Anopheles stephensi is an invasive disease vector, originally endemic in Asia and adept at transmitting malarial parasites Plasmodium falciparum and P. vivax.1,2 Native to south Asia and the Persian Gulf states,3 the first record in Africa of An. stephensi was from Djibouti in 2012,4 where it has been associated with an over 30-fold increase in malaria since 2013.5,6 Subsequent discoveries in Ethiopia7 and Sudan,8 in 2019 in Somalia9 and in 2020 in Somaliland9 (since confirmed in multiple sites in each country10–12), suggest An. stephensi is becoming established in the region and could spread further, with potentially serious consequences for malaria control and elimination.3,13
Unsurprisingly, the introduction of An. stephensi into the Horn of Africa has generated substantial global concern and calls for action14; this is primarily because of its potential to elevate malaria in densely populated urban environments where transmission has typically been considered lower than in rural communities.15,16 Unlike most Anopheles genera which tend to lay eggs in natural water sources and are relatively rarely found in urban settings, An. stephensi, in the same manner as Aedes aegypti, frequently lay their eggs in man-made water containers, including waste or polluted waters.17,18 Anopheles stephensi are thus well adapted to breeding sites created by urbanisation17 and thrive in such settings.1,2 Whilst the spread of An. stephensi in the Horn of Africa has attracted much recent attention, its invasion presents a threat to any countries with large populations located within habitats permissive for successful establishment. In sub-Saharan Africa and countries within the Middle East an increasing proportion of populations live in densely populated cities with insufficient water, sanitation, and hygiene (WaSH) and poor environmental hygiene, conditions in which An. stephensi may thrive and elevate malaria transmission.
Since 2000 the global Roll Back Malaria partnership has made significant progress in malaria reduction through a combination of scaling up access to insecticidal mosquito nets or indoor residual spraying and in access to effective rapid diagnostic testing and drug therapies. Resultingly, global malaria deaths declined steadily from 896,000 to 558,000 by 2019.19 However, over the last five to six years20 malaria deaths have increased again significantly to an estimated annual 627,000 by end-2020.19 This rise has occurred in parallel with a dramatic escalation in the number of conflicts and the number of internally displaced people (IDPs). By end-2021, an unprecedented 53.2 million people across 59 countries and territories were displaced as a result of conflict and violence – an increase of 5.8 million from 2020.21 The combination of increasing conflict, the destruction of urban areas and population displacement into urban settings appear inextricably linked to rises in cases of vector-borne diseases.22–25 Violence and widespread destruction of public and private infrastructure can mean populations have limited access to basic services such as healthcare, WaSH and education.21 This logically increases the risk of vector-borne diseases. Further, research has shown that where mass displacement of populations can increase the incidence of severe malaria26,27infection constitutes a large proportion of excess mortality and morbidity among displaced communities,28,29 particularly when they are immunologically vulnerable25; displaced children also appear at significantly higher risk than their non-displaced counterparts.30,31 This bears concern for outbreaks in densely populated, fragile urban areas.
The MENTOR Initiative (MI), an international non-governmental organisation, works in Aden city, Yemen, in partnership with the National Malaria Control Programme (NMCP) and the World Health Organization (WHO). As part of its operational research work into the control of Ae. aegypti, the vectors of dengue fever in Yemen’s cities, MI conducts entomological monitoring surveys of water containers across urban areas of the city, parts of which host IDPs. In 2021, as part of this monitoring, MI entomologically identified samples as An. stephensi, and here we report validation of these collections, confirming the presence of the vector in Yemen. This reports also contributes to those of others registered in the WHO Threats Map, enabling to map the vector’s spread across the country.9 Findings bear significance not just for the identification of invasive vectors and the expanding malaria risk across urban settings, but also for the fact that this vector was identified in an urban community hosting thousands of persons displaced within the country who are vulnerable to infection.