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Can irrigation be hazardous to your health?

Mostly, the Japanese Encephalitis virus does nothing at all. At worst, it might cause a headache. But, in less than one percent of cases, the patient's condition deteriorates. The headache persists, joined by fever and vomiting. The victims begin to suffer weakness, cognitive impairment or other symptoms as their brains become inflamed – the encephalitis the disease is named for. This can lead to seizures, which in turn may lead to a coma. 20-40% of those who go this route survive with brain damage. About a quarter die.

Despite the long odds, Japanese Encephalitis kills more than 15,000 people a year. The disease's range provides a clue as to how the virus afflicts enough people to reach this figure: it is found in about 20 Asian countries (though no longer common in Japan, where it was first recognized). These countries are covered by a total of about 145 million hectares of flooded rice paddies – standing water that hosts billions of potentially virus-carrying mosquitoes.

In a world trying to expand food production without much land to expand into, irrigation is viewed as a source of hope. The UN's Food and Agriculture Organization's most recent yearbook says there is "considerable scope" for increasing irrigation and cites it as a basic method for increasing crop yields. But farmland is just land rearranged for human convenience – and what is convenient for us tends to be convenient for our diseases. Irrigation, and the convenient breeding ground it offers to parasites, is a dramatic example of the dilemma this poses: on balance, does irrigation help in the fight against illness, or hurt?

Japanese Encephalitis is far from the only parasite that raises this question. Schistosomiasis, caused by flatworms and carried by a variety of freshwater snails, is one of the world's most important parasitic illnesses in terms of public health and economic impact, second only to malaria. The flukes cause a chronic infection that can lead to anaemia, liver damage or even paralysis, depending on where the flatworm's eggs end up. Since dam-building increased in the mid-20th century, they have undergone a global renaissance. The construction of Egypt's Aswan High Dam in 1965, for instance, led to a nearly fourfold increase in flatworm infections in the Nile delta.

Malaria, too, has been linked to water projects, with so-called "irrigation malaria" epidemics cropping up after dam-building efforts in Ethiopia and India in the 1990s. But the relationship here is more complex. A number of studies in sub-Saharan Africa have found that some villages near irrigation projects had lower levels of malaria than non-irrigated towns.

This is the irrigation catch 22: at least part of the explanation for this paradox, researchers concluded, was the irrigated villages' increased agricultural productivity. This was soon followed by residents having access to better healthcare, better nutrition, and a generally higher standard of living than their dryland neighbours.

Irrigation holds another critical health benefit. By improving crop yields, water projects may prevent farmland from spreading into wild areas, and humans from catching new diseases found there. 70% of the new pathogens that have emerged over the past three or four decades "are not, historically, 'human diseases,'" according to global health expert Anthony McMichael, Professor Emeritus of Population Health at the Australian National University College of Medicine. Instead, they come from wild animals, and their jump to humans "reflects human encroachment on the natural environment," he says. “Intensive, versus extensive, agriculture will be part of the solution."

It has often been a struggle to incorporate health issues into global environmental change programmes, although projects on water and food security have seen notable success. To date, mapping some of the subtleties and complexities has been a primary focus of the Global Environmental Change in Human Health (GECHH) project, headed by Ulisses Confalonieri and Mark Rosenberg (and formerly co-chaired by McMichael). Devoted to training and research at the intersections of ecology, economics and medicine, the GECHH research agenda specifically mentions dams and irrigation as among its "main health-impact issues."

Now, Future Earth leaders are beginning the discussion on how to bring projects like GECHH under one roof around a common research agenda.

Future Earth chair Mark Stafford Smith, science director of Australia’s CSIRO Flagship Climate Adaptation programme says  if we want to have it both ways – irrigation's benefits without its risks – careful management on a global scale is key. “A growing population coupled with growing interconnectedness means that these health issues will arise ever more frequently. Future Earth will provide the platform to bring together the right groups – specialists in health, agriculture and water, along with policymakers and engineers – to deliver the global innovation system to keep up with these growing rates of change.”

One priority must be creating a system for effective knowledge transfer, because there are existing projects that have successfully combatted such threats in the past.  Public works projects like the 1933 Tennessee Valley Authority – the network of dams that provided electricity and flood control to much of the American southeast – managed to prevent mosquitoes from breeding in shoreline plants by regularly altering reservoir water levels. Many newer irrigation projects keep parasite vectors from gaining a foothold with inhospitable concrete-lined channels and variable flow rates. Some rice farmers are abandoning a millennia-old flood irrigation strategy and leaving their fields dry for portions of the growing season. This "alternate wetting and drying," or AWD, promises to significantly reduce mosquito larva populations while potentially increasing crop yields.

These are not easy fixes. AWD, for instance, is popular in China, but has little traction in countries like Bangladesh where unreliable electricity, unpredictable water pumping schedules and fixed-rate irrigation payment schemes make it difficult for farmers to control their own irrigation. Even the best plans are meaningless until they are put into action.

All these findings provide hope that the irrigation-disease link can be, if not severed, at least held in check. Stafford Smith says, “They highlight how solutions in some sectors – agriculture and health – may depend on synergies with others – such as reliable energy supplies for all in this case.  Future Earth aims to provide the integrative thinking that can assist in creating such cross-sectoral synergies to the benefit of all.”