The Epidemiological Transition (or What We Died, Die and Will Die From)

What is it? 

A baby born in Spain in 1900 could hope to live an average of 35 years. In 2011, he/she can expect to live up to 82 years, more than twice. Over the last two centuries, not only the life expectancy has doubled (or even tripled) across the world. The causes of death have also changed. The baby born in 1900 would have likely died of an infectious disease – pneumonia, tuberculosis or a gastrointestinal infection. The one born in 2011 will most likely die of a chronic non-communicable disease such as cancer or cardiovascular disease.

The change in disease patterns and causes of death is known as an epidemiological transition

This change in disease patterns and causes of death – where a pattern of high child mortality and infectious epidemics shifts to one with high prevalence of chronic degenerative diseases – is known as an epidemiological transition, and has important consequences on the design of public health policies. 

Source: Global Burden of Disease 2015 Compare Visualisation - Institute of Health Metrics and Evaluation  

Why?

This epidemiological transition is the result of a series of interrelated factors:  

• Demographic changes: the reduction in childhood mortality leads to a decrease in fertility rates. As a consequence, a higher percentage of the population reaches the adult age and develops adult-related diseases. 
• Changes in risk factors: this includes changes in the prevalence, distribution and/or virulence of pathogenic organisms, environmental changes – frequently linked to human activity- that can cause disease, and social and cultural factors such as lifestyle and diet. 
• Modern medicine practices: vaccines are without doubt the greatest public health success in history – every year they avoid 2 to 3 million deaths by diseases such as diphtheria, tetanus and whooping cough, they have allowed the eradication of smallpox and practically that of polio. The discovery of penicillin in the 20th century was another milestone in modern medicine and, ever since, antibiotics have saved hundreds of millions of lives.  

Source: World Health Organization (WHO). 2015. Deaths in millions.

How is it measured? 

The mortality rate is not enough to measure the health of a population. The number of healthy years lived is also important

The mortality rate is not enough to measure the health of a population. The number of healthy years lived is also important. Thus, the disability adjusted life years (DALYs) is an overall measure that sums the impact of years lost to premature death and those lost to disability. A surprising result when applying this measure is the magnitude it ascribes to neuropsychiatric diseases (led by depression) that are not lethal but represent more than 15% of total disease burden in developed countries.  

The double burden of disease

Despite the health achievements of the last century and the global tendencies described above, the world’s most vulnerable populations continue dying of preventable infectious diseases, a burden amplified by malnutrition and poverty. For many low and middle income countries (LMICs) this means a double burden of disease: “old” health problems including infectious diseases and high maternal and child mortality in addition to “emerging” health problems due to chronic diseases associated to a western lifestyle and the ageing of the general population. 

For many low and middle income countries, there is a double burden of disease: “old” health problems including infectious diseases and “emerging” health problems due to chronic diseases

These differences are evident when comparing developing and developed countries, but also exist within a same country, where deaths by infectious diseases are much higher among people living in the poorest quintile. Ironically, these same populations, whose common denominator is poverty, will also be victims of obesity, cardiovascular disease and diabetes, while the wealthier persons – those that first suffered from them - have already started to adopt treatments and habits to prevent them. For example, a study last year showed that obesity prevalence and body mass index increased linearly with income in developing countries whereas in more developed countries, obesity was inversely related to income.    

Global Burden of Disease 2015, The Lancet 

Not so simple…  

In 1969, US Surgeon General William Stewart famously declared that it was “time to close the book on infectious disease as a major health threat”. Obviously, he did not foresee the global HIV/AIDS epidemic, or the epidemics caused by emerging pathogens such as Ebola or Zika, or the threats posed by antimicrobial resistance. Today’s reality is that malnutrition and poverty, together with environmental deterioration, demographic explosion and globalization, are fertile ground for the emergence of new pathogens (limited before to sylvatic cycles) or the increase of infections that were previously under control (for example, tuberculosis due to multidrug resistant strains, or dengue, due to vector re-emergence in the Americas).

Although we are indeed living an epidemiological transition towards chronic non-communicable diseases, infectious diseases caused by emerging (particularly vector-borne viruses) and re-emerging (multidrug resistant bacteria) pathogens pose a real and present threat at the global level.  

A further level of complexity is added by the fact that the distinction between infectious and chronic disease is not that clear

A further level of complexity is added by the fact that the distinction between infectious and chronic disease is not that clear. In fact, infectious agents and the inflammatory process they trigger play a key role in the origin of chronic diseases such as cervical cancer (linked to infection by the human papilloma virus), gastrointestinal ulcer (linked to the bacterium H. pylori) and cardiovascular disease or type 2 diabetes (linked to low-grade chronic inflammation), and evidence of inflammation has been found in many other non-communicable diseases including autism and other mental disorders.   

Source: A Short History Of Humans And Germs: Early Encounters | Goats & Soda | NPR

What to do?

This involves designing a series of interventions tailored to the reality and risks of each population

All this highlights the need to think beyond the epidemiological transition in order to meet the challenges in global health. This involves designing a series of interventions tailored to the reality and risks of each population. Infectious diseases that affect the poorest populations can be tackled with available interventions and with a high cost-effectiveness. The double burden of disease multiplies the number of diseases that potentially affect every person and therefore the interventions must be more varied and involve multiple sectors and actors. Finally, the strategies to reduce risks associated to a sedentary lifestyle and “hypernutrition” should not overshadow the present and real threat of emerging and re-emerging infectious diseases.