Disease collides with changing climates Mark Johnson and Ryan Patterson
In Brazil, an outbreak of yellow fever could be a global danger sign
SÃO JOÃO PEQUENO, BRAZIL Two years of drought had been hard on Valdemar Braun and his three grown sons. They lived in the hilly, picturesque village of São João Pequeno, and when the rains quit, the coffee would not grow. The farmers were forced to sell some of their cows.
Then at last the showers returned, and 2017 dawned full of promise for the plantations.
Valdemar had given each son two alqueires of land (almost 11 acres). In mid-January, one son helped another clear out forest to plant more coffee.
The portion of forest belonged to Edson Braun, who had recently divorced. He wanted to transfer the land to his exwife so she could provide for their daughters. His brother, Virlei, agreed to help.
Virlei, 30, with pale blue eyes and a handsome face, had his own family to provide for: a wife and toddler son. On the day he went to help his brother, Virlei had worked on the farm for 14 straight days. Never in his life had he been to a doctor.
That day in the forest, relatives believe, a mosquito bit Virlei.
In just 10 days, he would die, doctors desperately trying to lower his fever by packing his abdomen in ice, his mother crying out, “God, don’t take my son. Don’t take my son.”
“Go back home and help
raise my child,” Virlei told her. “I’ve already put myself in the hands of God.”
Brazil, hit hard by the Zika virus in 2015 and 2016, is once again in the throes of a devastating mosquito-borne disease.
The illness that killed Virlei and has claimed at least 263 other Brazilians is yellow fever, a virus that can cause victims to vomit blood, suffer liver damage and descend into organ failure and coma. In some of Brazil’s forests, the virus recurs every six or seven years.
The current outbreak is the nation’s worst on record; yellow fever deaths in the first four months of 2017 already exceeded all those from 1989 through 2008.
At the epicenter is a group of states that had just recovered from their worst droughts in 80 years. This intersection of drought and disease raises a complex and troubling question for scientists:
Is our changing climate contributing to flare-ups of infectious diseases?
“Yes, this is a factor that is present in our modeling,” says Márcia Chame, a researcher who has been examining the outbreak for the Oswaldo Cruz Foundation in Rio de Janeiro.
But climate alone cannot account for Brazil’s latest bout with yellow fever, says Chame, coordinator of the foundation’s biodiversity research unit. Other contributors include the clearing of forests for farms and plantations, which brings humans into areas thick with mosquitoes; the grinding rural poverty that makes insect repellent a luxury for many villagers; and the reluctance of many Brazilians to receive the yellow fever vaccine.
Still, it is clear that the recent climate in the areas most affected by yellow fever — severe drought followed by rainfall — benefits the forest mosquitoes. Their eggs can survive dry weather in a state of suspended animation “for years and years,” says Michael Osterholm, co-author of the new book Deadliest Enemy: Our War Against Killer Germs.
When rains do come, they unleash several years’ worth of mosquito offspring. Whether this outbreak is linked to climate change “is unclear,” Osterholm says. “It wouldn’t surprise me, but I don’t think we can say that.”
So far, yellow fever has been confined to rural, wooded areas, mostly in four states on Brazil’s eastern flank: Minas Gerais, Espírito Santo, São Paulo and Rio de Janeiro. But the virus already has spread much farther than in previous outbreaks, raising an unsettling possibility.
“If this thing takes off in the urban areas of Brazil, we’re in big trouble,” says Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
Experts say it is unlikely the U.S. will see a comparable outbreak of yellow fever, in part because air conditioning and window screens are commonplace. The U.S. has not experienced an outbreak of yellow fever in more than a century; the 1905 epidemic in New Orleans that killed more than 430 people was the last. Yet the past 20 years have seen the appearance or reappearance of several other mosquito-borne diseases:
In 1999, it was West Nile virus, which arrived in the U.S. in New York and has since spread through almost the entire country.
In 2001, it was dengue fever, thought to have been eliminated from the U.S. 30 years earlier. Hawaii, Texas and Florida have all reported outbreaks of dengue, a virus that produces flu-like symptoms but can lead to severe illness and death.
Last year, it was the Zika virus making its first appearance in the U.S. in South Florida and Brownsville, Texas, a port city on the Mexican border.
Osterholm notes one parallel between Brazil’s latest bout with yellow fever and the appearance of West Nile virus in New York: “In 1999, when West Nile virus broke in the U.S., lack of rainfall favored the mosquito.” There is another parallel. With West Nile, animals fell sick before humans did. Tracey McNamara, head pathologist at the Bronx Zoo at the time, noticed crows dropping dead in and around the zoo. Soon afterward, doctors began seeing patients with symptoms resembling encephalitis, including fever, dizziness and fatigue.
In Brazil, monkeys served as sentinels for the latest outbreak of yellow fever. In April 2016 — eight months before any people became sick — a single monkey was found dead on a farm in Montes Claros, about 530 miles north of Rio de Janeiro.
Even in areas where monkeys are plentiful, it is unusual to find one dead. Their bodies generally decompose quickly or are consumed by scavengers.
In this case, health officials came to Montes Claros to collect the monkey’s remains for testing. Waldney P. Martins, a professor at Universidade Estadual de Montes Claros who studies monkeys, says it took four months to determine the cause of death: Yellow fever. FRIGHTENING SIGNALS José Luis Machado, housekeeper for Fazenda Macacos, the “Farm of Monkeys,” was born just 2 miles away in the village of Itapina, about 400 miles northeast of Rio de Janeiro.
He has been there so long, he says, that he feels like part of the forest, much like the howler monkeys he used to watch feasting on mango leaves. A group of eight to 10 monkeys were permanent residents. They clambered through the trees. Sometimes their shouts could be heard clear across the Rio Doce, or Sweet River, a mile away.
“This was full of monkeys,” Machado says, staring at a hollow of empty trees. Like many of the Brazilians interviewed for this story, he speaks through an interpreter.
“They were very happy,” he says. “They make the house happy, too.”
But on this morning in early April, the house and forest are quiet.
The property’s owner found the first dead monkey on the last day of 2016. Soon after, Machado watched other monkeys fall ill.
“When they were already very sick,” he says, “they would fall down from the tree and die on the (forest) floor.”
Back east in São João Pequeno, Valdemar Braun also had been wondering about the monkey population. Two dozen or so used to come right onto his covered porch to eat juicy guava.
“They have all disappeared,” he says.
Although he cannot remember precisely, he believes the monkeys vanished around the end of last year, before his son Virlei grew ill and died. INSECTS HOLD THE KEY The idea that climate and disease are related dates back at least 2,000 years to the Greek physician Hippocrates. He wrote:
“Whoever wishes to investigate medicine properly should proceed thus: in the first place to consider the seasons of the year and what effects each of them produces. … Then the winds, the hot and the cold, especially such as are common to all countries, and then such as are peculiar to each locality.”
For some of the most serious diseases, including yellow fever, it is not so much the effect of climate on humans that matters; it is the effect on insects.
“There are some people that argue that global warming ’s greatest threat may also be the smallest, and, of course what we’re thinking about are insectborne diseases,” says Jonathan Patz, director of the Global Health Institute at the University of Wisconsin-Madison.
“And we all know that insects are cold-blooded, unlike us, and when the temperature changes a little bit, their body temperature changes with it.”
Decades of research has established that Aedes aegypti, a species of mosquito that carries yellow fever, Zika, dengue and chikungunya, thrives in warmer climates. The mosquitoes are more active, reproduce more frequently and enjoy a longer breeding season, though there’s a catch. If the climate becomes too hot and dry, it can shorten their lifespan.
Mosquitoes that transmit West Nile virus show a similar sensitivity.
“Every degree above 70 degrees exponentially expands the mosquito’s ability to transmit West Nile virus,” says McNamara, the former Bronx Zoo official and now a professor of pathology at Western University of Health Sciences in Pomona, Calif.
“Forget the cockroach inheriting the Earth. It’s going to be the mosquito.”
Already, warmer temperatures have helped mosquitoes settle into new regions. Aedes albopictus, another of the mosquitoes that carries the viral diseases dengue and chikungunya, “has undergone a dramatic global expansion facilitated by human activities,” according to the European Centre for Disease Prevention and Control.
Found originally in the tropical forests of Southeast Asia, Aedes albopictus, commonly known as the Asian tiger mosquito, has spread into Europe, the Middle East, Africa and North and South America — largely in the past 30 years. The mosquito, first discovered in the United States in Houston in the mid-1980s, has since spread to 37 states.
Researchers believe the Asian tiger mosquito’s rapid advance has been fueled by international transport of old tires and bamboo, objects that retain water, which makes them ideal places for mosquitoes to lay eggs.
Once carried overseas, however, the mosquitoes find the warming climate to their liking. In a 2013 paper in the journal PLOS
ONE, researchers said the Asian tiger mosquito “is poised to significantly expand its range in the northeastern United States in the next few decades primarily due to warming winter temperatures.”
Where the mosquitoes migrate, disease often follows.
But few scientists, if any, attribute the spread of such diseases to climate change alone.
Studies suggest other likely factors include global reductions in pesticide use and massive increases in waste plastics, such as bags, suitable for breeding by mosquitoes. In a larger sense, the growth and spread of the world’s population into rural areas is undoubtedly bringing more humans onto the turf of mosquitoes, ticks and other parasites and insects.
“Forget the cockroach inheriting the Earth. It’s going to be the mosquito.” Tracey McNamara, Western University of Health Sciences in Pomona, Calif.