Malaria morbidity and mortality have significantly decreased from over 5 million clinical cases in early 2000 to about 1 million in 2018. Encouraged by the significant 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 efficient 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 [23–31]. 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 [33–35]. Civil unrest in northwestern Ethiopia caused the setting up of many internally displaced people’s camps and the influx 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 [37–39]. 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 five 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 [40–42], 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 efficient trapping method, but it must be thoroughly evaluated under semi-field and field conditions. In any case, the sampling method(s) with less bias and better efficiency for both native African malaria vectors and An. stephensi need to be carefully investigated.
The WHO also recommends prioritizing research; specifically, 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 find 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 artificial 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 significantly 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 difficult 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 efficient 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.