This study determined the annual average incidence of malaria and identified climatic variables that affect its occurrence in highland and lowland areas of northwest Ethiopia.
The average annual malaria incidence rate was 29.9 per 100 populations. This is greater than studies conducted(9.7%) in the northwest Ethiopia(18), (4.3%) in Amhara region(5), (6.8%) in the southern nation and national states(4) and (10.4–13.5%) in Gambella; (7.6–14.1%) in Tigray, (0.9%) in Oromiya and (5.4%) in Southern Nations Nationalities People’s Regions (19) and (4.5%) in Ethiopia(13). However, it is smaller than the research carried out in the Tigray region in 2011 (43%) and 2014 (33%)(20). The higher incidence of malaria in the study area might be related to a long rainy season and temperature variability(13, 20). The majority, 69.8%, of the malaria cases were due to plasmodium falciparum species. This is consistent with other studies from the southwest part of Ethiopia (19). The high burden of plasmodium falciparum and plasmodium vivax in the region was also documented in another study(13). The cause could be the temperature that favors their growth; temperatures more than 18C0 for p. falciparum and more than15 C0 for p. vivax is suitable for the growth of these two species. The temperature is higher than the minimum indicated in most parts of Ethiopia, particularly in Northwestern Ethiopia(18). Research in Tanzania also endorsed this idea(21).
The malaria victims that accounted for about 71.8% are adults over 15 years old. This is consistent with other researches(22, 23). However, another research has shown that malaria infection decreases in this age group(24). The high occurrence of malaria in adults could be attributed to the high mobility of people in this age group to malaria-risk areas for farming and other reasons.
In early April, the significant transmission time was noted and reaching its peak at the end of June and July. Between October and November, the second peak was noted. Other surveys have shown that there are two peak transmission periods in Ethiopia during the summer, but the incidents of the disease are variable(25). However, whatever the point of departure, everybody agreed that the first wet (summer rain) season was high in malaria transmission, with the wet season following the end(25).
Keeping the other variables constant in the model, the estimated incidence rate ratio of malaria was increased by 1.04 for a unit increment of relative humidity. It is supported by other studies conducted in Mali, Bangladesh, and Mozambique (26–28). Humidity between 55–80% is suitable for the completion of the Plasmodium falciparum and Plasmodium vivax malaria parasite life cycles (26). It seems that humidity plays an important role in the life cycle of the mosquito. In the presence of high humidity values, the parasite would complete the necessary life cycle to increase the transmission of the infection. When there is a unit increment of altitude, the rate of malaria incidence was reduced by 0.95 times. These findings are in agreement with other studies (27, 29–31).
The estimated risk of malaria incidence was increased by 1.07 times for a unit increment of average maximum temperature. This is in line with other studies (32–35). It might be the effect of temperature for the development of mosquitoes, their survival, and reproduction (36). A study in the highlands of eastern Africa disclosed that a rise of 1% in minimum temperatures over 1–2 months and a rise of 1% in peak temperatures over 2–5 months led to a rise of 80%-95% in the proportion of outpatients diagnosis of Malaria (37). However, another study conducted in Bangladesh found that temperature and numbers of malaria cases have no significant relationship(38). The development of the mosquito larvas and Plasmodium parasites is accelerated at high temperatures. A higher number of mosquito generations and greater abundance occur with enhanced growth rates. As temperatures rise, the transmission of malaria increases up to a rarely exceeded limit of 37C0(24, 25). Higher temperature also increases the feeding rate of adult female mosquitos, which can increase the likelihood that the malaria parasite may transmit to uninfected human hosts(39, 40). The sporogenic cycle of plasmodium growth, which occurs in the mosquito, is also shortened by the fact that the temperature rises to an ideal rate (25, 41). Furthermore, insect longevity is temperature-sensitive and there are limited temperatures above which mosquito mortality increases and minimum temperatures below which insects become inactive(39).
The effect of rainfall on malaria incidence is somewhat controversial. In this study, rainfall had no significant relationship with malaria incidence. The finding is supported by other studies that studied across different countries (30, 42). But, many other studies have found a positive correlation. Studies in Angola, Mozambique, Mali, Zambia, Uganda, Botswana, and Thailand, for example, have found that greater precipitation values are linked to greater malaria incidence (26, 31, 43–46). In contrast, other studies have shown that precipitations are an important contributor to malaria reduction in some regions of India and Seri Lankas (26, 47–51).
The incidence rate of malaria was 2.75 times higher when a unit scale increment of normalized difference vegetation index. Other studies have also found a strong link between natural vegetation and malaria transmission (52–55). NDVI can serve as a surrogate variable that represents available water or conditions that are consistent with a higher likelihood of standing or pooled water. As precipitation falls to the ground and is absorbed by plant life, plants increase their biomass and therefore reflect radiation in the near-infrared region of the electromagnetic spectrum over a larger area than in times of vegetation senescence. It is therefore reasonable that if the water is widespread in order to enhance vegetation, there is a higher probability that water for mosquito reproduction will be accessible. Thereby probability of being infected, along with malarial transmission, rises as the mosquito population rises.