Hostname: page-component-78c5997874-g7gxr Total loading time: 0 Render date: 2024-11-05T19:53:47.214Z Has data issue: false hasContentIssue false
  • English
  • Français

On the interpretation of age—prevalence curves for schistosome infections of host snails

Published online by Cambridge University Press:  06 April 2009

M. E. J. Woolhouse
M. E. J. Woolhouse
Affiliation:
Department of Pure and Applied Biology, Imperial College, Prince Consort Road, London SW7 2BB
Get access Rights & Permissions [Opens in a new window]

Summary

The prevalence of schistosome infections in intermediate host snails varies with snail age. The relationship between age and prevalence, the age—prevalence curve, is complex and may vary in space and time, and among parasite—host species. Field studies show that the shape of the age—prevalence curve may be seasonally variable, and that at some times there may be a decline in prevalence among older snails. This paper attempts to explain these observations in terms of the underlying epidemiological processes. A discrete-time version of Muench's catalytic model for age-dependent infection is developed. Model simulations were carried out using life-history and epidemiological parameters derived from studies of Schistosoma haematobium—Bulinus globusus in Zimbabwe. Analysis of model behaviour identifies aspects of the schistosome— snail interaction that affect the shape of the age—prevalence curve. The following features can result in a decline in prevalence among older snails. (1) A decrease in the survival rate of patent infected snails with age. (2) A decrease in the force of infection with age. (3) A high rate of loss of infection. (4) A heterogeneity in the snail population such that the probability of infection is correlated with snail fecundity. (This would occur if there existed a spatial correlation between force of infection and fecundity, or if there were a correlation between fecundity and susceptibility.) The evidence for the occurrence of these features in the field is assessed. Survival rate is related more closely to the duration of patent infection than to age per se. The evidence for age-dependent force of infection is equivocal. Significant loss-of-infection rates have yet to be demonstrated. Heterogeneities in force of infection and fecundity have been reported and, for the Zimbabwe data, this mechanism can explain seasonally in the age—prevalence curve as a function of known seasonal variation in the force of infection and snail fecundity.

Keywords

Age—prevalence curvesBulinus globosusschistosome infectionsmodel simulationsepidemiologySchistosoma haematobium.
Type
Research Article
Information
Parasitology , Volume 99 , Issue 1 , August 1989 , pp. 47 - 56
Copyright
Copyright © Cambridge University Press 1989

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

REFERENCES

Anderson, R. M., & Crombie, J., (1984). Experimental studies of age—prevalence curves for Schistosoma mansoni infections in populations of Biomphalaria glabrata. Parasitology 89, 79–104.CrossRefGoogle ScholarPubMed
Anderson, R. M., & May, R. M., (1979). Prevalence of schistosome infections within molluscan populations: observed patterns and theoretical predictions. Parasitology 79, 63–94.CrossRefGoogle ScholarPubMed
Anderson, R. M., Mercer, J. G., Wilson, R. A., & Carter, N. P., (1982). Transmission of Schistosoma mansoni from man to snail: experimental studies of miracidial survival and infectivity in relation to larval age, water temperature, host size and host age. Parasitology 85, 339–60.CrossRefGoogle Scholar
Kassim, O. O., & Richards, C. S., (1979). Host reactions in Biomphalaria glabrata to Schistosoma mansoni miracidia, involving variations in parasite strains, numbers and sequences of exposure. International Journal for Parasitology 9, 565–70.CrossRefGoogle Scholar
Minchella, D. J., & Loverde, P. T., (1983). Laboratory comparison of the relative success of Biomphalaria glabrata stocks which are susceptible and insusceptible to infection with Schistosoma mansoni. Parasitology 86, 335–44.CrossRefGoogle ScholarPubMed
Muench, H., (1959). Catalytic Models in Epidemiology. Cambridge, Massachusetts: Harvard University Press.CrossRefGoogle Scholar
O'Keeffe, J. H., (1985). Population biology of the freshwater snail Bulinus globosus on the Kenya coast. II. Feeding and density effects on population parameters. Journal of Applied Ecology 22, 85–90.CrossRefGoogle Scholar
Shiff, C. J., (1964 a). Studies on Bulinus (Physopsis) globosus in Rhodesia. I. The influence of temperature on the intrinsic rate of increase. Annals of Tropical Medicine and Parasitology 58, 94–105.CrossRefGoogle Scholar
Shiff, C. J., (1964 b). Studies on Bulinus (Physopsis) globosus in Rhodesia. III. Bionomics of a natural population existing in a temporary habitat. Annals of Tropical Medicine and Parasitology 58, 240–55.CrossRefGoogle Scholar
Shiff, C. J., Coutts, W. C. C., Yiannakis, C., & Holmes, R. W., (1979). Seasonal patterns in the transmission of Schistosoma haematobium in Rhodesia, and its control by winter application of molluscicide. Transactions of the Royal Society of Tropical Medicine and Hygiene 73, 375–80.CrossRefGoogle ScholarPubMed
Sturrock, B. M., (1966). The influence of infection with Schistosoma mansoni on the growth rate and reproduction of Biomphalaria pfeifferi. Annals of Tropical Medicine and Parasitology 60, 187–97.CrossRefGoogle ScholarPubMed
Sturrock, B. M., (1967). The effect of infection with Schistosoma haematobium on the growth and reproduction rates of Bulinus (Physopsis) nasutus productus. Annals of Tropical Medicine and Parasitology 61, 321–5.CrossRefGoogle ScholarPubMed
Sturrock, B. M., (1968). Resistance of Bulinus (Physopsis) nasutus productus to infection by Schistosoma haematobium. Annals of Tropical Medicine and Parasitology 62, 393–7.CrossRefGoogle Scholar
Sturrock, R. F., (1973 a). Field studies on the population dynamics of Biomphalaria glabrata, intermediate host of Schistosoma mansoni on the West Indian island of St. Lucia. International Journal for Parasitology 3, 165–74.CrossRefGoogle ScholarPubMed
Sturrock, R. F., (1973 b). Field studies on the transmission of Schistosoma mansoni and on the bionomics of its intermediate host, Biomphalaria glabrata, on St. Lucia, West Indies. International Journal for Parasitology 3, 175–94.CrossRefGoogle Scholar
Sturrock, R. F., & Webbe, G., (1971). The application of catalytic models to schistosomiasis in snails. Journal of Helminthology 45, 189–200.CrossRefGoogle ScholarPubMed
Woolhouse, M. E. J., (1989). Estimation of the effects of patent schistosome infections on the mortality rates of Bulinus globosus and Biomphalaria pfeifferi. Annals of Tropical Medicine and Parasitology (in the press).CrossRefGoogle Scholar
Woolhouse, M. E. J., & Chandiwana, S. K., (1989). Spatial and temporal heterogeneity in the population dynamics of Bulinus globosus and Biomphalaria pfeifferi and the epidemiology of their infection with schistosomes. Parasitology 98, 21–34.CrossRefGoogle ScholarPubMed