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April 03, 2009

How maize moves malaria in Ethiopia

by Vicky Waltz

From 1973 to 1975, James McCann worked as a Peace Corps volunteer in Bure, a small farming village in the highlands of northwestern Ethiopia. At the time, malaria was ravaging much of the country, killing thousands, but McCann didn’t bother to take the antimalarial drug chloroquine. Bure was more than three miles above sea level, and villagers believed its chilly air and high altitude kept mosquitoes at bay.

But when McCann, now a College of Arts & Sciences professor of history, returned to Bure 20 years later, he found a village devastated by malaria. “Churches and schools were abandoned,” he says. “People locked the doors of houses and said, ‘They’re all dead in there.’”

The Bure district’s 1998 malaria outbreak infected nearly 42,000 people, almost 40 percent of the population, and killed 700. Shocked and deeply troubled by the tragedy, McCann set out to determine why an area with no history of malaria had suddenly become a hotspot for the disease.

He already had a suspicion. While researching his 2005 book, Maize and Grace: Africa’s Encounter with a New Crop, McCann learned from a colleague at the Harvard School of Public Health that laboratory results had indicated a possible connection between increased levels of malaria and maize cultivation. He wondered if the crop that was meant to sustain Ethiopia’s people could also be helping to kill them.

During the summer of 2003, McCann met with epidemiologist Asnakew Kebede (GRS’08,’13), an Ethiopian Ministry of Health and World Health Organization employee, and the two traveled to 21 rural villages throughout the Bure district where cultivation had shifted to maize from more traditional crops, such as teff, barley, and sorghum. They found that the rate of malaria transmission among maize farmers was 10 times higher than among those who farmed other crops.

Laboratory results confirmed the explanation for the difference: the powdery yellow substance that contains the plant’s reproductive cells. A maize field produces large amounts of pollen — about 300 pounds for every 2.5 acres. And mosquito larvae thrive on it. “Maize pollen is an accelerant for mosquito populations,” McCann says. “They just explode. You may as well be putting kerosene on a fire.”

Kebede learned that mosquitoes that feed on maize as larvae tend to have a longer lifespan and are more likely to transmit malaria simply because they bite more people more times. “It’s unlikely that you’ll contract malaria after one or two bites,” Kebede says. “But these mosquitoes are living long enough to bite multiple times.”

McCann knew that maize had been grown for centuries, primarily as a garden vegetable. It wasn’t until the 1980s, when Ethiopia’s government pressed for broader cultivation, that it replaced more traditional grains as the country’s major crop. Between 1993 and 1998, maize cultivation increased by 79 percent nationwide, he says, and the incidence of malaria rose with it. “As maize fields grew closer and closer to homesteads,” he says, “more and more farmers became infected.”

It was also around this time that the Ethiopian government introduced a new variety of maize, which sheds its pollen in late August and early September, weeks later than other varieties and at the precise time when the temperature and moisture are ideal for mosquito breeding. By appearing later in the rainy season, McCann and Kebede explain, the pollen is less likely to be washed away at the time when mosquito larvae have stable breeding sites.

The pollen produced healthier and more mobile mosquitoes, whose evolution coincided with a slight warming of the climate, leading to what McCann describes as “a perfect storm,” an ideal habitat for mosquitoes to breed and feed. Today, he says, while farmers and the Ethiopian government are aware of the connection between maize and malaria, they are reluctant to abandon high yields of maize for lesser yields of traditional crops.

One solution would have farmers grow a genetically modified variety of maize, whose pollen contains an insecticide that would kill mosquito larvae. But the cost of that is beyond the means of local farmers, according to McCann. Another possible solution involves “detasseling” — removing the pollen-producing tassel from a maize plant by hand to prevent pollination, but that is time-consuming.

A third option, he says, is encouraging farmers to grow a maize variety that sheds pollen earlier in the season, prior to peak mosquito development. Farmers could also plant fields at least 65 yards from their homesteads, he adds, to create a buffer zone between the breeding grounds and houses.

But the most logical solution, says McCann, is to encourage farmers to grow local varieties of self-pollinating grains, such as teff, or vegetables such as red pepper. “We are not suggesting that farmers stop growing maize,” he says. “We are simply suggesting that they grow it away from breeding grounds.”

McCann and Kebede hope to apply their Ethiopian research to other countries where both maize and malaria are prevalent. “Malaria is spreading to new areas every day,” McCann says. “Within the next decade, it will be the number-one crop in the world. And while maize doesn’t cause malaria, it most certainly accelerates it.”

“If we can do anything to help stop it,” Kebede says, “we will.”

This article appears in the Spring 2009 issue of Bostonia.

Boston University

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