Girl with yellow fever (Wellcome Images)
“Episode de la fièvre jaune”

By analyzing hepatitis C virus genome sequences, you can trace the virus’s history through its spread by the slave trade, and linked 19th-century health models in different countries to viral spread and transmission. Similarly, by looking at leprosy DNA, you can track its spread along the Silk Road and along slave routes.

Yellow Fever was one of the most dreaded plagues of the 18th and 19th centuries, waning only after it was understood to be mosquito-borne, so that mosquito control pushed the virus back. It’s still prevalent in Africa and in some parts of South America, though. Yellow Fever virus, too, originated in Africa and was spread to the New World through the slave trade:

The most commonly cited hypothesis of the origin of YFV in the Americas is that the virus was introduced from Africa, along with A. aegypti,1 in the bilges of sailing vessels during the slave trade. … 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.2

Mosquitoes aren’t merely passive carriers of the Yellow Fever virus. The virus actively infects the mosquitoes as well as their mammalian host, entering the insect gut, replicating and multiplying in various organs until it reaches the saliva, from which it can re-infect mammals3 when the mosquito bites and injects its anticoagulant saliva.

Mosquitoes - Harper's Weekly 1873
“Latest from the front — our friends the mosquitoes preparing and off for the summer campaign”
(Harper’s Weekly, 1873)

Another pattern is possible: The virus could also be spread vertically, from the mosquito to its egg, infecting the newborn mosquito before it hatches. However, although this was shown to happen as long ago as 1905,4 just after mosquitoes were proven to be carriers, it hasn’t been very clear if this is a significant part of the natural viral cycle or if it’s more of a lab curiosity:

Although transovarial transmission of YFV has been demonstrated, the relative importance of this in maintaining the transmission cycle is unknown. 5

Now, genome sequence analysis suggests that in fact transovarial spread of Yellow Fever virus may well be common and important in the viral life cycle.6

This was based on comparisons of Yellow Fever virus genome sequences over time, with those of a close relative, Dengue virus. Dengue and YFV probably arose about the same time, in the same area, and were both spread along the slave trade. But Dengue seems to have diversified much more than YFV:

… it is intriguing that the overall age of YFV (emergence within the last 2,500 years) is broadly similar to the time of origin of the four DEN viruses. Hence, YFV and DENV seem to have radiated at approximately the same time. However, since this time, DENV has differentiated into four antigenically distinct viruses while YFV is still classified as a single serotype.6

(This is actually clinically very significant, because the most severe form of Dengue disease is caused by sequential infection with two different Dengue serotypes.) In fact, in general YFV shows a much slower rate of evolution over time than Dengue — about 5-fold slower per year. The authors consider a reject a number of explanations for this — it’s not that they have different mutation rates, because their raw mutation rates are probably quite similar; it’s not that they infect different hosts, because they have very similar insect and mammalian hosts; and so on — and finally suggest that the difference may be because YFV spends a significant part of each year lying more or less dormant in mosquito eggs:

In particular, it is possible that a mechanism of vertical transmission, such as transovarial transmission where the virus may remain quiescent in mosquito eggs for many months, plays a more important role in YFV than in DENV6

As a result of this quiescent period, YFV would simply have fewer replication cycles per year than does Dengue, and so it appears to evolve more slowly. For this to be detectable at this level, transovarian transmission would have to be a fairly common event, not just a once-in-a-while half-accidental option.

  1. A. aegypti is the mosquito that is most involved in spreading the virus[]
  2. Bryant, J., Holmes, E., & Barrett, A. (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[]
  3. Mainly primates, for functional transmission[]
  4. Marchous E, Simond PL. 1905. La transmission hereditaire du virus de la fievre jaune chez la Stegomyia fasciata. C. R. Soc. Biol. 59:259[]
  5. Barrett, A., & Higgs, S. (2007). Yellow Fever: A Disease that Has Yet to be Conquered Annual Review of Entomology, 52 (1), 209-229 DOI: 10.1146/annurev.ento.52.110405.091454[]
  6. Sall, A., Faye, O., Diallo, M., Firth, C., Kitchen, A., & Holmes, E. (2009). Yellow Fever Virus Exhibits Slower Evolutionary Dynamics than Dengue Virus Journal of Virology, 84 (2), 765-772 DOI: 10.1128/JVI.01738-09[][][]