Possible contributing factors to the paucity of yellow fever epidemics in the Ashanti region of Ghana, west Africa.

Yellow fever virus vectors identified in the Ashanti region of Ghana included Aedes aegypti, Aedes africanus, Aedes luteocephalus and Aedes vittatus. Other mosquito species, unrelated to yellow fever transmission, identified in this study included Culex tigripes, Culex thalassius, Culex decens, Culex tarsalis, Anopheles gambiae, Anopheles stephansi and Toxorynchites brevipalpis. Factors generally known to influence yellow fever transmission were also studied in the Ashanti region. These included Aedes mosquito larval indices, biting or man-contact rates, rainfall, relative humidity and duration of sunshine. Calculated values for these factors were found to be far below internationally accepted threshold values, due, perhaps, to the vast distribution, resilience and preferential predatory propensity of the larvae of T. brevipalpis, a mosquito species we found exclusively in the Ashanti region of Ghana, for A. aegypti larvae. Other predators of mosquito larvae encountered included Notonecta (Nepa species), Hydromerta, Culex tigripes, Belostoma and Lispa. The observed paucity of yellow fever outbreaks in the Ashanti region of Ghana may, in the main, be due to the preponderance resilience and selective predatory propensity and preference of T. brevipalpis for A. aegypti larvae. Furthermore, the observed presence of other predators which prey on A. aegypti larvae in the study areas, the low larval indices and the low man-vector contact rates recorded as well as the high prevalence of Group B antibodies found in the blood of the population of this region may also be contributory to the paucity of yellow fever outbreaks in the Ashanti region.