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Present-day malaria seasonality (1960 to 2000)

MSM present-day simulation

The present-day ensemble runs for 1960 to 2000 of the REgional MOdel (REMO) (Paeth et al. 2007; see page "Data input") have been used for the calculation of the present-day climate conditions. The MSM was finally driven by means of this climate data.

Length of the malaria season

In agreement with the annual precipitation amounts, the two-dimensional MSM simulations show a decrease in the malaria season length from the Guinea Coast towards the Sahel for the period 1960 to 2000 (Fig. 1a). Malaria transmission is year-round in the equatorial tropics in the area of the largest precipitation amounts, e.g. in southern Cameroon, Gabon, Congo, and Uganda.
The presence of highlands in East Africa is causing a more complex pattern of the distribution of the malaria season length. Parts of Ethiopia, Kenya, Tanzania, Rwanda and Burundi are covered by highlands and are therefore characterised by lower temperatures than nearby plain areas. Note that temperatures in the order or even lower than the sporogonic temperature threshold prohibit the transmission of the malaria disease. Those are the reason for short malaria seasons or even the lack of malaria seasonality in the East African highlands. By contrast the lack of a malaria season around the Horn of Africa is due to low annual precipitation amounts.

Onset and end of malaria transmission

In West Africa malaria transmission starts shortly after the onset of the rainy season. At the Guinean coast the malaria season therefore starts in March and April and finally ends in November during the retreat of the West African monsoon. Because of the late start of the monsoon in the Sahel, the start of the malaria transmission is delayed to about July/August and quickly ends between September/October.
Due to partly two annual rainy seasons in East Africa the MSM simulates partly two malaria seasons. The first (second) season starts in boreal spring (autumn) and ends in boreal summer (winter).
Most regions south of the equatorial tropics are characterised by malaria transmission between boreal autumn and early boreal summer. However, due to the presence of high topography in parts of Angola and Zambia a complex pattern of malaria seasonality is simulated by the MSM. In such areas transmission of malaria is partly restricted between November and January. Whereas other parts show two distinct malaria seasons, one between November and January as well as another short season between April and Mai.

Comparison with MARA maps

The malaria seasonality corresponding to MSM simulations based on data from REMO are comparable to the maps produced by the MARA project (cp. Fig. 2). However, it was found that the onset of the West African monsoon was delayed in the REMO simulations. For this reason malaria season starts partly one month later than indicated by the MARA maps. Moreover at the Guinean coast transmission ends later in the MARA map than in the performed MSM simulations. This fact is again related to the REMO precipitation data.

Seas SSeas ESeas
b) c)
Fig. 1: MSM simulations of malaria seasonality based on climate conditions from three REMO ensemble runs between 1960 and 2000. (a) Length of the malaria season (Seas in months). (b) Start (SSeas) and (c) end month (ESeas) of the malaria transmission (cont: year-round transmission). (d) And (e) same as (b) and (c) but for the second malaria season. Note, in (d) and (e) are only areas with two annual rainy seasons and therefore two distinct malaria seasons considered.

a) b)
Fig. 2: (a) Length of the malaria season as predicted by the MSM (Fig. 11 in MARA/ARMA 1998). (b) Onset (left) and end month (right) of the malaria transmission. In areas with two annual rainy seasons, the maps refer to the first rainy season (Fig. 12 in MARA/ARMA 1998). Results in (a) and (b) correspond to the climate conditions between 1920 and 1980 (Hutchinson et al. 1996).