Can rain reduce the risk of malaria?

Rain might reduce the risk of malaria, since the mosquito larvae suffers higher mortality during rainfall. Photo courtesy of photocase.com.

Intuitively, we certainly would not expect rainfall to have any positive influence on the number of malaria cases, on the contrary. Malaria, one of the world’s most damaging diseases with more than 300 million cases per year and more than a million deaths, is transmitted by mosquitoes in the genus Anopheles. Since the mosquito larvae live in small ponds, it would be expected that the more rain the more water-filled ponds and crevices, the more mosquitoes and malaria. However, findings reported by the scientists from the Unversity of Wageningen in the Netherlands and Kenya Medical Research Institute suggest that increased rainfall will have a negative effect on mosquito larval populations and thus potentially on Malaria.

The scientists placed water basins with Anopheles gambiae mosquito larvae outdoors in Kenya and observed the changes in the number of larvae after dry and after rainy nights. They found that rainfall has a significant adverse effect on young mosquito larvae, which are either washed away by the rain or suffered higher mortality. Larvae gathers in the surface so rain and wind can be responsible for washing the larvae into small streams away from the habitat. These small streams are likely to dry out during the following day and thus washing away will probably result in the death of the larvae. The mortality was also directly observed to increase, presumably because the resultant turbulent water makes it more energy demanding and more difficult for the larvae to stay in the air-water interphase. Overall the experiments showed an increase in outwash and mortality of 18% for first instar larvae and 5% for fourth instar larvae after a night with heavy rain.

However, as the scientists are well aware these results may be counter-acted by an increase in habitats after rain or in an increase in the growth and survival of the remaining larvae as the larval density and thus the competition for resources decreases.

If the researchers’ results do work on a life-history and population level, then at least there may be one positive side effect of global warming. The Intergovernmental Panel on Climate Change (IPCC) predicts that global warming will result in increased rainfall in Eastern Africa.

Source: Paaijmans KP, Wandago MO, Githeko AK, Takken W (2007) Unexpected High Losses of Anopheles gambiae Larvae Due to Rainfall. PLoS ONE 2(11):  1-7.

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