the effect of environmental modification on Malaria epidemiology and anopheles bionomics in Ethiopia

Abstract

Rain-fed agriculture has affected African livelihood by making people vulnerable to climate- related drought. Construction of dams and initiating irrigation schemes has therefore been widely recognized as key solutions to ensure food security and enhance economic growth in drought prone regions. In this regard, Ethiopia has embarked extensive dam constructions and irrigation expansion to promote economic development. However, such development activities may cause environmental modifications that could adversely affect the spread of vector-borne diseases such as malaria. To date, there has been limited data evaluating the impact of environmental modifications on the epidemiology of malaria and its vector bionomics. Therefore, this study was done to evaluate impact of environment modifications on distribution and ecology of malaria vector mosquitoes at Arjo-Dedessa Sugarcane Irrigation Scheme in southwest Ethiopia. The study had three main objectives: i), it aimed to determine the existing malaria transmission dynamics in the area (Chapter 4). ii), it aimed to identify breeding habitats of malaria vector mosquitoes in the area (Chapter 5), and iii), it aimed to determine the effects of change in the agroecosystem on survivorship and development of Anopheles gambiae s.l, the main malaria vector in the area (Chapter 6). Methods Epidemiological and entomological surveys were conducted between 2017 and 2020 in Arjo- Dedessa area of Southwestern Ethiopia. Retrospective data of malaria for the period between 2008 to 2017 were obtained from health facilities near the Arjo-Dedessa Sugarcane Irrigation area. Malaria positivity rate, incidence rate, parasite species proportion, seasonality, age structure and sex distribution were analysed and discussed (Chapter 4). Entomological surveys were also conducted that encompasses; repeated cross-sectional survey for Anopheles larval ecology study and life-table experiment for survivorship and development study. Anopheles mosquito larvae were collected seasonally from two agroecosystems, ‘irrigated sugarcane plantation’ (‘irrigated area’ hereafter) and ‘non-irrigated mixed crop-covered’ areas (‘non-irrigated area’ hereafter) during the dry (December 2017 – February 2018) and wet (June XIV 2018 – August 2018) seasons. Mosquito habitat diversity and distribution, and larval abundance were compared between the two agroecosystems and discussed (Chapter 5). Life-table experiments were conducted to examine the effect of environmental modification on survivorship of An. gambiae s.l both immatures and adults in irrigated and non-irrigated areas. The pupation rate and development time of the immatures and adult longevity and fecundity were compared between the two agroecosystems and discussed (Chapter 6). Results The epidemiological profile of malaria for the study area was mapped (Chapter 4). Over 10 years, 54,020 blood films were collected for malaria diagnosis in the health facilities at the area, of which 18,049 (33.4 %) were confirmed malaria cases by both microscopically and RDT. Plasmodium falciparum, P. vivax, and mixed infection (P. falciparum and P. vivax) accounted for 8,660(48%), 7,649(42.4%), and 1,740(9.6%) of the malaria cases, respectively. The study also revealed that P. vivax was the predominant over P. falciparum for four years (2010, 2014, 2015 and 2016). Malaria has been reported in all age groups, but age distribution showed that the vast majority of cases were adults above 15 years of age (73.7%). In all age groups, males were more significantly affected than females. Moreover, malaria positivity rate showed a strong seasonality. However, malaria cases were reported in all seasons across the 10 years data analyzed. The association between environmental modification due to irrigated agriculture and Anopheles mosquito larval habitat diversity and distribution, and larvae occurrence is discussed in Chapter 5. In this study, 319 aquatic habitats were surveyed during the study period. Around 60% (n = 152) of the habitats were positive for Anopheles mosquito larvae, of which 63.8% (n = 97) and 36.2% (n = 55) were from irrigated and non-irrigated areas, respectively. The number of Anopheles positive habitats was two-fold higher in irrigated than non-irrigated areas. Anopheles larval abundance in the irrigated area was 16.6 % higher than in the non-irrigated area. Pearson’s chi- square analysis showed that season, agroecosystem, and turbidity had a significant association with larval Anopheles occurrence. The effect of environmental modification on mosquito survivorship and development is discussed in Chapter 6. The estimated mean survival time of female An. gambiae s.l in irrigated and non- XV irrigated area was 37.9 and 31.3 days, respectively. The estimated mean larval-to-adult development time of An. gambiae s.l larvae was not found to be different in both irrigated and non-irrigated areas. A survival analysis indicated that adult female An. gambiae s.l in the irrigated area live significantly longer than those in the non-irrigated area. Females mosquitoes showed higher longevity than males in both irrigated and non-irrigated areas. Conclusions and recommendations In general, malaria positivity showed a declining trend over 10 years period in the area. However, in recent years of study, it showed a slight rise, which indicates that the area needs attention to intensify the existing interventions to sustain control and enhance malaria elimination efforts. This study found out a higher Anopheles mosquito breeding habitat diversity, larval occurrence and abundance in the irrigated than non-irrigated areas in both the dry and wet seasons of the year. This indicates that irrigation development activities contribute to the proliferation of suitable mosquito breeding habitats that could increase the risk of malaria transmission. Incorporating larval source management into routine malaria vector control strategies could help reduce mosquito population density and malaria transmission around irrigation schemes. Adult An. gambiae s.l survivorship was found to be enhanced in irrigated area than non-irrigated area. Longer survival of adult mosquito in irrigated sugarcane plantation area may have important implications in malaria transmission. Thus, routine monitoring of entomological indices and environmental parameters in line with land use change is needed to detect any change in malaria epidemiology and device toiler-made interventions