The yellow fever virus appeared deep in the forests of Africa. “It’s a virus that began to emerge in humans about 1,500 years ago. And mosquitoes have made this epidemiological bridge between non-human primates and human primates, ourselves “, explains Serge Morand, CNRS researcher in health ecology.
But curiously, it is on the new continent that will appear in the 17th century the first epidemics. The slave trade will cause millions of deaths on the plantations.
“Around the years 1647-1648, nearly 700,000 African slaves will make the very first epidemic yellow fever in the Yucatán Peninsula and the Caribbean. In the middle of the 17th century, the disease was completely endemic in quite a few places, including Havana, where there were 5 million Africans living in plantations, “says the specialist.
Serious illness in 20% of cases
Because if the mild form of the disease, some headaches, muscle pain and a little fever, disappears within a week, in 20% of cases, Maladie “vomits black”, as it is sometimes called, is serious.
“It’s a very severe hemorrhagic fever with vomiting of blood. The most affected organ is the liver. As in liver disease, there is the yellow color of the skin. Then the disease progresses and affects organs such as the heart and kidneys and can lead to death, “emphasizes Giovanna Barda Spaeth, of the Institut Pasteur.
And yellow fever has even sometimes changed the course of history. “This is one of the reasons why the French army, for example, lost Haiti, not only for the slave revolts, but also because the colonial troops were particularly decimated by yellow fever”, explains Serge Morand.
It will be necessary to wait the end of the 19th century for researchers to begin to understand how the disease is transmitted. After the visit of the American doctor Walter Reed to Cuba. “In Havana, they will demonstrate that it is a virus and that it is transmitted by mosquitoes. It is an essential discovery. From 1905, thanks to the fight against mosquitoes, yellow fever will disappear from the United States “, explains Jean-François Saluzzo, expert at the WHO and author of The vaccine saga.
The virus isolated in 1927 then attenuated in 1936
Then in the 1920s, the quest for the vaccine will finally begin after the crossing in Africa of an important stage. “Around 1927, researchers succeeded in isolating the virus from the Asibi strain, named after a Ghanaian patient suffering from a not too serious form of the disease “, summarizes Giovanna Barda Spaeth.
The quest for the vaccine will intensify. Several researchers will die while manipulating the virus. In 1936, Max Theiler a young doctor at the Rockefeller Foundation develops an effective vaccine drawing inspiration from the work of Pasteur. He will attenuate the viral strain by growing it on chicken embryos. And it works.
“Pasteur had multiplied the rabies virus on animals. And he will do the same, with the idea that if we use animals that have nothing to do with the natural cycle of yellow fever, this virus will go away. adapt. And maybe by mutating it will become less pathogenic and not cause more disease. It will pass through embryonated eggs, which give birth to chicks. And it has a very interesting model to test the pathogenic model. of this virus, the monkey. The Asian monkey is very susceptible to yellow fever. When injected with the virus, it dies in 4 to 8 days. It passes serially and between the 80th and 100th passage, the virus no longer kills monkeys, “explains Jean-François Saluzzo.
We then vaccinate in Brazil. Immediately, wherever one vaccinates, the disease disappears. Max Theiler will receive the Nobel Prize for his discovery. Today, we still use the same vaccine. One injection is enough. 500 million people have already been vaccinated worldwide. But the disease cannot be eradicated because of the presence in tropical forests of monkeys and mosquitoes. Regularly, new epidemics claim hundreds of victims, for example in Angola in 2016 and in Brazil in 2018.
