Title: Rainfall
Course: Communicable Diseases Part 1 - Ethiopia (Full)

Rainfall

As you learned in Study Session 5, anopheline mosquitoes breed in water. So the right amount of rainfall is often important for them to breed. Different anopheline mosquitoes prefer different types of water bodies in which to breed. In Ethiopia, water collections that support vector breeding appear mainly after the rains, and therefore malaria transmission is highest following the rainy season.

Note that the anopheline mosquitoes that transmit malaria do not breed in foul-smelling polluted water.

Of course, too much rainfall can flush away breeding habitats temporarily, but mosquitoes start breeding as soon as the rain stops. In most cases, flushing has a bigger impact on vector breeding habitats in the highlands and hilly areas than in the lowland plains. Not all water collections are suitable for the mosquito life cycle. In Ethiopia, rain water collections are the most important breeding ground, as the anopheline mosquitoes prefer to breed in fresh water collections created after the rainy season. Such water bodies may be clear or muddy (Figure 6.2) but they are not polluted.

Figure 6.2 A muddy rainwater collection can support mosquito breeding if it is not polluted. (Photo: Dr Daddi Jima)

There are also places where less rainfall and drought can favour mosquito breeding and malaria transmission. Such places are usually covered by vegetation throughout the year and streams and rivers often flow rapidly. When the rains fail or are delayed, the flow of streams is interrupted and pooling occurs along the stream. Pooling creates a favourable environment for mosquito breeding. Malaria vectors mainly breed in stagnant water collections, rarely in slightly moving waters and never in rapidly flowing rivers and streams.

In drier areas, rainfall can also affect malaria transmission indirectly through its effect on humidity. Vegetation cover increases after rainfall, which in turn increases the relative humidity of the environment. The effect of humidity on malaria transmission is considered below.