Some factors contributing to the paucity of yellow fever in the Ashanti Region of Ghana

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Mosquitoes occurring in an urban and two rural areas of Ashanti were identified in a study to find out why the Ashanti Region of Ghana has nearly always escaped Yellow Fever epidemics that have swept through the country since 1900. Several species of the Yellow Fever mosquitoes were encountered, namely Aedes (Stegomyia) aegypti Linnaeus, Aedes (Stegomyia) africanus Theobald, Aedes (Stegomyia) luteocephalus Newstead, Aedes (Stegoiuyia) vittatus Bigot. Other mosquitoes were Culex (Culex) decens Theobald, Culex (Culex) thalassius Theobald, Culex (Lutzia) trigripes Granpre, Anopheles gambiae S.I. and Toxorhynchites brevipalpis Theobald. The mean Aedes mosquito indices throughout the research work were as follows: Biting rate 0.53; House Index 8.85; Container Index 3.67 and Breteau Index 11.45. Although the values are considered capable of promoting the transmission of Yellow Fever, they are remarkably lower than the International Threshold values of Biting rate 2; House Index 35; Container index 20; Breteau Index 50 and therefore unlikely to promote Yellow fever transmission by Aedes aegypti in the urban cycle. Rainfall and Relative Humidity (%) at 1500 hours GMT were remarkably correlated with larval and Biting indices. The low larval indices which may have reflected on the low man—vector contact rates were probably influenced by the vast distribution, resi1nce and predatory propensity of Toxorhynchites brevipalpis. Other minor predators found on the Aedes mosquito were Culex fLutzia) tigripes, Notonecta, Nepa sp. (Water scorpion), Hydrometra (Water Stick), Belostoma (giant water bug) and Lispa (anthomyid fly). Toxorhynchites preferred feeding on Aedes aegypti to other mosquito species such as Culex decens and Anopheles gambiae. No Toxorhynchites were found in other regions bordering Ashanti where Yellow Fever epidemics have been recorded.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirements for the award of the Degree of Master of Philosophy in Clinical Microbiology, 1988