Vectorial drivers of malaria transmission in Jabi Tehnan district, Amhara Regional State, Ethiopia

Abstract

Among the factors affecting the effectiveness of malaria control is poor knowledge of the entomologic drivers of the disease. We investigated anopheline populations as part of a baseline study to implement house screening of windows and doors as a supplementary malaria control tool towards elimination in Jabi Tehnan district, Amhara Regional State of Ethiopia. The samples were surveyed monthly using CDC light traps between June 2020 and May 2021. Mosquito trap density (< 3 mosquitoes/trap) was low, however, with a high overall Plasmodium sporozoite rate (9%; indoor = 4.3%, outdoor = 13.1%) comprising P. falciparum (88.9%) and P. vivax (11.1%). Anopheles gambiae s.l., mostly An. arabiensis, comprised > 80% of total anopheline captures and contributed ~ 42% of Plasmodium-infected mosquitoes. On the other hand, morphologically scored Anopheles funestus s.l., constituting about 6% of anopheline collections, accounted for 50% of sporozoite-infected mosquitoes. Most of the infected An. funestus s.l. specimens (86.7%) were grouped with previously unknown or undescribed Anopheles species previously implicated as a cryptic malaria vector in the western Kenyan highlands, confirming its wider geographic distribution in eastern Africa. Other species with Plasmodium infection included An. longipalpis C, An. theileri, An. demillioni, and An. nili. Cumulatively, 77.8% of the infected mosquitoes occurred outdoors. These results suggest efficient malaria parasite transmission despite the low vector densities, which has implications for effective endpoint indicators to monitor malaria control progress. Additionally, the largely outdoor infection and discovery of previously unknown and cryptic vectors suggest an increased risk of residual malaria transmission and, thus, a constraint on effective malaria prevention and control.

Keywords: Cryptic vectors; Highland areas; Malaria surveillance; Outdoor biting; Residual malaria transmission.

Figure 1

Map of the study area, Jabi Tehnan, a district in the West Gojjam Zone, Amhara regional state, Northwestern Ethiopia and its agroecological zones represented as follows, yellow = semi-arid/desert, green = subtropical dry mountain highlands and light green = subtropical plateau highlands.

Figure 2

Anopheline mosquito composition based on morphology, collected in Jabi Tehnan district, Amhara Regional State, Northwestern Ethiopia. The number of trap nights (n) were indoors: plateau highland = 69, dry mountain = 61, semi-arid = 64; outdoors: plateau highland = 82, dry mountain = 70, semi-arid = 90.

Figure 3

Mean captures (± se) of dominant anopheline species encountered in Jabi Tehnan district, Amhara Regional State, Northwestern Ethiopia. Comparisons are made between indoor and outdoor captures in each agroecology zone for each species, * significant difference at p < 0.05; ns, non-significant difference; se = standard error.

Figure 4

Neighbour-joining tree for sequences of mosquitoes (259–496 bp) morphologically scored as An. funestus group infected with P. falciparum sporozoites. Bootstrap support values are indicated at relevant nodes. Filled circles are sequences generated in this study from P. falciparum infected specimens; the novel cryptic species undescribed but previously reported in literature (pink/black), clustering with described species (blue/grey/green) or unreported species (red). Sequences of specimens harbouring Plasmodium sporozoite infection were deposited in GenBank (PP419038- PP419052).