Trachoma mapping at district level
In this step we describe the tools for mapping trachoma at the local level and explore the resources and support for mapping trachoma.
By the end of this presentation you should be able to: - Describe the use of epidemiological tools for mapping trachoma at the local level, and - Understand the resources and support for mapping trachoma provided by the WHO led initiative, Tropical Data. The first step in trachoma mapping is deciding when it is needed, and when it is not. In general, we need to map trachoma in populations when it is likely that trachoma is a public health problem and interventions are needed.
That is when any of the following conditions apply: - Invalid data indicates that the population is very likely to be trachoma endemic. Invalid data is more than 10 years old or current but from an adjacent area - The population has poor access to water, sanitation and hygiene - Trichiasis surgery is being performed by local healthcare providers or trichiasis cases are being identified. Mapping should not be done as an exercise to confirm that trachoma is not present. National programmes take on the overall responsibility of carrying out trachoma mapping. They often work closely with a WHO led initiative called Tropical Data which assists with survey design, field team training, survey implementation and data processing.
In general, mapping is done at the district level. The district is the normal administrative unit for health care management. For a trachoma survey, the population is first divided into appropriate evaluation units (EUs). For the practical implementation of trachoma elimination activities after mapping, it is helpful if these EU boundaries correspond with the boundaries of existing administrative divisions. However, in many cases we need to divide or combine locally-defined prefectures, counties, zones, districts or regions to produce population groupings of appropriate size. If a combination of local administrative units is needed to form an EU, only adjacent units which have similar socio-economic and environmental characteristics are combined.
To estimate the prevalence of active trachoma (TF) with reasonable accuracy in each EU, the survey teams need to select and examine about 1,019 children aged between 1 and 9 years old, from between 20 to 30 communities. The chosen communities are known in epidemiological terminology as clusters. To make surveys as reproducible as possible, the teams select and examine all the residents of selected households in each cluster rather than a fixed number of people in each cluster.
The number of clusters needed in each EU is calculated by dividing 1019 by the product of the number of households to be included per cluster - for example 30 - and the average number of 1-9 year-old children per rural household - for example 1.6 children. This figure is obtained from the most recent census. We inflate this number by 1.2 to account for non-response. In other words, the fact that not everyone who lives in the houses selected will be at home when the survey team calls. If the number of clusters is calculated to be less than 20, then 20 clusters should be included in each EU.
The number of households included per cluster is the number that one survey team can comfortably see in one day. A survey team usually consists of one certified grader, one certified recorder, one local guide, and one driver. In many places it’s possible for a survey team to see 30 households in a single day’s work. This makes the survey efficient. Sometimes, for example if the households are far apart, the team will not be able to see as many as 30 households per day. In these cases, we reduce the number of households to be included per cluster and recalculate the required number of clusters in each EU.
To reduce delays and inefficiencies, the number of households included per cluster is a key number for the survey. And care should be taken to make sure that it suits local conditions. Once the total required number of clusters per EU is determined, it is important to involve an epidemiologist. They will assist with the random selection of the actual clusters to be visited in each EU. There are three good methods for choosing
households in a selected cluster: random sampling, systematic sampling, and compact segment sampling. Random sampling requires a list of all households in the community. We write the name of each household head on a piece of paper, and place them into a bag or hat. Then, without looking, we draw out the 30 households to be visited. This is random selection. Another method is systematic sampling. This also requires a list of all households in the community. This time they are ordered geographically so that houses close to each other are listed together.
If there are 90 households in the community, and 30 households need to be sampled, we choose the first house by drawing lots, and then select every third household on the list after that. The third method used is compact segment sampling. A rough map may be available or it can be drawn with the help of community leaders. We then draw lines on the map, dividing the community into segments. In each segment there are approximately 30 households. We then choose one segment by drawing lots. Only one sampling method is used throughout a survey. In each household, every resident individual aged 12 months or above should be invited to enrol in the survey, and be examined by a certified grader.
Each team should be trained to conduct the survey and examination within households. After the initial training, it’s critical that teams continue to be supported in the field through supervision from trained and motivated supervisors. Ideally, one supervisor is needed for every 7-10 teams. The supervisor’s tasks are to actively and respectfully check
that: - Trachoma grading is being done well in the field - The team is recording data accurately - The team as a whole is happy, healthy and appropriately resourced. In addition to in-person supervision, continuous monitoring of household GPS data allows confidence that data are being collected from the correct locations - rather than, for example at the home of the team’s grader.
Data collected by the team is: - GPS data on household location - Information on household level access to water and sanitation - Demographic and trachoma examination data on all consenting adults. This information enables the programme to estimate the prevalence of TF and TT within each EU, amongst other things.
In conclusion, mapping trachoma at the district level: - Has to be clearly justified - Is a major undertaking and must be epidemiologically sound - Involves trained and certified teams, with good supervision, who examine persons within their households - Is done at the level of evaluation units (EUs) which are based on local administrative divisions - And it provides information on prevalence of active trachoma and trachomatous trichiasis at the EU level.
Generally speaking, trachoma mapping is carried out when it is reasonable to think that trachoma is a public health problem in a district.
Trachoma mapping measures the prevalence of trachomatous inflammation—follicular (TF) and trachomatous trichiasis (TT) in a population. Mapping also collects information on population demographics, and access to water, sanitation and hygiene (WASH). These data are then used to guide SAFE strategy interventions for trichiasis surgery, mass drug administration and measures to achieve facial cleanliness and environmental improvement.
Mapping is justified if any of the following conditions are true:
- If the population is very likely to be trachoma endemic based on: historical trachoma data, current trachoma data for adjacent areas, data on socioeconomic conditions, WASH data
- If trichiasis surgery is being performed by local healthcare providers
- If individuals with trichiasis are presenting to local healthcare providers
- If individuals with trichiasis are being identified as part of community outreach campaigns.
It’s also important to know when not to map! Mapping is not justified if any of the following conditions are true:
- If all the conditions which justify mapping (as outlined above) are not present
- If valid trachoma data, collected within the last ten years, are already available. The Global Trachoma Mapping project provided valid data for 1,546 suspected trachoma endemic districts across 29 countries between 2012 and 2016.
- If undertaking mapping might put survey teams at risk
- If the responsible authorities do not prioritise elimination of trachoma as a public health problem.
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