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Background:  Malaria is endemic in Sierra Leone, with stable and perennial transmission in all parts of the country. At present, the main prevention and control measures for mosquito vectors here involve insecticide treated nets (ITN) and indoor residual spraying (IRS). The most recent entomological surveillance was conducted prior to the civil war, between 1990 and 1994. Therefore, a new entomological surveillance required to support targeted malaria control strategies.   Methods:   Anopheles  mosquitoes were collected between June and December 2019 using the light trap method. On these, we conducted species identification, analyzed seasonal fluctuation and  Plasmodium  infection rate, and monitored insecticide resistance.   Results:  Surveillance of seasonal fluctuation showed that there were two peak of  Anopheles  density in July (mean 13.67 mosquitoes/trap/night) and October (mean 13.00 mosquitoes/trap/night). Meanwhile, the lowest  Anopheles  density was seen in early September. Ninety-one representatives of  Anopheles gambiae  s.l. were selected and identified as  An. coluzzii  ( n  = 35) and  An. gambiae  s.s. ( n  = 56) using PCR.  An. coluzzii  and  An. gambiae  s.s. were found to be heterozygous resistant to the knockdown resistance ( kdr ) L1014F mutation (100%). Meanwhile, the East African mutation ( kdr  L1014S) was absent in the tested mosquitoes. Three mosquitoes that tested positive for the parasite, had an individual  Plasmodium falciparum  infection rate of 12.50, 16.67, and 14.29%. The sampling dates of positive mosquitoes were distributed in the two periods of peak  Anopheles  mosquito density.   Conclusion:  This study identified the dominant  Anopheles  species in Freetown as  An. gambiae  while the predominant species within the  An. gambiae  complex was  An. gambiae sensu stricto . Surveillance of seasonal fluctuations and high  P. falciparum  infection rates in  Anopheles  indicate that the alternation of drought and rainy seasons from June to July, and from October to November, are the key periods for malaria control and prevention in Freetown, Sierra Leone. The high frequency of  kdr  allele mutations in  An. gambiae  calls for close monitoring of vector susceptibility to insecticides and tracing of resistance mechanisms in order to develop more effective vector control measures and strategies.

Entomological and Molecular Surveillance of Anopheles Mosquitoes in Freetown, Sierra Leone, 2019