WORLD HEALTH ORGANIZATION STRATEGIC AND TECHNICAL ADVISORY GROUP FOR NEGLECTED TROPICAL DISEASES WORKING GROUP ON MONITORING AND EVALUATION DESIGN AND VALIDATION OF A TRACHOMATOUS TRICHIASIS-ONLY SURVEY
Design and validation of a trachomatous trichiasis-only survey
Strategic and Technical Advisory Group for Neglected Tropical Diseases Working Group on Monitoring and Evaluation
WHO/HTM/NTD/PCT/2017.08 © World Health Organization 2017 Some rights reserved. This work is available under the Creative Commons Attribution-NonCommercialShareAlike 3.0 IGO licence (CC BY-NC-SA 3.0 IGO; https://creativecommons.org/licenses/by-nc-sa/3.0/igo). Under the terms of this licence, you may copy, redistribute and adapt the work for non-commercial purposes, provided the work is appropriately cited, as indicated below. In any use of this work, there should be no suggestion that WHO endorses any specific organization, products or services. The use of the WHO logo is not permitted. If you adapt the work, then you must license your work under the same or equivalent Creative Commons licence. If you create a translation of this work, you should add the following disclaimer along with the suggested citation: “This translation was not created by the World Health Organization (WHO). WHO is not responsible for the content or accuracy of this translation. The original English edition shall be the binding and authentic edition”. Any mediation relating to disputes arising under the licence shall be conducted in accordance with the mediation rules of the World Intellectual Property Organization. Suggested citation. Design and validation of a trachomatous trichiasis-only survey. Geneva: World Health Organization; 2017. Licence: CC BY-NC-SA 3.0 IGO. Cataloguing-in-Publication (CIP) data. CIP data are available at http://apps.who.int/iris. Sales, rights and licensing. To purchase WHO publications, see http://apps.who.int/bookorders. To submit requests for commercial use and queries on rights and licensing, see http://www.who.int/about/licensing. Third-party materials. If you wish to reuse material from this work that is attributed to a third party, such as tables, figures or images, it is your responsibility to determine whether permission is needed for that reuse and to obtain permission from the copyright holder. The risk of claims resulting from infringement of any thirdparty-owned component in the work rests solely with the user. General disclaimers. The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of WHO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted and dashed lines on maps represent approximate border lines for which there may not yet be full agreement. The mention of specific companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by WHO in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. All reasonable precautions have been taken by WHO to verify the information contained in this publication. However, the published material is being distributed without warranty of any kind, either expressed or implied. The responsibility for the interpretation and use of the material lies with the reader. In no event shall WHO be liable for damages arising from its use.
Contents
Abbreviations ......................................................................................................................................... iv Acknowledgements................................................................................................................................. v 1. 2. 3. 4. 5. 6. 7. 8. Background ................................................................................................................................. 1 Simulations with existing data .................................................................................................... 2 Conjunctival scarring, and lower lid trichiasis............................................................................. 5 Draft design ................................................................................................................................. 5 Validating the draft design: precision ......................................................................................... 6 Validating the draft design: cost ............................................................................................... 13 Discussion.................................................................................................................................. 15 Recommendations .................................................................................................................... 16
References ............................................................................................................................................ 17
iii
Abbreviations TF TT TS WHO trachomatous inflammation—follicular trachomatous trichiasis trachomatous scarring World Health Organization
iv
Acknowledgements This report was prepared by Rebecca Mann Flueckiger, Paul Courtright, David C. W. Mabey, Rachel L. Pullan and Anthony W. Solomon. Field work was conducted by the health ministries of Cameroon, Chad, Uganda and the United Republic of Tanzania. Support to and supervision of fieldwork was provided by Lucienne Bella Assumpta, Gilbert Baayenda, Jérôme Bernasconi, Epée Emilienne, George Kabona, Mathias Kamugisha, Edward Kirumbi, Upendo Mwingira, Jeremiah Ngondi and Patrick Turyaguma. Christopher Fitzpatrick, Katherine Gass, Charles Opondo and Rebecca Willis contributed to methodological development and data analysis.
v
1. Background 1.1 The Fifty-first World Health Assembly adopted resolution WHA51.11 in 1998, which targets the global elimination of trachoma as a public health problem by 2020 (1). The strategy recommended to achieve that goal is encapsulated by the acronym “SAFE”, which represents: Surgery for individuals with trachomatous trichiasis (TT; the late blinding stage of trachoma); and Antibiotics, Facial cleanliness and Environmental improvement (2). The A, F and E interventions are delivered to entire districts in which active (inflammatory) trachoma is common in order to treat ocular infection with Chlamydia trachomatis, the causative organism of trachoma, and sustainably reduce its transmission. 1.2 At a series of global scientific meetings on trachoma (3–6), elimination thresholds for trachoma were defined as a prevalence of the active trachoma sign “trachomatous inflammation—follicular” (TF) (7) of < 5% in children aged 1–9 years, and a prevalence of TT (7) unknown to the health system of < 0.2% in adults aged ≥ 15 years (8). The prevalence of these signs should be measured at district level, where districts are “the administrative unit for health care management”, which “for purposes of clarification, consists of a population unit between 100 000–250 000 persons” (5). 1.3 The World Health Organization (WHO) endorses the use of population-based prevalence surveys for estimating the prevalence of trachoma (9). In general, the prevalence of TF in children aged 1–9 years and the prevalence of TT in adults aged ≥ 15 years are measured at the same time in any district being surveyed. This was the approach of the Global Trachoma Mapping Project (10), which undertook baseline surveys in > 1500 districts worldwide in order to provide the data required to start interventions where needed (11). 1.4 The survey design recommended by WHO is a two-stage cluster random sample survey, which uses probability proportional to size sampling to select 20–30 villages (9), and random, systematic or quasi-random sampling to select 25–30 households in each of those villages (10). In most surveys, everyone aged ≥ 1 year living in selected households is examined. 1.5 Usually, surveys are powered to estimate the prevalence of TF in 1–9-year-olds (9,10). TF is most common in young children, whereas TT becomes increasingly common with increasing age (12–15); it is also, in the population as a whole, a much less common sign than TF. Because of this, and because the number of adults aged ≥ 15 years resident in a group of selected households is often not much more than the number of 1–9-year-olds resident in those households, the number of adults examined in a survey is generally not sufficient for estimating TT prevalence with good precision. These surveys simply accept poor precision in estimating TT (6,9,10). 1.6 Because TT is the blinding stage of trachoma, appropriate clinical management of TT (16–18) is the priority of trachoma elimination programmes. Obtaining precise data on TT prevalence helps programmes to plan surgical services, monitor progress and assess whether the trichiasis component of trachoma elimination has been successfully achieved. 1.7 There are four scenarios in which a TT-only survey may be warranted. 1) If at baseline survey, the estimated prevalence of TF in 1–9-year-olds is < 5% and of TT in adults is ≥ 0.2%, an impact survey to again measure the TF prevalence is not indicated; after interventions, a TT-only survey to re-estimate the TT prevalence is indicated. 2) If at surveillance survey, the estimated prevalence of TF in 1–9-year-olds is < 5% and of TT in adults is ≥ 0.2%, further surveys to again measure the TF prevalence are not indicated; after interventions, a TT-only survey to re-estimate the TT prevalence is indicated.
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3) If a survey at any stage of the programme estimated the prevalence of TT with a questionable methodological approach, the programme may wish to conduct a TT-only survey. 4) If at baseline survey, the estimated prevalence of TF in 1–9-year-olds is ≥ 30% and of TT in adults is ≥ 0.2%, at least 5 years of A, F and E interventions are recommended before an impact survey to again measure the TF prevalence. During this time, the programme may wish to undertake a TT-only survey to assess progress in addressing the TT backlog, facilitating adjustments in delivery of S interventions, if needed. 1.8 The work described in this report was commissioned by WHO to guide recommendations for optimizing the design of a TT-only survey. Doing that work provided an opportunity to also evaluate the precision of TT prevalence estimates in general.
2. Simulations with existing data 2.1 Simulations were undertaken to better understand two of the key parameters that influence the design of a TT-only survey: the age distribution of TT, and the extent to which observations of the presence or absence of TT correlate within clusters, expressed as the design effect. 2.2 Health ministries from Benin, Malawi and Nigeria kindly provided datasets from 491 surveys undertaken between 2012 and 2016 with the support of the Global Trachoma Mapping Project (10,19–26). Each of these surveys employed the population-based prevalence survey methodology (10) outlined in paragraphs 1.3–1.5 above. All surveys were conducted prior to the addition of data collection on the presence or absence of trachomatous scarring (TS) of the conjunctiva (7) in eyes with trichiasis (6) within the Global Trachoma Mapping Project’s training and fieldwork systems (27,28): these datasets therefore include data on all trichiasis, irrespective of the presence or absence of TS, and it is not possible to make presumptions as to the etiology of the cases. Included datasets represented a diversity of epidemiological situations for trichiasis (Table 1). Table 1. Summary of survey data used in simulations Country ([State], where Number of surveys applicable) Benin 27 Malawi 24 Nigeria [Bauchi] 20 Nigeria [Benue] 23 Nigeria [FCT] 6 Nigeria [Gombe] 11 Nigeria [Jigawa] 4 Nigeria [Kaduna] 23 Nigeria [Kano] 44 Nigeria [Katsina] 34 Nigeria [Kebbi] 2 Nigeria [Kogi] 4 Nigeria [Kwara] 8 Nigeria [Niger] 25 Nigeria [Sokoto] 3 Nigeria [Taraba] 13 Range of trichiasis prevalences in adults aged ≥ 15 years (%) 0.1–1.9 0.0–0.6 0.1–3.3 0.0–0.4 0.0–0.3 0.5–3.9 1.9–3.1 0.0–0.8 0.0–2.9 0.0–3.6 0.4–1.8 0.0–0.0 0.0–0.2 0.0–0.4 0.3–1.0 0.0–0.8
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2.3 Using the sqldf package in R (29), trichiasis prevalences by age and gender were calculated using the same approach as that used by the Global Trachoma Mapping Project (10). Raw data were grouped by cluster, then age and gender. For each cluster, the number of individuals examined and the number observed to have trichiasis were determined for each age and gender group; then the proportion of individuals with trichiasis in that group was weighted by the proportion of residents expected to have that age and gender (with underlying population data derived from www.worldpop.org, using the zonal statistics tool in ArcGIS 10.3 (30)). The sum of weighted proportions within a cluster produced the age- and gender-adjusted cluster-level proportion for adults aged ≥ 15 years. The mean of the cluster-level proportions was calculated to determine the adjusted survey-level prevalences summarized in Table 1 and which are presented in more detail elsewhere (19–26). The mean of the age-specific trichiasis proportions across all clusters was calculated to explore the age distribution of trichiasis in each survey, and the mean of the agespecific survey-level prevalences was calculated to generate age-specific prevalence curves for each country (Fig. 1). 2.4 These data indicate that trichiasis is first apparent in these populations at an age of about 30–40 years, and increases with increasing age thereafter, but that there is at least moderate heterogeneity between settings. Fig. 1. Age-specific prevalences of trichiasis by survey (evaluation unit, EU) and country (Benin, Malawi and Nigeria), Global Trachoma Mapping Project, 2012–2016 60% 50% Average Nigeria
Trichiasis prevalence
40% 30% 20% 10% 0% <15 15-1920-2425-2930-3435-3940-4445-4950-5455-5960-6465-6970-7475-79 80+
Average Malawi Average Benin EU level curve
Age group (years)
3
Table 2. Proportion of trichiasis cases within different age groups, by country (Benin, Malawi and Nigeria), Global Trachoma Mapping Project, 2012–2016 Proportion (%) of all trichiasis cases in subjects, by age Country ≥ 15 years ≥ 30 years ≥ 40 years Malawi 100 92 89 Benin 99 95 85 Nigeria 97 92 83 Between countries, the proportion of all trichiasis cases in those aged ≥ 40 years varies more than the proportion of all trichiasis cases in those aged ≥ 15 years (Table 2). 2.5 The design effect (for observations of trichiasis in ≥ 15-year-olds) arising from the clustersampled design was calculated for each survey as