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.

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.

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 Nigeria⁷, forschistosomiasis, STH and trachoma in Togo⁶; for LF, loiasis, schistosomiasis and STH in South Sudan⁸, ⁹, for trachoma, LF, schistosomiasis and STH in Mali and Senegal¹; 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