Fifty-two people have died of yellow fever so far, in an outbreak that began with the onset of Southern Hemisphere summer in December, and another 80 deaths are being investigated as suspected yellow fever fatalities, according to the Ministry of Health. The Ministry says that a further 667 suspected cases of the disease are under analysis. Public health officials have inoculated five million people in the central state of Minas Gerais, where almost all the cases have been concentrated, and ordered 11.5 million more doses of the vaccine for two more areas where a handful of cases have been confirmed, including Sao Paulo, the country’s most populous state. A suspected yellow fever case has also been reported in Bahia, a coastal state that has not recorded the disease since 1985.
Most people who contract yellow fever have no symptoms; only about 10 per cent develop the fever, severe headache, nausea and vomiting. Of these, about half will die. Yellow fever is still a major killer in Africa, where it originated, and it was once a scourge throughout the Americas after it was imported on slave ships. It decimated Napoleon’s army in Haiti, nearly scuppered the construction of the Panama Canal, and took hundreds of thousands of lives before a vaccine was found in the late 1930s. Since then, deaths have dropped steadily; the disease typically flares up Brazil in seven-year cycles, but with fewer than 20 deaths. This outbreak, however, is already much more lethal, and experts appear divided on whether it is now fully controlled.
Yellow fever is caused by a flavivirus (the same family as Zika and dengue fever) and is transmitted by mosquitoes of the Aedes and Haemogogus species. The mosquitoes feed on primates – human and otherwise – and the first sign that the disease was back in Brazil was the discovery of clusters of dead monkeys. At present, epidemiologists say, Brazil has a “wild” outbreak: the virus was in a reservoir of monkeys that passed it on to mosquitoes that bit them, which then spread it to more monkeys, and then more mosquitoes. Biologists call this “amplifying” the virus. Monkeys are its primary host, but some of those infected mosquitoes bit people, who were working in or passing through the forest homes of the monkeys.
The top priority now is to stop the outbreak from becoming “urban,” Brazil’s Health Minister, Ricardo Barros, said recently, urging people to seek vaccinations. If the virus gets firmly established in urban areas in mosquito species that live primarily around humans, and can rapidly spread the virus between them (as Aedes aegypti does with Zika virus), it raises the possibility of a huge epidemic of the kind Brazil has not seen for nearly a century, he said. The three deaths recorded in Sao Paulo so far were among people who were infected in Minas Gerais, the Health Minister said.
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