THE IMPACT OF MALARIA ON EARLY AFRICAN DEVELOPMENT: Evidence from the sickle cell trait

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poster “Keep out malaria mosquitoes repair your torn screens”. U.S. Public Health Service, 1941–45

While malaria historically claimed millions of African lives, it did not hold back the continent’s economic development. That is one of the findings of new research by Emilio Depetris-Chauvin (Pontificia Universidad Católica de Chile) and David Weil (Brown University), published in the Economic Journal.

Their study uses data on the prevalence of the gene that causes sickle cell disease to estimate death rates from malaria for the period before the Second World War. They find that in parts of Africa with high malaria transmission, one in ten children died from malaria or sickle cell disease before reaching adulthood – a death rate more than twice the current burden of malaria in these regions.

 

According to the World Health Organization, the malaria mortality rate declined by 29% between 2010 and 2015. This was a major public health accomplishment, although with 429,000 annual deaths, the disease remains a terrible scourge.

Countries where malaria is endemic are also, on average, very poor. This correlation has led economists to speculate about whether malaria is a driver of poverty. But addressing that issue is difficult because of a lack of data. Poverty in the tropics has long historical roots, and while there are good data on malaria prevalence in the period since the Second World War, there is no World Malaria Report for 1900, 1800 or 1700.

Biologists only came to understand the nature of malaria in the late nineteenth century. Even today, trained medical personnel have trouble distinguishing between malaria and other diseases without the use of microscopy or diagnostic tests. Accounts from travellers and other historical records provide some evidence of the impact of malaria going back millennia, but these are hardly sufficient to draw firm conclusions. Akyeampong (2006), Mabogunje and Richards (1985)

This study addresses the lack of information on malaria’s impact historically by using genetic data. In the worst afflicted areas, malaria left an imprint on the human genome that can be read today.

Specifically, the researchers look at the prevalence of the gene that causes sickle cell disease. Carrying one copy of this gene provided individuals with a significant level of protection against malaria, but people who carried two copies of the gene died before reaching reproductive age.

Thus, the degree of selective pressure exerted by malaria determined the equilibrium prevalence of the gene in the population. By measuring the prevalence of the gene in modern populations, it is possible to back out estimates of the severity of malaria historically.

In areas of high malaria transmission, 20% of the population carries the sickle cell trait. The researchers’ estimate is that this implies that historically 10-11% of children died from malaria or sickle cell disease before reaching adulthood. Such a death rate is more than twice the current burden of malaria in these regions.

Comparing the most affected areas with those least affected, malaria may have been responsible for a ten percentage point difference in the probability of surviving to adulthood. In areas of high malaria transmission, the researchers’ estimate that life expectancy at birth was reduced by approximately five years.

Having established the magnitude of malaria’s mortality burden, the researchers then turn to its economic effects. Surprisingly, they find little reason to believe that malaria held back development. A simple life cycle model suggests that the disease was not very important, primarily because the vast majority of deaths that it caused were among the very young, in whom society had invested few resources.

This model-based finding is corroborated by the findings of a statistical examination. Within Africa, areas with higher malaria burden, as evidenced by the prevalence of the sickle cell trait, do not show lower levels of economic development or population density in the colonial era data examined in this study.

 

To contact the authors:  David Weil, david_weil@brown.edu

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