Profile: The Dar Es Salaam Health and Demographic Surveillance System (Dar es Salaam HDSS)

The world urban population is estimated to reach 66% (2.5 billion) by 2050 from 54% in 2014. Nearly 90% of this future population growth is concentrated in developing countries, where Asia and Africa are expected to experience the highest growth.1 The majority of the urban dwellers are projected to reside in small settlements and about 1 in 8 will live in mega-cities with more than 10 million inhabitants. Dar es Salaam - the capital of Tanzania - where this HDSS is located is one of the cities expected to reach mega-city status by 2030.1 Local data show that Dar es Salaam was among the regions that grew the most in the country (by 75% from 2002 to 2012) with an average inter-censal growth rate of 5.6%; and has the highest population density in the country (1786 per km2).2,3 The increase is attributed to rural-urban migration and natural increase, although it is difficult to assess the impact of changes in fertility and mortality on the population growth rate due to lack of a reliable civil registration and vital statistics system.2 The lack of accurate and timely health data to monitor and plan evidence-based health interventions in Africa is a recurrent problem repeatedly described in the literature.4,5 Health and Demographic Surveillance Systems, if used effectively, offer an alternative source of public health data including vital statistics in areas that lack a comprehensive national civil registration and vital statistics systems.6 The Dar es Salaam Health and Demographic Surveillance System (commonly referred to as DUCS - Dar es Salaam Urban Cohort Study) was established to fill this gap by providing essential information on health disparities stemming from socioeconomic, urban-living and environmental influences on health and health behaviours through routine data collection and incorporated nested studies. Data from the Dar es Salaam HDSS will be used to supplement information from the other existing HDSS in the country (Ifakara, Rufiji and Magu) to provide a better national picture of health of the Tanzanian population.7–9 The dual burden of infectious and non-infectious disease observed in Dar es Salaam has been confirmed in the recent Lancet Global Burden of Disease series,10–12 emphasizing the need for monitoring trends and development of more target-specific interventions to address ill health. A multi-site analysis of HDSS data from Africa and Asia reported 35.6% of all deaths reported in the sites were attributed to non-communicable disease, with urban sites reporting a higher proportion (46.6% in the Ouagadougou, Burkina Faso urban HDSS) than rural sites.13 The DHDSS thus has an important role to play in supporting evidence-based health planning by providing much-needed accurate and timely data on the health status of a fast-growing urban population, possible risk factors and their dynamics in causing ill health among this population. The platform was designed to also allow for nested studies that can assess the effects of numerous exposures and interventions and draw inferences on the health of the population. The Dar es Salaam HDSS regularly collects information on demographic events, household wealth, water supply and sanitation, food insecurity and environmental exposures. In addition, the DHDSS recently started to collect data on household expenditure, labour stocks and fertility preferences and contraceptive use in women 15–49 years of age. We also assess knowledge on sickle cell disease and enumerate known sickle cell-affected children in the area. The Dar es Salaam HDSS is in Tanzania, which is geographically located in East Africa bordering Kenya and Uganda to the north; Rwanda, Burundi and the Democratic Republic of Congo to the west; Zambia, Malawi and Mozambique to the south; and the Indian Ocean to the east. Dar es Salaam, the financial capital of Tanzania, lies along the Indian Ocean coast and is divided into three administrative regions - Kinondoni, Ilala and Temeke. The demographic surveillance area (DSA) is in Ilala region in the Ukonga and Gongo la Mboto wards, about 20 km from the city centre (Figure 1). The DSA covers seven administrative streets (Gongo la Mboto, Guluka kwa lala, Mwembe Madafu, Markaz, Mazizini, Mongo la Ndege and Ulongoni) spanning a 9.91 km2 area selected due to its well-demarcated borders. To the south the DSA is bordered by the Tanzania-Zambia railway (TAZARA); to the west by a rough road dividing the DSA from Pugu ward; to the east the DSA is bordered by Kipawa ward near the Julius K. Nyerere International Airport; and tothe north it is separated from Kinyerezi ward by the river Zimbili (although not very clearly, as the river sometimes dries up during the dry season, leading to change of direction due to human activity). The DHDSS receives health care from 14 private dispensaries spread across the two wards. There are 16 primary and four secondary schools in the area. The DSA hosts a large prison, a military barracks, one textile factory, an abattoir and the Tanzania-Zambia oil pipeline (TAZAMA). Map of the Dar es Salaam HDSS. The Dar es Salaam HDSS covers all residents in the defined area. To identify eligible residents, all houses - defined as a structure made out of permanent building material - in the DSA were identified, and the number of households residing in each house were determined and enumerated using a unique seven-character identification system. This number was written on the door frame of each household using a permanent marker and later using a piece of wood engraved with the unique identification number, to withstand damage due to weather changes. A house with multiple households has the range of unique identification numbers listed on the main entrance to notify the enumerators the number of households in each house. A household was defined using the International Network for Demographic Evaluation of Populations and Their Health (INDEPTH) definition as members who ‘share a common pot’.14 Any household member considered as having primary dwellings within the household and having lived in the household for the previous 3 months preceding the census was eligible for inclusion. This included for example: students attending boarding school who travel back to the household for vacation. The DHDSS was established in a stepwise manner with three streets (Mwembe Madafu, Markaz and Mazizini) surveyed in January to September 2011; the remaining four streets were added in May to December 2012 (Figure 2). The study population is visited twice every year, with update rounds running from January to June and from July to early December. Due to financial constraints, no updates were carried out in 2013. Over 100 000 individuals living in 21 000 households have been enumerated through 30th June 2015. Mwembe Madafu is the most populated street, consisting of 24.5% of the HDSS population, and Markaz the least (6.3%). The population age and sex structure is presented in Figure 3. The median age of the population is 20 (interquartile range: 8, 31.5) years and majority of the population is female (52.5%). A road in one of the neighborhoods in the Dar es Salaam HDSS. Population pyramid of the Dar es Salaam HDSS population. Source: Dar es Salaam HDSS. The Dar es Salaam HDSS collected baseline data on: socio-demographic characteristics; house construction material; asset ownership; food insecurity; hygiene and sanitation practices; and use of insecticide-treated bed nets (Table 1). The Demographic and Health Survey tools were adapted to collect most of the household and individual information. Food insecurity was measured using an 18-item food insecurity questionnaire adapted from the Household Food Insecurity Access Scale (HFIAS) that has been validated in Tanzania.15 Birth or death events, change in marital status, and residency status data are collected for all enumerated household members during every update, and household information is updated every other year. Key informants (locally known as ‘ten cell leaders’ who oversee a defined set of 10–15 houses) have been recruited from the DSA to actively report any birth or death event in their areas within a month of occurrence of the event. The information is registered in the data room and awaits subsequent rounds for validation. During update, enumerators are provided with this information in the household registration books (HRB) and enquire about the occurrence of the event from a household member. Any discrepancy is reported to the field manager for deliberation and final decision making. Information collected at baseline and in the follow-up rounds Information collected at baseline and in the follow-up rounds Every pregnant woman is registered in the DHDSS, and information on use of an insecticide-treated bed net during pregnancy and on antenatal clinic attendance is acquired. In subsequent visits, data on the birth (live birth, stillbirth or miscarriage/abortion), mode of delivery and whether it was a health-facility delivery are collected. In addition, enumerators enquire about any birth event that has occurred in the household for all registered female members of the household, with a particular focus on women aged 15–49 years. Key informants actively report any death event occurring in their areas. For a death to be recorded, the deceased must be registered in the HDSS and be considered living in the DSA at the time of death. The Dar es Salaam HDSS collects information on both internal and external migration of its residents. Moving in or out of the DSA is recorded if the move has lasted for a period of 90 days. A short threshold (compared with other HDSS) was adopted to ensure accurate and timely capture of change of residency status, given the modest-high mobility in the area. Migration events that occur within the DSA are reconciled for both individuals and households. Neighbours and mobile phones of known household members are often used to track the migrant individual or household. Information on where the migrant moves to or from, and whether this is rural or urban, is recorded. All changes in marital status of any member of the HDSS are recorded in the follow-up surveys. Marriages that ended in divorce and later reconciled are also captured in the updates. Two nested studies have been carried out in the HDSS, addressing hypertension16 and healthy ageing. Both studies included adults aged 40 and above. The Dar es Salaam HDSS database is constructed in a three-level hierarchical tier (individual, household and street) using the household registration system-2 (HRS-2).17 The DHDSS database is thus a relational database system that exploits object-oriented programming to simplify the process of data management system for a diverse collection of longitudinal household studies. This approach is used in a number of demographic surveillance studies across Africa and Asia.7,9,18–20. Data quality checks are built in to the Dar es Salaam HDSS data collection and entry procedures. Spot checks are carried out by field supervisors on 10% of all interviews, selected randomly from the HRS-2. A series of logic checks are imposed during data entry to identify errors or inconsistencies and are immediately returned to the field for correction. Files are merged, summary statistics are run and values outside the agreed range are identified and reported to the data entry room and field supervisors. An error report is created before the end of a survey round and discussed by the data manager and field supervisors. All data issues are resolved before the end of a survey round. Clean data are then encrypted and archived in a server located at the Muhimbili University of Health and Allied Sciences. A total of 110 882 individuals living in 21 000 households have been enumerated in the Dar es Salaam HDSS up to June 2015. Of these 74% (82 342) are residents still living in the DSA. The average size of a household is six individuals (interquartile range 4, 8). Women comprise 52.5% of the population and 20% of the households are headed by women. Almost all households have a toilet facility (98%) and 94.5% of the households reported at least one household member owning a mobile phone, but only 54% have electricity in the house. About 58% of the household reported experiencing some level of food insecurity in the 4 weeks preceding the baseline survey (Table 2). Demographic characteristics of the Dar es Salaam HDSS population, 2011–15 Demographic characteristics of the Dar es Salaam HDSS population, 2011–15 The age distribution of the Dar es Salaam HDSS (de jure) population is summarized in Figure 3. The age and sex structure of the Dar es Salaam HDSS reflects a low-income country where the proportion of the dependent population is high (55%) - 38% of the population is less than 15 years old and 1.5% is 65 years or older. We observe a dependency ratio that is lower than the national estimates (92%; 44%aged less than 15 years and 3.9% aged 65 years or older), but slightly higher than that of the entire Dar es Salaam region (51%; 32% aged less than 15 years and 2% older than 65 years).3 There is a bulge at 15–34 years that is more pronounced in women, and the proportion of women aged 30–70 years is less than that of men. This distribution is similar to the age and sex profile for Dar es Salaam described in the 2012 National Census.3 The distribution may be explained by the differential mobility patterns for men and women where on average men start to migrate at an earlier age (10–14 years) peaking at 25–29 years, whereas women start to be mobile at 25–29 years, exceed men by 30–34 years and sustain higher rates through their 70s (Figure 4). Migration patterns by age and sex in the Dar es Salaam HDSS, 2011-2015. We observe a lower total fertility rate in the HDSS (1.9) compared with the 3.7 for urban areas and the national 5.4 reported in the Demographic and Health Survey of 2010.21 This difference was sustained even after adjusting for education levels. Although fertility rates in Tanzania vary across regions, with lower rates generally observed in urban areas, differences in population structure between the DHS and the HDSS may explain the lower fertility rate observed in the HDSS Of the female population in the HDSS, 58.3% are aged 15–49 years compared with only 22.1% in the DHS. Compared with the whole of Dar es Salaam region, the proportion of women aged 15–49 years in the HDSS is slightly lower (58% vs 62%) and there is a modest to high population mobility (in-migration 57.1 per 1000 and out-migration 98.6 per 1000). Also, Dar es Salaam being the largest urban area in the country may attract women who are more educated (22% have secondary education and above) and are less likely to be married (77% of women aged 15–49 are married). The differences described above may partly explain the low total fertility rate observed in the HDSS. Although our conclusion may not change, we acknowledge that there may be an underestimation of the number of births that occur in this population and in particular those that occurred in 2013 when no updated rounds were carried out. The 5-year (2011–15) crude birth rate was 50.4 per 1000 and the crude death rate was 4.8 per 1000 (Table 3). Vital rates of the Dar es Salaam HDSS population, 2011–15 aNumber of children born per woman if they were to pass through childbearing according to current schedule of age-specific fertility rates. Vital rates of the Dar es Salaam HDSS population, 2011–15 aNumber of children born per woman if they were to pass through childbearing according to current schedule of age-specific fertility rates. The HDSS plans to investigate further the frequency and pattern of population mobility and their effects on various health outcomes. The platform will also examine maternal mortality patterns and how various national and target-specific interventions have influenced these rates over time. The HDSS will continue to investigate the dynamics of non-communicable diseases, in particular cardio-metabolic diseases (hypertension, diabetes), injuries, cancer and ageing, and how they evolve in low-income settings. Studies on environmental exposures such as pollution and green space are also of interest to the study area. Despite the modest number and scattered distribution of HDSS in Africa and Asia, there is still need for new HDSS sites to be established to improve the socioeconomic coverage of the continent and countries, due to the great heterogeneities that exist.22 A majority of the HDSS in Africa are biased towards rural or semi-urban areas, with only a few located in large urban cities.18, 23–25 Notwithstanding the importance of these areas, the Dar es Salaam HDSS will be able to contribute to the discourse on urban health within the projected increase in urban populations, the limited infrastructure available to absorb the population growth and the sustained rural-urban migration flows. The Dar es Salaam HDSS uses a data collection approach similar to other HDSS, allowing for multi-site comparisons across other HDSS sites located in Tanzania. The design of the platform permits nesting of observational and experimental studies to explore the effects of various exposures and interventions on health in an urban population. The site is well staffed (17 field enumerators and seven clerks, three supervisors, a field manager, a data manager and 20 local key informants) to collect and enter data. The Dar es Salaam HDSS operates in a context of frequent population mobility within, in and out of the DSA. This creates a great challenge to follow-up, in particular timely and valid reconciliation of all movements within the DSA. To ensure complete follow-up of the population, the HDSS has introduced a number of measures to ensure all movements are identified and recorded in a timely manner by: (i) using a short threshold (90 days) to qualify a migration event is conservative, but sensitive to frequent movements including residential mobility within the DSA; (ii) use of proxy respondents, especially neighbours, to assist in a migrant individual or mobile numbers of at least two household members for each household, which has been to and of a migration for all who have not been reconciled by the have to be for timely identification of migration events and reconciliation of these in each update round. The surveillance system not out although information on deaths is collected. The Dar es Salaam HDSS plans to start by the end of of houses with a unique identification system the and weather of Dar es Salaam, and have to be a We have introduced engraved that are door or entry to all households to The Dar es Salaam HDSS data are the of the for to the data are available a Data are in or for to the data can be to the The Dar es Salaam HDSS has an to the International Network for Demographic Evaluation of Populations and Their Health we the the HDSS will including the public use of data the The Dar es Salaam HDSS over km2 seven administrative streets in the financial capital of Tanzania. The platform was established to provide information on and its effects on health in one of the cities in East The baseline census was from January to May to June a total of 110 882 individuals living in 21 000 households have been The population has been visited six with no updates in 2013. June the frequency of updates is twice Data on household and socioeconomic characteristics as well as vital events are collected. studies on in healthy ageing, and sickle cell disease in have been carried out. In with the the surveillance system is the of fertility in the for for data of a are The the for the of the site through a to the of We acknowledge the of all those who in the collection and of this information. We are most for the we from the local and the population of Ukonga and Gongo la Mboto and for the from Muhimbili University of Health and Allied and of of