The spread of Anopheles stephensi (An. Stephensi) mosquitoes poses a significant threat to malaria elimination in Africa, according to a 2019 vector alert from the World Health Organization (WHO). This malaria vector, formerly o­nly seen in South Asia and parts of the Arabian Peninsula, was detected for the first time in Africa in 2012, in the city of Djibouti. Over the last decade, An.stephensi has been expanding, with cases reported in five different countries o­n the African continent. To respond to this challenge, the WHO recently announced a new initiative aimed at halting the spread of this invasive mosquito species in the region. 

Unlike other mosquito vectors of malaria in Africa, An. Stephensi is able to thrive in urban areas. The expansion of this species in sub-Saharan Africa, where the burden of malaria is highest and where more than 40% of the population lives in urban areas, has sparked concerns about the effect this vector may have o­n the prevention and control of malaria in the area. Moreover, many conventional anti-malaria tools are not as effective against An. Stephensi, which has been found to be resistant to several insecticides.

Number of reported malaria cases in Djibouti, 2010?2020

Although large-scale monitoring of the vector is still in its early stages, and more research and studies are needed to determine the scale and scope of the problem, the rapid spread of An. Stephensi is a threat to progress against malaria. In light of this, the WHO's new initiative aims to support a comprehensive response to the mosquito's expansion through a five-pronged approach, which includes:

• Increasing collaboration between actors conducting monitoring, research and control of An. Stephensi;
• Strengthening entomological surveillance to determine the extent of the spread of this malaria vector and its role in malaria transmission;
• Improving information exchange and raising awareness of An. Stephensi in the areas and communities most affected;
• Developing evidence-based guidance for national malaria control programmes, and prioritizing research to evaluate the impact of vector control interventions.
  

Strategies against invasive vectors

Generally, in responding to invasive species, three main strategies can be considered:

While the feasibility of these strategies is not yet known when it comes to An. stephensi in Africa, it is essential that action be taken to build an evidence base to assess their validity while responding to the task at hand of halting the spread of this invasive vector across Africa.

Building and maintaining an integrated response

National responses to An. stephensi should be part of a comprehensive response to malaria vectors, guided by the WHO Global technical strategy for malaria 2016 - 2030. Where feasible, integration with efforts to control other vector-borne diseases should be explored, as for example in the area of breeding site surveillance in urban and in peri-urban areas. The WHO Global Vector Control Response 2017 - 2030 provides a framework for investigating and implementing such integration across vector-borne diseases.

Its four pillars of action are:

According to the WHO Malaria Threats Map, the presence of An. Stephensi in Africa is o­ne of the multiple biological challenges with the potential to reverse malaria control and elimination efforts. Rather than focusing o­n An. Stephensi as a stand-alone threat, the organization recommends that affected countries ensure their responses are part of a comprehensive approach and are integrated with existing efforts to control malaria and other vector-borne diseases.

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