What is yellow fever virus?

Yellow fever is an acute viral disease. It is an important cause of hemorrhagic illness in many African and South American countries despite existence of an effective vaccine. The yellow in the disease name refers to the jaundice that affects some patients.

Yellow fever has been a source of several devastating epidemics. During one of Napoleon's campaigns to Haiti in 1802, the troops were attacked by yellow fever. More than half of the army perished due to the disease. Outbreaks followed by thousands of fatalities occurred periodically in other Western Hemisphere locations until research which included human volunteers (some of whom died) led to an understanding of the method of transmission to humans (primarily by mosquitos) and development of a vaccine and other preventative efforts in the early 20th century.

Despite the costly and sacrificial breakthrough research by Cuban physician Carlos Finlay, American physician Walter Reed and many others, over 100 years later, unvaccinated populations in many developing nations in Africa and Central and South America continue to be at risk. As of 2001, the World Health Organization (WHO) estimates that yellow fever causes 200,000 illnesses and 30,000 deaths every year in unvaccinated populations.

Symptoms

After a 3 to 6 day incubation period, the typical symptoms that arise are fever, muscle aches, headache, and backache. Other symptoms may include a red tongue, flushed face, and reddening of the eyes. In a portion of cases there is also involvement of internal organs including the liver, kidneys, and heart. There may be hemorrhage from the gastrointestinal tract, causing a bloody vomit with a black colour called coffee ground vomiting (which explains the name black vomit). Later, the disease is sometimes complicated by jaundice with liver failure (which causes yellow skin colour, hence the name "yellow fever") and/or kidney insufficiency with proteinuria. If the disease progresses, delirium, seizures and coma ensue. Hypotension and dehydration are also common.

Epidemiology

Yellow fever occurs only in Africa, South and Central America, and the Caribbean. Most outbreaks in South America are to people who work within the tropical rain forests and have direct contact with the organisms within the rainforest.

The disease can remain locally unknown in humans for long periods of time and then suddenly break out in an epidemic fashion. In Central America and Trinidad, such epidemics have been due to a form of the disease (jungle yellow fever) that is kept alive in Red Howler monkey populations and transmitted by Haemagogus mosquito species which live only in the canopy of rain forests. The virus is passed to humans when the tall rainforest trees are cut down. Infected woodcutters can then pass on the disease to others via species of Aedes mosquitoes that typically live at low altitudes, thus triggering an epidemic.

Cause

Yellow fever is caused by an arbovirus of the family Flaviviridae, and is one of the smallest RNA viruses isolated. Human infection is begun after deposition of viral particles through the skin in infected arthropod saliva. Mosquitos are the primary vector in transmission of the disease from forest monkeys to humans and in person-to-person transmission. The mosquitos involved are Aedes simpsoni, A. africanus, and A. aegypti in Africa, and the Haemagogus and Sabethes genera in South America.

Prevention

A vaccine for yellow fever was developed which gives a 10-year or more immunity from the disease and effectively protects people traveling to the affected areas whilst being a means to control the disease at the same time. Woodcutters working in jungle areas should be particularly targeted for vaccination. Insecticides, protective clothing and screening of houses are helpful but not always enough. In affected areas mosquito control methods have proved effective in decreasing the number of cases.