M.S. Chan

Hookworm infection in pregnancy.

Hookworm infection is a recognized precipitating factor for iron deficiency anaemia (reviewed by SCHAD & WARREN 1990). In developing societies where hookworm infection is prevalent diets are marginal in quality and fertility rates are high it is probable that hookworm infection during pregnancy is a significant contributor to the burden of disease in women. Direct estimation of the disease burden is not possible using currently available data. This analysis pursues the simpler goal of providing an order of magnitude estimate of the number of women who are both pregnant and infected with hookworm burdens of a magnitude that is likely to be associated with disease. A basic sensitivity analysis is undertaken for data for sub-Saharan Africa before attempting a global estimate. (excerpt)

EPIFIL: the development of an age-structured model for describing the transmission dynamics and control of lymphatic filariasis.

Mathematical models of transmission dynamics of infectious diseases provide a useful tool for investigating the impact of community based control measures. Previously, we used a dynamic (constant force-of-infection) model for lymphatic filariasis to describe observed patterns of infection and disease in endemic communities. In this paper, we expand the model to examine the effects of control options against filariasis by incorporating the impact of age structure of the human community and by addressing explicitly the dynamics of parasite transmission from and to the vector population. This model is tested using data for Wuchereria bancrofti transmitted by Culex quinquefasciatus in Pondicherry, South India. The results show that chemotherapy has a larger short-term impact than vector control but that the effects of vector control can last beyond the treatment period. In addition we compare rates of recrudescence for drugs with different macrofilaricidal effects.

The development of an age structured model for schistosomiasis transmission dynamics and control and its validation for Schistosoma mansoni.

Mathematical models are potentially useful tools to aid in the design of control programmes for parasitic diseases. In this paper, a fully age structured epidemiological model of human schistosomiasis is developed and parameterized, and used to predict trends in infection prevalence, intensity and prevalence of heavy infections over age and time during several rounds of mass and age targeted treatment. The model is validated against data from a Schistosoma mansoni control programme in Kenya.

The development and validation of an age-structured model for the evaluation of disease control strategies for intestinal helminths

Epidemiological modelling can be a useful tool for the evaluation of parasite control strategies. An age-structured epidemiological model of intestinal helminth dynamics is developed. This model includes the explicit representation of changing worm distributions between hosts as a result of treatment, and estimates the morbidity due to heavy infections. The model is used to evaluate the effectiveness of different programmes of age-targeted community chemotherapy in reducing the amount of morbidity due to helminth infection. The magnitude of age-related heterogeneities is found to be very important in determining the results of age-targeted treatment programmes. The model was verified using field data from control programmes for Ascaris lumbricoides and Trichuris trichiura, and was found to provide accurate predictions of prevalence and mean intensities of infection during and following different control regimes.

Methods for estimation of associations between multiple species parasite infections.

Human populations are often infected with more than one species of parasite, especially in developing countries where overall rates of parasitism are high. Infections with multiple parasite species may not necessarily be independent within an individual as physiological, immunological or ecological factors may result in positive or negative associations between infections with different parasite species. A general framework for estimation of these associations is presented. Data from over 215000 individuals are analysed and the associations between geohelminth (Ascaris lumbricoides, Trichuris trichiura and hookworm) and malaria species are investigated. A method is presented for analysing data from multiple communities and testing whether the associations in different communities are equal. Overall estimates of the associations between species are obtained for each country and continent where data were available. Associations between geohelminth species were, in general, found to be positive whilst both positive and negative associations were found between the different Plasmodium species. There was evidence for significant geographical heterogeneity between the associations. A method for using these parameter estimates to predict the distribution of multiple infections when only marginal prevalence data are available is described and demonstrated.

Control of Ascaris infection by chemotherapy: which is the most cost-effective option?

Cost-effectiveness analysis is used to predict the optimal design of mass chemotherapy strategies in controlling Ascaris lumbricoides infection. The question of who to treat, how many to treat, and how often to treat are addressed using a population dynamic model of helminth transmission that assesses effectiveness in terms of disease reduction, combined with cost data from an actual control programme. Child-targeted treatment can be more cost-effective than population treatment in reducing the number of disease cases. The model also implies that, in the circumstances described here, enhancing coverage is a more cost-effective approach than increasing frequency of treatment.

Dynamic aspects of morbidity and acquired immunity in schistosomiasis control

In schistosomiasis control, rational planning of chemotherapy programmes is complicated by the dynamic interactions between treatment and levels of acquired immunity and morbidity in the community. In this paper, mathematical models that address the development of acquired immunity and the prevalence of morbidity are incorporated within an age-structured transmission framework to explore some of the dynamic complexities of long-term chemotherapy programmes. As well as illustrating some of the potential problems inherent in predicting the consequences of control measures, the model provides insights into the dynamics of schistosomiasis transmission and the parameters that need to be measured to further improve the design of community-based control programmes.

Predicting the impact of school-based treatment for urinary schistosomiasis given by the Ghana Partnership for Child Development

Mathematical models can be used to predict the impact of interventions to control infectious diseases. In this paper, an epidemiological model is used to predict the impact of chemotherapy of school-age children infected with Schistosoma haematobium, in a programme conducted by the Ghana Partnership for Child Development in the Volta Region, Ghana. Existing data were used to validate the predictions of the model, demonstrating convincingly the ability of the model to make correct predictions. Predictions of trends in mean egg count, infection prevalence and prevalence of heavy infection (> 50 eggs/10 mL urine) were then made for the period 1997-1999, and will be compared to the data collected in the programme in the future.

The practicality and sustainability of vaccination as an approach to parasite control

The development of a successful vaccine depends not only on the production of the vaccine itself, but also on the design pf the vaccination programme. This involves a better understanding of the epidemiology of the disease to be controlled, the delivery system to be used and the costs involved. This review focuses on current understanding of parasite population dynamics and epidemiology, and on the logistic experience gained from immunization programmes using existing viral and bacterial vaccines. The feasibility and sustainability of any new vaccine would greatly benefit from research into epidemiology and health systems which are conducted in parallel, rather than sequential to, vaccine development.