Malaria vector dynamics and utilization of insecticide-treated nets in low-transmission setting in Southwest Ethiopia: implications for residual transmission

Dynamics of malaria vector composition and Plasmodium falciparum infection in mainland Tanzania: 2017-2021 data from the national malaria vector entomological surveillance

BACKGROUND

In 2015, Tanzania National Malaria Control Programme (NMCP) established a longitudinal malaria vector entomological surveillance (MVES). The MVES is aimed at a periodical assessment of malaria vector composition and abundance, feeding and resting behaviours, and Plasmodium falciparum infection in different malaria epidemiological strata to guide the NMCP on the deployment of appropriate malaria vector interventions. This work details the dynamics of malaria vector composition and transmission in different malaria epidemiological strata.

METHODS

The MVES was conducted from 32 sentinel district councils across the country. Mosquitoes were collected by the trained community members and supervised by the NMCP and research institutions. Three consecutive night catches (indoor collection with CDC light trap and indoor/outdoor collection using bucket traps) were conducted monthly in three different households selected randomly from two to three wards within each district council. Collected mosquitoes were sorted and morphologically identified in the field. Thereafter, the samples were sent to the laboratory for molecular characterization using qPCR for species identification and detection of P. falciparum infections (sporozoites). ELISA technique was deployed for blood meal analysis from samples of blood-fed mosquitoes to determine the blood meal indices (BMI).

RESULTS

A total of 63,226 mosquitoes were collected in 32 district councils from January 2017 to December 2021. Out of which, 39,279 (62%), 20,983 (33%) and 2964 (5%) were morphologically identified as Anopheles gambiae sensu lato (s.l.), Anopheles funestus s.l., and as other Anopheles species, respectively. Out of 28,795 laboratory amplified mosquitoes, 13,645 (47%) were confirmed to be Anopheles arabiensis, 9904 (34%) as An. funestus sensu stricto (s.s.), and 5193 (19%) as An. gambiae s.s. The combined average entomological inoculation rates (EIR) were 0.46 (95% CI 0.028-0.928) for An. gambiae s.s., 0.836 (95% CI 0.138-1.559) for An. arabiensis, and 0.58 (95% CI 0.165-0.971) for An. funestus s.s. with variations across different malaria transmission strata. Anopheles funestus s.s. and An. arabiensis were predominant in the Lake and South-Eastern zones, respectively, mostly in high malaria transmission areas. Monthly mosquito densities displayed seasonal patterns, with two peaks following the rainy seasons, varying slightly across species and district councils.

CONCLUSION

Anopheles arabiensis remains the predominant vector species followed by An. funestus s.s. in the country. Therefore, strengthening integrated vector management including larval source management is recommended to address outdoor transmission by An. arabiensis to interrupt transmission particularly where EIR is greater than the required elimination threshold of less than one (< 1) to substantially reduce the prevalence of malaria infection.

Systematic review of sporozoite infection rate of Anopheles mosquitoes in Ethiopia, 2001-2021

BACKGROUND

Adult mosquitoes of the genus Anopheles are important vectors of Plasmodium parasites, causative agents of malaria. The aim of this review was to synthesize the overall and species-specific proportion of Anopheles species infected with sporozoites and their geographical distribution in the last 2 decades (2001-2021).

METHODS

A comprehensive search was conducted using databases (PubMed, Google Scholar, Science Direct, Scopus, African Journals OnLine) and manual Google search between January 1 and February 15, 2022. Original articles describing work conducted in Ethiopia, published in English and reporting infection status, were included in the review. All the required data were extracted using a standardized data extraction form, imported to SPSS-24, and analyzed accordingly. The quality of each original study was assessed using a quality assessment tool adapted from the Joanna Briggs Institute critical appraisal checklist. This study was registered on PROSPERO (International Prospective Register of Systematic Reviews; registration no. CRD42022299078).

RESULTS

A search for published articles produced a total of 3086 articles, of which 34 met the inclusion criteria. Data on mosquito surveillance revealed that a total of 129,410 anophelines comprising 25 species were captured, of which 48,365 comprising 21 species were tested for sporozoites. Anopheles arabiensis was the dominant species followed by An. pharoensis and An. coustani complex. The overall proportion infected with sporozoites over 21 years was 0.87%. Individual proportions included Anopheles arabiensis (1.09), An. pharoensis (0.79), An. coustani complex (0.13), An. funestus (2.71), An. demeilloni (0.31), An. stephensi (0.70), and An. cinereus (0.73). Plasmodium falciparum sporozoites accounted 79.2% of Plasmodium species. Mixed infection of Plasmodium vivax and P. falciparum was only reported from one An. arabiensis sample.

CONCLUSIONS

Anopheles arebiensis was the dominant malaria vector over the years, with the highest sporozoite infection proportion of 2.85% and an average of 0.90% over the years. Other species contributing to malaria transmission in the area were An. pharoensis, An. coustani complex, An. funestus, An. stephensi, and An. coustani. The emergence of new vector species, in particular An. stephensi, is particularly concerning and should be investigated further.

The interplay between malaria vectors and human activity accounts for high residual malaria transmission in a Burkina Faso village with universal ITN coverage

BACKGROUND

Mosquito and human behaviour interaction is a key determinant of the maximum level of protection against malaria that can be provided by insecticide-treated nets (ITNs). Nevertheless, scant literature focuses on this interaction, overlooking a fundamental factor for efficient malaria control. This study aims to estimate malaria transmission risk in a Burkina Faso village by integrating vector biting rhythms with some key information about human habits.

METHODS

Indoor/outdoor human landing catches were conducted for 16 h (16:00-08:00) during 8 nights (September 2020) in Goden village. A survey about net usage and sleeping patterns was submitted to half the households (October-December 2020). A subsample of collected specimens of Anopheles gambiae sensu lato was molecularly processed for species identification, Plasmodium detection from heads-thoraxes and L1014F pyrethroid-resistance allele genotyping. Hourly mosquito abundance was statistically assessed by GLM/GAM, and the entomological inoculation rate (EIR) was corrected for the actual ITN usage retrieved from the questionnaire.

RESULTS

Malaria transmission was mainly driven by Anopheles coluzzii (68.7%) followed by A. arabiensis (26.2%). The overall sporozoite rate was 2% with L1014F estimated frequency of 0.68 (N = 1070 out of 15,201 A. gambiae s.l. collected). No major shift in mosquito biting rhythms in response to ITN or differences between indoor and outdoor catches were detected. Impressive high biting pressure (mean 30.3 mosquitoes/person/hour) was exerted from 20:00 to 06:00 with a peak at 4:00. Human survey revealed that nearly all inhabitants were awake before 20:00 and after 7:00 and at least 8.7% had no access to bednets. Adjusting for anthropological data, the EIR dropped from 6.7 to 1.2 infective bites/person/16 h. In a scenario of full net coverage and accounting only for the human sleeping patterns, the daily malaria transmission risk not targetable by ITNs was 0.69 infective bites.

CONCLUSIONS

The high mosquito densities and interplay between human/vector activities means that an estimated 10% of residual malaria transmission cannot be prevented by ITNs in the village. Locally tailored studies, like the current one, are essential to explore the heterogeneity of human exposure to infective bites and, consequently, to instruct the adoption of new vector control tools strengthening individual and community protection.

Detection of temporal, spatial and spatiotemporal clustering of malaria incidence in northwest Ethiopia, 2012–2020

Malaria is one of Ethiopia's most targeted communicable diseases for elimination. Malaria transmission varies significantly across space and time; and Ethiopia had space–time disparity in its transmission intensities. Considering heterogeneity and transmission intensity at the district level could play a crucial role in malaria prevention and elimination. This study aimed to explore temporal, spatial, and spatiotemporal clusters of malaria incidence in northwest Ethiopia. The analysis is based on monthly malaria surveillance data of districts and collected from the Amhara public health institute. The Kulldorff's retrospective space–time scan statistics using a discrete Poisson model were used to detect temporal, spatial, and space–time clusters of malaria incidence with and without adjusting the altitude + LLIN arm. Monthly malaria incidence had seasonal variations, and higher seasonal indices occurred in October and November. The temporal cluster occurred in the higher transmission season between September and December annually. The higher malaria incidence risk occurred between July 2012 and December 2013 (LLR = 414,013.41, RR = 2.54, P < 0.05). The purely spatial clustering result revealed that the most likely cluster occurred in the north and northwest parts of the region while secondary clusters varied in years. The space–time clusters were detected with and without considering altitude + LLIN arm. The most likely space–time cluster was concentrated in northwestern and western parts of the region with a high-risk period between July 2012 and December 2013 (LLR = 880,088.3, RR = 5.5, P < 0.001). We found eight significant space–time clusters using the altitude + LLIN arm. The most likely space–time cluster occurred in the western and northwestern parts of the region in July 2012–December 2013 (LLR = 886,097.7, RR = 5.55, P < 0.05). However, secondary clusters were located in eastern, northwestern, western parts of regions, which had different cases and relative risks in each cluster. Malaria transmission had temporal, spatial, and space–time variation in the region at the district level. Hence, considering these variations and factors contributing to malaria stratification would play an indispensable role in preventing and controlling practices that ultimately leads to malaria eliminations.