Preface
Three decades ago, Advances in Parasitology published an article on ‘Helminth infections of humans: mathematical models, population dynamics, and control’, which attempted to summarize progress at that time in the development of mathematical models for the transmission dynamics of helminths that cause disease in human communities, and their use in evaluating the impact of mass drug administration (MDA) to control infection (Anderson & May, 1985). Expanded interest in the control of the so-called Neglected Tropical Diseases (NTDs) in recent years, spurred on in part by drug donations from the pharmaceutical industry to the World Health Organization to be used in resource-poor settings with endemic infection, and other international initiatives, led us to conclude that it is a sensible time to review again progress on model development and associated applications in the design of control policies, not just for the helminthiases, but for other infections falling under the remit of the NTDs.
The launch of the London Centre for Neglected Tropical Disease Research (LCNTDR) (
http://www.londonntd.org/) in January 2013—a collaboration between the London School of Hygiene and Tropical Medicine, Imperial College London, the Natural History Museum and the Royal Veterinary College to further research on how best to control the NTDs—provided an opportunity to discuss the idea for a new review of mathematical models with the present editor of
Advances in Parasitology, Professor David Rollinson. The launch of the LCNTDR took place on the 30th of January 2013, exactly one year after the London Declaration on Neglected Tropical Diseases (
http://unitingtocombatntds.org/resource/london-declaration) and the announcement by the Director-General of the World Health Organization, Dr Margaret Chan, of a roadmap to accelerate work to overcome the impact of NTDs on the 30th of January 2012. The roadmap set targets for the period 2012–2020—with interim milestones to be achieved by 2015—providing an unprecedented impetus for improved control (reduction or elimination of the public health burden) and, where feasible, the national or regional elimination (interruption of transmission) of prioritized NTDs (
http://www.who.int/neglected_diseases/NTD_RoadMap_2012_Fullversion.pdf). The release of this volume is, therefore, well timed in the sense that it coincides with the establishment of a consortium of researchers funded by an initiative of the Bill and Melinda Gates Foundation (B&MGF) to support mathematical modelling studies on these NTDs.
The term ‘Neglected Tropical Diseases’ (NTDs) was first used in 2005 (
http://blog.wellcome.ac.uk/2012/01/10/neglected-tropical-diseases-the-campaign-trail/) to increase both awareness and the global resources available for tackling a group of bacterial, viral, protozoan and helminthic diseases that collectively impose a substantial burden of morbidity and mortality among the poorest populations of our world. In the past, they received much less attention than the so-called ‘Big Three’ (malaria, HIV/AIDS and tuberculosis) from the global scientific, R&D, funding and health policy communities. The World Health Organization (WHO) has prioritized, for increased control efforts, 17 NTDs that are endemic in 149 countries and affect more than 1.4 billion people, costing developing economies billions of dollars every year (
http://www.who.int/neglected_diseases/diseases/en/).
Two main strategies have been endorsed by the WHO for reducing the health impact of NTDs, namely, preventive chemotherapy and intensified disease management. The former (PC) refers to the large scale, regular and prolonged delivery to populations (in the form of MDA) of single-dose medicines efficacious for the treatment of trematode and nematode helminthiases and trachoma, which contribute to averting the morbidity associated with these infections (e.g. the schistosomiases, the intestinal or soil-transmitted helminthiases and the filariases). The latter (IDM) are directed at NTDs for which similar simple tools and treatments are not as yet available (e.g. Buruli ulcer, yaws, leprosy, the leishmaniases, African and American trypanosomiasis, and cestode infections). Complementary measures to support these strategies include control of insect vectors or intermediate hosts, improved water and sanitation infrastructure, coordination with veterinary public health, health education and capacity building.
The Disease Reference Group on human helminthiases (DRG4), convened by the UNICEF/UNDP/World Bank Special Programme for Research and Training in Tropical Diseases (TDR), hosted at the WHO, has published a collection of articles entitled ‘A research agenda for the control and elimination of human helminthiases’ (
http://www.ploscollections.org/static/pntdCollections). This document noted that although it is generally accepted that mathematical models have an important role to play in our understanding of the transmission dynamics of NTDs in general and helminth infections in particular, the potential of models to provide critical insights to inform and support decision making in ongoing control and elimination programmes has not yet been fully realized. Three years later, most authors of the chapters we now present argue, in similar ways, that to fulfil this role, models still need to be further developed, parameterized, and most importantly, validated against epidemiological data. In general, the unifying theme is a call for a closer dialogue between modellers and the wider scientific and stakeholder communities. Progress in this area has been achieved in other infectious disease fields such as malaria, influenza and HIV.
The B&MGF-supported NTD Modelling Consortium (
http://www.ntdmodelling.org/) will be crucial in helping to coalesce and energize the NTD modelling community towards helping to achieve the WHO 2020 control and elimination goals. The motivation of the consortium lies in the recognition that many urgent policy issues concerning the control and elimination of NTDs can only be answered through the use of quantitative tools, and that this can only be truly achieved through strong collaborations between modellers, epidemiologists, policy makers and field epidemiologists.
It is in this spirit that we present the first part of this thematic volume of Advances in Parasitology. Initially envisaged as a single, stand-alone volume, we found that the very enthusiastic response from contributors warranted the publication of two volumes. This present volume brings together a range of articles looking at transmission dynamics, mathematical model definition and analysis, statistical tools for parameter estimation, molecular epidemiological approaches and health economic perspectives.
Part A starts with a chapter by Manoj Gambhir and colleagues on the role of the Allee effect in the elimination of NTDs, defined in the context of positive density dependence (the positive correlation at low population densities between population size and mean individual fitness—often measured as per capita population growth rate). These authors examine the various density-dependent processes that regulate populations of the mosquito-borne filarial nematode Wuchereria bancrofti—the causal agent of lymphatic filariasis—and discuss how these affect two important epidemiological outcome variables that relate to control and elimination programmes, namely, the parasite transmission breakpoint (or extinction threshold) and the reproduction fitness, measured as the effective reproduction ratio. They conclude that although the operation of a single positive density-dependent process can introduce a reasonable chance of achieving elimination, this chance is not appreciably increased by the operation of additional positive density dependencies. Reports by other published work of the possible existence of an Allee effect in trachoma are reviewed.
The paper by David Blok and co-workers focuses on transmission models for leprosy, one of the diseases identified by WHO for intensified case detection and case management. Two compartmental, population models and one individual-based model have been described in the literature, with the latter examining transmission within households and the impact of case finding among contacts of new leprosy patients. The models highlight that among the most relevant factors affecting epidemiological outcomes are contact heterogeneity, heterogeneity in susceptibility and spatial heterogeneity. However, existing models have only been applied to data from a limited number of countries. Parameterization of the models for other areas, particularly those with high incidence, is essential to support current initiatives for the global elimination of leprosy.
Continuing with the theme of IDM diseases, the paper by Kat Rock and colleagues reviews mathematical models of human African trypanosomiasis (sleeping sickness), a vector-borne infection transmitted by tsetse flies. The disease is usually fatal if untreated and transmission occurs in foci across sub-Saharan Africa. Mathematical modelling of African trypanosomiasis began in the 1980s, chiefly based on extensions of the classical Ross-Macdonald malaria model, but progress has been slow and renewed modelling efforts are badly needed. The existing deterministic compartmental models have captured salient epidemiological features and allowed examination of intervention effectiveness (treatment of humans and control of tsetse fly populations) but have, by and large, overestimated infection prevalence and ignored transient dynamics. The authors argue that there is still a need for improved models validated with enhanced data collection, which can provide...
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