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Anaemia in Ugandan preschool-aged children: the relative contribution of intestinal parasites and malaria
- HELEN K. GREEN, JOSE C. SOUSA-FIGUEIREDO, MARIA-GLORIA BASÁÑEZ, MARTHA BETSON, NARCIS B. KABATEREINE, ALAN FENWICK, J. RUSSELL STOTHARD
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- Journal:
- Parasitology / Volume 138 / Issue 12 / October 2011
- Published online by Cambridge University Press:
- 08 August 2011, pp. 1534-1545
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Anaemia is a severe public health issue among African preschool-aged children, yet little effective progress has been made towards its amelioration, in part due to difficulties in unravelling its complex, multifactorial aetiology. To determine the current anaemia situation and assess the relative contribution of malaria, intestinal schistosomiasis and infection with soil-transmitted helminths, two separate cross-sectional epidemiological surveys were carried out in Uganda including 573 and 455 preschool-aged children (⩽6 years) living along the shores of Lake Albert and on the islands in Lake Victoria, respectively. Anaemia was found to be a severe public health problem in Lake Albert, affecting 68·9% of children (ninety-five percent confidence intervals (95% CI) 64·9–72·7%), a statistically significant higher prevalence relative to the 27·3% detected in Lake Victoria (95% CI: 23·3–31·7%). After multivariate analysis (controlling for sex and age of the child), the only factor found to be significantly associated with increased odds of anaemia in both lake systems was malaria (Lake Albert, odds ratio (OR)=2·1, 95% CI: 1·4–3·2; Lake Victoria, OR=1·9, 95% CI: 1·2–2·9). Thus intervention strategies primarily focusing on very young children and combating malaria appear to represent the most appropriate use of human and financial resources for the prevention of anaemia in this age group and area. Looking to the future, these activities could be further emphasised within the National Child Health DaysPLUS agenda.
Mathematical modelling of parasitic infections: from data and parameter estimation to evolutionary implications
- MARÍA-GLORIA BASÁÑEZ, L. H. CHAPPELL
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- Journal:
- Parasitology / Volume 135 / Issue 13 / November 2008
- Published online by Cambridge University Press:
- 13 November 2008, pp. 1487-1488
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Rates of microfilarial production by Onchocerca volvulus are not cumulatively reduced by multiple ivermectin treatments
- CHRISTIAN BOTTOMLEY, VALERIE ISHAM, RICHARD C. COLLINS, MARIA-GLORIA BASÁÑEZ
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- Journal:
- Parasitology / Volume 135 / Issue 13 / November 2008
- Published online by Cambridge University Press:
- 03 October 2008, pp. 1571-1581
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Regular distribution of ivermectin reduces onchocerciasis transmission and morbidity by killing, within humans, the microfilarial stage of the parasite (microfilaricidal effect). In addition, ivermectin exerts a so-called embryostatic effect by which microfilarial production by the adult female worm becomes suppressed during a number of weeks after treatment. To assess the overall effect of ivermectin on onchocerciasis transmission and evaluate the likelihood of local elimination of the infection it is important to estimate the magnitude of the anti-fertility effect over the course of a treatment programme. Estimates of the effect of repeated drug treatments on the production of microfilariae by adult Onchocerca volvulus were obtained by developing a model that was fitted to data collected from three hyperendemic communities in Guatemala, where eligible residents received ivermectin twice per year for two and a half years. The data consist of microfilarial load measurements in the skin, collected just before each six-monthly treatment during the programme. The model that is developed describes the dynamics of an individual host's expected microfilarial load over the 30-month study period. We adopt a Bayesian approach and use Markov chain Monte Carlo (McMC) techniques to fit the model to the data. Combining estimates from the three villages, average microfilarial production in the first six months post-treatment was reduced by ~64% of its pre-treatment level, regardless of values chosen for the pre-ivermectin fertility rate within plausible ranges. Increased adult worm death rate after treatment (to mimic removal of macrofilariae via nodulectomy during the programme) resulted in a smaller estimated magnitude of the embryostatic effect (rate of microfilarial production was reduced by ~58% of pre-ivermectin value). After subsequent treatments, the rate of microfilarial production appeared to be similarly decreased. The data and analyses therefore do not support the hypothesis of a cumulative effect of multiple ivermectin treatments on microfilarial production by female worms.
Consumer-Resource Dynamics. By William W. Murdoch, Cheryl J. Briggs & Roger M. Nisbet, pp. 462. Monographs in Population Biology, 36. Princeton University Press, Princeton, USA and Oxford, UK, 2003. ISBN 0 691 00658 X (hbk) and 0 691 00657 1 (pbk). £55.00 (cloth) and £24.95 (paper).
- MARíA-GLORIA BASáÑEZ
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- Journal:
- Parasitology / Volume 131 / Issue 4 / October 2005
- Published online by Cambridge University Press:
- 07 September 2005, pp. 579-580
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The issue of species persistence may be a central theme in population ecology, but it is also crucial to infectious disease control. While ecologists aim at preserving biodiversity, disease eradication seeks to drive selected pathogens to global extinction; understanding the mechanisms explaining species persistence at local and regional levels underpins both goals.
In Consumer-Resource Dynamics, Murdoch, Briggs & Nisbet attempt to provide a unifying theory of population dynamics which, starting from the most fundamental of interactions, the consumer-resource duo in a homogeneous environment, expands into multispecies interactions, collapses into single population dynamics, or ventures into unpredictability and spatial heterogeneity. The fact that disease-host systems are hardly mentioned does not detract from the relevance this book can have to parasite population biologists, field parasitologists, and those interested in pathogen and pest control. The following synopsis highlights some salient points.
Chapters 1 and 2 provide conceptual and empirical contexts for the notion of population dynamics, focusing on population regulation and persistence, and discussing the concepts of deterministic and stochastic persistence and the importance of scale. Chapters 3 and 4, on simple predator-prey models, explore the reasons behind these being inherently unstable interactions, which nonetheless embody the various manifestations of population regulation. In particular, Chapter 3 discusses Lotka-Volterra (continuous time) models, whilst Chapter 4 deals with Nicholson-Bailey (discrete time) models, and introduces discrete generation parasitoid-host systems. Chapters 5 to 7 incorporate successive layers of biological realism into the latter, with Chapter 5 introducing stage-structure, Chapter 6 exploring the dynamical effects of various parasitoid life-history strategies, and Chapter 7 bringing aspects of behavioural and evolutionary ecology in focus with population dynamics theory. Chapter 8 is dedicated to competition and multispecies interactions, leading to Chapter 9 on biological control, which ends with a discussion on the need for a resurgence of past interest in placing such control in an appropriate and rigorous ecological framework (the same can be said about disease control programmes). Chapter 10 concentrates on the dynamical consequences of making space explicit, with an insightful discussion on the origins of instability and persistence in single-species vs consumer-resource metapopulations. Chapter 11 develops a ‘phylogeny’ of models by presenting the common origin of frameworks for the description and analysis of predator-prey, parasitoid-host, pathogen-host, and herbivore-plant interactions. It is also argued that few-species models may be appropriate to describe the dynamics of populations living in many-species food-webs (by virtue of decoupling the consumer-resource interaction). The striving for a unifying and coherent population dynamics theory, present throughout the book, culminates in Chapter 12's hierarchy of models, which draws together preceding insights and points towards future theoretical and empirical research directions.
Although not presupposing a mathematical biology background, some familiarity on the part of the reader with simple population ecology models may provide a useful backdrop. The uninitiated will find the various appendices on stability analyses most helpful. The book is very well written, with interspersed tables and boxes that list the various models and their stability properties, and concluding remarks at the end of each chapter that summarize main messages and lead naturally into following chapters. On a more personal note, I found this book profoundly stimulating and found myself often discussing the various insights gained through its reading with my students and colleagues. I particularly enjoyed the sections about age- and/or stage-structured models and the advantages and pitfalls of the (often implicit) assumption of exponentially distributed waiting times. The dynamical differences between constant maturation rates and fixed maturation times are relevant to the problem of incorporating latency in parasite-host models. Ratio-dependency in models, where consumer attack rate depends on the ratio of consumers to prey, is akin to the formulation of the vector to host ratio times the biting rate in (dipteran) vector-borne disease models, most of which do not link vector abundance and biting rates to host abundance. Incorporation of overdispersion in parasitoid-host systems is effected through the widely used negative binomial distribution (May-Hassell models), with the degree of overdispersion mainly independent of host or parasitoid density. The development of stochastic models in which the distributional properties of parasitoid or parasite populations emerge from model results clearly remains a research priority. The question as to whether few-species models can appropriately describe the dynamics of many-species systems is also relevant to the topical theme of multiparasitism and the detection and importance of interspecific interactions in shaping parasite communities. If these interactions were less important than intraspecific effects in determining transmission dynamics, the traditional approach to single-species parasite-host models would be adequate. Finally, the results of the Jansen-de Roos spatial versions of the Rosenzweig-MacArthur model (with logistic growth prey and saturating predator attack rate), in which restricted movement leads to substantial decreases in local and global fluctuations, may have important implications for understanding arthropod-borne disease dynamics, as some vectors have limited mobility (ticks, mites), whilst others may connect to a greater extent pathogen-host subpopulations.