Rice is a staple crop in sub-Saharan Africa. But it relies on flooded paddies, which provide breeding sites for mosquitoes. Photograph: Corbis/ Gideon Mendel
Raising agricultural productivity is a priority in much of sub-SaharanAfrica, where some 70% of people live in rural areas and rely on farming. But 90% of the estimated 660,000 global deaths caused by malaria in 2010 were also in Africa, and agricultural development can play a role in its transmission. How should this be reconciled to create a more "malaria smart" approach to agricultural growth?
But irrigation can also be conducive to mosquitoes, which transmit malaria. Rice, for example, is a staple crop in sub-Saharan Africa and important for food security. It also relies on flooded paddies, which provide breeding sites for mosquitoes such as Anopheles gambiae, one of the principal vectors of malaria.
That can be problematic, but the irrigation-malaria link is complex and varied. For example, research has shown that even where the introduction of irrigation systems increases mosquito density in a given area, malaria transmission can actually decline.
In other words, more mosquitoes may not automatically mean more malaria. This is known as the 'paddies paradox'.
"In my view there are two possible explanations for this," says Martin Birley, a former lecturer at the Liverpool School of Tropical Medicine and an expert in health impact assessments.
"Either the irrigation scheme attracts mosquitoes that are less able to transmit malaria, or the people who live on the irrigation schemes are better able to avoid getting malaria, because their income is growing. Poor people, when provided with additional wealth, tend to have an improvement in their health, and that applies everywhere."
But the effect isn't universal: studies of the 'paddies paradox' distinguish between areas where malaria is 'stable', or sustained, and areas where it is 'unstable', or episodic. The paradox is seen in the stable areas, where there is a higher level of immunity in the population.
In areas of unstable transmission, immunity tends to be low or non-existent, and malaria is a threat to adults as well as children, which in turn threatens adult worker productivity. The introduction of irrigation systems in such areas – and the associated increase in mosquitoes – may be much more serious. Well designed irrigation systems can help control this.
"The important thing is to reduce surface pooling in the irrigation schemes, so there's not too much water in the fields," says Steve Lindsay, a disease ecologist at the University of Durham and malaria expert.
"Make sure the irrigation channels do not leak and fill in areas which puddle. In Khartoum there is local legislation that requires the farmers to have dry fields one day per week, and that has helped reduce malaria. Ultimately malaria is a problem of surface water. Remove the breeding sites, you remove the problem."
Where populations are most vulnerable, irrigation projects need to take this on board and build malaria awareness into the scheme from the start. But too often, according to Kwadwo Asenso-Okyere, former director of the eastern and southern Africa regional office of theInternational Food Policy Research Institute and now a consultant to its Ghana programme, rural populations themselves lack understanding of malaria transmission and policymakers also fail to act.
"Most irrigation projects do not incorporate such externalities," he says. "They need to internalise this externality and incorporate malaria control – education, provision of bednets, screening of doors and windows, aerial spraying of insecticide, early diagnosis and treatment – as a component of the project."
Irrigation is not the only potential 'bridge' between agriculture and malaria. Mosquitoes are known to feed on maize pollen, and a 2005 study reported an association between intensified maize cultivation and higher malaria transmission in an area of Ethiopia, where a high-yielding variety introduced in the mid-90s replaced more traditional crops such as teff, barley and sorghum.
Previously a marginal garden crop, maize became intensely cultivated close to homesteads, with correspondingly high levels of pollen. The new variety also released its pollen a few weeks later in the season, coinciding perfectly with mosquito breeding times.
This doesn't mean maize causes malaria, but it shows how certain agroecological changes can accelerate it in perhaps unforeseen ways. According to James McCann, professor of history at Boston University and a specialist in the agricultural and ecological history of Africa, who co-authored the research in Ethiopia, this association is now recognised – even if policies haven't responded.
"The director of a key agency's crop programme – the Ethiopian Institute of Agricultural Research – came to a workshop on maize and malaria and stated that he was convinced of the link, but then honestly advised us that his primary mandate from his bosses was food production and that the government was committed to maize as the engine of that growth," says McCann. "Health implications were not a consideration."
Greater policy alignment in achieving health and agricultural objectives is therefore the challenge. One approach is for health impact assessments to be built into irrigation projects, just like environmental impact assessments (EIA) often are. This happens, but isn't yet standard practice, particularly for smaller schemes.
"The bigger ones have it," says Birley. "So where an EIA would look at the impact of an irrigation project on many aspects of the environment, the health impact assessment looks at unintended health consequences. But most national governments don't have regulations for health impact assessments."
In the example of maize, fairly simple changes such as growing other crops near mosquito breeding grounds and maize further away is one way of addressing the problem. Maize can also be de-tasselled by hand to remove the pollen-producing tassels, or maize breeders could potentially select for different pollen shedding patterns in their breeding strategies.
But for such 'malaria-smart' policy responses to happen, there needs to be broader awareness of how agricultural productivity and other objectives overlap, says McCann. This is where advocacy by development actors could make a difference.
"Malaria-smart investment would need to build NGO awareness and a broader view within the major agriculture efforts from Gates and Rockefeller that their efforts to increase yields and returns also have important ecological implications around health, crop disease resistance, and biodiversity," he says.
"The food/seed production system has continued to emphasise yield as the overwhelming priority. Changing that policy would be like turning the Queen Mary. These things move very slowly. The mosquitoes and the parasites respond much more quickly than human bureaucracies.
After service in the British SAS Regiment the author became a physician and then an orthopaedic surgeon.
He has held professorial positions in Canada, Vietnam and the United States, practiced and taught orthopaedic surgery in three continents and in several wars.
He has extensive experience as an expert witness in court. Somewhere along the way, time was found to operate a four hundred acre mixed farm, a one hundred seat restaurant and to obtain a licence as a flying instructor.
The author's books are available from bookstores, the publishers, or from on-line bookstores such as Amazon, Barnes and Noble, and Indigo/Chapters.