldwide. Despite their medical importance, an extensive understanding of IH snail ecology remains absent. Especially data on the tolerance limits to different abiotic factors are fragmented and incomplete. Consequently, the construction of accurate species distribution models to identify snail habitats and guide targeted snail control efforts remains difficult. Here, we compiled a summary on the tolerance limits to abiotic factors of African IH snails of human schistosomiasis. A systematic search on Web of Science, PubMed, and Embase identified 45 relevant studies. Synthesis of these studies indicates that research efforts differ greatly between IH snail species, life stages, and abiotic factors. The importance of each abiotic factor in determining snail presence and abundance is discussed. Furthermore, attention was drawn to knowledge gaps and the lack of standardised experimental designs, which impedes comparisons between studies. This in turn prevents us from making firm conclusions and calls for best practices adopted by all malacologists. In doing so, IH snail ecological data could serve as a basis to assess schistosomiasis risk and guide snail control efforts in order to support schistosomiasis control.

A call for standardised snail ecological studies to support schistosomiasis risk assessment and snail control efforts

Bulinus globosus snails inhabiting ephemeral rivers and ponds in sub-Saharan Africa experience seasonal water-level fluctuations and habitat desiccation. These environmental changes can instigate the aestivation of snails as a survival strategy. However, the net effect of these changes on snail population size following a period of desiccation and resuscitation is poorly understood. To lessen this knowledge gap, we evaluated the effects of desiccation on the fecundity and longevity of Bulinus globosus snails under laboratory conditions. The periods of desiccation used were 57, 64, 71 and 78 days. Our results showed that an increase in desiccation time from 57 to 78 days was associated with a twofold reduction in fecundity. Furthermore, the survival time of B. globosus snails decreased linearly with desiccation time. The median survival time of snails exposed to desiccation for 57 and 78 days was 63 days and 35 days, respectively. Reduction in the fecundity and snail survival post-desiccation may lead to population shrinkage of B. globosus.

Effects of laboratory-induced desiccation on fecundity and survival of Bulinus globosus (Gastropoda: Planorbidae)

(1967). The survival time and fecundity of Bulinus truncatus after desiccation in mud. Annals of Tropical Medicine & Parasitology: Vol. 61, No. 2, pp. 139-143.

J. Massoud

Papers

The survival time and fecundity of Bulinus truncatus after desiccation in mud