Decline of infant and child mortality rates in rural Senegal over a 37-year period (1963–1999)

Valérie Delaunay, Jean-François Etard, Marie-Pierre Préziosi, Adama Marra and François Simondon

Niakhar Project, Institut de Recherche pour le Développement.

Valérie Delaunay, IRD, Harvard Centre for Population and Development Studies, 9 Bow Street, Cambridge, MA, 02139, USA. E-mail: vdelauna{at}hsph.harvard.edu

Abstract

Background In spite of an improving trend, childhood mortality in rural sub-Saharan Africa remains high and has recently risen in some countries. The factors associated with the long-term decline in childhood mortality are poorly known, due to a lack of data.

Methods A Senegalese rural population has been under demographic surveillance since 1963. Infant and under-5 mortality rates were calculated for different periods to generate a long-term trend in childhood mortality. Evolution of age and seasonal patterns of mortality were observed.

Findings During the observation period (1963–1999), infant and under-5 mortality rates decreased from 223{per thousand} to 80{per thousand} and 485{per thousand} to 213{per thousand}, respectively, with a constant annual rate of decline in the probability of dying since the 1960s (–3.7% and –3.1%, respectively). The age pattern of the under-5 mortality changed drastically , with a large decrease in the death rate between 6 and 24 months of age (from 321{per thousand} to 87{per thousand}). This change took place during the 1970s. The seasonal variation, characterized by a greater proportion of deaths during the rainy season, was very marked during the 1960s, then decreased during the 1980s but it has tended to increase again in the 1990s, particularly among children 1–4 years old.

Conclusion This study confirms the long-term trend of decrease in child mortality in rural West Africa. Historical knowledge on healthcare developments suggests that immunizations have contributed to the decrease and the change in the age pattern. The re-emergence of malaria seems the most likely explanation for the recent rebound in seasonal variation. Attention to immunization and malaria should continue to be a priority.

Keywords Child mortality, infant mortality, past trend, age distribution, season, rural population, Africa, Senegal

Accepted 26 April 2001

The level of childhood mortality in sub-Saharan Africa remains one of the highest in the world. In 1998, the infant mortality rate (IMR) was 94{per thousand} compared to 56{per thousand} in Asia, 35{per thousand} in Latin America and 9{per thousand} in Europe.1 However, mortality rates have decreased drastically over the last decades. The decline of child mortality in Africa has been studied extensively and different theories, recently discussed, have been proposed to explain key determinants.2–4 An important review showed that the under-5 mortality in sub-Saharan Africa decreased in all countries (where data are available) before World War II, independently of economic development and in spite of the inadequacy of the healthcare system.5 Senegal is one of the sub-Saharan African countries where the longest mortality series are available. Recording of vital events by the government has been continuous since 1915 in the capital city of Dakar and the city of Saint-Louis. Their analysis showed that crude death rates started to decrease as early as 1920 in Dakar and 1930 in Saint-Louis.6,7

The level of mortality rate varies according to rural/urban area. In Senegal, the 1997 IMR reached 79.1{per thousand} in rural areas versus 50.2{per thousand} in urban areas.8 The decrease started earlier in cities and later in rural areas, following the same process but with a time lag. The date of the decline depended upon the distance from Dakar and the level of the healthcare system. Health policies and knowledge diffusion have been identified as the main determinants of mortality decline.9

However, the decline in childhood mortality in sub-Saharan Africa has not been sufficiently assessed with long-term, reliable and detailed data. In this article, we report mortality trends from the study zone of Niakhar, a rural area in Senegal, where a demographic surveillance has been conducted since 1962 by scientists from the ‘Institut de Recherche pour le Développement’ (IRD). We present here the evolution of child mortality over a 37-year period and provide some explanations for the observed trends by analysing age-specific death rates and their seasonal pattern, along with historical knowledge on the economic and healthcare developments within the study zone.

Population and Methods

The study zone of Niakhar
Population and environment
The current study zone of Niakhar, located in the Department of Fatick, Region of Fatick (Sine-Saloum), 135 km East of Dakar, comprises 30 villages and about 30 000 residents as of 1 January 2000. Rainfall decreased from 808 mm per year for the period 1921–1967 to 520 mm for 1968–1987 and to 463 mm for 1988–1999.10 The annual rainy season is short (July–October).

The population, mainly of Sereer origin (96.4%), lives traditionally on one food crop (millet), one cash crop (groundnuts) and cattle raising. The education level is low: in 1997, 59% of the men and 80% of the women aged 15–24 years had no primary education. To cope with the agricultural crisis in the Sahel and demographic pressure (85 inhabitants per km2 in 1966, 127 inhabitants per km2 in 1997), new activities arose: meat production and temporary migration to urban centres.11

The number of bore-holes and drinking-fountains has increased over the past decades: 60% of the households have now access to tap-water. Modern constructions, made of concrete and corrugated iron, tend to replace traditional houses. The use of latrines is more recent: only 22% of the households have access to them.

The first school opened in 1951 and, today, nine public and two private schools can be found in this district.

Healthcare system and baseline medical data
Three health posts within the study zone (the first opened in 1953, the last in 1983) and two outside, provide basic services to the study population: curative care, immunization, prenatal care, delivery, oral rehydration therapy and malnutrition management. The health structures are staffed by nurses and matrons. There is no permanent physician, no midwife, no laboratory facilities and no emergency transportation. Physicians from IRD also provide some assistance in relation to research programmes. The closest health centre is located in Fatick, 20–30 km from the study zone. Surgery, caesarean sections and blood transfusions are only available in the regional hospitals of Kaolack and Diourbel, 60–70 km from the study zone. Malaria transmission is mainly restricted to the months of August to October.12 Initiatives for vector control were undertaken in the 1950s.13 The Expanded Programme on Immunization (EPI) was launched in 1979 in Senegal, starting with the Region of Casamance and was generalized between 1982 and 1984.14 Fertility remains high with a total fertility rate of 7.1 between 1994 and 1996, despite some early signs of demographic transition.15

The demographic surveillance system
In December 1962, Pierre Cantrelle launched the first demographic survey in response to a demand issued by the Government in its first social and economic development programme. He initiated a demographic surveillance survey of the 65 villages forming the ‘arrondissement’ of Niakhar. In 1969, the surveillance was restricted to eight villages, ‘the Ngayokhem zone’, which have been continuously followed until now. In 1983 the surveillance area was extended to a total of 30 villages, 19 of whom were part of the first survey (Table 1Go). The history of the project from 1962 to 1991 has been published.16


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Table 1 Area and population under surveillance per period, 1963–1999
 
From 1962 to 1987, censuses took place annually or bi-annually. From 1987 to February 1997, vaccine trials required weekly household visits and, since March 1997, follow-up visits have been made on a quarterly basis. Residents are interviewed for incident events since the preceding visit, thus limiting the recall period and minimizing omissions, particularly important when early deaths are recorded retrospectively. In addition, the follow-up of pregnancies allows recording of their outcomes and increases confidence in early death registration.

The operational definition of residency has changed over the study period. At the very beginning (1962–1966), an individual was considered as emigrated if he/she stated his/her willingness to leave definitively or if he/she had been away for 4 years. This period of absence was shortened to one year between 1966 and 1983 and to 6 months after 1983. In addition, the residency definition allows seasonal workers and schoolchildren to keep their residency status if they spend at least one month per year in the study zone. However, the participation time took into account the date at which individuals were last known to be alive.

Currently, the zone is split into five areas with one interviewer assigned per area. Three supervisors, based at the field station in the village of Niakhar, collect the field logbooks, perform a first data check for consistency and make corrections if necessary with the help of the interviewers. Data are then sent to the main laboratory in Dakar where five data managers assign codes, enter the data into temporary files and run a second check procedure for completeness and consistency. If necessary, a new field visit is organized to correct a particular item. After keyboard entry, several automated checks are performed and, again, new field visits or checks of logbooks can be done. The database is then updated.

This research conforms to the Helsinki Declaration principles and has received approval from the national authorities.

Data
We have used already published indicators and have estimated new indicators when raw data were available, i.e. mainly for the more recent periods. The age-specific probabilities of dying have already been published for the entire 1962–1983 period, based on data from the only eight villages under demographic surveillance.9 They are presented per 5-year period resulting in a smoothing of the mortality trend. For the period 1984–1999, we have computed annual probabilities of dying, based on data from the 30 villages of the Niakhar area.

Quarterly probabilities of dying between 1963 and 1965 have already been estimated, based on data from the whole ‘arrondissement’ of Niakhar.17 We have calculated the same indicators based on data from the Ngayokhem area for the two following periods*: 1963–1971 and 1972–1980; and based on data from the Niakhar area for the three subsequent periods: 1984–1988, 1989–1993 and 1994–1999. Seasonal variations in the number of deaths were also calculated over the same periods.

The death rate (nMx) between age x and age x + n was computed as the ratio of the number of deaths to the person-time accrued over the same period. Death rates were transformed into probabilities of dying (nqx) between age x and x + n using the usual equation:18

where nax is the mean number of person-years lived in the interval (x, x + n) by those dying in the interval.

The rate of change in childhood mortality was estimated by regressing the Ln of the annual probability of dying on the year.

Results

Time trend
Levels of infant and child mortality calculated in life tables for 5-year periods from 1984 to 1999 (Table 2aGo) can be compared to the published probabilities of dying for earlier periods (Table 2bGo). In the 1960s the IMR was very high (1963–1967: 223{per thousand}) and almost half of the children had died by age 5 (1963–1967:5q0 = 485{per thousand}). For the most recent period, 1994–1999, mortality decreased to the following figures: 1q0 = 80{per thousand}; 5q0 = 213{per thousand}. This represents a total decrease of 64% and 56%, respectively, over the past four decades.


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Table 2a Life table before 5 years of age in Niakhar zone per period (30 villages, 1984–1999)
 

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Table 2b Long-term trend in infant and child mortality in Ngayokheme zone per period (8 villages, 1963–1980)
 
Indicators have been calculated for each of the years from 1984 to 1999 (Table 3Go). A regression of the Ln of the infant and under-5 mortality rates (Figure 1Go) shows a strong linear relationship (R2 = 0.84 and 0.66, respectively) and suggests a constant annual rate of decline in the probabilities of dying since the 1960s: –3.7% and –3.1%, respectively. The relationship is greater if we omit data from the two latest years (R2 = 0.92 and 0.88 respectively; rate of decline = –4.1% and –3.9%, respectively).


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Table 3 Infant and child probability of dying in Niakhar zone per year (30 villages, 1984–1999)
 


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Figure 1 Trend in infant and under-5 mortality in Ngayokhem and Niakhar, 1963–1999

Sources: before 1984: Ngayokhem, 8 villages after 1984: Niakhar zone, 30 villages, including the 8 above.

 
The yearly probability of dying between 1984 and 1999 shows fluctuations with two distinct peaks corresponding to outbreaks in 1985 and 1998–1999.

Quarterly death rates
Quarterly death rates have been calculated for different periods. In 1963–1965 data from the arrondissement of Niakhar17 showed that the death rate increased abruptly during the second and third quarter of life before decreasing slowly. Our estimations based on the 1963–1971 Ngayokhem data, more erratic due to a smaller population, confirm this pattern and indicate that this particularity disappeared from the early 1970s, resulting in a totally different age pattern of under-5 mortality (Figure 2Go).



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Figure 2 Quarterly probability of dying before 5 years of age per period, 1963–1999

Sources: 1963–65: 65 villages of Niakhar ‘arrondissement’301963–80: 8 villages of Ngayokhem zone since 1984: Niakhar zone, 30 villages, whose 19 are included in the above 65.

 
Indeed, the reduction in child mortality during the period was primarily due to a large decrease in the mortality before 24 months of age: mortality between 6 and 24 months fell from 321{per thousand} in 1963–1965 to 87{per thousand} in 1994–1999. After 1984, probabilities of dying between 3 to 28 months are consistently lower for the two more recent periods (1989–1993 and 1994–1999) as opposed to the early years (1984–1988).

Seasonality
In order to compare the monthly distribution of deaths between periods, we plotted the standardized number of deaths per month for 1200 annual deaths (Figure 3Go). A strong difference in the distribution could be observed between the rainy and the dry season, whatever the period. The sharp peak in childhood mortality in the rainy season during the 1960s decreased in the 1980s before a rebound in the 1990s. This is particularly marked for 1–4 year old children.




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Figure 3 Seasonal variations of the number of deaths over the periods 1963–70, 1984–88, 1989–93 asnd 1994–99

Sources: before 1984: Ngayokhem, 8 villages after 1984: Niakhar zone, 30 villages, including the 8 above.

 
Discussion

Methodological issues
The boundaries of the study area have varied over the periods discussed here. However, since this region is very homogeneous with respect to economic, cultural and social context, this should not have affected the observed changes in mortality rates. What might be questioned is the quality of death registration during the 37-year period in which lies the strength of these results. The quality of the demographic surveillance system lies in the completeness of birth and death recording. This system differs from retrospective assessments in that the interviewers can refer, at each follow-up visit, to previous information on the same subjects. A household listing allowed recording of deaths between two visits without relying only on people's statements. In addition, recording pregnancies as soon as possible (the fourth month in general) permitted a focus on the issue of the pregnancy and, hence, minimization of the omissions of early deaths. Very few events are likely to be omitted with weekly visits, as could be the case with annual visits. One cannot, therefore, exclude an underestimation of the number of deaths in 1962–1986 when the periodicity of the visits was yearly or half-yearly. In addition, calculation of the death rates needs an assessment of the person-years lived, which in turn depends on residency status. In the first decade of the demographic surveillance, 4 years of absence without declaration of emigration was necessary before a resident was withdrawn. This time has been shortened to 6 months since 1984. The resulting effect of these two sources of mismeasurement could be an underestimation of the death rates for the first period. Therefore, the true decrease has been at least as important as we have measured. However, the under-5 mortality was already so high that this bias cannot be very large.

Long-term trend
This study confirms the long-term trend of decrease in child mortality in rural West Africa.5,19,20 In addition, it suggests a constant rate of decline over almost four decades, in contrast to what has happened in other African countries where widespread HIV/AIDS has resulted in a recent increase in childhood mortality.21 The date of the beginning of the decrease in the study zone is still not certain. The available data do not allow us to go back beyond 1960. The data on childhood mortality in Senegal are amongst the richest and most plentiful and ‘indicate a substantial early fall in childhood mortality from 1940 to 1960, following by a plateau until the early 1970s, and then a renewed sharp decline to the mid-1980s'.22 As in other rural areas, childhood mortality decline in Niakhar started later than in urban areas like Dakar, where mortality started decreasing from the first half of the century. Adult mortality appears to have declined from the early 1970s both in urban and rural areas.9

Before the decline, national probabilities of dying were in the range of 400{per thousand}. Results from the Ngayokhem zone show a greater level of childhood mortality in the 1960s. During this period, mortality levels in the Ngayokhem and Niakhar areas remained higher than the national level, higher even than the rural national level.23–27 We propose two reasons for explaining this difference: first, the methodology adopted in Niakhar area ensured fewer omissions in death registration; second, the mortality level in Niakhar is greater than other rural areas: in the Mlomp area, the probability of dying before 5 years was less than 400{per thousand} before 1965, and less than 100{per thousand} in late 1980s.20 The decrease in mortality in Mlomp area was associated with an improvement in health services and programmes as early as 1960s, followed by an improvement in delivery conditions, and infant mortality started to decline before mortality in the 1–4 year age group. This pattern did not appear in Niakhar area where infant and child mortality began to decrease at the same time.

Recent rebounds in mortality in 1985 and 1998–1999 can largely be explained by several outbreaks of cholera and meningitis which have struck the whole country.28,29

Fertility behaviour (birth intervals, breastfeeding practices, delivery conditions) has not really changed during the period and seemed to have played a small role in mortality trends.

Age pattern
To explain the pattern of excess mortality between 6 and 24 months, obvious in the 1960s, some authors have suggested that the rainy season (the most critical period of the year for children), was particularly risky for children aged 9–15 months.30–32 This pattern, which characterized the 1960s, disappeared during the 1970s. Measles vaccine trials in the late 1960s, a major vaccination campaign against measles from 1978 to 1982 and the introduction of the EPI in the early-mid 1980s contributed to a decrease in the incidence of measles, other childhood infectious diseases and even of other diseases.33,34 In 1987, a few years after the generalization of the EPI in Senegal, the proportion of fully immunized children among those aged 12 to 23 months attained 33% in the Department of Fatick.35 This proportion increased to 61% in 1990 at the Region level before levelling out at 51% in 1991 and 47.6% in 1999.14,27,36 Within the study zone, however, measles and pertussis vaccine trials between 1987 and 1997 allowed maintenance of a much higher immunization coverage, which probably explains the lower probabilities of dying in young children for the more recent periods.37 Another major determinant of childhood mortality is the nutrition status. The first published data on nutrition relates to the 1980s, thus preventing study of the effect of a change in malnutrition on mortality patterns. During the 1980s and 1990s, it does not appear that the nutritional status improved or deteriorated. In 1983–1984, the prevalence of wasting (weight-for-height <-2 z-score) reached 9.5% and 7.7% among 6–17 month old and 18–35 month old children, respectively.38 Over the period 1990–1996, it increased slightly to 11.6% (9–10 months).39

Seasonality
The seasonal pattern of childhood mortality has also changed. The rainy season mortality rebound in the 1990s (while the overall mortality was decreasing), can be explained by a change in the distribution of the causes of death in favour of diseases for which death rates peak during the rainy season. A resurgence of deaths due to malaria is likely. This hypothesis is supported by the increase in the malaria-specific death rate observed between 1991 and 1995.40 An analysis of the causes of death for the following years will confirm this hypothesis. One of the determinants proposed to explain this emergence of malaria is chloroquine resistance which appeared in the early 1990s.41

Conclusion

The Niakhar area, as for other longitudinal surveillance systems, remains an excellent setting for studying trends in childhood mortality. Further analyses are planned to refine the analysis of causes of death and their evolution since the 1980s.

This study confirms that there is a broad set of factors acting together to explain the decline in mortality. Immunization certainly played a great role in the 1980s and 1990s in the Niakhar childhood mortality decrease, as in most parts of Africa, but a recent decrease in immunization coverage combined with the emergence of malaria may challenge three decades of progress. Efforts in these fields should continue to be a priority.

Moreover, the HIV/AIDS epidemic in Sub-Saharan Africa may contribute to a rise in childhood mortality, as already noted in certain countries.21,42 In the Niakhar area, however, the prevalence of HIV is very low.43

Other factors should be considered for future decrease in mortality. Fertility behaviour, such as birth spacing, infant feeding, breastfeeding, which is beginning to change now in Niakhar area, could become an important factor in the continuing decline in child mortality. Efforts should thus be maintained in reproductive health to provide access to family planning and to improve medical care during pregnancy and delivery at the appropriate level of the health system.

Contributors

Valérie Delaunay contributed to the study design, supervized the data collection and management from 1990 to 2000, analysed the data and wrote the article. Jean-François Etard co-ordinated the Niakhar Project from 1996 to 2000, analysed the data and wrote the article. Marie-Pierre Préziosi contributed to the study design, supervized the data collection between 1991 and 1997, contributed to the interpretation of the results and reviewed the draft. Adama Marra contributed to the study design, has been supervizing the data collection since 1992, contributed to the data management and the analysis and reviewed the draft. François Simondon coordinated the Niakhar Project from 1989 to 1995, contributed to the study design, supervized the data collection from 1990 to 1995, contributed to the interpretation and reviewed the draft.


KEY MESSAGES

  • Constant annual rate of decline in the probability of dying since 1960s (3–4%).
  • Change in age-pattern of infant and child mortality.
  • Change in seasonality of infant and child mortality.
  • Impact of immunization on infant and child mortality levels.
  • Re-emergence of malaria.

 



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Children in Senegal. Photograph kindly supplied by the authors.

 
Acknowledgments

The source of support was the Institut de Recherche pour le Développement. We thank the whole study population of Niakhar for their participation and welcome. We are grateful to Dr Pierre Cantrelle and Dr Michel Garenne for having initiated this longitudinal study. We thank all the staff of the Population and Health Laboratory from IRD, Dakar, who contributed to the data collection and management, particularly Ernest Faye who has been co-ordinating the fieldwork for many years. We are most grateful to Dr Anouch Chahnazarian (died 1993) who actively participated in the study design and initial data analyses.

Notes

* Longer periods were required due to the small size of the population. Back

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