In the early Americas, nothing scared people more than when Yellow Jack came knocking at the door of their city. Yellow Jack, or as we know it better today Yellow Fever, has rightly been called the plague of the Americas.
It has long been assumed that yellow fever came to the Americas with its vector, Aedes aegypti, in the hold of slave ships. These ships would have been an irresistible feast to the mosquito. Yet, little was known about the origin, locations, and dates of transmission to South America. Juliet Bryant, Edwarld Holmes and Alan Barrett (2007) looked to DNA analysis of yellow fever virus (YFV) strains from 22 countries ( 14 African and 8 South American) to resolve and date the phylogentic tree for YFV. They analyzed 133 isolates from humans and animal hosts collected over a 75 year period.
Bryant, Holmes and Barrett (2007: e75) made four clear observations.
- The American strains represent a single clade (monophyletic).
- There are two distinct sub-clades in east and west South America respectively.
- The South American clade is most similar to the West African isolates.
- The East African clade is the most distinctive.
These observations support an east or central African origin for the Yellow Fever Virus dominated by enzootic transmission. Its development parallels the transmission of its vector Aedes aegypti.
The split between the east and west African clades has been calculated to an average distance of 723 years (roughly 1284 AD). The West African isolates are the most diverse in Senegal, suggesting this was an early focus for West African YSF. From West Africa Yellow Fever was transmitted to Brazil a calculated average of 470 years ago (roughly 1537 AD). Early Portuguese seamen frequented this part of Africa and Brazil was their largest colony, founded in 1500. This suggests that Yellow Fever was transmitted to Brazil virtually from the beginning of the Portuguese colony. It is possible that Yellow Fever was one of the imported diseases brought by the Portuguese that decimated native Brazilians before large-scale importation of Africa slaves. The South American clade split into eastern and western populations when it was transmitted to Peru a calculated average of 306 years ago (roughly 1700). There is no evidence of transmission back to Africa or other areas where Aedes aegypti have spread in Asia. Byrant, Holmes and Barrett (2007) argue that sylvatic transmission is the primary means of maintaining YSF in South America. They note that there hasn’t been an urban epidemic of YSF in South America since 1928, unlike the annual urban outbreaks in West Africa.
Auguste et al (2010) confirmed the overall structure of the YSF phylogenetic tree in the Americas, including its Brazilian origin in the Americas. Their analysis of strains collected over the last decade also confirm that Brazil is the reservoir and origin for most strains in the Americas today with the Peruvian strains remaining primarily localized in Peru and neighboring Bolivia. The analysis of Auguste et al (2010) also supports enzootic maintenance and local evolution in areas of spread from Brazil such as Trinidad and Columbia.
What I find most surprising about the YSF tree is its relative youth. This all suggests that Yellow Fever originated in the Middle Ages and probably did not circulate outside of local areas of central Africa until the late medieval period. We still have a lot of learn about the landscape epidemiology of yellow fever including possible vertical transmission among mosquitoes and the importance of difference primate species as reservoirs. Although we have had an effective vaccine for decades, yellow fever is still a very clear and present danger in both the Americas and Africa.
J E Bryant, E C Holmes, & A D T Barrett (2007). Out of Africa: A Molecular Perspective on the Introduction of Yellow Fever Virus into the Americas PLOS Pathogens, 3 (5) : doi:10.1371/journal.ppat.0030075
Auguste, A.J., Lemey, P., Pybus, O.G., Suchard, M.A., Salas, R.A., Adesiyun, A.A., Barrett, A.D., Tesh, R.B., Weaver, S.C. & Carrington, C.V.F. (2010). Yellow Fever Virus Maintenance in Trinidad and Its Dispersal throughout the Americas, Journal of Virology, 84 (19) 9977. DOI: 10.1128/JVI.00588-10