Malaria Transmission in Africa Worsened by Small Dams

Judy Stone, MD

August 05, 2021

A recent study published in Nature's Scientific Reports suggests that the size of dams in Africa is an important and overlooked factor in malaria transmission.

The report's major finding was that between 0.9 million and 1.7 million malaria cases each year were attributable to dams, and more than 75% of these were due to small dams. The incidence of malaria per kilometer of shoreline was 2-7 times higher for small dams than larger ones. Unsurprisingly, those living closer to the reservoir (a breeding habitat for Anopheles mosquito species, which transmit malaria) had a higher incidence of malaria.

The researchers examined four major river basins in Sub-Saharan Africa. They identified water reservoirs and determined whether small dams (< 15 m high or roughly < 2.5 acres or < 3 million m3) or large dams held them back. They then mapped the distribution of Anopheles mosquitos around the reservoirs to look for differences. Finally, they compared the malaria incidence in the different regions using dam sizes as the reference. Malaria cases were compared for 2000, 2005, 2010, and 2015.

Researchers tallied 258 large dams and 4907 small dams. A population of 14.7 million people lived within 5 km of the reservoirs.

Dr Solomon Kibret

Over the past 20 years, numerous dams have been built to improve water storage, irrigation, and hydroelectric power as a part of economic development. Investigator Solomon Kibret, PhD, of the University of California, Irvine, told Medscape Medical News that at least 160 large dams are in progress and "there has been no time where such large-scale large-dam building occurred at one time." He noted that "Small dams are being utilized efficiently by communities; large dams, you need experts." Further, "Maintaining large dams for a number of years is difficult" (and requires more expertise), but "small dams...can be managed by communities" and there is a greater likelihood that everyone in the community might have access to the benefits of irrigation.

There are numerous small details that dam planners — who are focused on hydrology and engineering — tend to overlook. For example, small dams tend to have a gentle or shallow slope that promotes water pooling. This is further exacerbated as farmers bring their cattle to the water, compacting the soil. Kibret further explained, "And then there are so many hoof prints, and those were the main breeding sites for mosquitoes."

Kibret concluded, "Our goal was for large dam financiers, such as the global bank, to consider malaria while they build such magnificent dams...or large-scale irrigation schemes. There should be discussion between health authorities and water authorities...Those things may have adverse public-health impacts. So, the public-health person should be on board while designing, implementing, and running these projects.

Edward "Ned" Walker, PhD, professor of entomology and microbiology at Michigan State University, East Lansing, who was not involved with the study, told Medscape Medical News that, although the study findings were reasonable, it left out important context. "The larger context is the developmental issue and the problem of meeting food security needs. And the sacrifice there is that you increase malaria. And from my perspective, the intersectoral conversation is the one that really needs to take place."

He added, "Irrigation steepens the age-specific infection rate, so that proportionately more in the under fives are infected. That is not good because the under fives are the most vulnerable to malaria for morbidity and mortality." There is a conflict between "the need to develop to grow out of poverty and deprivation, but that the development required involves manipulation of water in such a way that worsens malaria (and probably other problems too). This brings up the conceptual problem: Does malaria make poverty, or does poverty make malaria?...To control malaria, we need to reduce poverty. One can argue that is really how malaria 'went away.'"

Hiral Shah, PhD, a senior policy analyst at the Center for Global Development in London, UK, told Medscape Medical News that Kibret's article was good and clearly showed the risk of small dams. We're at this "key sort of balance now where you need to improve conditions for those people living there, and [the] cost is increased malaria." He added, "Agriculture [and] dams for irrigation, all of these do increase the risk of disease, whether it be malaria or other neglected diseases." Mitigation efforts can include varying the water level, providing bed nets, spraying insecticides, and removing vegetation at the shore.

All three researchers agree that, moving forward, it is essential to have a better multidisciplinary approach to development and to include public health practitioners in policymaking. Shah concluded, "Every decision has a cost and benefit, and at the end of the day, people are obviously taking decisions on what sort of dams to make, but human health is not being considered."

Kibret, Walker, and Shah have reported no relevant financial relationships.

Sci Rep 2021;11:13355. Full text

Judy Stone, MD, is an infectious disease specialist and author of Resilience: One Family's Story of Hope and Triumph Over Evil and of Conducting Clinical Research, the essential guide to the topic. You can find her at drjudystone.com or on Twitter @drjudystone.

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