Assessment of malaria transmission intensity and insecticide resistance mechanisms in three rural areas of the Moyen Ogooué Province of Gabon

Dynamics of Plasmodium species and genotype diversity in adults with asymptomatic infection in Gabon

OBJECTIVES

We investigated the diversity and dynamics of Plasmodium infection in serially collected samples from asymptomatic participants of a clinical trial assessing the efficacy and safety of ivermectin in Gabon. We checked whether the baseline sample reflected the P. falciparum genotype and Plasmodium species diversity seen over 7 days of follow-up.

METHODS

Blood samples were collected at inclusion, every 8 hours until hour 72, daily until day 7, and on day 14. Plasmodium species was determined by qPCR and pfmsp1 length polymorphism was assessed for P. falciparum genotyping.

RESULTS

In 17/48 (35%) individuals, all pfmsp1 genotypes identified during the assessed period were detected at baseline; in 31/48 (65%), new genotypes were found during follow-up. Additional sampling at hour 24 allowed the identification of all genotypes seen over 7 days in 50% of the individuals. Ivermectin did not impact the genotype dynamics. Mixed Plasmodium spp. infections were detected in 28/49 (57%) individuals at baseline, and detection of non-falciparum infections during follow-up varied.

CONCLUSIONS

Our results reveal complex intra-host dynamics of P. falciparum genotypes and Plasmodium species and underscore the importance of serial sampling in clinical trials for antimalarial drugs with asymptomatically P. falciparum-infected individuals. This might allow a more accurate identification of genotypes in multiple infections, impacting the assessment of drug efficacy.

Contribution of Anopheles gambiae sensu lato mosquitoes to malaria transmission during the dry season in Djoumouna and Ntoula villages in the Republic of the Congo

BACKGROUND

Mosquitoes belonging to the Anopheles gambiae sensu lato complex play a major role in malaria transmission across Africa. This study assessed the relative importance of members of An. gambiae s.l. in malaria transmission in two rural villages in the Republic of the Congo.

METHODS

Adult mosquitoes were collected using electric aspirators from June to September 2022 in Djoumouna and Ntoula villages and were sorted by taxa based on their morphological features. Anopheles gambiae s.l. females were also molecularly identified. A TaqMan-based assay and a nested polymerase chain reaction (PCR) were performed to determine Plasmodium spp. in the mosquitoes. Entomological indexes were estimated, including man-biting rate, entomological inoculation rate (EIR), and diversity index.

RESULTS

Among 176 mosquitoes collected, An. gambiae s.l. was predominant (85.8%), followed by Culex spp. (13.6%) and Aedes spp. (0.6%). Three members of the An. gambiae s.l. complex were collected in both villages, namely An. gambiae sensu stricto (74.3%), Anopheles coluzzii (22.9%) and Anopheles arabiensis (2.8%). Three Plasmodium species were detected in An. gambiae s.s. and An. coluzzii (Plasmodium falciparum, P. malariae and P. ovale), while only P. falciparum and P. malariae were found in An. arabiensis. In general, the Plasmodium infection rate was 35.1% (53/151) using the TaqMan-based assay, and nested PCR confirmed 77.4% (41/53) of those infections. The nightly EIR of An. gambiae s.l. was 0.125 infectious bites per person per night (ib/p/n) in Djoumouna and 0.08 ib/p/n in Ntoula. The EIR of An. gambiae s.s. in Djoumouna (0.11 ib/p/n) and Ntoula (0.04 ib/p/n) was higher than that of An. coluzzii (0.01 and 0.03 ib/p/n) and An. arabiensis (0.005 and 0.0 ib/p/n).

CONCLUSIONS

This study provides baseline information on the dominant vectors and dynamics of malaria transmission in the rural areas of the Republic of the Congo during the dry season. In the two sampled villages, An. gambiae s.s. appears to play a predominant role in Plasmodium spp.

TRANSMISSION

Entomological indicators of Plasmodium species transmission in Goma Tsé-Tsé and Madibou districts, in the Republic of Congo

BACKGROUND

Malaria remains a major public health problem in the Republic of Congo, with Plasmodium falciparum being the deadliest species of Plasmodium in humans. Vector transmission of malaria is poorly studied in the country and no previous report compared rural and urban data. This study aimed to determine the Anopheles fauna and the entomological indices of malaria transmission in the rural and urban areas in the south of Brazzaville, and beyond.

METHODS

Indoor household mosquitoes capture using electric aspirator was performed in rural and urban areas during raining and dry seasons in 2021. The identification of Anopheles species was done using binocular magnifier and nested-PCR. TaqMan and nested-PCR were used to detect the Plasmodium species in the head/thorax and abdomens of Anopheles. Some entomological indices including the sporozoite infection rate, the entomological inoculation rate and the man biting rate were estimated.

RESULTS

A total of 699 Anopheles mosquitoes were collected: Anopheles gambiae sensu lato (s.l.) (90.7%), Anopheles funestus s.l. (6.9%), and Anopheles moucheti (2.4%). Three species of An. gambiae s.l. were identified including Anopheles gambiae sensu stricto (78.9%), Anopheles coluzzii (15.4%) and Anopheles arabiensis (5.7%). The overall sporozoite infection rate was 22.3% with a predominance of Plasmodium falciparum, followed by Plasmodium malariae and Plasmodium ovale. Anopheles aggressiveness rate was higher in households from rural area (1.1 bites/night) compared to that from urban area (0.8 ib/p/n). The overall entomological inoculation rate was 0.13 ib/p/n. This index was 0.17 ib/p/n and 0.092 ib/p/n in rural and in urban area, respectively, and was similar during the dry (0.18 ib/p/n) and rainy (0.14 ib/p/n) seasons.

CONCLUSION

These findings highlight that malaria transmission remains high in rural and urban area in the south of Republic of Congo despite the ongoing control efforts, thereby indicating the need for more robust interventions.

Anopheles gambiae s.s. resistance to pyrethroids and DDT in semi-urban and rural areas of the Moyen-Ogooué Province, Gabon

BACKGROUND

Pyrethroids are the main insecticides used in vector control for malaria. However, their extensive use in the impregnation of long-lasting insecticidal nets (LLINs) and indoor residual spraying has led to the development of resistance, threatening its success as a tool for malaria control. Baseline data prior to large scale distribution of LLINs are important for the implementation of efficient strategies. However, no data on the susceptibility of malaria vectors is available in the Moyen-Ogooué Province in Gabon. The aim of this study was to assess the susceptibility to pyrethroids and organochlorides of malaria vectors from a semi-urban and rural areas of the province and to determine the frequency of insecticide resistance genes.

METHODS

Larvae were collected from breeding sites in Lambaréné and Zilé and reared to adults. Three to five-day old female Anopheles gambiae sensu lato mosquitoes were used in cone tube assays following the WHO susceptibility tests protocol for adult mosquitoes. A subsample was molecularly identified using the SINE200 protocol and the frequency of Vgsc-1014 F and - 1014 S mutations were determined.

RESULTS

Anopheles gambiae sensu stricto (s.s.) was the sole species present in both Lambaréné and Zilé. Mosquito populations from the two areas were resistant to pyrethroids and organochlorides. Resistance was more pronounced for permethrin and DDT with mortality lower than 7% for both insecticides in the two study areas. Mosquitoes were statistically more resistant (P < 0.0001) to deltamethrin in Lambaréné (51%) compared to Zilé (76%). All the mosquitoes tested were heterozygous or homozygous for the knockdown resistance (Kdr) mutations Vgsc-L1014F and Vgsc-L1014S with a higher proportion of Vgsc-L1014F homozygous in Lambaréné (76.7%) compared to Zilé (57.1%).

CONCLUSION

This study provides evidence of widespread resistance to pyrethroids in An. gambiae s.s., the main malaria vector in the Moyen-Ogooué Province. Further investigation of the mechanisms underlining the resistance of An. gambiae s.s. to pyrethroids is needed to implement appropriate insecticide resistance management strategies.