As a veterinary doctor working in municipal government, Dr. Haroldo Greca Junior is always on the lookout for new diseases that might jump from animals to humans. Greca Junior works in São José dos Pinhais, Brazil, a municipality whose boundaries stretch from the densely populated suburbs of Curitiba, the largest city of Paraná state, into thickly forested slopes of Atlantic Rainforest. The municipality is located in a subtropical climate, so the scourge of yellow fever was never on his radar. Yellow fever is a seasonal disease spread by mosquitoes and normally confined to the tropics. There had been no cases in Paraná for decades.

In the late 2010s, however, the regional health system issued an alert: Yellow fever outbreaks were moving southward in Brazil. After waves of transmission in 2016-2017 and 2017-2018 that infected and killed people in the states immediately north, Paraná was now in the crosshairs.

“Yellow fever was something I had studied but never seen personally,” Greco Junior said. “It’s clear that it was going to come one way or another, so what are we going to do?”

In January 2018, Greca Junior attended a training course on how to deal with monkeys, who frequently fall prey to yellow fever infections during outbreaks. Dead monkeys, in turn, occasionally pass the disease to humans. Some of the training was routine, like demonstrating how to safely handle a dead monkey using a mask and gloves. But one component stood out: Fiocruz, Brazil’s leading public health research institute, had recently developed a smartphone application, the SISS-Geo platform, that allows members of the public to easily report dead wildlife with a photograph and geotagged location.

“We are going to need this platform,” Greco Junior said. He works with a team of three biologists and one veterinarian who monitor for zoonoses, or disease that can pass from animals to humans, but they are responsible for covering a geographic area three times larger than Curitiba with two-thirds of the surface area located in the forest.

“I found it very interesting, but I kept asking myself how we were going to use this thing,” Greca Junior said. “The rural community is stubborn and not so technology- friendly.”

Greca Junior’s team quickly determined that hosting a course or lecture would not reach their target audience, the municipality’s 27,315 rural residents, many of whom are farmers living in isolated communities. Instead, they started making daily field visits in April 2018 to conduct time-consuming but effective one-on-one outreach to inform residents of an impending yellow fever outbreak, encourage vaccinations, and teach them how to use SISS-Geo.

“You have to be informal and make it sound like a chat between friends,” Greca Junior said. “It’s like building a wall brick by brick.”

The municipality registered its first monkey-borne yellow fever case in December 2018. “2019 was a tough year for us,” Greco Junior said. “You encounter families of howler monkeys all dead. It’s very sad.”

While the municipal disease monitoring team spent nearly every day in the forest, the terrain was too vast and rugged to cover on their own. As the first line of defense, they relied on residents like Rosângela Scrippe de Oliveira, who lives with her husband on two hectares that abuts old-growth rainforest, some 27 kilometers from the urban center of São José dos Pinhais, where she works in administration.

“I have so many apps so I didn’t give [SISS-Geo] much importance at first,” she said, although she did seek an inoculation against yellow fever on the officials’ advice. “We have WhatsApp groups among the rural neighbors who would take photos of dead monkeys.”

Around her property she sees and hears cascavel rattlesnakes, many species of monkeys, tucans, and parrots. When the sound of howler monkeys disappeared from the jungle chorus in 2019, Oliveira took notice.

“With time, after the yellow fever situation, I became more interested,” she said. “Today I see the importance.” She estimates that she accesses the app once per week and sends in roughly five wildlife photos from her weekends working on her property.

The app has registered around 10,000 submissions since it was introduced three years ago, with spikes on Friday through Sunday when urban residents visit their second homes or spend weekends at eco-tourism lodges. The COVID-19 pandemic also enhanced public awareness of zoonotic diseases, which led to more users among the general public. Through October 2020, the app helped city staff locate 55 dead primates in São José dos Pinhais. Greco Junior said that São José dos Pinhais’ early adoption has inspired the other municipalities in metro Curitiba to begin using the platform, which will soon become standard across Paraná.

Marco Antônio Barreto de Almeiuda, a biologist and epidemiologist with the Rio Grande do Sul State Center for Health Monitoring, applauds these outcomes. “In other places where they did not have the time or resources to directly train the primary source of information, which are the people living in proximity to primates, the majority of SISS-Geo users are health workers,” he said. “These people work in primate death monitoring and are already in the field investigating information that arrives from other sources, and in these situations they have lost the opportunity to be notified immediately, which is the great triumph of SISS-Geo.”

In the end, the city only recorded one human case of yellow fever during the outbreak that temporarily decimated monkey populations. While it is difficult to prove scientifically that SISS-Geo was a significant determining factor -- vaccinations are a vital component for preventing human infection -- the experience has left a lasting outcome for citizen science.

“I’m proud because we contributed to collecting the data and informing the municipal team,” Oliveira said. “And today, the howler monkey sounds are back.”