The yellow fever virus was sleeping in the Amazon rainforest when it left for the densely populated south of Brazil, worn by monkeys and mosquitoes, around July 2016.
With an estimated speed of 3.3 kilometers per day, the virus went to the cities of Sao Paulo and Rio de Janeiro, where it had not circulated for decades and where more than 35 million people who had not been vaccinated against this disease were staying. , which can lead to death in less than ten days.
Two years later, 676 people died in the worst epidemic of yellow fever in Brazil in a century.
For the first time, the virus route was reconstructed by an international team from the University of Oxford and Fundação Oswaldo Cruz (Fiocruz) in Rio de Janeiro, which carried out an authentic search for the genetic, geographical and epidemiological treasure of which the results were published on Thursday in the American journal Science.
"This is the first time that we are able to calculate the rate of propagation of the virus in space and time," explains Nuno Faria, professor at the Oxford Department of Zoology.
The work rejected the hypothesis that was feared for a long time for a return of the transmission of the virus "from person to person", ie through the urban mosquitoes, the famous Aedes aegypti.
People are infected with "wild" mosquitoes, the Haemagogus and the Sabethes, who apparently had bitten infected monkeys in the jungle. These people were mainly infected because they lived or lived near the habitat areas of these monkeys.
First indication: the researchers reconstructed the geographical distribution of the virus and realized that the cases of infection in the apes preceded human cases in just four days.
But the virus moved faster than the normal speed of primates, suggesting that it was people who probably transported the disease through illicit trafficking in monkeys or carrying infected mosquitoes in vehicles.
They also discovered that 85% of the cases were men between 35 and 54 years old, who go to places near the jungle earlier because they are the ones who work, for example, as a truck driver or agricultural worker. They generally lived less than 5 kilometers from the jungle.
Finally, they collected and analyzed the genomes of infected monkey and human viruses, and strengthened their conclusions about the origin of the epidemic.
For scientists, this method will serve to analyze and take measures in real time in the following epidemics.
"It's a flying job," said David Hamer, a contagious disease expert at the University of Boston when he discusses the study with AFP, which he did not participate in.
"His approach has great potential, but requires a large amount of data," said the professor, adding that developing countries, especially in Africa – another continent often hit by yellow fever – do not have the required infrastructure. to create a monitoring and warning system such as the referenced system.
– Vaccination –
The best tool against yellow fever is still the vaccine, discovered in 1938. The Brazilian government started a big campaign in January of this year, which began in the regions of Sao Paulo, Rio and Bahia.
But there was a shortage. For researchers, the lesson is that, in addition to the eradication of wild mosquitoes, priority should be given to vaccination of the populations that are most at risk in rural areas and peri-urban areas.
In the state of Minas Gerais, epicenter of the epidemic in the southeast, 85% of the cases were men and the highest incidence in the age of 40 to 49 years. "We have to reach these populations and vaccinate them," Faria insisted.
The fact that the virus has not entered an "urban cycle" and that the areas in the center of Sao Paulo and Rio have remained free should not let the guard down, he warned.
"We are now beginning to realize that many of the most worrying infectious diseases are caused by deforestation and the increasing proximity of humans to animals," said Peter Hotez, dean of the School of Tropical Medicine at the university. Baylor College in Houston, who also named the Ebola virus, the SARS and MERS corona viruses in China and Saudi Arabia respectively, and even the Nipah virus recently in India.
© 2018 AFP