Out of Africa: A Molecular Perspective on the Introduction of Yellow Fever Virus into the Americas

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Abstract
Yellow fever virus (YFV) remains the cause of severe morbidity and mortality in South America and Africa. To determine the evolutionary history of this important reemerging pathogen, we performed a phylogenetic analysis of the largest YFV data set compiled to date, representing the prM/E gene region from 133 viral isolates sampled from 22 countries over a period of 76 years. We estimate that the currently circulating strains of YFV arose in Africa within the last 1,500 years and emerged in the Americas following the slave trade approximately 300–400 years ago. These viruses then spread westwards across the continent and persist there to this day in the jungles of South America. We therefore illustrate how gene sequence data can be used to test hypotheses of viral dispersal and demographics, and document the role of human migration in the spread of infectious disease. Throughout the 18th and 19th centuries, yellow fever was one of the most dreaded of diseases in New and Old world port cities. Large-scale epidemics of yellow fever helped shape colonial expansion in both the Americas and in Africa, and the medical and scientific developments associated with control of the virus have been a favored topic of historians for many years. The most commonly cited hypothesis of the origin of yellow fever virus (YFV) in the Americas is that the virus was introduced from Africa, along with Aedes Aegypti mosquitoes, in the bilges of sailing vessels during the slave trade. Although the hypothesis of a slave trade introduction is often repeated, it has not been subject to rigorous examination using gene sequence data and modern phylogenetic techniques for estimating divergence times. Herein we have assembled a comprehensive data set of gene sequences for YFV, which we used to infer the time-scale and evolutionary history of YFV. These data show that the spread of YFV to the Americas corresponds closely with the routes and timing of the slave trade. Overall, this study demonstrates how molecular epidemiological studies can provide new insight into debates on the origin and spread of infectious disease.