Background
The classic framework of epidemiological transition describes the shift from infectious diseases to non-communicable diseases (NCDs) as population income increases and mortality levels decline [
1‐
4]. However, many low- and middle-income countries are facing a double burden of disease with persisting high levels of infectious disease, coupled with an increasing prevalence of NCD [
5,
6]. The 2010 Global Burden of Disease study reported that infectious diseases together with maternal, neonatal and nutritional disorders were amongst the leading causes of premature death in sub-Saharan Africa, accounting for 76% of mortality in under 80 year-olds [
2]. Cardiovascular disease burden nevertheless increased in sub-Saharan Africa, with death from cardiovascular disease rising by 81% between 1990 and 2013 [
7,
8]. Fast-changing health and connected demographic transitions are underway in these settings altering population dynamics and patterns of health and disease [
4]. Empirical evidence concerning the dynamics of epidemics and related population trends has highlighted marked regional and national variability [
2,
7], however, these processes have been insufficiently documented due to a scarcity of appropriate data.
It is well established that health transitions and epidemic dynamics are influenced by socioenvironmental and economic conditions [
9]. Understanding the shifting cause-pattern of disease and mortality [
1], as well as the mechanisms through which changing socioeconomic and environmental contexts may influence health is therefore crucial. Migration, which involves a change in environment and health exposure, is one such mechanism. Migration has been recognised as a key risk factor for infectious as well as chronic disease, and operates through complex pathways.
The migration and health relationship is multi-faceted and bi-directional. Health status can drive a decision to migrate, but such relocation may in turn have an influence on health. A number of hypotheses concerning the relationship between migration and health have been proposed in the literature, each relating to a stage of the migration process [
10]. The most utilised hypothesis of selection holds that migrants are often amongst the healthier in the population when they undertake a migration (the “healthy migrant hypothesis”) [
10]. A paradox is present since migrants who return to their places of origin have been observed to be amongst the less healthy, resulting in a “salmon bias” effect, first described with respect to the return migration of Hispanic migrants from the United States [
11]. Similarly studies of internal migrants in sub-Saharan Africa have described the phenomenon of migrants “returning home to die” as their health in destinations deteriorates [
12‐
14].
While migrants may be relatively healthier when they undertake a move, health status may be affected following migration as a result of disruption and stress, behavioural change, unstable employment and possible exposure to disease in a new environment [
15‐
18]. Migrants may encounter barriers in accessing health care, or be exposed to changing lifestyle factors that negatively affect their health and well-being at destinations [
16,
19]. These potential risks may be offset by improved employment and education opportunities, or superior health services and infrastructure at migrant destinations - conditions often associated with urban environments [
18]. Nevertheless, the balance between potential negative and positive effects of relocation may vary considerably across settings [
20].
Duration of residence in a new location has been found to reduce these effects as migrants adapt to their new environment. The hypothesis of adaptation suggests that over time, migrants’ health and behaviours may assimilate to non-migrants in a destination area, resulting in no discernible difference in health status between migrants and non-migrants [
10,
16,
21].
Migration, in particular circular migration, has further been linked to the spread of health conditions or behaviours between origin and destination areas. Studies have revealed how infectious diseases such as HIV may be diffused through a population via migration [
22,
23]. This has been attributed to the transmission of infection through physical contact and increased sexual risk behaviours associated with mobility [
22,
24]. Non-communicable disease risk has been associated with the process of urbanisation where movement to urban areas may expose migrants to negative lifestyle factors such as unhealthy diets or lower levels of physical activity [
19,
25,
26], and these lifestyle behavioural changes may potentially spread to other areas via return migration. The role of migration in the propagation of disease will depend on the stage of the epidemic, the prevalence of the disease in an area, and patterns of movements linking origins and destinations [
22,
24]. Evidence on the association between migration and epidemic dynamics, and the pathways linking movement to disease is much needed to improve current understanding of interactions between disease and the social and environmental context [
27]. Indeed, intergovernmental organisations and researchers have highlighted the importance of focused regional studies, and the triangulation of varied data sources on burden of disease and surrounding processes such as migration in informing the development of appropriate public health policy and interventions [
7,
28].
This paper follows from a previous study of the relationship between internal migration and all-cause mortality in nine Health and Demographic Surveillance System (HDSS) sites in sub-Saharan Africa [
21]. The study confirmed that premature adult mortality differed strongly by migration status. It also confirmed the adaptation hypothesis by showing that in the ten-year period after migration, no health differences between migrants and non-migrants were observed in these HDSS populations.
This paper explores the relationship between internal migration and mortality by cause of death through an analysis of four HDSS sites situated in Kenya and South Africa. These are two sub-Saharan African countries that present with high rates of premature adult mortality associated with two primary causes: AIDS/TB and NCDs. The study asks how internal migration status relates to patterns of AIDS/TB and NCD mortality in the context of the double burden of disease in these countries. The paper has two objectives. The first objective is to define for each geographical area, the patterns of AIDS/TB and NCD mortality for a period of nine to fourteen years. The second objective is to test whether the mortality of migrants converges to that of non-migrants as the epidemic evolves over the period of observation.
We hypothesise that health consequences of migration are associated with the extent of diffusion of an epidemic in a particular local population. At the initial phase of the epidemic, we would expect the health of migrants to diverge from that of the host population. As the epidemic is diffused, we anticipate that the health of migrants would converge with that of non-migrants as both origin and destination areas have been affected. This pattern of divergence and convergence would represent an association between migrant status and the dynamics of the epidemic. The alternate hypothesis is that the health consequences of migrants are not associated with the dynamics of the epidemic. This would be observed as a non-convergence of health outcomes between migrants and non-migrants over time. In this instance, we would attribute the persisting differential health consequences amongst migrants and non-migrants to structural determinants that are independent of the epidemic dynamics. These may be individual, household or community factors, or factors relating to health and social systems that result in discriminatory barriers to prevention or treatment.
These convergence or non-convergence patterns may apply to both infectious and non-communicable diseases. Vector-borne diseases may spread more easily through physical contacts but behaviours and differential health care also contribute to the spread of diseases whether communicable or not. We would expect that migrants are more subjected to the divergence-convergence cycle in the case of infectious diseases because of the assumed greater role of the physical environment (transmission of viruses, bacteria, and parasites). However the divergence-convergence cycle may also apply to transmission of health behaviours (e.g.: eating and drinking habits) across social environments through migrants. Conversely the divergence-convergence pattern may not apply at all if migration status is unrelated to physical or social propagation. The epidemic dynamics would then be independent of the migration pattern.
Discussion
This is the first study to analyse the extent to which internal migrant status is associated with patterns of mortality by cause of death. The results of the study shed light on both the dynamics of the AIDS/TB and NCD epidemics underway in these South African and Kenyan local areas, and the extent to which the mortality of migrants converges to that of non-migrants as the epidemic evolves. In all four HDSS populations, AIDS/TB accounts for a significant proportion of total deaths. In the Agincourt HDSS, there is evidence of a gradual increasing trend in NCD mortality. In general, the migration effect on mortality over the period of observation presents similar patterns in relation to both infectious and non-communicable diseases, and shows a migrant mortality disadvantage and no convergence in mortality risk between migrants and non-migrants.
In the Agincourt HDSS no convergence or divergence by migrant status is observed – either in relation to AIDS/TB or NCD mortality. In-migrants and even more so, return migrants have a persistent health disadvantage, which has been corroborated in previous studies [
12,
21]. This holds for both males and females, and is independent of the diffusion of diseases. In-migrants to the Agincourt HDSS area generally originate from surrounding rural areas that carry similar health risks [
13,
39]. Return migrants most commonly move to metropolitan areas or secondary cities to access employment, and may return home with physical or mental health issues. The higher mortality risk among return migrants is of particular concern in this population.
In the Kisumu HDSS, the main features of the migration-mortality relationship are the lack of significant differences between in-migrants and return migrants, and the greater risk of AIDS/TB mortality amongst female migrants. One hypothesis for the similarity between in-migrants and return migrants is that their exposure to health risks is similar within and outside the HDSS area. Although the Kisumu HDSS is built upon a strong health provision infrastructure run by the Ministry of Health and its collaborative partners, which emphasises ART and malaria treatment [
40], research has suggested that mobile females from the Kisumu area who are HIV positive may experience interrupted ART as a result of access issues and mobility [
41].
In two instances, the trends by migration status are less stable over time. This is indicated by an increase in AIDS/TB mortality risk amongst females from the AHRI HDSS from 2004, and a divergence in AIDS/TB mortality risk amongst Nairobi females in the most recent period. It is well established that female return migrants to the AHRI HDSS face health challenges as they are significantly more likely to die, particularly from HIV-related conditions [
42], as compared with residents of both sexes [
43]. Poor linkage to regular ART may help to explain the higher AIDS/TB-related mortality amongst female migrants. A number of studies originating from the ARHI study site point to proximity to primary care, and gender inequality as major barriers to accessing care among this vulnerable population [
44]. Female return migrants who spend more time away from the AHRI study site have been observed to have a greater likelihood of HIV acquisition compared with males [
45].
In the Nairobi HDSS, AIDS/TB and NCD mortality is higher amongst females as compared with males, while external causes of death are consistent with the expected (i.e.: males are at higher risk). This corroborates a study by Mberu et al. [
46] that found a number of gender variations in causes of death in the Nairobi HDSS that were inconsistent with the literature. For cardiovascular diseases, Mberu et al. [
46] suggest that misperception by health practitioners may lead to underestimated risk and under-diagnosis amongst females, resulting in higher mortality. They may be more vulnerable and often stigmatised, with less economic opportunities to sustain themselves and their children [
47]. Both married and single mothers are often tied down to a particular location in the slums and are therefore less mobile [
48]. Thus for females in Nairobi, the observed patterns may be explained by conditions and circumstances of migrants in the urban areas. The hypothesis is that in case of an adverse health event, females are less likely than males to leave their households in the slums to seek treatment or care. However, the complexity of risk and selection factors is likely to contribute to the instability of the relationship between migration status and mortality in Nairobi.
HDSSs provide a versatile platform to study a diverse range of settings, and offer detailed measures of migration and mortality dynamics. The value of a comparative perspective is that it highlights the differences but also the commonalities in the relationship between internal migration status and changing disease patterns in these local areas. A study limitation is that the analysis does not include information about reasons for movement or specific migrant destinations. Further detail on social and economic conditions that would explain the differences in mortality between migrants and non-migrants would add value. These more detailed contextual dimensions are being explored as part of site-specific studies aimed at collecting detailed survey data by following migrants who leave the HDSS areas.
Conclusion
In conclusion, the study findings do not confirm the hypothesis of mortality convergence between migrants and non-migrants. There is no apparent association between migration status and epidemic dynamics as no period convergence by migrant status is observed. A convergence (or divergence) would indicate the reduction (or increase) in adverse health conditions for migrants. Rather, the stability by migration status over time and over a range of settings, for both infectious diseases and NCDs, is suggestive of a general health care deficit for migrants. Findings suggest that structural issues rather than epidemic dynamics explain difference in mortality risk by migrant status. Factors such as poor access to health care at destinations, poor social integration, inadequate living conditions at destinations, employment status, stress experienced as a result of relocation, or behavioural factors associated with migration may all contribute to these observed differences by migration status. The findings also suggest that female migrants may be particularly vulnerable in certain contexts.
Despite the differences observed across sites, a similar policy message may apply. Recent migrants should be identified and targeted by the health systems to improve their access to treatment at all stages of the epidemic, whether infectious or non-infectious. Interventions aimed at educating people who intend leaving an area may be an effective means of mitigating potential risks at migrants’ destinations. Further research into the circumstances at migrant destinations in these various contexts is a priority going forward.
Acknowledgements
We greatly value the contribution of each participating HDSS centre in providing data for this multi-centre study. We further acknowledge institutional support from the School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, South Africa; Centre de Recherche en Demographie et Societes, Universite Catholique deLouvain, Louvain-la-Neuve, Belgium; and the African Population and Health Research Centre, Nairobi, Kenya, as critical bases for the MADIMAH project leadership.