Search PubMed⌕ Search

Biomedical subjects

M E Woolhouse

Publications and source records attributed to M E Woolhouse.

98 records · Page 6Linked to original sources
PubMed1989

Spatial and temporal heterogeneity in the population dynamics of Bulinus globosus and Biomphalaria pfeifferi and in the epidemiology of their infection with schistosomes.

Populations of Bulinus globosus and Biomphalaria pfeifferi were studied in a river habitat in Zimbabwe over a period of 12 months. Data were obtained on the prevalences of infections of Schistosoma haematobium (also S. mattheei) and S. mansoni respectively. Population parameters showed the following patterns for both snail species. (1) A patchy distribution correlated with the distributions of aquatic plants. (2) Life-expectancies of only a few weeks. (3) Recruitment rates correlated with water temperature and showing a distinct seasonal peak. (4) Spatial variation in recruitment. (5) A redistribution of snails during the rainy season. Epidemiological parameters showed the following patterns. (1) Large seasonal variations in the prevalence of patent infections. (2) Evidence from size-prevalence curves that suggests a variable force-of-infection from man to snail, correlated with water temperature. (3) Prevalences of infection that were higher in the vicinity of (+/- 60 m from) major water contact sites. Local prevalences of infection for B. globosus sometimes exceeded 50% and may have approached 100% if pre-patent infections are included. Snail numbers may limit transmission at these locations. Attention is drawn to the need to make field observations at an appropriate spatial scale and also to the implications for the effectiveness of focal snail control as a means of reducing transmission.

Animals↗
PubMed1989

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

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 seasonality in the age-prevalence curve as a function of known seasonal variation in the force of infection and snail fecundity.

Animals↗
PubMed1989

The effect of schistosome infection on the mortality rates of Bulinus globosus and Biomphalaria pfeifferi.

Schistosoma infections of Bulinus globosus increased the per capita mortality rate in the laboratory from 0.231 to 0.406 per week. Schistosoma mansoni infections of Biomphalaria pfeifferi increased the per capita mortality rate in the laboratory from 0.055 to 0.177 per week. Calculations based on data from previous work by another author indicate an increase in the mortality rate of B. pfeifferi associated with prepatent S. mansoni infection.

Animals↗
PubMed1988

A mark-recapture method for ecological studies of schistosomiasis vector snail populations.

Mark-recapture methods have been used only sporadically in studies of the population biology of the snail vectors of schistosomiasis, yet mark-recapture has potential advantages for obtaining estimates of absolute abundance and recruitment and loss rates for snail populations in the field. A case study is described that uses Bailey's triple catch design to estimate these parameters for Bulinus globosus and Biomphalaria pfeifferi in a small river in Zimbabwe. The assumptions inherent in the design and tests of these assumptions are described. The parameter estimates obtained are biologically plausible, suggesting significant recruitment into the population at the time of the study and a life expectancy for B. globosus of about three weeks. The major drawback is the lack of precision of these estimates. Means of improving precision are discussed, and mark-recapture is evaluated as a research tool for the study of vector snail population biology.

Animals↗
PubMedDate not supplied

The interpretation of intensity and aggregation data for infections of Schistosoma haematobium.

The relationships between mean intensities of infection, the aggregation of infection among hosts, and host age are analysed using data from 2 large (> 3000 individuals) field studies of Schistosoma haematobium infection. The data show a convex relationship between mean intensity and age, a convex relationship between an inverse index of aggregation and age, and an age-dependent relationship between the mean and aggregation of infection intensity when levels of infection are high. These patterns are qualitatively compared with the output of mathematical models based on an immigration-death process (model I), and incorporating age-dependent changes in the distribution of exposure to infection as suggested by field data (model II), or reductions in the rate of infection as a function of either current (model III) or cumulative (model IV) parasite burdens, that is, density-dependent processes without or with memory, respectively. Models II and III were able to reproduce observed patterns, but model IV, which is a possible representation of acquired immunity, was not. These results are consistent with the following assumptions: (i) age-related patterns of aggregation can be generated without recourse to density-dependent processes; (ii) the epidemiological impact of density-dependent processes depends on whether these act with or without memory; and (iii) any acquired immunity to human S. haematobium infection may be significantly less than life-long.

Adolescent↗
PubMedDate not supplied

Epidemiological features of epidemic cholera (El Tor) in Zimbabwe.

Epidemics of cholera have been frequent in southern Africa since the reintroduction of the disease to the continent in 1970. In late 1992, following a severe drought and an influx of refugees from Mozambique, cholera reappeared in Zimbabwe for the first time since 1985 and rapidly spread through the rural areas of the country. Data relating to symptomatic cholera infection collected during 2 large outbreaks on the eastern border of the country showed that host age and sex were important factors relating to symptomatic infection, as were population density and access to water. Epidemic profiles for the 2 study areas differed in that one of the profiles exhibited a distinct second phase epidemic. This unusual pattern was compared qualitatively with the output of a series of simple mathematical models to examine the contribution of different epidemiological processes to the pattern of disease observed. Model output suggested a complex disease process, in which the dynamics may have been influenced by spatial components. Statistical analysis of these unusual data showed that the observed pattern was independent of the effects of host age or sex, and provided compelling evidence of a marked spatial component of the second phase epidemic.

Adolescent↗