Models of health transition

Changing health in low- , middle- , and high- income countries

[Just a quick note before the post. The number of subscribers continues to grow (3.5k) – welcome to all – many it seems unconnected to the UK education system.

And to be a bit of a ‘broken record’ – you will find Substack a much better read in the Substack app on your phone or tablet. Also, can I strongly recommend the Substacks of climate scientist Hannah Ritchie (who also has an excellent Podcast – Solving for climate) and the UK Sky journalist Ed Conway? For my new North American subscribers, the latter has a recent post on why you are likely to be paying a bit more for your tinned Campbell’s soup (with a UK connection to Port Talbot, South Wales). You can find it here.]

Introduction

It has long been recognised that societies experience a change in the underlying patterns of illness and death with increasing levels of economic development. In general terms, economic development is associated with an overall reduction in all causes of illness and death, a significant shift in the dominant causes of illness and death, and an increase in life expectancy.

The epidemiological transition.

In 1971, a model relating to population, health and disease was put forward by Omran - The Epidemiological Transition model. The model states that societies undergo three ‘ages’ of health:

· an age of pestilence and famine: a period in which mortality is high with the principle causes of death being infectious diseases and poor maternal conditions reinforced by nutritional deficiencies. Famines, wars and pandemic diseases are common.

· an age of receding pandemics: socio-economic improvements and advances in medical science and healthcare mean infectious diseases are reduced and life expectancy increases. Examples of such improvements and advances include better public water supplies and the discovery of penicillin.

· an age of degenerative and ‘man-made’ diseases: as infectious diseases are controlled and people live longer, there is an increased visibility of degenerative diseases (cancers, heart disease). Diseases associated with modernisation and industrialisation (obesity, diabetes) begin to increase (some geographers have described these as ‘diseases of affluence’).

· some writers have added a fourth stage – an age of delayed degenerative diseases. Here the causes of death are generally the same as the third stage (although dementia is more prevalent); they just occur later in the life cycle as life expectancy increases.

Different parts of the world have progressed through these stages (‘ages’) at different times and at different rates. In western Europe, the transition occurred over an extended period of several centuries. For many low- and middle-income (economically developing) countries, the transition was delayed until the mid-twentieth century and continues through to the present day.

The Omran model parallels the Demographic Transition (see a previous post here ) from high fertility and mortality rates, with young populations with high levels of infectious disease to societies with low fertility and mortality rates and ageing populations where non-communicable diseases predominate. For this to occur a medical and healthcare revolution must take place which tackles and/or controls infectious diseases. The Omran model therefore provides us with a useful reference point for exploring the linkages between health and development.

The situation in the twenty-first century

The World Health Organization (WHO) generates estimates of deaths in each of its member countries for the purposes of health policy development and resource allocation. These estimates are grouped into three broad categories of condition:

■ category I - communicable, including infectious and parasitic diseases, along with maternal, perinatal and nutritional conditions

■ category II - non-communicable, including chronic diseases such as cancers, cardiovascular and cerebrovascular diseases, neurological conditions and substance (drugs and alcohol) use

■ category III - injuries, both unintentional (accidents) and intentional (self-harm, interpersonal and collective, including those arising from military conflict).

The first two categories correspond to the principal causes of death in the early (category I) and late (category II) stages of the epidemiological transition. Category II conditions of late transition now account for almost three-quarters (74%) of all deaths worldwide; most of the remainder (18%) are due to category I conditions of early transition.

For the period 2000 to 2021, WHO stated:

· Non-communicable diseases have become more prominent with Alzheimer’s disease and diabetes entering the top 10 causes of death whereas communicable diseases are on the decline with HIV dropping out.

· The world’s biggest killer is ischaemic heart disease, responsible for 13% of the world’s total deaths. Since 2000, the largest increase in deaths has been for this disease, rising by 2.7 million to 9.0 million deaths in 2021.

· As a newly emerged cause of death, COVID-19 was directly responsible for 8.7 million deaths in 2021.

· Lower respiratory infections remained the world’s most deadly communicable disease other than COVID-19, ranked as the fifth leading cause of death in 2021, although the total number of deaths from lower respiratory infections has decreased.

Patterns

Economically developing countries still experience mostly category I conditions, with the low- and lower-middle-income countries of sub- Saharan Africa (40% or more of all deaths) and south and southeast Asia (20% or more of all deaths) being the largest. This contrasts with the high-income countries of North America, Europe and Australasia, where category II conditions account for the overwhelming majority (80% or more) of all deaths.

Between 2000 and 2021 the proportion of deaths due to category I conditions fell by 10% or more in many countries of sub-Saharan Africa and south and southeast Asia, with a corresponding increase in the proportion of deaths due to category II conditions. These developments were particularly pronounced in the countries of south and southeast Asia, with the largest reductions in category I conditions in Nepal (33%), North Korea (33%) and Bangladesh (28%).

Lower respiratory infections and diarrhoeal disease are the fourth and fifth most significant causes of death and ill health globally and rank second and third in low-income countries. Household air pollution from cooking and heating homes with solid biomass fuels (for example, wood, charcoal, dung or crop residue) contributes to illness and deaths from respiratory illnesses, cancers and heart disease. However, approximately three billion people rely on such fuels for cooking and heating.

Diarrhoeal disease is strongly linked to poor drinking water, sanitation and handwashing facilities. Infants bear a disproportionate share of environmental risk linked to poor water, sanitation and hygiene access, which is estimated to kill over 2,000 children daily - more than AIDS, malaria and measles combined. Almost 1.7 billion people lack basic sanitation systems, with almost 500 million defecating in the open.

Around 2.3 billion lack adequate handwashing facilities and over 750 million lack basic drinking water services. The situation may worsen seasonally if water supplies dry up, freeze or if flooding restricts access and increases the risk of faecal contamination.

The environmental risk transition

Geographical changes in the causes of death imply a shift in the underpinning risks of contracting different diseases. The Environmental Risk Transition model was developed by Kirk Smith in the 1990s and builds on the Epidemiological Transition model by providing a framework for understanding the environmental basis of changing disease risks.

The model (Figure 1) below outlines how, in low-income countries, environmental risks associated with poor access to water, sanitation and clean household energy occur primarily at the household level.

With increasing economic development, environmental risks such as ambient particulate matter air pollution (for example, outdoor air pollution associated with industrial facilities) and lead exposure (associated with, for example, motor vehicles) tend to become more prevalent at the community level. (Note: This part of the model reflects the Kuznets curve, which suggests that environmental problems increase and later decline as economic growth proceeds.)

With further economic development however, global level environmental risks become more significant, with low-income populations bearing the brunt of climate change risks such as temperature extremes, food insecurity and natural disasters.

Figure 1. The Environmental risk transition

The disease burden and disability-adjusted life years (DALYs)

The burden of disease is calculated using the disability-adjusted life year (DALY). One DALY represents the loss of the equivalent of one year of full health due to ill-health. DALYs for a disease or health condition are the sum of years of life lost due to premature mortality (YLLs) and/or years of healthy life lost due to disability (YLDs) due to prevalent cases of the disease or health condition in a population.

For the period 2000 to 2021, WHO stated:

• DALYs due to communicable diseases such as HIV/AIDS and diarrhoeal diseases have dropped by over 50% since 2000.

• DALYs from diabetes and Alzheimer’s disease more than doubled between 2000 and 2021.

Figure 2 shows the changes in health conditions that are linked to each level of the Environmental Risk Transition model in each of low-, middle- and high-income countries.

Figure 2. The proportion of disability-adjusted life years (DALYs) attributed to environmental risks in low-, middle- and high-income countries in 1990 and 2019.

[Note Diagram B below should be labelled Community-level risks]

Source: Geography Review

Household-level risks

Figure 2A shows the decline in DALYs linked to water, sanitation and handwashing and household air pollution with increasing income level. Due to reduced reliance on biomass fuels for cooking and household energy, DALYs linked to household air pollution have declined quite sharply in middle-income countries, while DALYs linked to water, sanitation and hygiene have fallen more in low-income countries over the same period. This is consistent with the decline in deaths from category I conditions in many low- and middle-income countries.

Community-level risks

Figure 2B shows how community-level environmental risks have tended to worsen before improving as countries progress to higher levels of economic development. The proportion of DALYs linked to ambient particulate matter air pollution rose in low- and middle-income countries between 1990 and 2019 but fell in high-income countries.

DALYs linked to lead exposure show a similar, if less pronounced, pattern. This is consistent with the increase in deaths from category II conditions in low- and middle-income countries.

Global-level risks

Estimates of the proportion of DALYs linked to global-level environmental risks are currently limited to measures of high and low non-optimal temperature. (Note, these estimates do not capture the health impacts of changing crop yields, extreme weather, sea level rises and changes in disease vector distribution.)

Figure 2C shows high optimal temperatures are linked to 0.5% of DALYs globally, with higher levels in low- as compared to high-income countries. Low optimal temperatures account for 1% of DALYs globally, with greater impacts in higher- compared to lower-income countries.

To summarise:

Household level environmental risks account for a significant proportion of deaths and ill health among the poorest due to the main factor of where they live. The impacts are especially pronounced among young children. Financial insecurity limits access to decent housing and the infrastructure needed to supply sanitation and clean water. Unaffordable healthcare also hinders recovery from exposure to environmental risks.

However, the poorest tend to benefit most quickly from improvements in environmental health. Relatively small gains in access to vital services like clean drinking water and sanitation can significantly reduce the burden of ill-health and death faced by the poorest and the youngest. This in turn translates into improved life expectancy, lower infant mortality and livelihood benefits as fewer days are lost to sickness.