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Biomedical subjects

M E Woolhouse

Publications and source records attributed to M E Woolhouse.

At least 91 records · Page 5Linked to original sources
PubMed1993

Epidemiology of trypanosome infections of the tsetse fly Glossina pallidipes in the Zambezi Valley.

The epidemiology of trypanosome infections of Glossina pallidipes was studied at a riverine site in the Zambezi Valley, Zimbabwe for a period of 13 months. Over 9000 flies were captured using a single trap. These flies were dissected, screened for trypanosome infection, sexed, and aged using both wing fray and (for females) ovarian category indices. Midgut infections were identified to species using recently developed DNA probes. The overall prevalence of mature infections was 5.5%, comprising 3.1% Trypanosoma vivax-type and 2.4% T. congolense-type (which included very low prevalences of T. brucei, T. simiae and another Nannomonas species). The prevalence of infection increased with age. For T. vivax-type infections in flies aged by ovarian category this relationship could be described by a simple 'catalytic' model assuming a constant per capita rate of infection. For T. congolense-type infections this model tended to over-estimate prevalence in older age classes, implying that the rate of infection decreases with age, and/or that infected flies have higher mortality rates, and/or that a significant fraction of the population is resistant to infection. Prevalences of infection also varied between months. This variation was more marked for T. vivax-type infections and was negatively correlated with both temperature and rainfall. The shape of the age-prevalence relationship, however, did not vary significantly between months. These observations are not fully explained by variation in the age-structure of the tsetse population and are consistent with temporal variation in the rate of infection (rather than in the trypanosome developmental period or in effects of infection on fly mortality). Possible causes of this variation are discussed.

Animals↗
PubMed1993

A theoretical framework for immune responses and predisposition to helminth infection.

Many field studies of human helminth infections have reported a positive correlation between parasite burdens and the rate of re-establishment of infection following chemotherapy, i.e., predisposition. Some studies have also reported the relationships between re-establishment and exposure and immune responses. The interpretation of these data is made difficult by the complexity of the underlying immunoepidemiological processes. In this paper, simple mathematical models are used to explore expected patterns, especially in relation to host age. These patterns are determined by rates of infection, parasite life expectancy, the level of immunological responsiveness, the duration of immunological memory, and may be greatly affected by the immunogenic roles of different parasite stages. In general, acquired immunity is predicted to reduce the degree of predisposition. This reduction is age-dependent and may generate 'negative predisposition' in some age classes. Age-dependent reductions in the correlation between re-establishment and exposure are also predicted. The correlation between re-establishment and protective immune responses is also predicted to be age-dependent, but may remain positive for all ages despite significant acquired immunity. The results suggest that great care is needed in the interpretation of immunoepidemiological data from treatment-reinfection studies.

Adolescent↗
PubMed1992

On the application of mathematical models of schistosome transmission dynamics. II. Control.

Mathematical models have considerable potential as aids to the design of schistosome control programmes. This is because of the complex nature of the schistosome transmission cycle and the variety of control measures available, which make the comparative effectiveness of different control options extremely difficult to predict. This review aims to demonstrate how control can be incorporated in models of schistosome transmission dynamics, and to make explicit the assumptions and limitations of the models and their relationships with field data. A basic model is described which considers changes in the mean number of schistosomes per person. The criteria for the eradication of endemic infection and the potential for reducing levels of infection are discussed. The treatment of various control measures within this framework is reviewed. These include: chemotherapy (mass, selective and targeted), molluscicide application (blanket and focal) and other snail control measures, larval stage control, improved water supplies and sanitation, and health education. The incorporation of economic variables is also discussed. The choice between different control options depends on the relationships between schistosome epidemiology and the costs and effects of control. The evaluation of cost-effectiveness is a dynamic problem, and the outcome will depend on local conditions and constraints. Very general recommendations for the design of schistosome control programmes are unlikely to prove useful.

Animals↗
PubMed1992

A further model for temporal patterns in the epidemiology of schistosome infections of snails.

The prevalence of Schistosoma mansoni infections of Biomphalaria pfeifferi shows seasonal variation. Field data from Zimbabwe show annual ranges from 0 to 7%. In this paper a mathematical model of B. pfeifferi population dynamics and S. mansoni epidemiology is used as a framework for analysis of these patterns. Snail fecundity is a function of snail age and of temperature, and is apparently affected by other seasonal factors. The pre-patent period is dependent on temperature. Infection affects snail fecundity and mortality. Parameter values are derived from previous field and laboratory studies. The force-of-infection is estimated from the analysis of size-prevalence data. Using observed temperatures, model output agrees well with field data on snail abundance and prevalence of infection over a 14-month period. Seasonal variation in prevalence largely reflects variation in the pre-patent period and in snail population age structure. The possible role of seasonality in the force-of-infection is discussed. Prevalence patterns are not greatly affected by year-to-year differences in temperature. Significant seasonal variation in snail-man transmission rates is expected.

Animals↗
PubMed1992

A theoretical framework for the immunoepidemiology of helminth infection.

Field studies of parasitic helminths in endemically infected human communities have provided quantitative information on the relationships between parasite burdens, immune responses and age. There are considerable difficulties in the interpretation of these immunoepidemiological data due to the complexities of the biological processes generating the observed patterns. In this paper simple mathematical models are used to explore the expected patterns of variation with host age in parasite burdens, the aggregation of parasites among hosts, levels of immune response, and the correlation between parasite burdens and immune responses. These relationships reflect rates of infection, rates of parasite mortality, the strength of the immune response, and the duration of immunological memory. The models generate some complex and counterintuitive patterns. The analysis suggests that some of these patterns might serve to (i) distinguish effects due to acquired immunity from effects due to age-dependent exposure, (ii) identify potentially protective immune responses, and (iii) identify the parasite stages important in the development of acquired immunity. The results imply that previous analyses of immunoepidemiological data may have been overly simplistic and, especially, that patterns believed to be inconsistent with protective immunity may have been incorrectly interpreted.

Adolescent↗
PubMed1991

Heterogeneities in transmission rates and the epidemiology of schistosome infection.

Previous theoretical studies have suggested that heterogeneities in transmission rates can have significant effects on the epidemiology of parasite infections. However, the magnitude of these effects in practice remains uncertain because of the difficulty of quantifying such heterogeneities under natural conditions. In this paper we consider the effects of heterogeneous rates of contact with infective water bodies on the basic reproductive rate, R0, of human schistosomes. In particular, we examine the theoretical effects of differences in the distribution of contacts among individuals and among sites. We find that an individual's contribution to R0 depends not only on the net contact rate but also on how these contacts are distributed among sites--the contact pattern. Our analysis of field data suggests a substantial impact of heterogeneous contact rates on R0 in practice. Significant reductions in R0 may be possible if schistosomiasis control efforts are targeted at certain sites and, especially, certain individuals.

Animals↗
PubMed1991

Acquired immunity and epidemiology of Schistosoma haematobium.

Human immune responses to schistosome infection have been characterized in detail. But there has been controversy over the relative importance of ecological factors (variation in exposure to infection) and immunological factors (acquired immunity) in determining the relationships between levels of infection and age typically found in areas where infection is endemic. Independent effects of exposure and age on the rates of reinfection with Schistosoma haematobium after chemotherapy have been demonstrated in the Gambia and Zimbabwe. This age effect could be the result of acquired immunity to infection. Indeed, allowing for variation in exposure and age, low rates of reinfection in the Gambia are correlated with high amounts of specific IgE antibodies--human IgE can kill S. mansoni schistosomulae in vitro. Further, animals can acquire immunologically mediated resistance to S. mansoni infection, although nonimmunological factors could also be involved. Acquisition of this immunity seems to be related to the cumulative effects of repeated infection and provides only partial protection. These characteristics are consistent with immuno-epidemiological data for both S. mansoni and S. haematobium infections of humans. We have now analysed age-prevalence data for human infection with S. haematobium, and find patterns of variation that are indeed consistent with the epidemiological effects of acquired immunity predicted by mathematical models.

Adolescent↗
PubMed1991

On the application of mathematical models of schistosome transmission dynamics. I. Natural transmission.

The many mathematical models of the transmission dynamics of schistosomes that have been published since 1965 have had little impact on field studies or on the design of schistosome control programmes. At least in part, this is due to limited interaction between theoretician and field worker, resulting in unrealistic models that are not easily applied to field data. This review aims to make explicit the assumptions and limitations of existing models and their relationships with field data. A basic model is described which considers the mean number of schistosomes per person and the prevalence of patent infections of snails. Various modifications to this model are introduced. These include: prepatent infections of snails; loss of infection of snails; the effects of snail population dynamics; the effects of miracidia and cercariae population dynamics; miracidia searching efficiency; reservoir hosts; heterogeneous patterns of transmission; seasonality; and predisposition to infection. Variation in levels of infection with age and the effects of acquired immunity to infection are also considered. Published models of schistosome transmission dynamics are reviewed within this framework. Approaches to the modelling of schistosome control measures are considered in a companion paper. It is suggested that future theoretical studies give greater attention to the details of snail population dynamics, heterogeneous patterns of transmission and the effects of acquired immunity. There is a need for field studies explicitly designed to provide estimates of transmission parameters and for studies of the epidemiological effects of acquired immunity.

Age Factors↗
PubMed1991

Heterogeneities in water contact patterns and the epidemiology of Schistosoma haematobium.

Variations in the amount of water contact made by individuals and in the amount of water contact made at different sites may have significant impacts on patterns of human schistosome infection. Previous studies have reported variations in the rate of water contact and differences in the sites used between age/sex classes, but there is limited information on variations in individual water contact behaviour. In this paper we report and analyse observations of essentially all water contacts made over a two week period by all individuals in a rural community in eastern Zimbabwe. The mean rate of water contact was 0.43 contacts/person/day. These data were over-dispersed, ranging from zero to 3.3 contacts/person/day; 90% of contacts were made by only 37% of the population. Contact rates were related to age (highest in 8 to 10-year-olds) but not sex, with substantial variation unaccounted for by these variables. Age and sex classes differed in types of water-related activities and the time of day of contact. A greater diversity of sites was used by children than by adults and by males than by females. Individual contact rates were correlated with intensities of infection, although the risk of infection per contact was estimated to be highest in 2 to 4-year-old children and higher for males than females. Five contact sites were used during the study period, with more than 50% of contacts occurring at just 2 sites. Different age and sex classes used different sites and there were additional site-related differences in types of activity and the time of day of use. The implications of these water contact patterns for schistosome epidemiology are discussed. In particular the results provide strong quantitative support for control programmes aimed at heavily used sites (e.g. focal mollusciciding) or at the minority of individuals making most water contacts (e.g. targeted chemotherapy).

Adolescent↗
PubMed1991

Factors affecting the intensity of reinfection with Schistosoma haematobium following treatment with praziquantel.

Infection with Schistosoma haematobium was studied in a rural community of approximately 500 persons in eastern Zimbabwe. The overall prevalence of infection, as determined by urine egg counts, was 40.1%, and of heavy infections (greater than or equal to 50 eggs/10 ml urine) was 11.0%. The prevalence of both heavy and all infections was highest in the 8 to 10-year-old age class. During 1987-88 data were obtained from 102 individuals on intensity of reinfection 14 weeks after treatment with praziquantel, the efficacy of treatment having been determined after 4 weeks. The water contact made by these individuals during 2-week periods immediately following treatment was recorded. The relative abundance of patent infected intermediate host snails, Bulinus globosus, was also monitored. An index of exposure was developed which weighted each water contact by its duration, the type of activity, the time of day, and the abundance of infected snails at the site used. The relationships between rates of reinfection, rates of exposure, and age were examined. Although only 13 individuals showed positive rates of reinfection, there were statistically significant and independent effects of both exposure and age on reinfection rate. Quantitative estimates of reinfection rates suggested that individuals aged 12 years or less acquired substantially more infection (measured as egg output) than individuals more than 12 years old.

Adolescent↗
PubMed1990

Population dynamics model for Bulinus globosus, intermediate host for Schistosoma haematobium, in river habitats.

A mathematical model is developed that describes the population dynamics of the freshwater snail Bulinus globosus, an intermediate host of Schistosoma haematobium. The model is developed from field data recording abundance changes, recruitment rates, and mortality rates for adult snails in river habitats in Zimbabwe. Rate of recruitment into the adult population is dependent on temperature, incorporating a time lag to allow for growth to adult size. Mortality rate is also a function of temperature. A temperature-dependent model provides a useful description of changes in abundance during the dry season. Long-term population fluctuations are greatly affected by spates (flooding) associated with heavy rainfall. An estimate is made of the frequency and effects of spates. A simulation model that allows variable annual rainfall predicts fluctuations in snail abundance over two orders of magnitude over timescales of ten or more years. The role of density-dependent factors and the long-term persistence of the population are discussed. B. globosus population dynamics can be described as 'density vague'.

Animals↗
PubMed1990

On the distribution of schistosome infections among host snails.

The distribution of Schistosoma haematobium infections in a population of Bulinus globosus snails collected from a field site in Zimbabwe is described. Cercariae of both sexes emerged from a proportion of patent infected snails, as was demonstrated by the detection of paired schistosomes in hamsters each exposed to cercariae from a single snail. These snails must have been infected by two or more miracidia. Analysis showed that the proportion of mixed-sex infections was higher than expected if infections were distributed at random among snails. An index of overdispersion was calculated. Overdispersion of infections results from heterogeneities in the exposure and/or susceptibility of snails to infection. The implications for the epidemiology and control of schistosomiasis haematobium are discussed.

Animals↗
PubMed1990

The epidemiology of schistosome infections of snails: taking the theory into the field.

Most field studies of the epidemiology of schistosome infections of snails have been essentially descriptive in nature. Although much use has been made of epidemiological models in the interpretation of experimental studies of schistosome-snail interactions, the models have had restricted application outside the laboratory. Here, Mark Woolhouse and Steve Chandiwana make use of a theoretical framework in discussing current understanding of schistisome-snail epidemiological patterns in the field, and then consider the implications of the analysis for the control of schistosomiasis.

Journal Article↗
PubMed1990

Temporal patterns in the epidemiology of schistosome infections of snails: a model for field data.

The prevalence of patent schistosome infections of intermediate host snails often shows seasonal variations. For schistosome, mainly Schistosoma haematobium, infections of Bulinus globosus field data from Zimbabwe show annual ranges in prevalence from 2% to 30%. In this paper a mathematical model is developed for B. globosus population dynamics and the epidemiology of schistosome infection. The model is a discrete-time analogue of a catalytic model, with an added recruitment term. Snail fecundity and survival are functions of age, infection, and temperature. The pre-patent period of infection is dependent on temperature. Parameter values, and relationships with temperature, are taken from earlier field and laboratory studies. The force-of-infection is estimated from analysis of age-prevalence data. Model predictions, making use of temperature data recorded in the field, are in good agreement with observations over a 2-year period at a study site in Zimbabwe. The key features of interest are a fall in the prevalence of patent infections during the cold season and a rapid rise to a peak early in the hot season. This pattern results from the accumulation of pre-patent infections which fail to develop at low temperatures, but mature en masse, together with new infections, after a few weeks at higher temperatures. Model analysis demonstrates that seasonal patterns in the prevalence of patent infections may be due largely to the influence of temperature on the pre-patent period. Seasonal and year-to-year variations in temperature may therefore have significant epidemiological effects.

Age Factors↗