The recent commitment of the Obama administration to establish the Global Health Initiative,
which is expected to increase to over US$100 million annually for neglected tropical
disease (NTD) control, provides the most significant investment and opportunity for
the global control of NTDs to date [1], [2]. These investments, together with commitments
by the British Department for International Development, the World Bank, and several
key private philanthropies, including the Bill & Melinda Gates Foundation, must be
guided by a strong evidence-based approach. First, the problem, and the resources
required to tackle it, need to be clearly quantified. Second, mass drug administration
(MDA) should be optimally targeted to communities with the highest prevalence of infection
and presumed greatest morbidity [3], [4]. Furthermore, for diseases targeted for elimination,
including lymphatic filariasis (LF) and onchocerciasis, it will become increasingly
important to determine whether MDA can be stopped, and, if so, when and where. In
the case of schistosomiasis, as control is scaled up, there is the additional requirement
of determining when and where to shift praziquantel treatment from once per year to
less frequent intervals.
New Diagnostic Tools
Better diagnostic tools and new methods of surveillance provide more affordable and
realistic opportunities to improve the planning, monitoring, and evaluation of NTD
control. Diagnosis of onchocerciasis was previously based on detection of microfilariae
in skin snips, but this invasive technique is gradually being replaced by antibody-based
tests, which can provide a simple and rapid method of diagnosis [5], [6]. The development
of a simple and rapid antigen detection test for Wuchereria bancrofti antigenaemia,
based on the immuno-chromatographic test (ICT card test), revolutionized LF surveys
since it avoided the need to collect blood at night and the time-consuming preparation
and examination of blood slides [7]. Ongoing efforts are investigating similar rapid
antigen detection tests for schistosomiasis [8]. This new generation of diagnostics
provides a sound foundation for developing reliable, up-to-date maps of the distribution
of different NTDs to guide and target resources efficiently. Without such maps, the
journey of NTD control will be difficult.
Mapping of NTDs
In the past, national reporting on NTDs has been incomplete and unreliable because
of weak disease surveillance systems, often necessitating dedicated surveys to be
undertaken. Perhaps the first attempt to develop evidence-based maps of any NTD was
that of the African Programme for Onchocerciasis Control (APOC), which developed the
rapid epidemiological mapping of onchocerciasis (REMO) approach to quickly and cheaply
identify priority areas for community-directed treatment with ivermectin (CDTI) and
estimate the number of individuals requiring treatment [9]. This approach stratifies
a country into areas that are suitable and unsuitable for transmission, based on known
geographic factors, and within suitable areas implements rapid assessment of communities
by screening individuals for onchocercal nodules. In Africa, a total of 23 countries
have been mapped for onchocerciasis. As countries have been successful in controlling
onchocerciasis, many are now conducting new REMO assessments to provide updated maps.
For LF, the rapid geographical assessment of Bancroftian filariasis (RAGFIL) method
[10] using ICT card tests enabled large-scale assessments of the boundaries of filaria-endemic
areas for identifying areas requiring MDA (with albendazole and ivermectin or diethylcarbamazine-citrate)
[11]. In 2009, all endemic countries in the World Health Organization (WHO)-defined
regions of the Eastern Mediterranean, South-East Asia, and Western Pacific have completed
mapping, whilst in Africa, 37 of 39 endemic countries have completed or are in the
process of mapping, with only Chad and Eritrea yet to start [12]. In the case of schistosomiasis,
one of the first steps of the Schistosomiasis Control Initiative (SCI) in designing
country programmes was to conduct national prevalence surveys of schistosomiasis to
identify communities and districts requiring mass treatment with praziquantel [13],
[14]. Outside SCI-supported countries, national surveys of schistosomiasis and soil-transmitted
helminth (STH) infections are often scarce and ad-hoc, with the exception of some
dedicated surveys, such as those supported by the Carter Center [15], [16] and most
recently by countries supported by the NTD Initiative of the US Agency for International
Development (USAID) [17]. Despite these efforts, there remains a considerable mapping
requirement to support global NTD control. Also, it is little appreciated that as
control is successful in reducing transmission, there will be an increasing requirement
to conduct mapping surveys to assess whether to stop MDA or switch to less frequent
treatment.
New Mapping Technologies
Future data collection efforts should take advantage of recent advances in recording
and processing data and converting data into maps. Increasingly, surveys utilize electronic
data capture systems, mainly based on the use of personal digital assistants (PDAs)
but also small laptops, to enter data at the point of collection [18] and almost instantaneously
transmit the information to a central database using mobile phone technology [19].
Once data have been collected and collated, geographical information systems (GIS)
provide a ready framework in which to manage and display the data. The distribution
of NTDs is particularly sensitive to climatic and environmental factors because of
the vulnerability of vectors, intermediate hosts, and free-living stages. The GIS
frameworks allow ready comparison between disease patterns and environmental data,
while remote sensing (RS) technologies can use high-resolution satellite data to provide
estimates of such variables as temperature, vegetation (as a proxy for various environmental
factors), and humidity [20]. The relationships between observed infection patterns
and environmental factors can be investigated using both traditional and spatially
explicit statistical approaches [21], allowing the spatial distributions of infection
prevalence to be predicted in unsurveyed areas. Such analyses are increasingly adopting
a Bayesian approach to statistical inference that provides a robust method for measuring
uncertainty in prediction [22], [23]. Finally, it is perhaps worth emphasizing that
this journal is itself an example of the power of the new technologies, since the
open-access format permits the data, analyses, and predictions to be published in
a way that maximizes access to the information.
Current Global Mapping Resources
Although many surveys of different NTDs have been conducted and many more are planned,
the survey data are only useful if available in a form that is accessible to policy-makers
and the managers of public health programmes. At present the most detailed maps available
are for onchocerciasis, LF, and schistosomiasis. For onchocerciasis, the country-level
REMO maps, which highlight the areas where CDTI is needed, are made available through
APOC's Web site [24]. For schistosomiasis, the most complete global resource remains
the 1987 Atlas of the Global Distribution of Schistosomiasis [25], now available online
[26]. For each country, a map is presented that shows the presence or absence of schistosomiasis.
Although these maps represent an important early source of information, the data are
at best 23 years old and do not include the wealth of prevalence data collected more
recently.
The preventive chemotherapy (PCT) databank was developed by WHO as a tool to map progress
on the implementation of NTD control efforts targeting LF, schistosomiasis, and STH
infections [27]. The PCT databank includes country profiles that present the estimated
population at risk of infection and requiring PCT and, where available, data on treatment
coverage for each NTD [28]. However, the PCT databank only includes data as reported
to WHO and excludes survey information from other sources, such as data collected
by academic researchers, nongovernmental organizations (NGOs), and other partners.
The data are presented at administrative level 1 (province or region) and the derived
maps inevitably cannot capture the fine-scale heterogeneity of infection patterns,
so they tend to overestimate the numbers of individuals requiring treatment.
A Global Atlas of Helminth Infection
In an attempt to provide open access to up-to-date information on schistosomiasis
and STH infections, a project has been undertaken to develop a Global Atlas of Helminth
Infection (GAHI) [29], [30]. The overall goal of this project is to provide an open-access,
global information resource on the distribution of STH and schistosomiasis, with the
specific aims of 1) describing the global distribution and prevalence of infection
of each species and 2) highlighting geographical areas for which further survey information
is required. The maps are grounded in structured searches of the formal and grey literature
for suitable survey data that are then collated into a single database, according
to specified inclusion criteria. The eligible surveys are geo-positioned using electronic
sources and maps are then developed using GIS. To date, more than 10,000 prevalence
surveys have been identified, catalogued, and mapped. The potential usefulness of
the data to identify “at risk” populations for which data are scarce is enhanced by
using Bayesian model-based geostatistics [23] to predict the prevalence of infection
with each species in as yet unsurveyed areas.
The GAHI Web site will be launched on August 12, 2010 at http://www.thiswormyworld.org/.
The Web site allows users to visualize the assembled data and models. Three types
of maps are presented for every country where these infections occur: (i) a Survey
Data Map showing the prevalence of infection based on survey data; (ii) a Predictive
Risk Map showing the probability that infection prevalence warrants MDA, according
to recommended WHO thresholds; and (iii) a Control Planning Map showing which districts
require MDA treatment or where further surveys would be helpful in defining risk.
As the URL suggests, inspiration for this project comes from the work of the American
parasitologist Norman Stoll, who, during his 1946 presidential address to the American
Society of Parasitologists, posed the question, just how much human helminthiasis
is there in the world? The resultant paper, This Wormy World [31], was the first systematic
attempt to measure the worldwide impact of human parasitism by helminths and provides
the foundation for subsequent attempts to define the burden of NTDs [32]. Building
on the GAHI, an ongoing project is developing a Global Atlas of Trachoma, expanding
earlier attempts to the map the global distribution of trachoma [33]. This work is
intended to provide an evidence base for allocating resources for trachoma control,
including surgery and administration of Zithromax [35].
Conclusion
Accurate and up-to-date maps of different NTDs can help improve the precision of decision-making
in NTD control. They can help increase the reliability of estimates of disease burden,
for example, as part of the ongoing revision of the Global Burden of Disease study
[34]. The maps can also establish a baseline against which to measure the impact of
NTD control efforts. Finally, they can provide an important planning tool for national
control programmes. Considerable effort is required to develop an integrated NTD atlas,
necessitating cooperation and collaboration of the different NTD communities. We hope
the Global Atlas of Helminth Infection will encourage this interaction and help progress
towards an open access Global Atlas of NTDs.