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. All of Sao Paulo state has been deemed at risk for yellow fever just weeks before Carnival celebrations, a huge draw for visiting foreigners. Public health specialists worry 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 over 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 47 countriesin Africa and the Americas, from which it periodically spreads, via mosquitoes, to humans. A modeling study estimated that yellow fever caused as many as 170,000 severe cases and up to 60,000 deaths globally in 2013.
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 recent confirmed cases in Brazil, 35 percent have died.
3. Where is incidence up?
Brazil has reported the most cases in at least 38 years since an outbreak began in December 2016. The scourge was declared over in September 2017, yet an upsurge of infections in monkeys and humans indicates the virus is rebounding. In the last weeks of 2017 and in early 2018, the number of confirmed human cases has tripled, mainly in the states of Sao Paulo and Minas Gerais. Since 2016, the Ministry of Health has confirmed 681 cases, with another 435 suspected and under investigation. Colombia, Bolivia, Ecuador, French Guiana, Peru, and Suriname have also reported yellow fever cases since 2016, representing the highest yellow fever activity in decades.
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 typically 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. Dozens of monkeys in Rio de Janiero are reported to have been killed this year by people believing erroneously that the animals can spread the disease directly to humans.
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. No human cases have been reported in the metropolitan areas of Sao Paulo and Rio de Janeiro, but some monkeys have been infected. 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. In Minas Gerais state, cases are reported in municipalities that have no recent history of human infections. In Sao Paulo state, Brazil’s economic powerhouse, the highest proportion of cases is reported in Maripora, an area 15 kilometers (9 miles) from the northern fringe of Sao Paulo, a city of more than 12 million people. New cases in Rio de Janeiro state are reported in the northern municipalities of Valenca and Teresopolis, which is 96 kilometers from Rio de Janeiro, Brazil’s second-most populous city. 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. Health authorities are especially concerned about the risk of spread during Carnival, one of the largest international mass gatherings in Brazil. The Ministry of Health recommends those attending the Feb. 9-14 event be vaccinated at least 10 days prior.
7. What’s Brazil’s response?
Brazil’s Ministry of Health began a mass immunization campaign in 69 municipalities in Rio de Janeiro and Sao Paulo states in early 2018. Almost 24 million people will be targeted in the world’s largest immunization campaign using so-called fractional doses of yellow fever vaccine, an approach aimed at stretching supplies. The state of Bahia plans to vaccinate 3.3 million people. When a public health emergency was declared in Minas Gerais in January 2017, 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, the Portuguese island of Madeira, off Morocco, and Fuerteventura, one of the Canary Islands. Since January 2017, three travel-associated cases of yellow fever have been identified among unvaccinated Europeans returning from Bolivia, Suriname and Brazil.
9. Should I get vaccinated?
- all of Espirito Santo state
- all of Rio de Janeiro state, including the city of Rio de Janeiro
- all of Sao Paulo state, including the entire city of Sao Paulo
- a number of cities in Bahia state
10. Is the yellow fever vaccine available?
Manufacturing problems have led to a total depletion of the only U.S.-licensed yellow fever vaccine, with new supplies not expected until mid-2018. In the meantime, the vaccine maker, Sanofi Pasteur, has worked with U.S. authorities to make an alternative yellow fever vaccine, Stamaril, available at select locations. 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.
11. How else can I protect myself?
The U.S. Centers for Disease Control and Prevention 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.
The Reference Shelf
- Anthony S. Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, and colleagues explain the latest yellow fever threatin the New England Journal of Medicine.
- Alan Barrett, director of the Sealy Center for Vaccine Development at the University of Texas Medical Branch in Galveston, explores issues with the yellow fever vaccine in the same journal.
- Information on yellow fever is regularly updated on the websites of the U.S. Centers for Disease Control and Prevention, the European Centre for Disease Prevention and Control, the Pan American Health Organization and the World Health Organization.