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
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Date
2009
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Abstract
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.
Description
A Thesis Submitted to the School of Graduate Studies of the Kwame
Nkrumah University of Science and Technology for Doctor of Philosophy
(Ph.D.) Degree in Biological Sciences,