Impact of ivermectin and vector control on onchocerciasis transmission in Togo: Assessing the empirical evidence on trends in infection and entomological indicators

Background The World Health Organization’s 2021–2030 Road Map for Neglected Tropical Diseases boosted global commitment towards the elimination of onchocerciasis, but the timeline to elimination will vary strongly between countries in Africa. To assess progress towards elimination in the Republic of Togo, we reviewed the history of control and time trends in infection. Methodology/principal findings We collated all available programmatic, entomological, and epidemiological data since the initiation of the Onchocerciasis Control Programme (OCP) in Togo through different data sources. We then visualised data trends over time, to assess the impact of interventions on infection and transmission levels. Vector control was initiated by OCP from 1977 (northern and central parts of Togo) or 1988 (southern regions) up to 2002 (most areas) or 2007 (“special intervention zones” [SIZ], parts of Northern and Central Togo). Between 1988 and 1991, Togo initiated ivermectin mass drug administration (MDA) in eligible communities. The impact of vector control was high in most river basins, resulting in low annual biting rates and annual transmission potential declining to very low levels; the impact was lower in river basins designated as SIZ. Repeated, longitudinal ivermectin mass treatments have overall strongly reduced onchocerciasis transmission in Togo. Epidemiological surveys performed between 2014 and 2017 showed that the prevalence of skin microfilariae (mf) and anti-OV16 IgG4 antibodies had declined to low levels in several districts of the Centrale, Plateaux, and Maritime region. Yet, relatively high mf prevalences (between 5.0% and 32.7%) were still found in other districts during the same period, particularly along the Kéran, Mô and Ôti river basins (SIZ areas). Conclusions/significance Trends in infection prevalence and intensity show that onchocerciasis levels have dropped greatly over time in most areas. This demonstrates the large impact of long-term and wide-scale interventions, and suggest that several districts of Togo are approaching elimination.


1
The following document describes the variables used in the six tab-pages of the S1 Database.
Please therefore read the following document for a detailed description of the tab-pages and variables.
Skin snip (mf) prevalence We used the EPICROSS (historical OCP epidemiological database) as backbone for this database (first tab, "1.Skin snip").Many of the variable names and codes remain the same.We hereby provide a variable list.

DATABASE
The original database of the data.The references are listed under the 'References' heading.
Pre.control Classification of data as pre-control (before initiation of vector control or MDA).This variable was added for analysis purposes.
Period Classification of data into three time periods (pre-control, end phase OCP, recent).If the record fell outside one the periods, the data points were not included in the mapping exercise.This variable was added for analysis purposes.

OV16 prevalence
The variables in this database corresponds to the skin snip prevalence database (first tab, "1.Skin snip").The variables that differ from the aforementioned list of variables, will be described here.

3.
Prevalence of other diagnostic measures (e.g., palpable nodules) The variables in this database are very similar to the skin snip prevalence database (first tab, "1. Skin snip").The variables that differ from the aforementioned list of variables, will be described here.

Comments
Additional comments relevant to the respective record.

Entomological database including vector control
Many variables in this database corresponds to the skin snip prevalence database (first tab, "1. Skin snip").The variables that differ from the aforementioned list of variables, will be described here.Variable added for the purpose of this study.

CODEPC
Infec_F.1000parousF Number of positive female blackflies out of the total number of parous female blackflies examined per a 1,000 flies from the respective collection point.Information only provided for more recent studies.Variable added for the purpose of this study.

MDA history at the village-level
Many variables in this database corresponds to the skin snip prevalence database (first tab, "1. Skin snip").The variables that differ from the aforementioned list of variables, will be described here.

MDA history at the district-level
We used the ESPEN (https://espen.afro.who.int/) as backbone for this database.Most of the variable names and codes remain the same, and can be found on the ESPEN portal [1].We added mean district-level therapeutic coverage of ivermectin MDA from other databases than ESPEN to this database for completeness (EpiCov).We only explain variables additionally added or so far unclear, for analysis purposes.

MDA_freq
Ivermectin MDA frequency per annum in the respective district (ADMIN2, Implementation Unit).

Cum_MDA
Cumulative number of MDA, as counted by the number of years that MDA is provided in the district.

Cum_MDA_rounds
Cumulative number of MDA, as counted by the number of MDA rounds provided in the district (taking account of multiple MDA rounds per year).

EffMDA
Coding for district and year when effective MDA was provided, as defined as therapeutic coverage ≥65% (0 = not effective; 1 = effective).

EpiCov
The district-level therapeutic coverage of ivermectin MDA, measured as the population treated over the population requiring treatment within a district (PopTreat ÷ PopReq × 100).For other sources that ESPEN, we calculated the mean district-level therapeutic coverage if population numbers were missing.

Source_thera_cov
The data source (database) from where we collated the records used to estimate the district-level therapeutic coverage of MDA.

MDA_geo_cov
The district-level geographic coverage of ivermectin MDA, measured as the number of villages treated out of the total villages eligible for treatment within a district.

Source_geo_cov
The data source (database) from where we collated the records used to estimate the district-level geographic coverage of MDA across eligible villages.

Comments
The ESPEN database, as downloaded on 11 th of May 2021, contained multiple records with unrealistic therapeutic coverages of MDA (>100%), Number of people examined during an epidemiological or researchspecific survey.POS Number of people positive by the respective diagnostic tool.PREV Prevalence as measured by the respective diagnostic tool (POS ÷ EXA × 100) DIAGNOSIS "MFT" (Membrane filtration technique) or "Nodules" (palpation-based detection of nodules that could be attributed to onchocerciasis with reasonable certainty).
Unique code of the blackfly collection point, as provided by the OCP.PC_NAME Name of the entomological collection point.PCLONGC Longitude decimal degrees (DD) of the blackfly collection point.PCLATIC Latitude decimal degrees (DD) of the blackfly collection point.Riverbasin A stratification of the large river basins into the main and commonly used names of the endemic river basins in Togo.This variable was added for analysis purposes.ANNEE Year of the entomological survey performed in the blackfly collection point (similar to Survey_year in database 1).ABR Annual Biting Rate, please be referred to Text S1 for definitions.ATP Annual Transmission Potential, please be referred to Text S1 for definitions.ATPCOR Corrected Annual Transmission Potential, ATP corrected for Onchocerca ochengi (i.e., non-pathogenic parasite species to humans).NBJRS "Nombre des jours", Number of days that blackflies were collected at the collection point.NBMOIS "Nombre des mois", Number of months that blackflies were collected at the collection point.Flies_exa Number of blackflies examined from the blackflies collected at the respective collection point.Information only provided for more recent studies.Variable added for the purpose of this study.Flies_pos Number of blackflies positive from the blackflies collected and examined at the respective collection point.Information only provided for more recent studies.Variable added for the purpose of this study.Prev_cr_flies_pos.1000Number of positive blackflies out of the total number of blackflies examined per a 1,000 flies from the respective collection point (Flies_pos ÷ Flies_exa × 1,000).Information only provided for more recent studies.
FOSA/MSTRHealth district (ADMIN3) where ivermectin Mass Drug Administration (MDA) may have been distributed from.MoisMonth that the MDA round took place in the respective village.RecensesNumber of people present/inhabiting a respective village according to the census (similar to REC_BASE) performed at the time of the survey.Traites Number of treated by ivermectin MDA within a respective village at the time of the survey.TheraCov Therapeutic coverage of ivermectin MDA of the respective village, calculated as (Traites ÷ Recenses × 100).Sourcefile Specific source file among the various excel files obtained from the Togo Ministry of Health.
Unique codes of surveyed villages as provided by EPICROSS.
GBALIBName of the major of river basin.PHASE_CODE Unique code designated by OCP to the various phases of implementation of interventions.PHANOM Name of the implementation phase of interventions, as designated by OCP.DIS_CODE Unique code of a respective district in which the village is located.DISNOM Name of the district (ADMIN2) in which the village is located.REG_CODEUnique code of a respective region in which the village is located.
(OV16 measured in children and young adults up to 20 years of age), or "All-age OV16" (OV16 measured in individuals of all ages).