researchPublished 2007
How yellow fever got to the Americas
Each year, yellow fever causes an estimated 200,000 cases of sickness and 30,000 deaths. Most of these cases occur in Africa; there are also some in South America. The disease is caused by an RNA virus in the same genus (Flavivirus) as viruses that cause dengue, Japanese encephalitis and West Nile disease. Like these congeners, Yellow Fever Virus is transmitted by mosquitoes, and can infect other animals besides humans (monkeys, for example).
A yellow fever vector
(a female Aedes aegypti)
takes a blood meal.
Image courtesy of the
Public Health Image
Library (no. 8936).
Yellow Fever has been recorded as a disease in the Americas for several centuries. The historical pattern of epidemics suggested that it was originally imported from Africa as a consequence of the slave trade, before becoming endemic. In an evolutionary analysis of the largest set of viral isolates to date, Eddie Holmes and collaborators have now found clear support for this "out of Africa" hypothesis. Specifically, by analyzing viral isolates collected from 22 countries over the last 80 years, they discovered that:
- Yellow Fever Virus falls into two geographic groups — an African clade and an American one; within this broad grouping, there are further divisions into East and West Africa and eastern and western South America
- Viral isolates from the Americas are more closely related to viruses from West Africa than those from East Africa
- South American and West African strains diverged approximately 470 years ago
- The two South American lineages diverged approximately 300 years ago
Together, these findings suggest that Yellowfever virus was introduced to the Americas from West Africa during the early sixteenth century, and has not moved successfully from one hemisphere to the other for at least some decades. The study also suggested that in Africa and in the Americas, Yellow Fever outbreaks are fueled by transmission from sylvatic (jungle) reservoirs.
» The researchers' findings are published in PLoS Pathogens.