Yellow fever, which can include symptoms like jaundice, high fevers and multiple organ failure, has killed at least 240 people in recent months, Brazil’s Health Ministry said. The disease, normally found in parts of the Amazon River Basin, has spread to the country’s most populous states: Minas Gerais, Rio de Janeiro and São Paulo.
As yellow fever made the leap out of the Amazon, the authorities in several areas of southeastern Brazil reported episodes in recent weeks of monkeys being killed illegally, involving a range of methods including poisoning and hunting down the primates with rifles or clubs.
The killings reflect broad misunderstanding in Brazil about how yellow fever spreads. Cases that are sylvatic, or in the wild, involve monkeys that are infected by mosquito species inhabiting the forest canopy.
The same mosquitoes then transmit the virus to humans who make incursions into jungles, like gold prospectors, hunters or loggers. Such outbreaks are often relatively small. While yellow fever is often asymptomatic in humans, patients who develop severe symptoms can die within seven to 10 days.
Credit Dado Galdieri for The New York Times
“People in regions where yellow fever is on the march often don’t realize how crucial monkeys are in informing us about the disease,” said Danilo Simonini Teixeira, the president of the Brazilian Society of Primatology.
Monkeys have died in much higher numbers than humans in Brazil’s current sylvatic outbreak. Brazilian authorities estimate that over 4,400 monkeys have died from yellow fever in recent months; subspecies like southern brown howler monkeys are especially vulnerable.
While the outbreak has many people in Brazil on edge, virus specialists fear the possibility that yellow fever could spin out of control by spreading rapidly around cities, stirring ghosts of epidemics that devastated urban areas like Rio de Janeiro more than a century ago.
In urban outbreaks, infected people often introduce yellow fever into heavily populated areas where the virus can be transmitted by Aedes aegypti mosquitoes, the same devilishly resilient species that spreads viruses like chikungunya, dengue and Zika.
While Africa has recently faced such urban outbreaks, including a 2016 epidemic in Angola, they are rare in South American cities. In 2008, epidemiologists found yellow fever transmission by Aedes aegypti in Paraguay’s capital, Asunción. The last confirmed urban outbreak of yellow fever in the Americas was previously thought to be during the 1940s.
Credit Roberta de Oliveira Resende
As the authorities grapple anew with yellow fever in Brazil, they are voicing relief that it has not evolved into an urban outbreak. Nearly 19 million doses of vaccine are being distributed in areas where the virus is spreading. The World Health Organization is also providing Brazil with 3.5 million doses of the vaccine from its emergency stockpile.
In addition to urging people to get vaccinated and to stop killing monkeys, Brazilian officials are telling people to eliminate places with stagnant water where mosquitoes can breed, like discarded tires or shower floors. Still, in impoverished parts of Brazil where yellow fever has hit hard, some say that the authorities have been slow to act.
“It’s clear that hunting monkeys is wrong, but when monkeys started showing up dead from yellow fever the vaccinations should have started,” said Dalila de Oliveira, 30, a manicurist. Her cousin Watila dos Santos, a 38-year-old construction worker, died in March from the virus in Casimiro de Abreu, an area in the interior of Rio de Janeiro State.
“He went to the public hospital and was mistakenly diagnosed with sinusitis,” Ms. Oliveira said. “The authorities could have been more aggressive about preventing this tragedy, but they preferred to minimize the situation.”
Brazil’s health minister, Ricardo Barros, acknowledged in an interview that fewer people would have died in recent months had the government responded more vigorously in the outbreak’s early stages.
Credit Dado Galdieri for The New York Times
“There may have been a failure in the vaccination blockade after the first monkeys began showing up dead in Minas Gerais,” said Mr. Barros, referring to the Brazilian state that has been hardest hit in the outbreak, with more than 160 fatalities.
Infectious disease specialists are trying to determine why yellow fever is emerging with greater ferocity this year in Brazil. Dr. Anna P. Durbin, a researcher at Johns Hopkins Bloomberg School of Public Health, said a combination of factors might be in play, including climate change and the deforestation of areas that serve as buffer zones between tropical jungles and urban areas.
“A big concern is for the virus to jump the Panama Canal” into Central America, Dr. Durbin said.
The virus may also have the potential to spread to Puerto Rico and cause travel-related cases in the continental United States, Dr. Anthony S. Fauci and Dr. Catharine I. Paules, both of the National Institute of Allergy and Infectious Diseases, said recently in The New England Journal of Medicine. In late April, the Centers for Disease Control and Prevention warned of a shortage of yellow fever vaccine in the United States because of recent manufacturing problems.
As epidemiologists monitor yellow fever’s advance into parts of Brazil, monkeys are still turning up dead, either at the hand of man or as a result of the virus. Researchers say the virus is threatening species already at risk of extinction, like the golden lion tamarin, which lives in the forests of Rio de Janeiro State.
Karen Strier, an anthropologist at the University of Wisconsin who has studied monkeys in the Atlantic Forest in Brazil since the 1980s, said she had never seen monkeys die from disease in such high numbers. She described a “sense of emptiness” in a reserve near Caratinga in Minas Gerais State, where howler monkeys had largely vanished.
An earlier version of this article misspelled the given name of a woman whose cousin died of yellow fever. She is Dalila de Oliveira, not Danila.