How a Pesky Parasite Impacts Africa

Lee Sanders, MD, and Marcella Alsan, MD, PhD, MPH

Stanford Assistant Professor of Medicine Marcella Alsan had always wondered why the mineral-rich African continent—with so many natural resources, diverse climates, and arable land—remains so poor.

She launched into extensive research while working on her PhD in economics and has now come up with an intriguing theory: A pesky parasite prevented many precolonial Africans from adopting progressive agricultural methods, a phenomenon that still impacts parts of the continent today.

The tsetse fly has plagued Africa for centuries—having sent millions of people into the confusing stupor of sleeping sickness, while killing the cows and other livestock needed to plow their fields and feed their families.

Alsan writes in a paper published in The American Economic Review that the tsetse fly, which today is found only in Africa, drove precolonial Africans to use slaves instead of domesticated animals for agriculture. This limited their crop yields and the ability to transport goods.

“Communicable disease has often been explored as a cause of Africa’s underdevelopment,” writes Alsan, who is the only infectious-disease trained economist in the United States and a core faculty member of the Center for Health Policy/Center for Primary Care and Outcomes Research.

“Although the literature has investigated the role of human pathogens on economic performance, it is largely silent on the impact of veterinary disease,” she notes. “This is peculiar, given the role that livestock played in agriculture and as a form of transport throughout history.”

The economic impact caused by the parasite of the trypanosome vector is estimated to be as much as $4 billion a year. The Food and Agricultural Organization estimates 37 African countries are affected by the tsetse fly and that its trypanosomosis kills around 3 million livestock per year.

The World Health Organization reports that the sleeping sickness delivered by the tsetse bite in humans is hard to diagnose and treat. Some 60 million people were once at risk with an estimated 300,000 new cases each year.

Sleeping sickness causes headaches, fatigue and weight loss; confusion and personality disorders occur as the illness progresses. If left untreated, people typically die after several years of infection.

Fortunately, sustained control efforts have reduced the number of new cases, dropping below 10,000 annual cases for the first time in 50 years in 2009. This is in part due to an eradication effort using radiation sterilization techniques adopted by the International Atomic Energy Agency.

But the lingering economic impact from the tsetse has been monumental.

For her research, Alsan used geospatial-mapping software to mine data gathered by missionaries and anthropologists in the 1800s. She found that farming methods used in other developing regions of the world—such as the agricultural revolution in England—were not widely adopted in Africa.

“They pulled plows and carried carts. Their manure was used for fertilizer,” Alsan said. “They helped transport people and goods across land.”

She found that ethnic groups inhabiting tsetse-prone African regions were less likely to use domesticated animals to plow their fields, turning instead to the slash-and-burn technique still used in many parts of the continent today.

“These correlations are not found in the tropics outside of Africa, where the fly does not exist,” she writes. “The evidence suggests current economic performance is affected by the tsetse through the channel of precolonial political centralization.”

The FAO estimates that the tsetse fly infects nearly 10 million square kilometers in sub-Saharan Africa. Much of this large area is fertile but left uncultivated, a so-called green desert not used by humans and cattle. Most of the tsetse-infected countries are poor, debt-ridden, and underdeveloped.

And this is what triggered Alsan’s interest in the tsetse fly: How its deadly bite has altered the socioeconomic impact of a continent.

“It’s incredibly important to shine light on issues that are Africa-specific and therefore may not garner as much attention as those economic and medical issues that affect wealthier regions of the world,” she said.

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