Between 2017 and 2019, a yellow fever epidemic killed roughly 30% of the population, leaving only 2,500 alive<\/a>. This small, endangered monkey, which nearly went extinct in the 1970s, is relegated to fragmented forests in the state of Rio de Janeiro, about 2% of their original territory<\/a>, according to conservation nonprofit Save the Golden Lion Tamarin.<\/p>\n But the tamarins (Leontopithecus rosalia) have made a significant recovery, rebounding to roughly 4,800 individuals according to a 2023 census<\/a>.<\/p>\n To better prepare for a possible outbreak in the future, a group of researchers wanted to find out how different environmental factors affected yellow fever transmission between tamarins. Their research identified high-risk areas<\/a>, which could guide conservationists aiming to vaccinate the population against future epidemics.<\/p>\n Researchers compared mortality data from 2014-2016, before yellow fever struck, with more than 1,000 deaths<\/a> that occurred during the outbreak in 2017-2019, information that was gathered by the nonprofit Golden Lion Tamarin Association<\/a> (known in Brazil as Associa\u00e7\u00e3o Mico-Le\u00e3o-Dourado). They then looked at how a series of variables in the environment itself, including human population, forest cover and habitat connectivity, might be linked to the deaths.<\/p>\n Notably, they found that areas with higher human density had significantly lower tamarin survival rates during the outbreak. This comes down to cross-species transmission, said Maxime Pierron, a University of Strasbourg ecologist and first author on the study. Yellow fever is a zoonotic disease: It can infect both humans and animals, specifically primates.<\/p>\n \u201cWe can both get [yellow fever], so it\u2019s easy \u2026 to share it,\u201d he said.<\/p>\n Habitat connectivity had a similar effect. Though it improved survival before the outbreak began, that outcome flipped once the epidemic got underway.<\/p>\n \u201c[Connectivity] reduces mortality because it improves the gene flow and it allows primates to evade threats more easily, so it\u2019s great,\u201d Pierron said. \u201cBut during an outbreak, it turns out that this also facilitates the movements of mosquitoes that are infected with yellow fever,\u201d which increases transmission, he added. The researchers found yellow fever caused more deaths in forests that were better connected.<\/p>\n Despite this, Pierron warned that the findings should not be interpreted as an argument against reforestation and restoring or relinking fragmented habitat.<\/p>\n \u201cReforestation is both welcome and urgently needed around the world,\u201d he wrote in an email to Mongabay, adding that \u201cin this context, it is essential to combine reforestation with landscape-informed disease monitoring and to integrate disease ecology into conservation planning.\u201d<\/p>\n Connected habitat could help the tamarin population recover from a future outbreak, Carlos Ruiz-Miranda, co-author of the study and board president of the Golden Lion Tamarin Association, told Mongabay in a voice message.<\/p>\n \u201cThe more connected the habitat is, the faster or more likely the recovery and recolonization,\u201d he said.<\/p>\n University of Wisconsin-Madison anthropologist Karen Strier<\/a>, who has studied monkeys in the Atlantic Forest since 1982, said the research brought to mind an old ecological debate between scientists known as SLOSS (single large or several small): a disagreement on the ideal structure of wildlife reserves. One of the arguments against larger areas, she said, was that diseases within them could spread.<\/p>\n \u201cConnectivity is important, but it has some costs too, and I think it\u2019s important to be aware of those costs,\u201d Strier said. \u201cNot because we should stop trying to connect habitats, but so that we can think as [the paper\u2019s authors] do about how to mitigate the risks.\u201d<\/p>\n A key part of that mitigation is vaccinating tamarins against yellow fever. AMLD staff began<\/a> that process in 2020. They\u2019ve immunized just over 500 individuals so far, according to Ruiz-Miranda.<\/p>\n According to Pierron, \u201cdeath risk during the outbreak can serve as a proxy of transmission risk.\u201d In other words, based on where deaths occurred during the outbreak, scientists can predict areas where the disease might spread more easily in the future. These high-risk areas are \u201cpriority zones\u201d for vaccination.<\/p>\n The team has already immunized some individuals from most populations, Ruiz-Miranda said, and plans to vaccinate individuals from the rest in the future. To vaccinate the monkeys, a team captures them and brings them to a field laboratory, he said, where they sedate the animals. They collect hair samples for DNA studies and take measurements before injecting the vaccine. The team tattoos numbers on each animal\u2019s inner thigh to make sure they don\u2019t vaccinate the same individual twice, he said, and then they release them where they were found.<\/p>\n According to Marcos da Silva Freire, a scientific advisor at Brazil\u2019s Oswaldo Cruz Foundation, who helped develop the yellow fever vaccine for golden lion tamarins, the immunization campaign should continue indefinitely. Though vaccinated animals will eventually die, keeping at least a percentage of the population protected helps \u201cavoid some disaster with yellow fever reemergence in the region,\u201d he said.<\/p>\n Banner image: Two golden lion tamarins in Brazil. Image by Henrique Bitencourt via Wikimedia Commons<\/a> (CC BY-SA 4.0<\/a>).<\/p>\n Alex Megerle is an intern writer at Mongabay, where he covers wildlife and conservation, and a student in the Massachusetts Institute of Technology Graduate Program in Science Writing.<\/p>\n More to explore:<\/p>\n Reconnecting \u2018island habitat\u2019 with wild corridors in Brazil\u2019s Atlantic Forest<\/a><\/p>\n The next great threat to Brazil\u2019s golden lion tamarin: Yellow fever<\/a><\/p>\n Citations:<\/p>\n Pierron, M., Ponchon, A., Ruiz\u2010Miranda, C. R., Martins, A., Sueur, C., & Romano, V. (2026). When conservation meets contagion: Mapping yellow fever risk to guide vaccination in an endangered primate. Journal of Applied Ecology, 63(6). doi:10.1111\/1365-2664.70463<\/a><\/p>\n Dietz, J. M., Mickelberg, J., Traylor\u2010Holzer, K., Martins, A. F., Souza, M. N., & Hankerson, S. J. (2024). Golden lion tamarin metapopulation dynamics five years after heavy losses to yellow fever. American Journal of Primatology, 86(7). doi:10.1002\/ajp.23635<\/a><\/p>\n Dietz, J. M., Hankerson, S. J., Alexandre, B. R., Henry, M. D., Martins, A. F., Ferraz, L. P., & Ruiz-Miranda, C. R. (2019). Yellow fever in Brazil threatens successful recovery of endangered golden lion tamarins. Scientific Reports, 9(1). doi:10.1038\/s41598-019-49199-6<\/a><\/p>\n FEEDBACK: Use this form<\/a> to send a message to the author or editor of this post. If you\u2019d like to post a public comment, you can do so at the bottom of the page.<\/p>\n Cite this articleAlex Megerle (2026). Researchers find \u2018priority zones\u2019 for yellow fever vaccination of golden lion tamarins. Mongabay Conservation news. DOI: https:\/\/doi.org\/10.66709\/news-324739<\/a> <\/p>\n ![\"The](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/07\/3840px-Duas_Embau_bas_no_Parque_-Natural_Municipal_do_Trabiju.jpg\")
The golden lion tamarin, an endangered species, is found exclusively in Brazil\u2019s Atlantic Forest, a seriously threatened biodiversity hotspot: only 7%-12% remains. Image by Themium via Wikimedia Commons<\/a> (CC0 1.0<\/a>).
\nMapping an outbreak<\/p>\n![\"Connected](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/07\/1785434408_130_5.jpeg\")
Connected habitat is crucial for wildlife survival, but a new study found that during the 2017-2019 yellow fever outbreak, more tamarins died in connected forest areas and areas where there were more people to pass it to them. Image by Alex Popovkin via Wikimedia Commons<\/a> (CC BY 2.0<\/a>).
\nThe value of connectivity<\/p>\n![\"The](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/07\/Golden_lion_tamarin_portrait_2.jpg\")
The golden lion tamarin is one of Brazil\u2019s most iconic conservation success stories, but yellow fever seriously impacted the population nearly 10 years ago. Image by Jeroen Kransen via Wikimedia Commons<\/a> (CC BY-SA 2.0<\/a>).
\nVaccination nation<\/p>\n![\"Golden](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/07\/3840px-Leontopithecus_ros-alia_in_Sao_Paulo_Zoo.jpg\")
Golden lion tamarins in the S\u00e3o Paulo Zoo in Brazil. Image by Wilfredor via Wikimedia Commons<\/a> (CC BY-SA 4.0<\/a>).<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/07\/92a8467a68ba6484f5372dc6c0c8580efd7d81cc823c1a1f46c9977026eb411e.png\")
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