The hotspot of malaria is governed by a large number of factors relating to the parasite, the vector and the socio-economic condition of the communities. Predominant among these are physical factors affecting habitat and breeding sites of the Anopheles mosquito vectors such as temperature, precipitation, humidity, presence of stagnant water pools, vegetation which are related with other factors such as elevation and slope of the area. Specifically, temperature and rainfall act as limiting factors on the development of Anopheles mosquitoes, which are the intermediate hosts in the transmission of malaria parasites. About 70–90% malaria risk is associated with environmental factors, which in turn influence the abundance and survival of the vectors(Saxena et al., 2009).
In this study, the physical factors identified in various researches to significantly contribute to the effects of malaria distribution are identified with great care and by critical analysis of studies conducted in Ethiopia. The standard to determine and rate the effect of each factor is also based on this literature (Aster & Seleshi, 2009; Aynalem, 2014; Olana et al., 2011). These factors are described below:
Temperature is one of the key environmental contributors to mosquito propagation. High temperature speeds up the development of the life cycle of a mosquito and accelerates the length of the development of the life cycle of malaria parasite within the mosquito host. The optimum temperature for development of malaria parasite is between 25°C to 30°C. At lower temperature less than 16°C, the larval and pupal stages of mosquitoes take longer time to complete sporogony cycle and below 16°C the sporogony ceases. However, with an increase in temperature sporogonic period become shortened and the Plasmodium parasite within the vector increase and effective up to about 30°C. Contrary to this, increased temperature above 30°C has negative impact on the survival of the vector(Mestewat, 2014).
In general, the temperature less than 16°C and above 30°C, are less conducive for malaria breeding. Therefore, based on these literatures the temperature based malaria hot spot of the study area classified as high and very high for 19-210C and 21-240C respectively; believing that the temperature increasing more than 250C will gradually decreases the breeding (The detail of the standard used is described in the Table 1).
The association between rainfall and malaria epidemics has been recognized in various literatures(Mestewat, 2014). But it can be seen in two ways. First, increasing precipitation may increase vector populations in many circumstances by increasing available anopheles breeding sites. Excessive rainfall in warm, arid areas can lead to increased transmission due to creation of vector breeding sites. Apart from creating mosquito breeding sites, rainfall also affects malaria transmission through increasing humidity, which in turn will help to increase the longevity of the adult vectors Second, excessive rains may also have the opposite effect by flushing out small breeding sites, such as ditches or pools or by decreasing the temperature, which in regions of higher altitude can stop malaria transmission.
Based on this information, it is possible to identify the significant amount of rainfall support malaria breeding while other physical factors are highly associated with its effectiveness. Areas with annual rainfall amount greater than 1000 mm are malarious and have intense malaria transmission, but areas with rainfall amount between 500 and 1000 mm have seasonal transmission (President’s Malaria Initiative, 2012).Moreover, very high rainfall is not suitable for vector immature stages, areas having > 1600 mm annual rainfall is unfavorable for mosquito breeding. In general, rainfall based mosquito breeding zones are classified as high and very high for rainfall amount of 1504–1600 mm and > 1600 mm respectively.
Malariologists working in the field in the first half of this century, in the decades following the elucidation of the malaria cycle in man and mosquitoes, appreciated that it was a focal disease and that the topography of the land was an important consideration in understanding the local epidemiological situation(Qayum et al., 2013). Topography (particularly altitude and slope) are identified by (Saxena et al., 2009) as an important factor in understanding the malaria epidemiological situation at local scale.
It is locally well-known that the prevalence of malaria parasites in people varies with altitude. People at low lands have significantly higher prevalence of malaria than those in middle and highlands. Therefore, altitude is significant in determining the distribution of malaria and its seasonal impact on many parts of the World. Based on altitude, traditionally Ethiopia is divided in five agro-ecological climatic zones. These are locally known as Baddaa Dilallaa, Baddaa, Badda Daree, Gammojjii and Gammoojjii Ho’aa. In Ethiopia the both Baddaa Dilallaa and Baddaa zone with elevation above 2500 meter above mean sea level is malaria free even though the dynamics is high due to present climate change and variability. However, malaria frequently occurs in areas below 2000 meters elevation and the transmission is very intense in areas below 1500 meters elevation (Aynalem, 2014). Particularly, in some parts of Badda Daree, and in most parts of Gammojjii. Contrary to this, the areas of Gammoojjii Ho’aa have an altitude < 500 meter above mean sea level and annual rainfall of < 900mm is not suitable for mosquito breeding as a result of low annual rainfall amount and very high temperature above 300C.Therefore, the study area is classified in to three classes as 1317- 1500masl, 1500–2000 m and > 2000 meter for very high, high and low, respectively.
Malaria breeding is affected by slope of the land. Mosquito larvae need stagnant water pools to survive, and these pools are less likely to form in steep slope areas. Moreover, larvae developing in water pools in sloped areas are more likely to be washed away during downpours. Thus, sloped areas make poor mosquito breeding ground, reducing the threat of malaria transmission because of low water stagnation. In general, the steeply the slope, the low mosquito prevalence and vice versa are well-known. Therefore, the slope classes 0–5%, 5–8%, 8–15%, 15–30% and > 30% are assigned for very high, high, moderate, low and very low slope based risk of malaria respectively.
Proximity to Water Bodies
Surface water provides the habitat for the juvenile stages (egg, larvae, and pupae) of malaria vectors. The state of small water bodies and wetlands is very useful source of malaria vectors. Seasonal variation in volume of water in Ethiopia results the variation in malaria breeding situation spatially and temporally. The rainy season for both the rivers and lakes lead to flooding which hinders vectors of malaria. However, during dry season their volume is decreased and they create different pockets of water body that is favorable ground for the breeding of mosquito. Therefore, even though the effect is varying seasonally, accessibility to water bodies aggravate the prevalence of malaria. The water body selected for this project could be river, lake and swamps. Accordingly, the value is assigned for these variables: Areas found < 1 km, 1–2 km and above 2–5 km from rivers and canals assigned as very high, high and low malaria risk level respectively.
Soil Moisture Holding Capacity
Soil moisture holding capacity/permeability is important factor determine mosquito breeding. Poorly drained soils is believed to facilitate water stagnation and create conducive conditions for mosquito breeding and thus, favorable for malaria outbreak. Well drained soil doesn’t allow water stagnation, so it creates unfavorable condition for anopheles breeding. Soil drainage classification of Ethiopia made by CSA, were used for this project. Accordingly, it is classified in to very poor, poor, moderately well-drained are assigned as very high, high and low malaria risk factor respectively.