YELLOW FEVER


What is Yellow fever?
Yellow fever is considered to be one of the most lethal viral diseases. Humans and monkeys are the the principle species to be infected and infection occurs through the bite of an infected mosquito. Transmission of yellow fever occurs in two main cycles, namely jungle and urban.  In the urban cycle man is the host and disease is spread by the Aedes aegypti mosquito which lives in urban areas. In the jungle cycle the transmission cycle occurs between monkeys and forest mosquitoes.
Yellow fever has an incubation period of 3 to 6 days. After this incubation period there are two disease phases although some individuals may not experience any symptoms at all. The first phase is characterised by fever, muscle pain, headache, shivers, loss of appetite, nausea and vomiting. After 3 to 4 days most patients improve and their symptoms disappear. However, within 24 hours of apparent recovery, 15-25% of patients progress to a more serious illness. This phase is characterised by jaundice, haemorrhagic fever and deterioration of kidney function. 20% to 50% of patients who develop this form of the disease die within 7-10 days after the onset.
Where is Yellow fever a problem?
Yellow fever occurs in tropical parts of South America and sub-Saharan America.  In recent years, outbreaks of yellow fever have been reported to the World Health Organisation most commonly from west Africa. In west Africa the most dangerous time of year is during the late rainy and early dry seasons (July-October). In Brazil, virus transmission is greatest during the rainy season (January-March). To date, no cases of yellow fever gave been reported in Asia despite the presence of appropriate primates and mosquitoes. 
Who is at risk?
Although ongoing cases and outbreaks are occurring in Africa and South America, the disease is preventable by vaccination and remains a very rare cause of illness in travellers. A traveller's risk of acquiring yellow fever is determined by immunisation status, travel destination, intensity of yellow fever transmission in area to be visited, season of travel, duration of travel and activities allowing exposure to mosquitoes.
Prevention of Yellow fever
Yellow fever can be prevented by mosquito bite avoidance and control, and immunisation. Nearly all affected countries require proof of yellow fever vaccination in the form of a Certificate of Vaccination. Failure to provide a valid certificate to port health authorities can result in travellers being quarantined, immunised or denied entry.