Climate Change and Yellow Fever in South America: Ecological Factors, Emerging Risks and Future Transmission Scenarios

1. Introduction

Yellow fever (YF) continues to be one of the most important arboviruses in the Americas due to its high lethality [1], its epidemic potential, and its close relationship with tropical and subtropical ecosystems [2,3]. Caused by the yellow fever virus (YFV), a flavivirus transmitted mainly by mosquitoes of the genera Haemagogus and Sabethes in the jungle cycle of South America, the disease maintains a complex ecological dynamic that involves non-human primates as the main amplifying hosts and multiple environmental factors that condition viral transmission [4,5,6]. Despite the availability of a highly effective and safe vaccine for decades, yellow fever continues to generate human outbreaks and epizootics in several South American countries, constituting a persistent threat to regional public health [7,8].

During the last decades, South America has experienced important environmental transformations associated with climate change, including sustained increases in average temperature, modifications in precipitation patterns, more frequent extreme events, hydrological alterations, and changes in vegetation cover [9,10]. These transformations are modifying the ecological conditions that regulate the distribution, abundance, and behavior of vectors, as well as the population dynamics of vertebrate hosts involved in YFV transmission [11]. As a consequence, there is growing concern about the potential impact of climate change on the geographical expansion of areas receptive to transmission, the intensity of enzootic cycles, and the emergence of new epidemiological scenarios [12,13].

The relationship between climate and vector-borne diseases has been widely documented [14,15,16,17,18]. Environmental temperature directly influences essential biological parameters such as vector survival, feeding frequency, larval development, and viral replication within the mosquito [19,20]. Likewise, rainfall determines breeding-site availability, while relative humidity affects vector longevity and activity. In the specific case of yellow fever, these factors acquire particular relevance due to the complexity of the forest ecosystems where the jungle cycle develops [21,22]. Even small environmental modifications can alter interactions between mosquitoes, primates, and humans, increasing the risk of transmission [23].

South America is home to some of the most biodiverse ecosystems on the planet, including the Amazon, the Brazilian Cerrado, the Atlantic Forest, and the Andean forests, regions that constitute historical areas of YFV circulation [24,25,26]. However, these ecosystems also face increasing pressures from deforestation, agricultural expansion, mining, urbanization, and habitat fragmentation [27,28,29]. These anthropogenic activities, along with climate change, can alter viral circulation patterns. Reducing ecological barriers between wild environments and human settlements increases opportunities for contact between people, infected vectors, and susceptible primates, facilitating the emergence of human cases and epidemic outbreaks [30].

In recent years, several South American countries have reported events suggesting changes in yellow fever epidemiology [31]. Between 2016 and 2018, Brazil experienced one of the largest epidemics in decades, expanding into regions previously considered low-risk [32,33,34,35]. Similarly, Colombia has seen a resurgence of human cases and epizootics in non-human primates, making epidemiological surveillance increasingly important [23,30,36,37,38]. These events highlight the need to understand better how climatic and ecological factors may influence disease re-emergence and the potential expansion of its geographical distribution.

Another growing concern is the potential risk of yellow fever re-emergence. Although urban transmission of YFV has not been established in South America since the mid-twentieth century, the wide distribution of Aedes aegypti, the increase in human mobility, and the persistent viral circulation in jungle environments maintain the possibility of reintroduction of the urban cycle [39,40,41,42,43,44]. Climate change could further support this risk by expanding climate suitability for urban vectors and prolonging transmission seasons across multiple regions.

Climate projections for the 21st century suggest that much of South America will experience significant changes in temperature, precipitation, and frequency of extreme events [45,46,47]. These scenarios could substantially modify vector distribution, ecosystem susceptibility, and human population exposure [14,48,49]. However, significant uncertainties remain about the magnitude and direction of these effects, especially because climatic, ecological, socioeconomic, and demographic variables interact. Therefore, it is essential to integrate multidisciplinary approaches that incorporate climate modeling, disease ecology, epidemiological surveillance, and public health strategies under the One Health framework [50,51].

In this context, this article reviews evidence on how climate change influences yellow fever epidemiology in South America, analyzes the main ecological factors involved, emerging risks to human and animal health, and possible future transmission scenarios. It also discusses implications for surveillance, vaccination, biodiversity conservation, and outbreak preparedness in a region particularly vulnerable to global climate change.

2. Climate Change and the Ecology of Yellow Fever Transmission

Yellow fever (YF) is a viral disease transmitted by mosquitoes whose epidemiological dynamics are closely linked to the environmental and ecological conditions that regulate interactions between vectors, vertebrate hosts, and humans [52]. In South America, yellow fever virus (YFV) circulates mainly in jungle cycles involving mosquitoes of the genera Haemagogus and Sabethes, as well as various species of non-human primates that act as amplifying hosts [2,53]. Due to this complex ecological dependence, the transmission of YFV is particularly sensitive to climatic alterations [54,55]. Global climate change is progressively changing temperature, precipitation patterns, ambient humidity, and the frequency of extreme weather events, factors that can significantly influence the geographical distribution, abundance, and competition of vectors, as well as primate ecology and the risk of human exposure [56].

Temperature is a key determinant of mosquito vector biology. Increasing ambient temperatures can accelerate larval development, increase blood-feeding frequency, and shorten the virus's extrinsic incubation period within the mosquito [57]. As a result, vectors can transmit the virus over shorter periods and for a greater proportion of their lifespan [58]. In addition, higher temperatures may favor the expansion of some species into previously less suitable geographic areas, including higher altitude regions in the Andes, where the risk of yellow fever transmission has historically been limited. These changes could expand the spatial range of viral circulation and increase the vulnerability of human populations previously considered low-risk [59,60,61].

Rainfall is another key factor in transmission ecology. Jungle mosquitoes associated with YFV use natural tree cavities, holes in trunks, bromeliads, and other temporary reservoirs of water for the development of their immature phases [62]. Changes in rainfall patterns can alter the availability and persistence of these breeding sites, directly influencing vector population density. Periods of moderate rainfall followed by favorable temperatures usually create optimal conditions for increased mosquito abundance [63,64]. However, extreme rainfall can also alter larval habitats, generating complex ecological responses that vary depending on the region and species involved [65].

Environmental relative humidity also exerts an important influence on the survival and activity of adult vectors [66]. High humidity levels promote greater longevity of mosquitoes and increase the chances that they will survive long enough to complete the viral transmission cycle [67]. Conversely, prolonged periods of low humidity can reduce vector survival [68]. Because vector capacity is highly dependent on insect survival, small variations in humidity can result in significant changes in transmission potential [69].

Extreme weather events represent another relevant component of climate change; prolonged droughts, floods, heat waves, and wildfires can profoundly modify the ecosystems where YFV circulates [50,70]. These events can alter the availability of habitats for mosquitoes, affect the distribution of non-human primates, and modify patterns of interaction between wildlife and humans [71]. For example, forest fires and deforestation associated with periods of drought can increase the fragmentation of ecosystems, favoring more frequent contacts between human populations and environments where the virus is actively circulating [72,73].

In addition to long-term climate trends, interannual climate variability plays an important role in yellow fever dynamics. The El Niño-Southern Oscillation (ENSO) phenomenon is one of the main climate modulators in South America [74]. El Niño events typically generate temperature increases and alterations in precipitation patterns over large areas of the continent, while La Niña events produce opposite effects in many regions [75,76]. These fluctuations can influence mosquito abundance, breeding-site availability, vector survival, and vector competition for acquiring and transmitting the virus. ENSO can also modify the productivity of forest ecosystems and the availability of food resources for non-human primates, affecting their movements, population densities, and patterns of contact with vectors [77].

Climate change can also affect yellow fever through indirect mechanisms. Climate alterations interact with other environmental and social processes, including deforestation, agricultural expansion, mining, urbanization, and human migration. These factors transform ecosystems and create new interfaces between humans, vectors, and wild reservoirs [78]. Forest fragmentation can increase occupational and community exposure to infected mosquitoes, while human displacement associated with extreme weather events can increase the number of susceptible people entering endemic areas. In addition, the geographic expansion of Aedes aegypti in numerous South American cities raises concerns about a possible increased risk of yellow fever redevelopment under future climate change scenarios [3,19].

Taken together, the available evidence indicates that climate change acts as an important ecological modulator of yellow fever transmission [79,80]. Its effects range from direct alterations in vector biology to complex modifications of ecosystems, animal populations, and human activities. Understanding these interactions is essential to anticipate changes in risk distribution, strengthen surveillance systems, and develop One Health-based adaptation and prevention strategies in the face of epidemiological challenges that could emerge in the coming decades [81,82,83].

3. Environmental Transformation and Human-Wildlife Interactions in South America

The epidemiology of yellow fever in South America is determined not only by climatic factors, but also by profound environmental transformations driven by human activities [84]. Over the past few decades, the region has experienced an accelerated modification of its natural ecosystems due to deforestation, forest fragmentation, agricultural expansion, mining, urbanization, and infrastructure development [27,28]. These processes have significantly altered the ecological relationships between vectors, non-human primates, and human populations, creating conditions that may favor the persistence, expansion, and re-emergence of yellow fever virus (YFV) [23]. The interaction between these environmental transformations and the effects of climate change is currently one of the main challenges in understanding and anticipating future patterns of disease transmission [85].

Deforestation represents one of the most important environmental modifications in South America, particularly in the Amazon, the Brazilian Cerrado, and other tropical regions [25]. The removal of large tracts of forest changes the structure of ecosystems, alters local microclimates, and reduces biodiversity [86]. While the loss of forest cover may decrease some favorable habitats for certain vector species, it also creates new ecological interfaces between wild environments and human settlements [87]. These transition zones favor contact between people, infected mosquitoes, and susceptible primates, increasing opportunities for zoonotic transmission of YFV [30]. In addition, activities associated with deforestation, such as logging, road construction, and settlement establishment, directly expose workers and communities to ecosystems where the virus is actively circulating [88].

Forest fragmentation produces equally significant effects. When continuous forests are divided into isolated patches, the movement and distribution patterns of both vectors and non-human primates are modified [23]. Many primate species are forced to concentrate in smaller areas or move through narrow ecological corridors, which can alter viral transmission dynamics [71]. At the same time, some mosquito vectors can adapt to forest edges, where interactions with human populations are usually more frequent. As a result, fragmentation can increase the likelihood of encounters between infected hosts, competent vectors, and susceptible individuals [89,90].

Agricultural expansion is another key driver of environmental transformation. The growth of monocultures, extensive cattle ranching, and other productive activities has driven the conversion of vast areas of forest into agricultural landscapes [91,92,93]. These changes modify the ecological composition of biological communities and generate new patterns of human mobility. Agricultural workers, gatherers, hunters and rural populations often carry out activities in proximity to jungle ecosystems where enzootic cycles of yellow fever persist [94,95,96]. In addition, certain agricultural practices may create suitable habitats for some vectors, indirectly contributing to persistent transmission risk.

Mining activities, especially those associated with extracting gold and other resources in Amazonian regions, are an additional concern. Mining is often accompanied by deforestation, alteration of watercourses, temporary settlements, and mass movements of workers [27,97]. These conditions increase human exposure to jungle mosquitoes and make it difficult to sustain surveillance and vaccination programs. Likewise, the opening of new mining areas can facilitate the entry of susceptible populations into territories where YFV circulates among primates and wild vectors [4,25,98].

Urbanization and infrastructure development also change the disease's ecological dynamics. Roads, dams, power lines, and development projects facilitate human penetration into previously isolated ecosystems. At the same time, urban expansion into peri-urban and rural areas increases the proximity between human settlements and forest areas [99,100,101]. These processes favor biological exchange between wild and anthropized environments, increasing the probability of exposure to infected vectors. The human mobility associated with these transformations can also facilitate the dispersal of infected people to densely populated regions [102,103].

These environmental modifications are usually amplified when they interact with climate change. Rising temperatures, alterations in precipitation patterns, and the increased frequency of extreme events can amplify the ecological consequences of land-use changes [104]. For example, deforestation can increase local temperatures and reduce humidity, creating microclimates that affect vector survival and distribution. Similarly, more intense droughts can increase wildfire risk, accelerating habitat loss and increasing wildlife movements. This interaction between climate change and environmental transformation generates complex scenarios where multiple factors converge to modify epidemiological risk [105,106,107].

The consequences for vectors and non-human primates are particularly important. Changes in vegetation cover and climatic conditions can alter the availability of breeding sites, modify the geographic distribution of mosquitoes, and affect the density of their populations [108,109]. At the same time, habitat alterations can influence the abundance, movements, and population structure of primates, the main amplifying hosts of YFV in South America [110]. The epizootics observed in different countries in the region reflect how these ecological changes can affect viral circulation and serve as early signs of risk to human health [3].

From a One Health perspective, yellow fever is a paradigmatic example of how human health depends on ecosystem and animal health. The processes of environmental degradation, biodiversity loss, and climate change not only affect wild species but also increase the probability of emergence and re-emergence of zoonotic diseases [111,112,113,114]. In this context, integrated surveillance of vectors, non-human primates, environmental factors, and human cases is essential for early risk detection. Likewise, prevention strategies must incorporate ecosystem conservation measures, sustainable territorial planning, environmental monitoring, and strengthening of vaccination programs [115,116]. Understanding the interaction between environmental transformation, climate change, and yellow fever transmission will allow for the development of more effective responses to the epidemiological challenges facing South America in the coming decades [94,117]. The major climatic and environmental determinants influencing yellow fever transmission in South America, their underlying ecological and biological mechanisms, potential epidemiological consequences, and implications for surveillance and prevention are summarized in Table 1.

4. The Resurgence of Yellow Fever in South America in the Context of Climate Change

Over the past decade, yellow fever (YF) has experienced a significant resurgence in South America, evidenced by the emergence of large-scale outbreaks, the increase in human cases and epizootics in non-human primates, as well as the spread of transmission to areas where the disease had been absent for decades [94,118]. Although yellow fever has remained endemic in several tropical regions of the continent, the events recorded since 2016 suggest important changes in its ecoepidemiological dynamics [119,120]. This phenomenon has coincided with processes of climate change, environmental transformation, expansion of human activities in jungle ecosystems, and persistent gaps in vaccination coverage, factors that together have contributed to increasing the risk of transmission [79].

Brazil has been the country most affected by this resurgence. Between 2016 and 2018, it experienced the largest yellow fever epidemic in decades, with thousands of suspected cases and a progressive expansion from Amazonian areas to densely populated southeastern regions, including Minas Gerais, Espírito Santo, Rio de Janeiro, and São Paulo [121,122]. The spread of the virus into the Atlantic Forest led to intense non-human primate mortality and put millions of previously unvaccinated people at risk [85]. More recently, during 2024 and 2025, Brazil continued to report numerous human cases, including events recorded outside traditional Amazon areas, particularly in São Paulo and Minas Gerais [123].

Colombia has also experienced a notable re-emergence. Since 2024, a sustained increase in human cases has been observed, reaching during 2025 one of the largest outbreaks recorded in the country in more than two decades [79]. Although historically transmission was concentrated in Amazonian regions, the Orinoquia and scattered jungle areas, important foci have recently been identified in departments such as Tolima, Putumayo, Caquetá, Meta, Guaviare, Cauca, and Caldas [36,37,38]. The emergence of active transmission in areas relatively close to major population centers has raised concerns about the potential geographic expansion of the risk.

Peru has historically maintained endemic circulation of the virus, particularly in Amazonian departments and Andean-Amazonian transition areas [124]. However, recent outbreaks have shown greater temporal persistence and wider geographic dispersion, especially in border regions with intense human mobility, extractive activities, and accelerated land-use change [125]. Bolivia has followed a similar pattern, with human cases and epizootics mainly associated with Amazonian and sub-Amazonian ecosystems, though evidence is growing of expansion into new risk areas [36].

In Ecuador, Venezuela, Guyana, Suriname and French Guiana, the epidemiological situation has been more heterogeneous, but equally relevant. Ecuador reported human cases during 2025 after several years with a very low incidence, while Venezuela has faced recurrent episodes of re-emergence in jungle regions with insufficient vaccination coverage and limitations in surveillance systems [36,126,127]. Guyana, Suriname and French Guiana continue to be part of the complex Amazon corridor of jungle transmission, where viral circulation persists among mosquitoes and non-human primates, with sporadic episodes of human infection associated with occupational and recreational activities in forest environments [128,129].

The spatial and temporal patterns observed over the last few years have remained consistent. Figure 1 illustrates the spatial distribution of deforestation across South America during 2021–2025, along with the geographical distribution of reported yellow fever cases and associated deaths during 2024–2026, highlighting regional patterns of environmental transformation and disease occurrence. Most human cases continue to be associated with jungle transmission and are concentrated in people who carry out work, extractive, agricultural, or recreational activities in forested areas [59]. However, the geographical distribution of outbreaks has shown a progressive expansion towards peri-urban regions and areas historically considered low risk [61]. Likewise, a marked seasonality has been observed related to periods of greater vector activity, generally associated with rainy seasons and favorable climatic conditions for the proliferation of mosquitoes [130].

Various lines of evidence suggest that climate anomalies have contributed to this resurgence. Sustained increases in temperature, modifications in precipitation regimes, and extreme events associated with phenomena such as El Niño have altered the ecological conditions that regulate the abundance and distribution of jungle vectors [131]. Higher temperatures can accelerate mosquito development and shorten the virus's extrinsic incubation period, while changes in rainfall modify the availability of natural breeding sites. In parallel, droughts, wildfires, and floods can alter the movements of non-human primates and increase opportunities for contact between wildlife and human populations [77,131]. Although the direct causal relationship between climate change and specific outbreaks continues to be investigated, accumulating evidence indicates that anomalous climatic conditions play an important role as modulators of epidemiological risk [132].

One of the most worrying aspects is yellow fever's expansion into new ecological and geographical areas [61]. The recent detection of cases outside the traditional Amazonian hotspots, including regions of southeastern Brazil and Andean and inter-Andean areas of Colombia, reflects changes in historical patterns of viral circulation [79,133]. Climate change, deforestation, habitat fragmentation, agricultural expansion, and human mobility are creating new interfaces between vectors, primates, and susceptible populations [94,132,134]. These processes increase the likelihood of viral shedding events and pose significant challenges for surveillance, vaccination, and control programs [135,136]. Consequently, the current resurgence of yellow fever should be interpreted as the result of multiple ecological, climatic, and social factors interacting in an increasingly dynamic and complex regional scenario, with significant implications for South America's public health over the coming decades [60,94,132,137].

5. Future Transmission Scenarios in the Context of Climate Change

Climate projections for the 21st century suggest that South America will experience substantial changes in the main environmental determinants of vector-borne disease transmission [138]. According to the scenarios developed by the Intergovernmental Panel on Climate Change (IPCC) [139], much of the continent will face sustained increases in temperature, alterations in precipitation patterns, a greater frequency of extreme weather events, and significant modifications in tropical and subtropical ecosystems [138,140]. The Amazon basin, considered the main ecological reservoir of yellow fever in the Americas, is among the regions particularly vulnerable to these changes [110]. Rising temperatures, combined with more frequent droughts, forest fires, and ongoing deforestation processes, could profoundly transform the environmental conditions that regulate the circulation of the yellow fever virus (YFV), generating epidemiological scenarios different from those observed historically [3,106,133].

Projections for the Amazon indicate that some areas could experience temperature increases of more than 2–4 °C by the end of the century, depending on the emissions scenario considered [138]. At the same time, various climate models predict a reduction in rainfall in certain Amazon regions and a greater frequency of extreme events associated with severe droughts [141,142]. These changes could affect both the structure of forest ecosystems and the distribution and abundance of key species involved in YFV transmission [143]. In turn, other regions of South America could experience increased rainfall or greater climate variability, creating favorable conditions for vector expansion into new geographic areas [144].

Expected changes in vector distribution are among the most relevant factors for the future epidemiology of yellow fever. The jungle species of the genera Haemagogus and Sabethes, responsible for maintaining transmission in forest environments, depend closely on factors such as temperature, humidity, and availability of suitable habitats to complete their biological cycles [145,146,147]. Global warming could allow some species to expand their altitudinal and latitudinal distribution, colonizing previously less climate-friendly areas. As a consequence, Andean regions, inter-Andean valleys and subtropical areas could experience a progressive increase in ecological receptivity for the transmission of the virus [50,148].

These changes could also affect vertebrate reservoirs. Non-human primates, the main amplifying hosts of YFV in South America, are especially sensitive to environmental alterations [71]. Changing precipitation patterns, habitat fragmentation, and shifts in food availability could alter their movements, geographic distribution, and population densities [149]. Some species may be forced to move to new areas or concentrate in smaller forest fragments, changing opportunities to interact with vectors. These ecological changes could affect the virus's persistence and spread, creating transmission patterns that are difficult to predict using only historical data [3,25,143].

In this context, ecological niche models and risk mapping tools have become increasingly important to anticipate future scenarios. These models integrate climatic, environmental, and ecological variables to estimate the potential distribution of vectors, reservoirs, and areas favorable for transmission [150]. Studies in South America suggest that climate change could expand ecological suitability for yellow fever vectors to regions that are currently marginal or free of transmission [94,132,151]. In addition, projected risk maps indicate that densely populated areas located near forest ecosystems could experience increased exposure over the coming decades. Although uncertainties persist associated with the complexity of ecological systems and differences between climate models, these tools allow the identification of priority areas for epidemiological surveillance and preventive strategies [138,152].

The possible emergence of new transmission sources is a major concern for regional public health. Historically, yellow fever has been concentrated in Amazonian and tropical jungle regions; However, recent outbreaks have shown that the virus can spread to different ecosystems when favorable environmental conditions, susceptible populations, and competent vectors converge [153,154,155]. Under future scenarios of climate change and environmental transformation, transmission hotspots could emerge in ecological transition areas, peri-urban regions, and mountainous areas previously considered low-risk; this phenomenon could increase the complexity of surveillance programs and make it difficult to identify emerging areas of viral circulation early [50].

Although sustained urban transmission by Aedes aegypti has not been documented in South America since the mid-twentieth century, current conditions differ considerably from those that existed during historical control campaigns [42,118]. The mosquito is widely distributed across nearly all countries in the region; many cities have high population densities, and vaccination coverage remains uneven. In addition, increased human mobility facilitates the transport of infected people from jungle areas to urban centers. Climate change could exacerbate this risk by expanding the geographic distribution of Aedes aegypti, prolonging favorable seasons for its activity, and increasing vector competition in certain regions [156,157,158].

Although the probability of re-establishing urban transmission remains relatively low compared with jungle transmission, the consequences could be extraordinarily serious because so many susceptible people live in South American cities [118]. Therefore, future yellow fever scenarios must contemplate not only the expansion of traditional jungle cycles, but also the possibility of focused urban events or peri-urban outbreaks favored by the convergence of climatic, ecological, and social factors [159]. Consequently, strengthening integrated surveillance, expanding vaccination coverage, and developing predictive systems based on climate and environmental information will be fundamental to reducing the disease's impact amid accelerated climate change [158,160]. The potential future scenarios of yellow fever transmission in South America under climate change, including the expansion into previously less receptive geographical areas, changes in sylvatic transmission dynamics, increased human–wildlife interactions, and the possible re-emergence of urban transmission, are summarized in Table 2, together with their epidemiological implications and recommended public health responses.

6. Strategies for Monitoring, Preparedness and Adaptation to a Changing Climate

The resurgence of yellow fever in various regions of South America and the growing influence of climate change on its transmission dynamics pose significant challenges for public health systems [161]. Changes in weather patterns, the expansion of vectors into new ecological areas, and changes in the interaction between humans, non-human primates, and forest ecosystems require more dynamic, integrated, and predictive monitoring and control strategies [36]. In this context, adapting to a changing climate requires strengthening epidemiological monitoring systems, incorporating new sources of environmental information, and developing response mechanisms capable of anticipating emerging risks before they translate into large-scale outbreaks [60].

Surveillance systems based on climate information are an increasingly relevant tool for preventing vector-borne diseases. The availability of meteorological and climate data from ground stations, remote sensing, and satellite platforms allows variables such as temperature, precipitation, humidity, vegetation cover, and drought indices to be monitored in near real time [162,163]. These variables can be used to identify favorable environmental conditions for vector proliferation and viral circulation [164]. The systematic incorporation of climate indicators into yellow fever surveillance programs would improve the early detection of high-risk periods and optimize the allocation of resources for prevention, vaccination, and control activities [165].

Integrating multiple information sources is another fundamental element in facing future challenges. Traditionally, yellow fever surveillance has relied mainly on the detection of human cases and, in some countries, on the notification of epizootics in non-human primates [11,23,30]. However, the increasing complexity of the factors influencing transmission calls for broader approaches that incorporate environmental, ecological, genomic, and epidemiological data [84]. Monitoring forest cover, land-use changes, vector distribution, and primate populations can provide valuable insights into how risk evolves in specific areas. Likewise, the integration of genomic surveillance tools makes it possible to track the spatial dispersion of viral lineages, identify propagation routes, and detect possible evolutionary changes associated with new ecological scenarios [50,166].

Wildlife surveillance plays a particularly important role in the early detection of viral circulation. Non-human primates act as amplifying hosts highly sensitive to YFV, so identifying epizootics is a critical warning signal for implementing preventive measures in human populations [71,115]. Strengthening primate mortality surveillance networks, combined with georeferenced environmental and epidemiological information systems, can contribute significantly to improving the capacity to respond to emerging events [6].

In this context, early warning systems and predictive models are among the most promising areas for climate change adaptation [167]. Models that integrate climatic, ecological, entomological, and epidemiological variables can estimate transmission probabilities and anticipate potential outbreaks weeks or months in advance [168,169]. Tools based on artificial intelligence, machine learning, and spatial analysis can help identify areas of increasing risk and generate prospective scenarios for decision-making [170,171]. Although these systems still face limitations in data availability and quality, their increasing implementation could significantly transform yellow fever surveillance strategies in the coming decades.

Vaccination strategies will also need to adapt to increasingly dynamic risk patterns. Historically, immunization campaigns have been mainly aimed at populations residing in areas considered endemic [94,154]. However, the geographical expansion of risk observed in recent years suggests the need to periodically review vaccination recommendations based on the evolution of epidemiological and climatic scenarios [36,52]. Ecological modeling tools and risk maps can help identify new priority areas for preventive immunization, particularly in regions where environmental changes favor vector spread or the emergence of new transmission sources. Likewise, it will be essential to maintain high vaccination coverage in exposed populations, travelers, and workers who carry out activities in forest ecosystems [61,158].

From a One Health perspective, the prevention and control of yellow fever requires recognizing the close interdependence between human health, animal health, and environmental health [172]. Climate change, deforestation, biodiversity loss, and land-use transformations simultaneously influence vectors, reservoirs, and human populations [173]. Accordingly, monitoring and adaptation strategies should promote effective collaboration between traditionally separate sectors, including public health, veterinary medicine, environmental conservation, meteorology, land management, and ecological sciences. This approach allows for a comprehensive understanding of the determinants that favor the emergence and re-emergence of the disease [81,82,83].

Regional cooperation is also essential. Yellow fever transcends national borders because of the Amazon's ecological continuity, wildlife movements, human mobility, and vector dispersion [25]. Therefore, the timely exchange of epidemiological, climatic, and genomic information among South American countries is essential to strengthen collective response capacity [60,137,143,156]. International bodies, regional surveillance networks, and cross-border cooperation mechanisms can play a key role in harmonizing monitoring, vaccination, and control strategies [174]. In an accelerating climate-change scenario, preparedness for future yellow fever threats will increasingly depend on the ability to integrate knowledge, coordinate actions, and develop resilient systems that anticipate and respond to emerging risks across the region [138,175].

7. Limitations

This review has several limitations that should be considered when interpreting its findings. First, as a narrative review, it did not use a systematic literature search or formally assess study quality, which may introduce selection bias and limit the reproducibility of the evidence synthesis. Second, the available literature on the relationship between climate change and yellow fever transmission in South America remains geographically heterogeneous, with substantial differences in surveillance capacity, ecological characteristics, and data availability across countries. Third, establishing direct causal relationships between climatic variations and yellow fever outbreaks is challenging because transmission is influenced by multiple interacting factors, including deforestation, land-use changes, vector ecology, non-human primate populations, human mobility, and vaccination coverage. Furthermore, ecological and climate-based predictive models frequently rely on incomplete entomological and epidemiological datasets, introducing uncertainty into projections of future transmission scenarios. Underreporting of human cases and non-human primate epizootics, particularly in remote Amazonian regions, may further affect assessments of disease distribution and emerging risks. Finally, long-term projections remain sensitive to assumptions regarding greenhouse gas emissions, environmental transformation, and socioeconomic development. Despite these limitations, this review provides an integrated perspective on the ecological and epidemiological challenges posed by climate change and highlights priorities for future research, surveillance, and One Health preparedness.

8. Conclusions

The evidence indicates that yellow fever is undergoing an epidemiological transformation in South America, driven by interactions among climate change, land-use change, biodiversity loss, and increasing human pressure on tropical ecosystems. The resurgence of outbreaks in several countries, the expansion of transmission into new geographic and ecological areas, and persistent vaccination gaps show that yellow fever remains a dynamic threat to regional public health. In this context, understanding how environmental changes affect the complex relationships among vectors, non-human primates, and human populations is a key priority for anticipating future scenarios and strengthening prevention and control strategies.

Despite progress in understanding yellow fever virus ecology, significant knowledge gaps remain. Better characterization of how climate change affects the distribution, abundance, and vector competition among different Haemagogus and Sabethes species, as well as the ecology and vulnerability of non-human primates involved in viral amplification, is required. Likewise, studies that integrate climatic, environmental, genomic, and epidemiological information are necessary to understand better the mechanisms linking climatic anomalies to outbreak emergence. Developing more robust predictive models that incorporate future climate change and environmental transformation scenarios is a strategic priority for identifying emerging risk areas and optimizing preventive interventions.

The implications for public health policies are profound. National yellow fever prevention programs will need to be adapted to a context where risk areas can change more rapidly than in the past. This means strengthening epidemiological and epizootic surveillance systems, incorporating climate and environmental indicators into decision-making, and expanding capacity to respond to emerging events. Vaccination will remain the main prevention tool, but immunization strategies will increasingly need to rely on dynamic ecological and climate risk assessments. Similarly, territorial planning, ecosystem conservation, and climate change adaptation policies should be considered essential components of yellow fever control strategies, particularly in regions where deforestation, agricultural expansion, and extractive activities favor human-to-human contact and jungle transmission cycles.

Future yellow fever prevention efforts should be grounded in integrated One Health approaches, recognizing that human health closely depends on animal and environmental health. Coordinated surveillance of vectors, non-human primates, and human cases, along with monitoring ecological and climate changes, will enable earlier detection of emerging threats. Strengthening regional cooperation among South American countries to share epidemiological, climatic, and genomic information, harmonize surveillance strategies, and ensure timely responses to transboundary outbreaks will also be essential. In an era of accelerating environmental change, effective yellow fever prevention will require resilient health systems, innovative predictive tools, and cross-sectoral policies that can simultaneously address the health, ecological, and climate challenges that will define this disease's future in South America.