caa a22 4500 386399212 CHVBK 20180307112151.0 cr unu---uuuuu 161130e198908 xx s 000 0 eng 10.1017/S0031182000061011 doi S0031182000061011 pii (NATIONALLICENCE)cambridge-10.1017/S0031182000061011 Woolhouse M. E. J. Department of Pure and Applied Biology, Imperial College, Prince Consort Road, London SW7 2BB On the interpretation of age—prevalence curves for schistosome infections of host snails [Elektronische Daten] [M. E. J. Woolhouse] 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. Copyright © Cambridge University Press 1989 Age—prevalence curves nationallicence Bulinus globosus nationallicence schistosome infections nationallicence model simulations nationallicence epidemiology nationallicence Schistosoma haematobium nationallicence Parasitology Cambridge University Press 99/1(1989-08), 47-56 0031-1820 99:1<47 1989 99 PAR https://doi.org/10.1017/S0031182000061011 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1017/S0031182000061011 text/html Onlinezugriff via DOI NATIONALLICENCE 100 1- Woolhouse M. E. J. Department of Pure and Applied Biology, Imperial College, Prince Consort Road, London SW7 2BB NATIONALLICENCE 773 0- Parasitology Cambridge University Press 99/1(1989-08), 47-56 0031-1820 99:1<47 1989 99 PAR CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence SWISSBIB 386399077 BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-cambridge