A study has found that conditions conducive to the spread of malaria are created when projects that cause land-use change and labour migration are kicked off.
Credit: sarangib/pixabay
When India was carving out her development trajectory post independence, dams were an important part of the narrative. Our first prime minister, Jawaharlal Nehru, had hailed them as the ‘temples of modern India’. In a country that faced drought and floods simultaneously, whose eastern and western borders straddled rainforests and desert, massive structures that could trap ephemeral water must have seemed magical. Dams and canals together even transported water over hundreds of kilometres to transform parched soil into verdant cropland.
The Indira Gandhi canal was one such enterprise. Conceptualised soon after independence, the project finally took off in the 1970s. Starting from the confluence of the Himalayan rivers Satlej and Beas in Punjab, the 650-km-long canal carries water to the Thar Desert in Rajasthan, and so from desolate sand dunes and thorny bushes, the desert landscape was transformed to alluvial soils and irrigation-intensive crops, dotted with water bodies and human settlements.
Along with the water, and even before the promised prosperity, came an unwelcome visitor: mosquitoes. The aridity the canal had sought to rid had in fact been the state’s shield against mosquitoes and the diseases they brought. The unwary settlements near the canal began to be racked year after year by epidemics of malaria and the deadlier cerebral malaria.
This is an example of ‘frontier malaria’, a change in malaria risk that follows changes in land use patterns in previously underdeveloped ‘frontier’ areas undergoing large-scale change in how they’re used. In a recently published paper, Andres Baeza, from the University of Maryland, and his colleagues investigated factors that triggered frontier malaria using models based on real data from India and the Brazilian Amazon. And they found that the land alone wasn’t responsible for the disease outbreaks even if it played an important role.
Modelling malaria in frontier regions
Malaria is a disease of humans caused by the Plasmodium parasite. Of its three main species in India – P. vivax, P. falciparum and P. malariae – P. vivax is responsible for than 60% of the country’s malaria incidence. This more common form starts off with flu-like symptoms and then develops like classic malaria: cycling between chills/shivering and then fever/sweating. P. malariae causes a relatively benign form of malaria; P. falciparum causes the most severe form, called cerebral malaria, that could result in neurological complications, coma and even death.
The malaria-causing Plasmodium parasite spends part of its life inside mosquitoes of the genus Anopheles and the rest of its life in humans. When a mosquito bite infects a person with Plasmodium, the insect is a vector: for having hosted the parasite and then injecting it into humans. India has many species of Anopheles. According to Aditya Dash and his colleagues at the National Institute of Malaria Research: “An. culicifacies is widely distributed and is the principal vector of rural malaria, An. stephensi is the primary urban vector, An. fluviatilis is a vector in the hills and foothills. An. minimus, An. nivipes, An. philippinensis, and An. dirus are vectors in the northeast and An. sundaicus is restricted to [the] Andaman and Car Nicobar islands. An. annularis and An. varuna are secondary vectors with wide distribution.”
So, the bewildering variety of parasites and mosquitoes makes the Indian situation quite complicated. And whatever the parasite-mosquito combination may be, frontier regions provide ideal conditions for malaria to spread. In an attempt to capture the complexity, Baeza and others incorporated three sub-models into their main model: epidemiology, or the state of and contacts between human and mosquito populations; demography – movement of people and changes in land use; and economics – the capital accumulated by the people.
“We started the model assuming a landscape without intervention, and we simulated the three sub-models together, assuming a transformation of the land from undeveloped to a more developed state,” Baeza explained.
It starts with a change in land use patterns in an area – like deforestation or irrigation – that triggers a sudden ecological change. If the temperature is warm enough and if there is abundant water, mosquitoes multiply. Labourers move into these areas seeking employment, such as in agriculture and construction. Because they’re migrating, many of them are not immune to the local malarial parasites. Moreover, their first settlements typically have poor living conditions that encourage mosquito-human contact. Add to this mix lack of access to good or timely healthcare and you have just the ingredients for a malaria outbreak. The researchers call this the transient period. It is succeeded by the decay phase, when the labourers’ living conditions improve and malaria cases drop.
How long the transient period lasts depends on how many mosquitoes can breed in the transformed land, how fast healthcare is provided and the delay before which the decay phase kicks in – among other factors.
In Rajasthan, for example, malaria epidemics paralleled the construction of the Indira Gandhi canal. Between the the first segment of the canal going operational in 1975 and 2002, there were thirteen epidemics, “particularly in the canalised irrigation areas under the Indira Gandhi Nahar Pariyojana,” as B.K. Tyagi, the Centre for Research in Medical Entomology, Madurai, wrote.
Heralding the decay phase
According to A.C. Dhariwal, director of the National Vector-Borne Disease Control Programme, “Irrigation, development projects and malaria is a known and recognised malaria paradigm.” When the Bargi dam was built across the river Narmada, more than 150 villages were submerged while the surrounding areas reported four-fold hike in the incidence of malaria.
Historical records show that, in the pre-Independence period, the eastern and western parts of erstwhile British Punjab (now in India and Pakistan respectively) had been centres of devastating malaria outbreaks – as well as the sites of major irrigation projects. Today, they are prosperous food-producing areas with low rates of malaria. Evidently, they went from the initial phase of ecological change and malaria onset, to the transition phase where were regular malaria outbreaks, to a stage where the malaria decayed away. If it sounds rosy in hindsight, it would be pertinent to remember that the transient phase lasted for more than half a century.
According to Baeza, “The decay in transmission occurs when economic conditions allows for people to accumulate capital and invest in malaria, in the short term by investing on medication, and in the long term by investing on infrastructure – housing education, etc.”
In other parts of the country, the decay phase is yet to show up – largely because of our reliance on the pesticide called DDT to get rid of mosquitoes. Some mosquitoes, including An. culicifacies, the major rural vector of malaria, evolved to resist the pesticide. When malathion was introduced in 1970, the same vector became resistant to it in a rapid three years. And just like that, India’s post-Independence malaria-free honeymoon was over.
An additional cause for concern today is that many populations of Plasmodium falciparum, which causes the more lethal cerebral malaria, are resistant to chloroquine, a frontline antimalarial drug. In neighbouring Myanmar, there are reports of malaria becoming resistant to artemisinin, the final frontier in malaria treatment.
Across the world’s tropics, there is mounting evidence linking development projects with malaria. A study published in 2005 estimated that about 800 million people worldwide were at risk of contracting malaria because they lived close to irrigation infrastructure.
What Baeza’s model provides is a framework to plan strategies to combat malaria when development projects are planned. During early stages, when settlements are still poor and not well-equipped, it is critical to provide immediate healthcare and curb the spread of malaria by tackling mosquito breeding and biting. Over the longer term, economic development and revamping of existing infrastructure can push malaria over the transition phase into decay.
Sandhya Sekar is a freelance science writer interested in evolution, ecology and wildlife.
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