What Is Yellow Fever?April 23, 2014
The Facts About Yellow Fever
Yellow fever is a virus transmitted by mosquitoes in the tropical and subtropical areas of South America and Africa. A viral haemorrhagic fever, it is transmitted to humans by infected mosquitoes through biting, and can be found in historic texts dating back 400 years. “Yellow” fever refers to the yellowing of skin and eyes or jaundice that appears in some cases. The World Health Organisation (WHO) estimates that up to 50% of severely affected persons without treatment die each year.
There are an estimated 200,000 yearly cases of yellow fever that cause 30,000 deaths, 90% of which occur in the tropical zones of Africa. To travel to various parts of Africa and South America a yellow fever vaccination certificate is required.
Yellow fever is part of the same Flaviviridae virus family that includes dengue and Japanese encephalitis. Over 900,000 people live in zones affected by the virus. WHO states that while the yellow fever is transmitted through different species of Aedes and Haemogogus mosquitoes, the virus is carried from host to host, primarily between monkeys, from monkeys to humans, and from person to person. Mosquitoes bite during daylight hours, especially around dawn and dusk, and once they are infected are infected for life (2-3 months).
Three types of transmission cycle exist in Africa: sylvatic, intermediate and urban, however only sylvatic and urban yellow fever occur in South America.
Sylvatic (or jungle) yellow fever:
Sylvatic transmission occurs in monkeys that are bitten and infected by wild mosquitoes in the jungle. The infected monkeys then act as carriers infecting other mosquitoes who bite them. Humans can then be infected as they enter the environment and are bitten by the mosquitoes. The largest demographic affected is young men working in the jungle.
Intermediate yellow fever:
Intermediate yellow fever refers to small-scale epidemics and can occur in tropical or sub-tropical parts of Africa, where mosquitoes who breed in the wild around villages can infect both monkeys and humans. Transmission occurs with increased contact and often affects a large percentage of people in a small area at once. Intermediate yellow fever is the most common type of outbreak in tropical Africa.
Urban yellow fever:
Urban yellow fever refers to large epidemics that occur when infected people introduce the virus into densely populated areas that have a high number of non-vaccinated people and Aedes mosquitoes.
Signs and Symptoms
Yellow fever has an incubation period of 3-6 days. The first phase causes muscle pain, backache, headache, nausea, vomiting, loss of appetite and fever. While most patients improve in 3-4 days, around 15% develop a second toxic phase of the virus that results in death for up to 50% of patients. Characterised by high fever and rapidly developing jaundice, patients can suffer from abdominal pain with vomiting, bleeding from the eyes, nose, mouth or stomach, and finally blood in faeces and vomit. Eventually kidney function deteriorates. 50% of the patients at the toxic stage recover without significant organ damage, however the remainder die within 10 to 14 days. Once an infected patient recovers, they retain lifelong immunity from the disease.
Diagnosis of yellow fever is often based on the patient’s location or recent dates and places of travel to affected zones and physical symptoms. Laboratory diagnosis of yellow fever by serum testing is also used to detect the virus and sometimes blood samples taken early in the disease may be able to determine whether the patient has contracted yellow fever or not.
Treatment for yellow fever is only symptomatic and is aimed at reducing the symptoms for the comfort of the patient.
Measures are being taken to eliminate the habitats of infected mosquitoes, however the most effective form of personal prevention is vaccination and to avoid getting bitten. For 60 years a highly effective live yellow fever vaccine has been available for persons living in or travelling to affected areas. Wearing light-coloured clothing that covers the skin, especially at dawn and dusk, using DEET based insect repellents as well as mosquito netting are also standard preventive procedures.