Resurgence of Clinical Malaria in Ethiopia in the Era of Anopheles stephensi Invasion

Background. The invasion of Anopheles stephensi into Africa poses a potential threat to malaria control and elimination on the continent. However, it is not clear if the recent malaria resurgence in Ethiopia has linked to the expansion of An. stephensi. We aimed to summarize the major achievements and lesson learnt in malaria control in Ethiopia from 2001 to 2022, to assess the new challenges and prospects for the control of An. stephensi. Methods and findings. We obtained the clinical malaria case reports, antimalarial drug treatment records, insecticide-treated and long-lasting insecticidal net (ITN/LLIN) distribution and utilization records, and indoor residual spraying (IRS) coverage data from the Ethiopian Ministry of Health (MoH) for the period 2001–2022. We analyzed clinical malaria hotspots using spatially optimized hotspot analysis. We investigated malaria outbreaks in 2022 and examined the potential role of An. stephensi in the outbreaks. Clinical malaria cases in Ethiopia decreased by 80%, from 5.2 million cases (11% confirmed) in 2004 to 1.0 million cases (92% confirmed) in 2018; however, cases increased steadily to 2.6 million confirmed cases (98% confirmed) in 2022. Plasmodium vivax cases and proportion have increased significantly in the past 5 years. Clinical malaria hotspots are concentrated along the western Ethiopian border areas and have grown significantly from 2017 to 2022. Major malaria outbreaks in 2022/23 were detected in multiple sites across Ethiopia, and An. stephensi was the predominant vector in some of these sites, however, it was absence from many of the outbreak sites. Conclusions. The malaria burden has been significantly reduced in Ethiopia in the past two decades, but in recent years it has increased substantially, and the cause of such increase is a subject of further investigation. Major gaps exist in An. stephensi research, including vector ecology, surveillance, and control tools, especially for adult mosquito control.


Background
Malaria morbidity and mortality have decreased signi cantly in the past two decades due to the scale-up of interventions, but malaria remains the most serious tropical infectious disease globally [1].Worldwide in 2022, there were an estimated 249 million malaria cases in 85 malaria endemic countries and areas, an increase of 5 million cases compared with 2021, deaths declined in 2022 to 608,000 from 631,000 in 2020 [1].In Ethiopia, malaria transmission has been a signi cant cause of public health issues due to its pronounced seasonal and regional uctuation.Ethiopia is one of the few countries in Africa where Plasmodium falciparum and Plasmodium vivax are both endemic [2], making the malaria control and elimination more complex than the other malaria endemic African countries.Reported malaria cases decreased from a peak of 5.2 million in 2004 to 1.0 million in 2018 [1,2].Encouraged by the progress made since 2000, Ethiopia envisaged the elimination of malaria and began a subnational program in 2017 and expanded to the national level in 2021, setting the goal of zero indigenous malaria cases by 2030 [3].Despite these major achievements, there has been an upsurge in malaria burden in Ethiopia in the past few years.The Ethiopian Ministry of Health (MoH) has con rmed 2.6 million malaria cases in 2022.In addition, the invasion of Anopheles stephensi Liston 1901 (Diptera : Culicidae), a malaria vector from South Asia, into Africa -with Ethiopia as the epicenter -and its rapid expansion in its new territory pose a signi cant threat to malaria control and elimination on the continent [4][5][6][7][8].Since it was reported from Djibouti in 2012, An. stephensi has been detected in Ethiopia, Eritrea, Sudan, Somalia, Kenya, Nigeria, and Ghana [9].The invasion of An. stephensi has been linked to malaria outbreaks in the urban areas of Djibouti and Ethiopia [10][11][12][13].The World Health Organization (WHO) has called for urgent action to halt the spread of An. stephensi in Africa [9].
The Ethiopian MoH has updated its vector control policy in response to the new threat.To contain the spread of An. stephensi, the Ethiopian MoH partnered with The President's Malaria Initiative (PMI) launched a larval source management (LSM) program using microbial larvicide for An.stephensi control in eight cities in northeastern and central Ethiopia [14].However, major knowledge gaps on An. stephensi ecology, behavior and its containment exist which may hamper vector control planning.
The aim of this study was to summarize the major achievements, challenges, and knowledge gaps in malaria control in Ethiopia from 2001 to 2022, to assess the changes in malaria epidemiology from 2017 to 2022, to document the new challenges in malaria control due to the invasion of An. stephensi, and to discuss the prospects for its control.The ndings will be useful for guiding policy updates to contain the spread of An. stephensi and to minimize its impact on malaria transmission in Africa and speci cally in Ethiopia, the current epicenter of the invasion.

Materials and Methods
We obtained the clinical malaria case reports, microscopy examination and rapid diagnostic test (RDT) results, antimalarial drug treatment records, insecticide-treated and long-lasting insecticidal net (ITN/LLIN) distribution records, and indoor residual spraying (IRS) coverage data from the Ethiopian MoH for the period 2001-2022.The nationwide clinical case reports included weekly microscopy and RDT con rmed cases, probable cases (clinically diagnosed), and parasite species at each woreda (administrative unit equivalent to district or county).Microscopy and RDT results included the number of tested cases, positive cases, and parasite species.Malaria treatment drugs delivered included artemisinin combination therapies (ACTs) and others.ITN/LLIN records included the number of nets distributed, and IRS data included population coverage.
To assess disease hotspots and their changes from 2017 to 2022, we analyzed woreda-level malaria incidence rates (cases/1,000 persons at risk/year) based on the weekly outpatient records and population projections from the Ethiopia o ce of the United Nations O ce for the Coordination of Humanitarian Affairs (https://data.humdata.org/organization/ocha-ethiopia).We selected the period 2017-2022 for two reasons.Firstly, the changes in malaria incidence distribution and malaria management policy in Ethiopia from 2013 to 2016 have been studied [2].Secondly and more importantly, An. stephensi was rst detected in Kebri Dehar in the Somali Regional State in eastern Ethiopia in 2016, and 2017 could be seen as a key turning point for urban malaria in Ethiopia, as An.stephensi was detected in 9 other sites across eastern Ethiopia in 2018 (Fig. 1) [5].We determined hotspots and coldspots of clinical cases using the Optimized Hot Spot Analysis tool of ArcGIS 10.8.2 (ESRI, Redlands, CA 92373, USA), which calculates Getis-Ord Gi* spatial statistics and z-scores at signi cance levels of 90%, 95%, and 99% [19,20].We analyzed malaria risk level changes from 2017 to 2022 based on the Ethiopian MoH risk classi cations using annual parasite incidence (API), i.e., malaria free API = 0, very low risk 0 ~ 5, low risk 5 ~ 10, moderate risk 10 ~ 50, and high risk ≥ 50 annual parasite infection rate per 1,000 people.
To analyze clinical malaria outbreaks, we randomly selected 40 districts representing different regions of Ethiopia, with at least three sites for each administrative region.We obtained the monthly con rmed malaria cases for the 40 sites for the period 2013-2022.We used Cullen's method to detect malaria outbreak months from 2018 to 2022 based on 2013-2017 case numbers adjusted for population growth [21].After initial evaluation of data completeness, 33 sites were included in the data analysis.The analysis of variance (ANOVA) with repeated measure was used to compare monthly clinical malaria incidences between 2022 and 2017-2021.We used the proportion of An. stephensi at each site to examine if An. stephensi played any role in malaria outbreaks in 2022.

Malaria epidemiology, diagnosis, treatment, and prevention in Ethiopia
Nationwide, reported clinical malaria cases (probable plus con rmed) remained unchanged from 2001 to 2013, although with great uctuations, and most of the reported cases were probable cases (Fig. 2A).This was followed by a signi cant decline from 2013 (2.65 million con rmed cases and 0.67 million clinical cases) to 2018 (0.96 million con rmed cases and 0.08 million clinical cases).However, clinical malaria cases have increased slowly but steadily since 2018, reaching 2.65 million cases (2.60 million con rmed cases and 0.05 million clinical cases) in 2022 (Fig. 1A).The proportion of P. falciparum malaria cases was stable from 2001 to 2014 and increased from 2014 to 2018, while the proportion of P. vivax cases increased from 10.6% in 2018 to 28.4% in 2022 (Fig. 2A), re ecting a 7-fold increase in vivax malaria cases from 2018 (0.10 million) to 2022 (0.73 million).The increase in clinical malaria cases was clearly not due to the lack of diagnostic tools, because the number of microscopically and RDT examined blood samples was stable from 2013 to 2021 (average of 6.5 million per year) and increased signi cantly to 9.8 million in 2022 (Fig. 2B).Distribution of antimalarial drugs uctuated but remained relatively stable from 2006 to 2022 (Fig. 2C).The number of ITNs/LLINs delivered uctuated greatly, with the major mass distributions occurring in 2007, 2010, 2015, and 2019 (Fig. 2D).Per Ethiopian government policy, IRS was implemented in epidemic-prone areas of the country.Therefore, the reduced IRS coverage from 2009 (28.37 million people) to 2022 (8.86 million people) is likely a re ection of reduced malaria risk levels rather than reduced effort (Fig. 2D).

Changes in clinical malaria incidence from 2017 to 2022
Malaria risk levels increased signi cantly in many districts from 2017 to 2022, based on the changes in API (Table 1, Figs. 3A, B).For example, in 2017 there were 47 districts and 7.19 million people free of malaria (API = 0); by 2022, malaria-free areas had decreased to 19 districts and 2.42 million people (Table 1).The number of districts with high malaria risk increased from 161 districts (8.33 million people) in 2017 to 267 woredas (15.88 million people) in 2022 (Table 1).It is important to note that not all districts had an increased risk.For example, 30 districts where malaria risks were high in 2017 had decreased risk levels in 2022, while 150 districts with lower risk levels in 2017 jumped to high-risk levels in 2022 (Table 1, Figs. 3A-C).Overall, from 2017 to 2022, malaria incidence rates decreased in 38 districts covering a population of 22,965,183 people, while malaria incidence rates increased in 780 woredas covering a population of 76,626,859 people.Malaria incidence remained unchanged (de ned as change in malaria incidence rate < 0.1 cases/1,000 people/year) in 38 woredas covering a population of 5,757,660 people.Clinical malaria hotspots in 2017 were concentrated in three focal areas along the western Ethiopia borders, and no clear coldspots were detected (Fig. 3D).By 2022, hotspot areas had expanded tremendously along the western borders and a large coldspot was detected in central Ethiopia (Fig. 3E), revealing an increase in clinical malaria incidence in western Ethiopia.Hotspots of increase in clinical malaria occurred in northwestern and southwestern Ethiopia (Fig. 3F).
Malaria outbreaks and the role of An. stephensi Ten-year dynamics of clinical malaria showed a strong heterogeneity (Figs. 4 & S1).Trend of clinical malaria dynamics can be classi ed into four categories (Figure S1), i.e., sudden recent outbreaks such as in Dire Dawa, declined trend with recent resurgence such as in Adama, stable trend with seasonality such as in Alamata, and overall declining trends such as in Abeshega and Yebelo (Figure S1).Malaria incidence rate increased signi cantly from 2017 to 2022 in many study sites, while it decreased signi cantly in other sites (supplement Table S1).For example, clinical malaria incidence rates in Dire Dawa increased 9.0-fold in 2022 compared to the average of 2017-2022 (ANOVA with repeated measure, P < 0.01), while in Kemise Town it decreased about 40% during the same period (P < 0.01, Table 2).Overall, malaria outbreaks were detected in 25 of the 33 selected sites (Figs. 4, 5 & S1, Table 2), indicating the severity of malaria outbreaks in Ethiopia in 2022.The link between An. stephensi and malaria outbreaks was complex (Table 2).The proportion of An. stephensi in some outbreak sites was high, for example, in Semera and Dire Dawa, An. stephensi accounted for 96-100% of all Anopheles adults collected in 2022, indicating a possible contribution of An. stephensi to the outbreaks (Table 2).Whereas, in Fik, no other Anopheles have been collected in 2022 except An. stephensi, there was a signi cant decrease in clinical malaria incidence in 2022 compared to 2017-2021 (Table 2).Furthermore, in many places where malaria outbreaks have been detected in 2022, for example, in Abobo and Gambella, no An.stephensi has been detected so far (Table 2), i.e., outbreaks in some sites were independent of the existence of An. stephensi.

Discussion
Malaria morbidity and mortality have signi cantly decreased from over 5 million clinical cases in early 2000 to about 1 million in 2018.Encouraged by the signi cant progress that has been achieved, the Ethiopian National Malaria Eradication Program (NMEP) has set the goal of achieving zero indigenous malaria in the country by 2030.However, the COVID-19 pandemic may have paused the declining trend in clinical malaria.The increase in clinical malaria since 2019 and the sudden malaria outbreak in Ethiopia in 2022-23 are alarming signs for malaria control.
Still more ominous is the emergence and rapid spread of the highly e cient and invasive vector An.stephensi, which may jeopardize the gains made in malaria control in the past decade.While native African malaria vectors breed mainly in rural natural habitats, An. stephensi demonstrates a robust ability to breed and thrive in urban environments, leading to malaria outbreaks in urban areas [12,13,22].Clearly, An. stephensi is not the cause of the 2022 malaria outbreak in Ethiopia because malaria outbreak occurred in many places where An. stephensi has not been detected, however, malaria outbreaks in some urban areas might be associated with An. stephensi, because An. stephensi was nearly the sole malaria vector in some areas.The recent upsurge in clinical malaria in Ethiopia may be multi-factorial including climate change, deterioration of healthcare system due to the COVID-19 pandemic, civil unrest in northwestern Ethiopia, and refugees in southwestern Ethiopia among other factors [23][24][25][26][27][28][29][30][31].Other possible causes for the 2022 malaria outbreaks include but are not limited to the possible K13 gene mutation related antimalarial drug resistance by malaria parasite and potential missed diagnosis of P. falciparum infections due to the PfHRP2/3 deletions, both need further investigations.Nonetheless, although the contribution of An. stephensi to malaria outbreaks in 2022 is a subject for further investigation [7,11], the outbreaks indicate the severity of the situation.
The WHO has reported a global increase in malaria cases in 2022; and Pakistan, Ethiopia and Nigeria were the three countries with increase of > 1 million malaria cases from 2021 to 2022 [32].The causes of the recent upsurge in clinical malaria in Ethiopia are worth of in-depth investigations.Although its contribution is a subject of further investigation in Ethiopia, climate change might have major impact on malaria transmission and risk globally [33][34][35].Civil unrest in northwestern Ethiopia caused the setting up of many internally displaced people's camps and the in ux of Sudanese refugees in southwestern Ethiopia (mainly in Gambella Region) led to the setup of refugee camps in the area, Plasmodium infections are prevalent in these camps which may serve as reservoirs for local transmission [30,31,36].More importantly, COVID-19 could have contributed to the malaria resurgence and outbreak by an accumulative effect [37][38][39].COVID-19 pandemic interrupted not only the services at health facilities but also peoples' malaria treatment seeking behavior.Without effective treatment Plasmodium parasite reservoir might have cumulated from 2020 to 2021 and eventually caused the malaria outbreaks in Ethiopia in 2022, this hypothesis needs to be investigated.Although Ethiopian MoH has implemented the primaquine 14-day low-dose radical treatment of P. vivax since 2021, there was an increased proportion of clinical vivax malaria cases in 2022, the causes of such an increase requires further investigation.Lastly, we cannot rule out the contribution of An. stephensi in malaria outbreaks in Ethiopia because An. stephensi was the predominant in come urban areas in Ethiopia [11,12,15].
To contain the spread of An. stephensi in Africa, WHO has announced an initiative to support an effective response to An. stephensi on the African continent in 2022 [9].The WHO initiative sets ve aims, including increasing collaboration among national malaria control programs, researchers, and funders to ensure sharing of knowledge, optimization of resources, and prioritization of key activities.The Ethiopian MoH, in partnership with PMI, has worked with the NMEP and researchers on enhanced surveillance to document the spread and vectorial capacity for malaria transmission in Ethiopia.They have launched LSM programs in eight cities in Ethiopia aiming to reduce An.stephensi populations and to slow and eventually prevent its spread.However, there are major knowledge gaps and policy implications to consider.
The WHO recommends strengthening surveillance, including entomological surveillance, which can determine the spread of An. stephensi and its role in transmission, and malaria case surveillance, which can be used to investigate the impact of An. stephensi on malaria, particularly in urban areas.However, the optimal sampling method for An.stephensi adult mosquitoes has not yet been established.For example, aspiration (suction applied by a human or machine) is often used for An.stephensi adult samplings [15], but this method is highly subjective regarding the selection of sampling locations, i.e., one may intentionally select potential An. stephensi resting places such as animal shelters.Human landing catches (HLC) is considered the gold-standard for African Anopheles adult samplings [40][41][42], it will have similar problems; e.g., should one sit inside/outside human dwellings or inside/outside animal shelters?Similarly, for larval surveys one may deliberately select container habitats instead of randomly selecting both man-made and natural habitats.More importantly, these sampling methods may affect the population dynamics assessments [15,16].Additionally, we must take into consideration of native African malaria and arboviral disease vectors, as they are present in some urban areas and may require different sampling methods than An.stephensi.Since we found most An.stephensi adults inside animal shelters, animal-baited traps may be an e cient trapping method, but it must be thoroughly evaluated under semi-eld and eld conditions.In any case, the sampling method(s) with less bias and better e ciency for both native African malaria vectors and An.stephensi need to be carefully investigated.
The WHO also recommends prioritizing research; speci cally, it recommends evaluating the impact of vector control interventions, particularly new tools, against An.stephensi and focuses on research which will enable malaria control programs to nd better ways to respond to this invasive vector.The Ethiopian government and PMI are conducting pilot larviciding programs using Bacillus thuringiensis var.israelensis (Bti) in a number of cities, aiming to generate data for strategic control of An. stephensi.This is a good starting point, as An.stephensi breeds mostly in man-made habitats such as arti cial water containers and construction pits, however it is hard for LSM to cover all larval breeding habitats.Therefore, adult control tools should also be developed and evaluated.Since we know that An. stephensi rests mainly in animal shelters or other outdoor structures in Ethiopia, we can develop new mosquito control methods utilizing this information [43].For example, targeting animal shelters with IRS may signi cantly reduce the population density of An. stephensi.Regardless, integration of different control tools and including both larval and adult interventions may help to greatly reduce the An.stephensi population if not eliminate it.
There are limitations regarding the clinical malaria case reporting and the investigations of the causes of the 2022 malaria outbreaks in Ethiopia.In Ethiopia, private clinics also provide service for malaria diagnosis and treatment, however, compared to the free malaria treatment at government-run public healthcare facilities, the cost at private clinics may prevent some patients from seeking treatment over their facilities.Therefore, the under-report of clinical malaria cases from private clinics may not be a major issue.The causes of the 2022 malaria outbreaks in Ethiopia may be multi-factorial, which requires further investigations, and this can be a major study itself.Another limitation is the clinical malaria case dynamics at each woreda, it would be better to used incidence rate based on population size, however, because the Ethiopian government has not done any census since 2007 and some woredas have been redraw in the past 10 years, therefore it is very di cult to calculate incidence rate over time.
In conclusion, while the malaria burden in Ethiopia has been greatly reduced in the past 20 years, the 2022-23 malaria outbreak has undercut control efforts.Although the overall contribution of An. stephensi to malaria transmission in Africa is unclear, it has been linked to the malaria outbreaks in some urban settings of Africa.Larviciding has been implemented in Ethiopia for controlling An. stephensi, but adult control strategies should also be developed.The rapid growth of many African cities and global climate change, coupled with the invasion and spread of this highly e cient and adaptable malaria vector and the knowledge gaps surrounding it, could undermine the malaria control and elimination efforts in Ethiopia and other African countries.Containing the spread of An. stephensi and eliminating malaria in Africa requires strong international collaboration, investment, and commitment.

Abbreviations
Figures

Figure 1 Distribution
Figure 1

Figure 2 A
Figure 2

Figure 4 Example
Figure 4

Table 1
Changes in at-risk populations from 2017 to 2022 ,859 † API values indicate the reduction/increase in API ranges.API: annual parasite incidence.

Table 2
Malaria outbreak detection and An.stephensi status and proportion in selected sites