Anopheline Mosquito Vectors and Malaria Transmission Dynamics Along an Altitudinal Gradient on the Highlands of Mambilla Plateau, Nigeria

Abstract

The presence of a suitable and efficient vector is a major determinant of local malaria transmission intensity.In the last few years, the Mambilla Plateau has witnessed an influx of people from different parts of the country who engage in various anthropogenic activities that could have profound consequence on vectors and malaria transmission in the region.Until now, no data exist for vector anopheline species and malaria transmission intensity for the Mambilla Plateau, a highland region prone to malaria epidemics. The study aimed to provide the baseline data on the anopheline vectors of malaria and malaria transmission on the highlands of Mambilla Plateau, Nigeria.The study was conducted in five locations along an altitudinal gradient namely; Nguroje (1,885m), Yelwa (1,674m), Gembu (1,584m), Kakara (1,496m) and Mayo Selbe (484m), above sea level. Collections were conducted once every month from November 2015 to October 2017. Adult mosquitoes were captured by the use of Centre for Disease Control (CDC) modified light traps and Pyrethrum Spray Catches (PSC). Larvae were sampled using the standard dipping methodand reared to adulthood. Larval habitats were identified and characterized according to the features observed in the field. The physicochemical parameters were also measured. The mosquitoes were identified morphologically by microscopy using taxonomic keys.Anopheles gambiae sensu lato (sl)where further identified genetically to sub species and molecular levels (M and S) forms by Polymerase Chain Reaction (PCR).Enzyme Linked Immunosorbent Assay (ELISA) was used to test for Plasmodium falciparum infectivity as well as source of bloodmeal.A total of 878 anophelines comprising of five species namely; Anopheles gambiae sl,An. coustani, An. funestus, An. pharoensis, and An. rufipes,were collected. An. gambiaeslwas the highest number collected in all the locations and seasons (757). An. gambiaesensu stricto was the only member of theAn. gambiae complex identified by PCR, the S molecular form of the vector dominated over the M form (520 and 192) respectively.There was a very strong positive correlation between the S and M forms, r2 = 0.94204.Mayo-selbe had the highest species abundance of 572 but lowest species diversity(H’= 0.24). Yelwa had the least species abundance (24) but very high diversity (H’=0.81). December recorded highest abundance of anophelines (235) but very low diversity (H’ = 0.28). A total of 60 larval habitats were sampled and characterized. Swamps were the highest number of habitats encountered (15) and tree trunk was the least (1). There was relatively low occurrence of positive breeding habitats along the altitudinal locations. Only six were positive. This could be due to the temporary nature of the habitats. Larvae were associated with still, clear and sunlit temporary habitats with a wide range of physicochemical parameters.There were two peak malaria transmission seasons in the Mambilla Plateau, in December and June. All malaria transmission indices namely; sporozoites rates (SR), man biting rates (MBR) and entomological inoculation rates (EIR) decreased with increasing altitude. These were in the range of SR (0.17-3.55%), MBR (0.002-2.56) and EIR (0.003-9.09 ib/p/n). Only 22.67% of the mosquitoes had fed on human blood, other sources of blood meal were unknown.The human blood index ranged from (0.00 to 0.22). Nguroje and Kakararecorded the lowest while Mayo-selbe recorded the highest.In the current study malaria transmission is occurring in the Mambilla Plateau and An. gambiae s. s is the major species of An. gambiaecomplex involved. However, other species that were collected have been reported to play secondary roles in transmission in other parts of Africa. Malaria control in the study area should therefore focus on An. gambiae s. s. There should also be close monitoring and surveillance of the other anophelines species that were encountered in the study area as these may become potential vectors in the future.This study has provided the baseline information about the anophelines malaria vectors and malaria transmission in the highlands of Mambilla Plateau, which would be helpful for the sustainable management of vector mosquitoes and also inform policy measures to prevent or counter malaria epidemics in the future.