Molecular identification and role in malaria transmission of Anopheles gambiae complex in dry season in Freetown, Sierra Leone,2019–2020

  Objective  To understand the distribution of Anopheles gambiae complex and its infection rate of Plasmodium in Freetown, Sierra Leone, and provide epidemiological evidence for the effective malaria prevention and control.

  Methods  Malaria vector surveillance was carried out at 10 surveillance sites in Freetown, the capital of Sierra Leone, from December 2019 to March 2020. Genomic DNA was extracted from the thorax and abdomen of each Anopheles gambiae mosquito. Mosquito species were identified by multiplex PCR. The positive rate of Plasmodium in Anopheles was detected by nested PCR. Excel 2019 and SPSS 21.0 were used to analyze the experimental results.

  Results  From December 2019 to March 2020, a total of 349 Anopheles mosquitoes were captured, with an average trapping density of 0.83 Anopheles mosquitoes/trap/night. A total of 324 Anopheles mosquitoes were trapped in the western rural areas, accounting for 92.84% of the total Anopheles mosquitoes captured. The average density was 2.00/night. The molecular identification of 192 An. gambiae complexes showed that 84 (43.75%) mosquitoes were An. gambiae s.s. S-form and 108 (56.25%) mosquitoes were An. gambiae s.s. M-form. Only Plasmodium falciparum was found in An. gambiae s.s. mosquitoes, but no Plasmodium vivax, Plasmodium malaria, and Plasmodium ovale were found. The positive rates of P. falciparum sporozoites and oocysts were 3.13% and 6.77%, respectively. Statistical analysis showed that there was no significant difference in the infection rate of P. falciparum sporozoites and oocysts between December 2019 and March 2020. The positive rates of P. falciparum sporozoites and oocysts of 108 mosquitoes of An. gambiae s.s. M-form were 3.70% and 7.41%, respectively; The positive rates of P. falciparum sporozoites and oocysts of 84 mosquitoes of An. gambiae s.s. S-form were 2.38% and 5.95%, respectively. The positive rates of P. falciparum sporozoites and oocysts of An. gambiae s.s. M-form were higher than those of An. gambiae s.s. S-form, but statistical analysis showed no significant difference.

  Conclusion  The predominant Anopheles species in Freetown was An. gambiae s.s. (M-form and S-form). Anopheles funestus and Anopheles coustani were also found. The density of Anopheles mosquitoes in the western rural areas was much higher than that in the western urban areas, suggesting that the western rural areas has a higher risk of malaria infection. The positive rate of P. falciparum oocysts was two times higher than that of sporozoites in different months in dry season. However, the monthly transmission of malaria was stable in dry season. This study results can be used as theoretical basis for the development of malaria prevention and control strategy in Sierra Leone.