With insight from Kallista Chan’s research, Charles explores new findings in research between the relationship of malaria transmission and rice cultivation in sub-Saharan Africa.

The ‘paddies paradox’ is the apparent lower risk of malaria infection experienced by communities residing close to irrigated rice fields compared to surrounding areas, despite an increase in malaria mosquito vectors. This theory postulates that irrigation schemes in Africa do not increase malaria risk, except in areas of unstable transmission. However, our understanding of malaria transmission in sub-Saharan African farming communities has recently shifted. New evidence published in The Lancet Planetary Health suggests that diagnosed cases are twice as prevalent in communities residing close to irrigated rice paddies. 

The relationship between agriculture and human disease dates back millennia, with vector-borne and zoonotic infections contributing 17% and 61% of infectious diseases known. Our species’ close relationship with agriculture has undoubtedly exacerbated our disease risk to many infectious agents. However, the vector-borne malaria parasite, which first infected humans some 50,000 years ago, had until recently been presumed low risk for communities living close to rice farms. 

Malaria is caused by the intracellular parasite Plasmodium, most notably of the species falciparum, vivax, and malariae. Annually, malaria kills more than half a million people, with the highest levels in a decade—more than 627,000 deaths—in the year 2020. These deaths rarely make the news as they form part of the long-standing disease prevalence. The infection typically consists of cyclic fevers (shaking chills), headaches, extreme tiredness, and in severe cases, death. These parasites are transmitted via the infectious bites of female Anopheles mosquitoes. 

A major malaria transmitting group of mosquitos inhabiting rice paddies, Anopheles gambiae sensu lato (s.l.), is highly effective at spreading the parasite and is abundant throughout sub-Saharan Africa. These vectors require pools of fresh sunlit water (2–10cm deep) that is still or very slow flowing, relatively clean containing silt or clay, and without suspended organic matter for the successful development of larvae. Most natural habitats for mosquito breeding are shallow pools made from animal footprints, tree hollows, and in a growing number of instances, receptacles found surrounding housing. However, one ecological niche previously  understood to be unassociated with increased malaria risk was irrigated rice paddies. This notion has since been challenged by new research that questions the validity of a twenty-year-old paradox into the connection between malaria and rice cultivation.

With the population of sub-Saharan Africa predicted to triple by 2100, the demand for rice is increasing, causing significant pressure to farmers across irrigated regions of sub-Saharan Africa. Additionally, this region accounts for 95% of the global 225 million malaria cases seen in 2020, leading to a high demand to investigate the connection between this ancient disease and the expanding rice fields.

Where rice cultivation occurs, vast numbers of malaria transmitting mosquito breeds. The question facing agronomists, entomologists, and public health experts is how can we maintain high rice yields without inadvertently creating vast breeding grounds for malaria transmitting mosquitoes?

Research teams based at the Africa Rice Centre, International Institute of Tropical Agriculture (IITA), and the London School of Hygiene and Tropical Medicine (LSHTM), attempted to determine whether the twenty-year-old dogma coined the ‘paddies paradox’ still holds. This paradox, first described in Ijumba and Lindsay’s 2001 publication implied that where malaria transmission is stable (holoendemic), the introduction of rice cultivation had negligible or even reduced impact on overall malaria cases. To support the redefinition of the ‘paddies paradox’ the groups looked at malaria rates in farming communities across sub-Saharan Africa following the introduction of irrigated rice cultivation. 

Kallista Chan, the study’s lead author and a doctoral candidate at LSHTM, spoke to Keppel Health Review (KHR) to explain the importance of re-investigating the ‘paddies paradox’ phenomena.

“We thought that this relationship needed re-investigation because the previous conclusions were thinking about the wrong counterfactual—that the mosquitoes produced at rice fields were harmless, when in fact they are harmful. In some places, rice fields are a major source of the most important malaria vectors in that region, for example, An. gambiae in sub-Saharan Africa and An. sinensis in central China.

Moreover, since malaria intervention coverage [antimalarial drugs, insecticide-treated bed nets, vaccines] has increased equitably across sub-Saharan Africa, we cannot keep assuming those in non-rice growing communities are less protected against malaria.”

The study compared malaria risk between rice-growing and non-rice-growing areas from 14 countries in sub-Saharan Africa including: Burkina Faso, Burundi, Cameroon, Côte d'Ivoire, Ghana, Kenya, Madagascar, Mali, Nigeria, Rwanda, Senegal, Sierra Leone, Tanzania, and The Gambia. 

The teams discovered that in the years following 2003, not only was the density of the mosquito vectors in irrigated rice communities 8-fold higher than those in non-irrigated communities (measured by bite per person per night), but that malaria prevalence was also 1.73-fold greater in said communities. Moreover, in 68% of quantitative analyses conducted, the entomological inoculation rate of An. gambiae s.l. (estimated as the number of infectious bites per person per year) was higher in irrigated rice growing communities.

In part, the ‘paddies paradox’ stood unchallenged for more than twenty years because it proved beneficial for agronomists and economists who were responsible for reporting annual rice yields and subsequent economic returns, but also because malaria across sub-Saharan Africa was at saturation conditions. Thus, quantifying the direct impact on increased malaria cases attributed to rice growing was near impossible.

It is important to recognise that since the 2000’s, when extensive vector control campaigns were launched, malaria mortalities fell by an astonishing 44.43% from 736,000 in 2000, to 409,000 by 2019. The huge efforts to reduce malaria vectors over the last 22 years allowed researchers to once again pinpoint rice cultivation as an emerging factor directly responsible for increased malaria cases.

The impact of this research is connected to our growing requirement as a species to produce enough food to survive and thrive. Rice is a staple food cereal crop estimated to provide 19% of our daily energy supply and sustains 3.5 billion people around the world. World rice production is set to reach 520 million tonnes in 2022—a 0.7% increase from previous years and new record yield. Despite renewed pressure from agronomists and economists across sub-Saharan Africa to help achieve these enormous international yields, only 22% of the rice growing regions are irrigated. The concentration of irrigated land represents a considerable habitat for the breeding Anopheles malaria vectors. 

Chan adds: “There are many lessons to be learned and can potentially be applied to African rice-growing communities, especially solutions which provide agriculture and health co-benefits.” 

The World Health Organization (WHO) recently marked the annual World Malaria Day (25 April 2022) with this year’s theme to “Harness innovation to reduce the malaria disease burden and save lives”. This broad theme can be interpreted in numerous ways. One interpretation that is applicable to the results of this study is agricultural and health co-benefit approaches. If bolstered with significant investment, these approaches could provide the urgent win-win solutions required so desperately by agronomists and public health experts to ensure high rice yields without growing malaria vectors. They include C4 rice cultivation (lower irrigation requirements), alternate wetting and drying irrigation, and encouraging cross-cutting strategies between the agricultural sector and public health spheres.

A concern argued by Ministries of Health, public health experts, and governments focuses on the continued and growing economic cost attributed to treating malaria. These concerns are justified considering the high economic burden attributed to protecting one person for one year against malaria, amounting to $5.70 for vector controls (long-lasting insecticide treated bed nets), $5.97 for chemoprevention, and per-case treatments as high as $89.93.

Chan explains: “There is an urgent need for win-win solutions that can simultaneously control vectors (reducing disease burden) whilst boosting agricultural productivity (enhancing farmer livelihood). There needs to be further studies on the links between rice growing and malaria—especially longitudinally (before-after starting irrigation schemes or rice cultivation) but also spatially. It would be particularly interesting to find out how distance away from rice fields (which can be measured in many ways) affects malaria risk.” 

Malaria is a manageable, preventable, and treatable infectious disease. The redefinition of the ‘paddies paradox’ is aptly timed with the WHO calling for innovative approaches to reduce the malaria burden and save lives. This innovation should come in the form of uniting agronomists, economists, governments, and public health experts to generate further evidence to establish the relationship between rice growing and malaria. Malaria can only be confined to history books if collaboration across a range of expertise is employed.