Els Mathieu
Alaine Knipes
Disease prevalence mapping allows countries to identify infected individuals and populations
in need of disease-control measures, such as mass drug administration (MDA) with preventive
chemotherapy. For countries with unlimited time and resources, disease-specific prevalence
mapping may be carried out in a slow, careful manner, by large teams of diagnostic
technicians, supervisors, and drivers. These mapping protocols, established by the
World Health Organization (WHO), are intended to be carried out independently, by
each respective disease-control programme, in order to assess prevalence of neglected
tropical diseases (NTDs) in individual populations. They work particularly well in
areas endemic for one NTD.
In reality, the countries most affected by NTDs have limited resources for mapping,
and are endemic for two or more NTDs, for example lymphatic filariasis (LF), and/or
trachoma, and/or schistosomiasis and/or soil transmitted helminths (STH). For these
countries, with efficiency and field-practicality in mind, the coordinated threshold
mapping (CTM) method was developed. The method enables coordinators from two or more
disease programmes to work together to determine the prevalence of two or more diseases
in a population, at one time. The CTM method achieves the same goals as the WHO's
disease-specific protocols, namely identifying the need for MDA with preventive chemotherapy.
It does not provide prevalence figures, but instead aims to determine whether a disease
has attained the threshold necessitating a public health intervention.
1
Therefore, rather than carrying out simultaneous, independent mapping efforts in areas
endemic for more than two NTDs, at a high cost to the national control programmes,
the CTM method saves countries precious time (smaller sampling size) and resources
(fewer survey team members). Table 1 gives a comparison of disease-specific prevalence
mapping and CTM.
After giving stool and urine samples (to detect soil-transmitted helminths and schistosomiasis,
respectively) the children are checked for signs of trachoma. TOGO
It is the responsibility of a national-level coordinator to work together with the
coordinators of the various NTD control programmes to compile historic data that will
determine areas where NTDs are suspected to be endemic. Once the areas in need of
NTD prevalence mapping have been identified, a protocol is agreed upon at the national
level. In the CTM method, each disease module uses the WHO-recommended standard indicators
and diagnostic methods (Table 2), and is field-tested and independent. As such, a
protocol is built to suit the particular mapping needs of a country simply by adding
each of the disease modules together as needed. Mapping needs within a country may
differ by region or district, which can easily be accommodated within the CTM.
Table 1.
Advantages and disadvantages of disease-specific prevalence and coordinated threshoid
mapping
Disease-specific prevalence mapping
Coordinated threshold mapping (CTM)
Advantages
Workers in each disease programme are accustomed to working independently, managing
their own budgets and personnel
Diseases differ in their geographical distribution; some are more localised (schistosomiasis,
onchocerciasis) while others occur widely (trachoma, soil-transmitted helminths, lymphatic
filariasis)
Produces disease prevalence data for trachoma
Reveals whether threshold for public health intervention has been surpassed
Workers share logistical responsibilities, reducing the burden on each programme and
allowing them to achieve disease-specific, non-mapping objectives
Employs smaller survey teams thus encouraging each team member to perform multiple
tasks – saving money and better utilising broadly trained technicians
Reveals and improves understanding of occurrence of co-infections among individuals
within the population
Unites NTD control programmes with respect to public
Encourages involvement of community members -building local capacity and local advocacy
Reveals whether threshold for public health intervention has been surpassed
Disadvantages
Mapping efforts must be mobilised independently
Each NTD control coordinator has responsibility of disease mapping
Does not produce precise prevalence data for trachoma
Increases responsibility of all survey team members
Next, a CTM team is formed, including: one supervisor (usually selected from among
the national-level NTD-control programme coordinators), and one or two diagnostic
technicians per NTD to be mapped. The team members are selected for their demonstrated
expertise and independence in NTD diagnosis, as well as for their prior field experience
and enthusiasm for collaboration. Team members will be trained in CTM methodology,
sampling, questionnaires, data collection tools, and in obtaining informed consent.
The CTM team (supervisor and diagnostic technicians) travels in one vehicle to survey
two villages in each subdistrict each day. Depending on which diseases are being mapped,
some combination of the following activities takes place: trachoma examinations, stool
collection, urinalysis, and LF testing (during the day) as well as examination of
stool samples (Kato-Katz method) in the evening. Community health workers, teachers,
and village chiefs are encouraged by the NTD control programme coordinators to assist
with bringing participants to a central location, then organising and registering
them. This participation of local volunteers has been shown to build local capacity
and spontaneous local advocacy for co-ordinated NTD control.
6
Co-ordinated (or integrated) mapping surveys have been successfully implemented in
several countries, including for schistosomiasis and trachoma in Nigeria
7
, forschistosomiasis, STH and trachoma in Togo
6
; for LF, loiasis, schistosomiasis and STH in South Sudan
8
,
9
, for trachoma, LF, schistosomiasis and STH in Mali and Senegal
1
; and forschistosomiasis and STH in Cameroon.
10
Although CTM offers several important advantages over the WHO's disease-specific protocol
for countries that are endemic for more than one NTD, the method is not without shortcomings
(Table 1). However, the various NTD programmes share the common goal of a healthier,
more productive population. By working together in disease mapping, this common goal
is more attainable. In our experience, the initial reluctance towards collaboration
among different NTD programmes was eased as members quickly noticed improved efficiency
compared to disease-specific methodologies.
1
Table 2.
Diagnostic methods thresholds used in coordinated threshold mapping (CTM). These are
the same as those recommended by the WHO, except for trachoma: the protocol for this
disease was adapted in collaboration with the International Trachoma Initiative
Disease
Diagnostic method
Thresholds for different public health interventions
Trachoma
2
Clinical examination using the WHO Simplified Trachoma Grading System
Follicular trachoma (TF) present in >10% of children examined (1-9 years old)
TF in >5 of children examined (1-9 years old)
Trichiasis present in >1% of adults (>15 years old)
Onchocerciasis
3
The prevalence of nodules is determined via the rapid epidemiological mapping method
(REMO); or the presence of Onchocerca volvulus microfilaria is determined via skin
snip
Forcontrol: presence of palpable nodules in >20% of adults tested (>15 years old)
For elimination: prevalence of palpable nodules in >5% of adults tested (>15 years
old)
Lymphatic filariasis(LF)
4
Immuno-chromatographic card test (ICT) to determine the presence of daytime antigenemia
Present in >1% of adults tested (>15 years old)
Soil-transmitted helminths (STH)
5
Kato-Katz method to look for presence of eggs in stool
Present in >50% of children tested (5-14 years old)
Present in >20% and <50% of children tested (aged 5-14 years)
Schistosomiasis
5
Schistosoma mansoni:
Kato-Katz method to look for presence of eggs in stool.
Present in >50% of children tested (aged 5-14 years) if based on parasitological methods;
or >30% if based on questionnaires for visible haematuria
Schistosoma haematobium:
Reagent strips (urinalysis) to look for blood in urine and administer questionnaire
or urine filtration to look for eggs in urine
Present in >10% and <50% if parasitological methods; or > 1% and <30% if based on
questionnaire forvisible haematuria in children aged 5-14 years
Present in >1% and <10% (if based on parasitological methods) in children aged 5-14
years