Diversity in Anopheles gambiae s.s and Wuchereria bancrofti, and the Distribution of Lymphatic Filariasis in Ghana
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2010
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Abstract
Lymphatic Filariasis (LF) is a disease transmitted by the filarial worm Wuchereria bancrofti. It is a disease of public health importance, and has been earmarked for elimination by the year 2020 through Mass Drug Administration (MDA) programs. In Ghana, the distribution of LF reveals a pattern of endemicity separated by a continuous area of non-endemicity. As such, this study hypothesized that genetic differences – probably not revealed by current identification methods, within LF vectors and parasite populations, may be responsible for the observed distribution pattern. The main aim of this project was therefore to explain the current observed distribution of LF based on the diversity in the Anopheles gambiae s.s and Wuchereria bancrofti in Ghana. Mosquito samples were collected from 14 sites across Ghana, divided into 4 ecological zones, and falling into endemic and non-endemic areas. W. bancrofti samples were picked from dissected mosquitoes -using dissecting pins, collected from previous studies in Ghana. The samples were morphologically and molecularly identified using routine methods. Using the PCR, the Cytochrome C Oxidase subunit 1 (COI) from W. bancrofti and An. gambiae s.s was amplified, sequenced and analyzed. To better understand the effects of environmental factors on the diversity in the An. gambiae s.s, the spatial distribution of the An. gambiae M and S molecular forms and associated environmental factors were examined, and their relationship with disease prevalence was determined. A total of 10274 mosquitoes were collected, out of which 6150 (59.9%) were morphologically identified as An. gambiae s.l; 1494 (24.3%) of the An. gambiae s.l were further classified into sibling species. The S form of An. gambiae s.s predominated in the middle belt, while the M form predominated in the northern and coastal Savannah belts. Bivariate correlation analysis between the M and S forms indicated that their prevalences were negatively correlated (-0.763). Spatial analyses carried out, indicated a positive spatial clustering for both the An. gambiae M (MI =0.19, Z score=4.2, P< 0.01) and An. gambiae S (MI =0.19, Z score=4.2, P< 0.01) forms. Multiple regression analyses of all data (n=70 sites), indicated that temperature was an important variable for both forms, explaining for An. gambiae M, 28% (R2=0.28, F=25.8, P<0.001) and for An. gambiae S, 36% (R2=0.36, F=37.9, P<0.001) of the variance in the model. An. gambiae M was significantly correlated with LF, and 2.5 to 3 times more prevalent in the high LF zone than low to medium LF zones. Phylogenetic analyses of An. gambiae s.s revealed the samples to be grouped according to endemicity and ecological zones. The equality of evolutionary rate between the consensus sequences from the endemic and non-endemic areas, revealed a χ2 test statistic of 3.71 (P = 0.054), rejecting the null hypothesis of equal rates between lineages. Seven and 15 W. bancrofti specimens from Gomoa District (in the South) and Bongo District (in the North) respectively were sequenced. There was a significant difference in the evolutionary rate between the consensus sequences of W. bancrofti from the North and the South, with a χ2 test statistic of 7.44 (P = 0.00637). The results of this study indicate that environmental factors, especially temperature, play an important role in the distribution of LF and its vectors in Ghana. The observed genetic differences in An. gambiae s.s and W. bancrofti populations in Ghana may affect the vector-parasite interactions in various areas, and may explain the observed distribution of LF in the country.
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A Thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of Doctor of Philosophy