CHAPTER ONE

Introduction     

1.1    Background of the Study

Yellow fever outbreaks are common in many regions of tropical Africa, especially in western Africa, and are responsible for thousands of cases annually. At least one yellow fever outbreak is reported to the World Health Organization (WHO) every year, and in the past 15 years, the incidence of yellow fever has steadily increased S.E. Robertson, B.P. Hull, O. Tomori, O. Bele, J.W. LeDuc, K. Es-teves[1996]. This is alarming, considering that a safe and efficacious yellow fever vaccine, 17D, has been avail-able for over 60 years. In addition, the maintenance cycle of yellow fever virus in the jungle and the epidemic transmission cycle in urban areas are fairly well understood, and urban yellow fever trans-mission is relatively easy to interrupt by mosquito control measures. Despite advances in preventive measures, yellow fever is still a major public health problem in many regions of tropi-cal Africa and South America. In the past 2 years alone, yellow fever outbreaks have been reported in five African countries (Nigeria, Ivory Coast, Liberia, Senegal and Guinea), and a total of 840 cases, including 216 deaths, were reported to the WHO. The recent epidemics in Africa indicate a break-down of yellow fever control measures, since most cases are in children under the age of 15, who were born after routine yellow fever vaccinations were abandoned J. Vainio, F. Cutts, [1998].

Yellow fever is a viral hemorrhagic fever. It is transmitted to humans and other vertebrate hosts by the bite of infected mosquitoes. The etiological agent is yellow fever virus. In humans, infections with the yellow fever virus cause a broad spectrum of disease, from mild symptoms to severe illness to a fatal disease. Clinical symptoms of yellow fever typically appear 3–6 d after the mosquito bite, but only about 15% of those infected develop clinical yellow fever, and the majority only have a mild disease and quickly recover. The clinical symptoms of yellow fever have been described in detail by Monath [1989].

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