Tropical forests are home to most of the planet’s biodiversity (Reed et al., 2020). The Amazon Forest, for example, accounts for around one quarter of all the species in the world (Betts et al., 2008), and the Atlantic Forest is on the list of biodiversity hotspots, areas that have a high concentration of endemic species and are suffering intense habitat loss (Myers et al., 2000). These areas are also responsible for providing what are known as ecosystem services, a wide range of benefits that mankind obtains from nature and which are fundamental for its well-being (MEA, 2005), such as climate regulation, the provision of water, pollination and disease regulation (Charles and Dukes, 2007).
Deforestation has direct impacts not only on biodiversity loss, but also on the provision of ecosystem services (Brockerhoff et al., 2017; Crespin and Simonetti, 2016; Mori, 2017), leading to a reduction in the quality of life and health status in human populations as a result of increased exposure to and contact with pathogens (Plowright et al., 2017), particularly those of zoonotic origin.
Zoonotic diseases are infectious diseases transmitted from animals to humans through exposure to zoonotic pathogen by direct contact, food, water or the environment (WHO, 2021) and correspond to 60% of existing infectious diseases (Jones et al., 2008). The increase in the number of epidemics of zoonotic origin over the last decades and the speed with which the COVID-19 epidemic evolved to become a pandemic indicate that regulating services provided by ecosystems are not working correctly and that man-made changes to the landscape, especially deforestation and changes in land use, are the ultimate causes of the increase in the number of these epidemics (Gottdenker et al., 2014; Jones et al., 2013; Mishra et al., 2021). Indeed, around 40% of recent zoonotic diseases were the result of deforestation and the associated changes in land use (Loh et al., 2015).
A number of studies have attempted to shed light on how vegetation cover and changes in land use affect the risk of transmission of zoonotic diseases. For example, deforestation across Southeast Asia has been suggested to be the main factor responsible for dispersal of Plasmodium knowlesi, one of the causative agents of malaria (Davidson et al. 2019). Changes in land cover together with climate changes have affected the abundance of the vectors and hosts of Japanese encephalitis virus (Le Flohic et al., 2013), and man-made changes to the landscape have led to increased abundance of mosquito vectors and changes in the transmission patterns of West Nile virus (Ferraguti et al. 2021). However, few studies have investigated this issue in the Atlantic Forest (Ogrzewalska et al., 2011; Prist et al., 2021a; Prist et al., 2017, 2016; Scinachi et al., 2017), mainly because of the complexity of the host-pathogen dynamics involved in zoonoses and landscape changes. Studies in this field involving YF and its causative agent are even scarcer. Worthy of note are the studies by Abreu et al. (2022), Childs et al. (2019), de Almeida et al. (2019), Ilacqua et al. (2021) and Prist et al. (2021b), of which the latter two identified the risk of transmission of YFV associated with thresholds of vegetation cover of 30–70% and forest edge density (ED) > 80 m/ha, and greater mobility of the virus associated with forest edges, respectively. In a recent study of Atlantic Forest patches, our research group found greater permeability to YFV in forest edges than in core areas classified as Water, Human-impacted, Urban and Forest (Wilk-da-Silva et al., 2022), corroborating earlier results reported by Prist et al. (2021b).
Changes in land use are a key driver of habitat reduction, with a consequent negative impact on biodiversity (Fahrig, 2003) and direct influence on the dilution effect. This effect predicts that the greater the species diversity, the less the chance of contact between host and vectors of a particular type of pathogen. A reduction in species diversity can therefore lead to increased abundance of host and reservoir populations, increasing the probability of host-vector interaction and consequently the likelihood of infection (Ostfeld and Keesing, 2000; Schmidt. and Ostfeld, 2001).
In the particular case of zoonoses caused by mosquito-borne pathogens, their distributions involve the presence of hosts, vectors and human populations (Dantas-Torres and Otranto, 2015). Hence, overlapping of these plays a determining role in spillover events, an excellent example being sylvatic YF, a disease that affects humans when they are bitten by infected mosquitoes after entering forest areas (Monath and Vasconcelos, 2015). The disease is caused by YFV, an arbovirus in genus Flavivirus (Flaviviridae) which circulates in the wild among non-human primates (NHPs) and mosquitoes in the family Culicidae, in particular genera Haemagogus and Sabethes in Central and Southern America and Aedes (subgenus Diceromyia and Stegomyia) on the African continent (Chippaux and Chippaux, 2018; Monath and Vasconcelos, 2015).
Deforestation also affects the risk of transmission of this disease as loss of forest cover usually leads to fragmentation of natural environments, resulting in greater forest edges (Fahrig, 2003). The edges have a warmer, dryer microclimate and favor habitat generalist species, such as one of the main vectors of YF, Haemagogus leucocelaenus (Dyar and Shannon, 1924) (Abreu et al., 2019a), which exhibits ecological plasticity to vegetation patches subjected to man-made disturbances (Gomes et al., 2010; Lopes, 1997; Silva and Lozovei, 1999), as well as other potential vector species (Couto-Lima et al., 2017; Wilk-da-Silva et al., 2020). Furthermore, the juxtaposition of remnant forest patches and modified areas can contribute to greater availability of sources of blood meals for these vectors (Mucci et al., 2015), leading to increased abundances. Edge environments also make it easier for humans to enter vegetation patches (Bloomfield et al., 2020) and consequently have more contact with animals and any pathogens these may carry.
As well as contributing to an increase in edge surface, habitat fragmentation is directly reflected in the incidence and abundance patterns of vector mosquito species (Zittra et al., 2017) and causes changes in interaction patterns between hosts, vectors and pathogens (Zohdy et al., 2019). In the specific case of YF, habitat fragmentation is a key factor in the maintenance and circulation of YFV, particularly because of a greater prevalence of the virus due to the increased density of NHP populations (Mbora and McPeek, 2009).
The Atlantic Forest, with its highly fragmented remnants (Rezende et al., 2018), therefore consists of areas in which YFV can easily spillover to human populations as these areas are home to hosts (NHPs) and vectors (mosquitoes of genus Haemagogus and Sabethes) of this virus (Abreu et al., 2022; Possas et al., 2018), with both sometimes overlapping in edge areas (Bolt et al., 2018; Lenz et al., 2014; Wilk-da-Silva et al., 2020). This scenario is corroborated by the virus’s capacity to persist in Atlantic Forest remnants during consecutive transmission intervals (Abreu et al., 2019b), together with the large number of human cases associated with these remnants, which are close to cities with high population densities.
Zoonoses are generally one of the main public-health problems in low-income countries, where they account for approximately one quarter of all infectious diseases, and have less impact in higher-income countries, as a result of which they tend to be neglected (Grace et al., 2012). An understanding of aspects of landscape involved in the maintenance and circulation of pathogens such as YFV is crucial to support epidemiologic surveillance services in order to ensure better allocation of human and material resources in the fight against epidemics.
The main aim of the present study was therefore to understand how landscape structure affected the dispersal of YFV in the state of São Paulo between 2016 and 2020 using an interaction network analysis approach. Our hypothesis is that municipalities that act as a source of the virus and are therefore responsible for disseminating it to other locations in the state have a more fragmented landscape with greater forest edge density and less isolated patches than municipalities where the virus is not disseminated. This occurs because a more fragmented yet more connected landscape can favor dissemination of the virus, whereas fragmented landscapes with fewer connections do not allow the virus to spread, instead acting as a dead zone and preventing YF outbreaks in the state.