Photographer: Dado Galdieri/Bloomberg

Why Yellow Fever Is Poised for a Comeback: QuickTake Q&A

Yellow fever, the scourge that killed hundreds of thousands of people in past centuries, is threatening to make a comeback. The sometimes-lethal disease has been contained to the tropical areas of Africa and, to a lesser extent, Central and South America. But an outbreak that began in Brazil has put South America at greater risk of a major epidemic than at any time in the past 50 years. What’s more, the growth of international air travel and the global spread of the mosquitoes that transmit the virus have public health specialists worrying that yellow fever may take root once again in North America and Europe and, for the first time, in Asia.

1. Why is yellow fever resurgent?

Mass immunization with a yellow-fever vaccine developed in the 1930s resulted in a dramatic decline of the disease. During the mid-20th century, the use of the insecticide DDT and other pest-control tools virtually wiped out the mosquito species that spread yellow fever. However, DDT was banned in much of the world in the 1970s and 1980s because of concerns about its effects on the environment and human health, and in the 1990s immunization campaigns lost momentum in many countries. Consequently, yellow fever re-emerged. The disease is endemic -- or constantly present -- among monkeys living in tropical areas of Africa and the Americas, from which it periodically spreads, via mosquitoes, to humans. Worldwide, an estimated 200,000 yellow fever cases each year result in about 30,000 deaths.

2. How does it affect humans?

Most people who are infected don’t develop symptoms. When they do, they typically experience fever, chills, headache, backache and muscle aches within three to six days. There’s no specific treatment. About 15 percent of people develop serious illness that can lead to bleeding, shock and organ failure. The “yellow” in the name refers to the jaundice that affects some patients. Among confirmed cases in Brazil, 34 percent have died.

3. Where is incidence up?

Brazil has reported the most cases in at least 37 years since an outbreak began in December. So far, the Ministry of Health has confirmed 729 cases, with another 663 suspected and under investigation. It has reported 249 deaths. Colombia, Ecuador, Peru, Bolivia and Suriname have also reported recent yellow fever cases.

4. How is yellow fever transmitted?

In forested and rural areas, Haemagogus and Sabethes mosquitoes spread so-called jungle yellow fever mostly to monkeys and sometimes to unvaccinated people working in or traveling through forested areas. Sporadic infections have occurred in people this way in South America since the virus was introduced there from Africa centuries ago. In towns and cities, urban yellow fever is spread among people by the Aedes aegyti mosquito, which breeds around houses. Epidemics start when a person infected with yellow fever in the jungle travels to an urban area and is bitten by an Aedes aegypti mosquito, which subsequently bites more people, starting a chain of human infections. Yellow fever can spread explosively among unvaccinated people in this situation. Fortunately, urban outbreaks are rare. Since Brazil’s last such outbreak in 1942, it’s only reported cases of jungle yellow fever.

5. How is the virus spreading in South America?

So far, all human infections in the current outbreak have been linked to jungle yellow fever, and there’s no evidence of an urban disease cycle. While yellow fever is a serious threat to humans, it’s potentially catastrophic for South America’s primates. Monkeys aren’t vaccinated against the virus and have little or no natural immunity to it. Thousands of them have died -- decimating some endangered species. Researchers have speculated that prolonged, wet weather in 2016 made it harder for monkeys to forage, weakening their immune systems and making them more vulnerable to disease.

6. What’s different about this outbreak?

Deforestation, which has reduced monkey habitat, and expanding urban sprawl have brought yellow fever closer to human populations. Minas Gerais, an interior state that supplies about half the nation’s coffee beans, is the epicenter of the outbreak. Espirito Santo, a state not previously considered at risk, confirmed its first locally contracted case since 1940. The virus’s spread to cities where yellow fever vaccine isn’t routinely administered raises concern that, for the first time in decades, urban transmission will occur in Brazil. Adding to that risk is the fact that the Aedes aegypti mosquito is present in all Brazilian states, and the insects are most active from December to July. Yellow fever cases have also occurred in the states of Para, Rio de Janeiro, Sao Paulo, and Tocantins.

7. What’s Brazil’s response?

Brazil’s Ministry of Health has dispatched about 25 million extra doses of vaccine to high-risk states this year. When a public health emergency was declared in Minas Gerais in January, the percentage of people who were already vaccinated against yellow fever varied from town to town from 9 percent to 100 percent.

8. How could yellow fever spread to other parts of the world?

Thanks to international air travel, an unvaccinated person can be infected with yellow fever in South America or Africa one day and journey thousands of miles the next. The risk is that such a traveler will bring the disease to a place where Aedes aegypti mosquitoes are present, get bitten and spread yellow fever that way. Such places include about half the U.S., the Middle East, the eastern coastal region of the Black Sea, Southeast Asia, the Pacific and Indian Islands, northern Australia and the Portuguese island of Madeira, off Morocco. At least four travelers have arrived in Europe from South America with the disease since late 2016. In these cases, the disease spread no further. Sick travelers carried the virus to China, Kenya and the Democratic Republic of Congo during an outbreak in Angola that started in December 2015, although no localized spread was reported.

9. Should I get vaccinated?

If you plan to travel to an area where the virus is circulating and haven’t had the shot, yes. The area for which vaccination is recommended in Brazil has recently been extended to the coastal area of Bahia State, to Rio de Janeiro State and to the urban area of Campinas in Sao Paulo state. The U.S. Centers for Disease Control and Prevention in Atlanta recommends that travelers to Brazil “practice enhanced precautions” to prevent mosquito bites such as covering exposed skin with long-sleeved shirts and pants, using an effective insect repellent and wearing pesticide-treated clothing.

10. Is the yellow fever vaccine available?

Manufacturing problems have led to a shortage of the only U.S.-licensed yellow fever vaccine, and supplies for the immunization of U.S. travelers are projected to run out by midyear. The vaccine maker, Sanofi Pasteur, is working with U.S. authorities to gain approval for an alternative shot. While yellow fever vaccine is considered to be safe, it has been known in rare cases to cause a severe allergic reaction known as anaphylaxis.

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