Retrospective analysis of malaria case is quite informative for the annual and inter-annual comparison of the disease burden among the different section of the society, so as topredict the future trend of the disease and the need to scale up effective interventions. The present study revealed that, malaria is still remaining a major public health problem in the study areas. Five-year trend analysis indicated higher malaria cases in Boset followed by Adama and Lume districts. Although the overall disease burden has decreased from 2016 to 2018, SPR and prevalence showed a slight increment in 2019 and 2020, respectively. During the study period, out of 286,647 symptomatic patients 47,016(16.4%) were microscopically/RDTs confirmed malaria cases (Table 1). This finding is much greater than trend analysis study conducted in Wolkite; southwestern part of central Ethiopia (8.56%) (Solomon et al., 2020), and Tigirai; northwestern of Ethiopia (6.96%) (Berhe et al., 2019), lower than similar study conducted in western part of Ethiopia; Guba district of Benishangul Gumuz Zone (51.04%) (Alkadir et al., 2020), also lower than the cross sectional study report from East Shawa Zone of Oromia Regional State in 2012 (25.2%) (Tadesse et al., 2018). Even though, the specific factors that accounted for such variation in malaria endemic regions need further investigation climatic factors and the status of malaria interventions in the region (Weiss et al., 2014; Berhane et al., 2019) might have greatly contributed.
In the previous studies, it was reported that age-standardized malaria incidence and prevalence in Ethiopia between 1990 and 2015 showed a declining trend (Deribe et al., 2017). However, in the present study maximum SPR was observed in 2016 (47.8%), and the minimum was in 2018 (7.9%). On the other hand, the prevalence also declined from 2016 to 2019, and slightly increased by 2020 (Fig. 2). Even though malaria prevalence has declined from (6 to 1%) in the study area, it is still higher when compared with the national malaria indicator survey conducted in 2007, 2011, and 2016 which ranged from (0.5 to 1.3%) (FMOH, 2016b). The reduction of SPR and prevalence was also not consistent in the study area. Although the major factors that contributed to such inconsistency in maintaining progressive decline of malaria incidence demand further investigation, the finding indicates the need for rigorous efforts to scale up the ongoing malaria control and prevention strategies in the region.
With respect to age, 60% of all malaria positive cases were individuals with ≥ 15 years of age, followed by 5 ≤ 14 age groups (28%), while children below the age of 5 were least affected. The higher malaria incidence rate was reported among group ≥ 15 years of age, which could largely be due to agricultural practices and other livelihood engagement of this productive section of the society in the study areas. Previous epidemiological studies from different localities of Ethiopia also reported more malaria cases among age group ≥ 15 years, followed by 5 ≤ 14 years (Chala and Petros, 2010; Gemechu et al., 2015; Legesse et al., 2015; File et al., 2019; Berhe et al., 2019; Hassen and Dinka, 2020; Alkadir et al., 2020).
With regard to sex, comparatively more malaria cases (57.4%) were observed among males than female counter parts (Fig. 4). This finding is in agreement with trend analysis study reported from Sibu Sire district of East Wollega Zone in western part of Ethiopia (Gemechu et al., 2015), southwestern of Ethiopia (Sena et al., 2014), and south-central Ethiopia (Yimer et al., 2015).This might be due to the work tradition in Ethiopian culture, where male individuals are commonly more responsible for outdoor activities than female. Since Anopheles mosquitoes generally bite more frequently outdoors than indoors (Kenea et al., 2016), it might have contributed to such a higher malaria burden in male individuals.
Concerning malaria cases detected by species from the blood film, the present study revealed that out of 47,016 confirmed malaria cases 53%, 41% and 6% showed P. falciparum, P.vivax and mixed infection, respectively. The present study finding is in agreement with the previous reports (Tesfaye et al., 2018; Berhe et al., 2019; Alkadi et al., 2020). The predominance of P. falciparum was consistent over the five years with slight seasonal variation. P.vivax cases during the minimal malaria season in the study area could be largely due to relapse.
Regarding seasonal transmission dynamics, year round malaria cases were observed in the three districts of the study areas. However, the highest peak was in the month of September and October (autumn), due to the formation of stagnant water and higher relative humidity suitable for mosquito breeding after the summer rainy season. This malaria peak season observed in the study areas is in agreement with similar studies conducted in different localities in Ethiopia (Hailemariam et al., 2015; Legesse et al., 2015; Tesfaye et al., 2018). The second minor malaria season was during May to June (spring) following short rainy season in the country, which is in agreement with the report of Sena et al. (2014) and Gemechu et al. (2015).
Determinants of such seasonal variation of malaria transmission in the study sites might be due to variation in seasonal temperature, topographic features, the condition of the residential areas (availability bushy gorges, shanty dwelling, untidiness, etc.) (Alemu et al., 2011; File et al., 2019), also the economic factors of the inhabitant population are some of the common epidemiological factors for malaria incidence in the study area.