Anopheles stephensi: a guest to watch in urban Africa.

Malaria vector control programs in Sub-Saharan Africa have invested many efforts and resources in the control of eight-sibling species of Anopheles gambiae complex and An. funestus group. The behaviour of sibling species of these vectors is well known and used for implementing the current intervention tools. The reports of An. stephensi in urban Africa with different habitats breeding behaviour is an alert on the success of malaria vector control efforts achieved so far. This communication intends to give an insight on what should be considered as a challenge for the management of An. stephensi in urban Africa to retain the achievement attained in malaria control.

Background

Malaria vectors have been managed well for the past two decades with significant progress in preventing malaria and related adverse outcomes [1]. From 2018 to 2019 the malaria mortalities have been stalled with an increase in 2020, the efforts done so far through the distribution of long-lasting insecticidal nets (LLINs), indoor residual spray (IRS) and urban larval source management have increased the coverage [1, 2]. The gradual changes in land use, interventions and climate changes have led to species shift and re-distribution [3-6].

For a decade now in different countries of Africa there are reports of An. stephensi invasion [7-9]. This vector has been for long a malaria vector in south-eastern Asia [10]. The countries reported having An. stephensi are Djibouti, Ethiopia, Sudan and Somalia [9]. These reports have been confirmed after the DNA molecular analysis [11]. Anopheles stephensi is quite different from An. gambiae s.l. (Table 1). This species invasion has prompted the author to make a commentary on An. stephensi in urban Africa and its control challenges.

Table 1

Differences between An. gambiae s.l. and An. stephensi

Factor	Differences	Comment
Oocyte prevalence	If both feed in the same infected blood meal source An. stephensi have higher oocyst development rate than An. gambiae s.s [12].	This means An. stephensi are more susceptible to parasites than An. gambiae s.l.
Breeding sites	An. stephensi breeds in containers and water cans indoors and outdoors while An. gambiae s.l. breeds in the natural habitats away from human dwellings	An. stephensi has advantage of transmissions of malaria based on breeding sites and man access point.
Feeding and Resting preferences	An. stephensi higher densities are found cattle sheds than human dwellings while for An. gambiae s.l. feeding and resting depend on species. An. stephensi rests both indoor and outdoor while An. gambiae s.l. depend on the species. Most of An. stephensi feed on cattle while An. gambiae s.l. depends on species	The feeding and resting behaviour of An. stephensi suggests having contribution to malaria transmission for been in contact with man either indoor or outdoor

Main text

The introduction of Anopheles stephensi in African countries from Asia has alerted the national malaria control programmes in re-designing vector control strategies. The author indicates the main factors which are expected to be challenges in the efforts to control the species. These challenges are;

An. stephensi is different from the current malaria vectors available in Africa with its breeding habitats mostly utilizing containers, holes in trees, water storage tanks and roof gutters used by Aedes aegypti species [13] (Table 1). Also, they were found to co-habit with culicine species in polluted habitats [13]. In Sri Lanka the An. stephensi has been found colonizing large water bodies breeding sites [14] which for larviciding are difficult to attend effectively. This vector possess a risk of occurrence in more countries Africa as a first case was reported in Djibouti in 2012 [15], Ethiopia in 2016 [16] and in Sudan 2019 [17]. The distribution rate of An. stephensi is very high covering a long distance Djibouti to Sudan in 6 years.

nsecticide resistance has been reported as the main challenge for insectides used in IRS and in LLINs for other documented existing vector species [18]. In An. stephensi, the insecticides resistance has been reported in Sudan and Ethiopia [8, 19, 20]. Insecticides resistance confirmation is important for the vector control insectides based tools selection.

The An. stephensi in Asia do feeding on human and bovines, resting indoors and outdoors [12]. Due to variations on host availability in Africa it’s not well known in which host apart from humans shall feed on. The An. stephensi resting and feeding behaviour in all reported areas has not been yet established in African countries.

Monitoring of anthropogenic factors. Due to high rural-urban migration areas in sub–Saharan Africa, the emerging of urban agriculture, unplanned settlements, and poorly organized drainage systems effective habitats have been created [21-24]. The new species of An. stephensi is well known to be urban and peri urban malaria vector.

The way forward

To strengthen the entomological surveillance system with the ability to capture the presence of this invasive An. stephensi mosquitoes.

To coordinate capacity building for laboratory and field entomologists in identification of An. stephensi. This is of priority to ensure sustainacy of achieved malaria vector species control and cases in two decades, 2000 to 2020.

To establish the continuous monitoring of insecticide resistance profile of An. stephensi where the species will be reported to avoid impairing the existing tool efficacy.

To identify the potential breeding habitats for An. stephensi in urban and peri urban for appropriate control design.

To establish the sentinel sites for continues data collection in all zones. These sentinels’ sites should operate on proposed standard operating procedures for species sampling, identification and insecticides resistance status.

v) To emphases on the use of personal protection tools such as repellents for protection outdoors.

Conclusion

The NMCPs of sub-Saharan Africa have been awaken on insuring that, the attained malaria control efforts are not compromised by the new invasive species. The way forward plans should be considered for proper management and control of this new species vector.