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“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.
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“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.
- A. aegypti is the mosquito that is most involved in spreading the virus[↩]
- 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[↩]
- Mainly primates, for functional transmission[↩]
- Marchous E, Simond PL. 1905. La transmission hereditaire du virus de la fievre jaune chez la Stegomyia fasciata. C. R. Soc. Biol. 59:259[↩]
- 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[↩]
- 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[↩][↩][↩]
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You said “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.”
I totally thought it’s carried simply through the blood. No wonder these diseases can spread so rapidly.
Pretty eye opening when you get the facts like this. Thanks.
So where does Yellow Fever stand these days? This article helps me realize a little more why everyone gets hysterical when a mosquito passed virus gets passed around a little. Once they get it is I can see the rough possibility of a pretty serious epidemic. But then again… I am talking about the media here. :-)
Ryan Jelley
It may require years of mosquito suppression to effect the yellow fever virus, because mosquito eggs are very long lasting, and when favorable environmental factors prevail, there is a local flush of mosquitoes that predictable wind patterns can push into a populated area, especially a topographically low lying one.
YFV is a terrible virus which has 3 epidemiologically different cycles from which the bite of the yellow fever mosquito Aedes aegypti is passed to humans. In 2009 a mass vaccination against yellow fever started in Afica whci has already cut in half the deaths of YFV which i thik is great news along with so countrys like asian demanding visitors from YFV infeccted countrys have been vaccinated and i really feel this also will have a positive effect on controlling the problem. I really enjoyed reading this post and thank you very much.
James
I was fascinated to see that “YFV shows a much slower rate of evolution over time than Dengue.” In a way that sounds good as far as controlling YF is concerned, but Dengue Fever is a serious problem in SE Asia where I live so that statement could be bad news. Having said that Filipinos in the know use a common weed, known locally as tawa-tawa or variation thereof, to brew a “tea” that quickly restores blood platelet levels to safe levels.
It’s a deadly disease. During last rainy season, it impacted hundreds in Mumbai city and few was died also. The main problem was initially people spread rumor that this is a different form of H1N1 infection. Actually 2 of employee was also suffered but thank God they cured quickly.
Rita
Nice Art work. I try to catch Art Openings and exhibits but lately I haven’t had the time. Thanks for the Blog.
Chris