Malaria Transmission Dynamics and Pyrethroid Insecticide Resistance Status of Anopheles Gambiae Sensu Lato Gilles (Diptera: Culicidae) in two Districts of the Brong Ahafo Region, Ghana

Malaria results from infection with Plasmodium species transmitted mostly through the bite of female Anopheles species. The Brong Ahafo Region (BAR) of Ghana is endemic for malaria with endemicity greater than 75%. The Ghana National Malaria Control Programme (NMCP) has prioritised the use of pyrethroid-treated bednet and/or indoor residual spraying as a key strategy for combating malaria transmission. Control of malaria transmission using insecticide treated bed-nets and/or IRS impacts on both morbidity and mortality due to malaria. Evidence of insecticide resistance in malaria vectors in different areas necessitates surveillance studies to allow prompt detection of resistance should it arise and thus enable its management. Also, in preparation for the future IRS expansion programme planned by the NMCP, data on indices of malaria transmission would be required from several parts of Ghana, including the BAR. Hence, density, diversity, biting habits, feeding behaviour, parity rates, survival rates, sporozoite rates and entomological inoculation rates (EIRs) as well as pyrethroid insecticide resistance in the main malaria vector were investigated in some communities in two districts of the BAR. Mosquito larvae were collected using the dipping technique and reared to the adult stage and used for the insecticide susceptibility bioassays. Adult mosquitoes were also collected using human landing catches. The susceptibility bioassays were carried out using the standard WHO diagnostic bioassay kits. Bioassays were performed on non-blood fed female mosquitoes 2- to 4-day old. Knockdown rates were recorded after 5, 10, 15, 20, 30, 40, 50, 60, and 80 minutes. Mortality rates 24 hours post-exposure were also noted. Anopheles gambiae complex mosquitoes were analysed using the An. gambiae species specific PCR protocol. DNA from specimens identified as An. gambiae s.s. were subjected to PCR assays for the identification of the M and S molecular forms, and the detection of the leucine to phenylalanine knockdown resistance (kdr) gene mutation. Ovaries of wild-captured adult mosquito samples were dissected to examine the ovarian tracheations in order to determine parity, survival rates and life expectancy. The heads and thoraces of the parous mosquito samples were tested for the presence of circumsporozoite antigens of P. falciparum using enzyme-linked immunosorbent assay. A total of 15,384 Anopheles was collected, 13,088 in the rainy season and 2,296 in the dry season. Three Anopheles species were identified in both seasons: An. gambiae s.l. Gilles (98.80%), An. hancocki Edwards (1.20%) and An. coustani Laveran (0.03%). Anopheles gambiae s.s. was the only member of the An. gambiae s.l. found, consisting of both the M and S molecular forms but the S form was predominant (72.30%). No M/S hybrids were found. The susceptibility tests showed resistance to deltamethrin and permethrin. That is, mortality rates below 80% were observed ranging from 45% to 59% for the 0.05% deltamethrin bioassays conducted at Ahafo Kenyasi and Sunyani respectively. Mortality rates for the 0.75% permethrin bioassays ranged from 27% to 35% for the Ahafo Kenyasi and Sunyani samples respectively. Lastly, 46% mortality was observed for the Ahafo Kenyasi samples tested with 1.5% permethrin. The leucine to phenylalanine kdr gene mutation was detected in the two molecular forms but it was more frequent in the S form. That is, approximately 80.1% and 31.0% of the S and M molecular forms of the An. gambiae s.s. respectively, had the mutation. Anopheles gambiae s.s had its peak biting activity in the third and fourth quarters of the night in the rainy season but activity in the dry season peaked in the third quarter. In the rainy season, human-biting rates did not differ significantly between Sunyani, Ahafo Kenyasi and Hwidiem but the rates in these communities were significantly higher than in Chiraa (p = 0.034). In the dry season, however, there were no significant differences in HBR between the study sites (p = 0.898). Overall biting rates were higher in the rainy season (p < 0.001). Parity and survival rate did not differ significantly between sites but they were higher in the dry season. The sporozoite rate of both the An. hancocki and An. coustani was zero. There were no significant differences in P. falciparum sporozoite rates of An. gambiae between sites and seasons, however, EIRs were higher in the rainy season. The average inoculation rate was 2.864 infective bites per man per night (ib/m/n) in the rainy season, which gives an estimated 85.92 infective bites per man per month (ib/m/m). Comparative figures for the dry season were 0.468 ib/m/n and 14.07 ib/m/m. An overall average inoculation rate was 1.649 ib/m/n, which gives an estimated 49.48 ib/m/m. The results of this study suggest that the An. gambiae s.s. populations are resistant to the pyrethroid (deltamethrin and permethrin) insecticides tested. The implication of this is that malaria vector control measures in these communities using pyrethroid insecticides may be compromised due to the existence of insecticide resistance in the main malaria vector, An. gambiae s.s. The results of this study also suggest that the present study communities are areas of high malaria transmission intensity with transmission occurring all year round even though it is significantly higher in the rainy season.

Source: KNUST