Here's Why Dams With Low Slopes Increase Risk of Malaria

One way that African countries are trying to address is by building dams, a report in Conversation states. Notably, number of large and small dams are currently under construction, particularly in sub-Saharan Africa.

However, the report emphasises that while dams could solve problems, it may also bring issues like malaria. Water is cruicial to the spread of the mosquito-borne disease because mosquitoes breed and lay their eggs in or near bodies of water. The insects prefer breeding in shallow pools for these purposes, and dams provide shorelines that offer just this: many shallow pools.

According to a study conducted by Solomon Kibret, a researcher from the University of California, Irvine, the number of people living close to (less than 5 km away) large dam reservoirs in sub-Saharan Africa increased from around 14 million to about 19 million between 2000 and 2015.

The study further finds that nearly 75 percent of the population stays in areas where there is malaria. During the same 15 year period, the number of large dams increased from 884 to 919 in Africa.

According to the University of California study, at least one million malaria cases can be attributed to large dams in Africa each year. But the relationship between dams and malaria is not entirely straightforward.

However, according to Kibret, dams don’t always increase malaria cases. When they do, the degree of increase is not uniform.

Using topographic data, study authors unpacked the relationship between large dams and their characteristics, like altitude and rainfall, linked to malaria in sub-Saharan Africa.

The study used satellite data to accurately measure elevation, slope and surface areas of African reservoirs in four time periods – 2000, 2005, 2010 and 2015.

They concluded that a number of factors that increases the risk of malaria in areas close to larger dams.

These included:

Altitude: While malaria used to be a lowland disease, evidence showed that climate change helped malaria encroach to highlands where the disease was uncommon.

Rainfall: Causes rain pools and puddles which are favourable habitats for mosquito breeding.

Temperature: This determines the rate of a mosquito’s growth in the water and the development of the malaria parasite inside adult mosquitoes.

Finally, Slope: According to study authors, this was the most important predictor of the level of malaria around dam. Analysis showed that in regions which are otherwise suitable for mosquito breeding, the slope of the reservoir shore played the largest critical role in determining the occurrence of malaria. Reservoirs with higher slope at shoreline had lower malaria impacts, while conversely, reservoirs with lower slope at shoreline had higher malaria impacts.

Researchers say that mosquitoes prefer to breed in shallow puddles created around shorelines which are often free of predators such as fish and tadpoles.

Lower slope generally corresponded to poor drainage, promoting formation of surface water bodies and stable pools that are good habitats for mosquito breeding.