A holistic ecological and evolutionary process perspective is required to understand the risk of spillover and spread of pathogens in humans and animals (Alexander et al. 2018). Pathogens are not fixed entities, and some pathogens carry a greater innate ability to evolve and spillover into new hosts than others (Johnson et al. 2015; Olival et al. 2017). Evaluation of pathogen evolution from initial spillover to establishment in the human population (e.g., simian immunodeficiency virus chimpanzee to human immunodeficiency virus; HIV-1) (Gao et al. 1999) is critical to understanding why certain pathogens can establish and others cannot. Viral surveillance in “real-time” is required to examine and track pathogen evolution. For example, the Global Influenza Surveillance and Response System, designed to monitor the quickly evolving and recombining influenza virus in a timely manner, has served for over half a century as a global alert mechanism for the emergence of influenza viruses with pandemic potential (World Health Organization 2019a). Knowledge of ecological scenarios surrounding the accelerated viral evolution of highly pathogenic avian influenza (HPAI) is vital to preparing for future outbreaks of the disease in humans and worthy of significant investment in China and elsewhere (Gao 2018). Monitoring of viral evolution in similarly relevant time scales for other quickly evolving pathogens, such as coronaviruses, lags further behind and is an area of research with much needed additional attention. For example, recent evolutionary analyses of Middle East Respiratory Syndrome (MERS) coronavirus have helped to elucidate viral transmission dynamics between camels and people (Dudas et al. 2018). We also now know that viruses sharing diverse vertebrate hosts are “worth watching” for potential emergence in humans, because host breadth (i.e., infecting a taxonomically diverse range of hosts) is a key factor associated with a virus’ likelihood of spillover and secondary human-to-human transmission and geographic spread (Johnson et al. 2015; Olival et al. 2017). Thus, expanding “real-time” surveillance of such pathogens would be a worthwhile investment for public health.

Analyzing epidemiological data using theory and methods from macroecology allows us to forecast impacts of ecological and environmental drivers on infectious disease incidence. Examples include how land cover and climate changes influence epidemics in wildlife (e.g., chytrid fungus, white-nose syndrome), livestock (e.g., foot-mouth-disease, African swine fever, bluetongue) and human (e.g., schistosomiasis, malaria) populations, as well as impact complete wildlife–livestock–human systems (e.g., avian influenza, tuberculosis) (Estrada-Pena et al. 2014; Peterson 2014; Purse et al. 2005). Large-scale, high-resolution data on environmental and climatic factors and populations of humans and wildlife facilitate the creation of dynamic distribution models for infectious agents and can help prioritize research and control efforts (Cohen et al. 2016; Carlson et al. 2017). Collaborative research between China and the USA could greatly advance spatiotemporal disease prediction schemes because both countries encompass large climatic gradients, have high capacity for ecological and climatic data collection, and share some overlapping vectors, and zoonotic pathogens (Liu et al. 2018; Estrada-Pena et al. 2012; Springer et al. 2015; Wu et al. 2013; Centers for Disease Control 2018).

A complete understanding of host–pathogen interactions and how and where to intervene requires an eco-system or “One Health” viewpoint that accounts for processes occurring at both macro- and micro-scales, including at the pathogen, host and environmental levels, as well as an integration of the effects of processes across these scales (Alexander et al. 2018; Forst 2010; Blackburn et al. 2019). Such multiscale “One Health” research requires the incorporation of many disciplines including, but not limited to, human medicine, veterinary medicine, public health, environmental science, ecology, conservation biology, nursing, social sciences, the humanities, engineering, economics, education and public policy (Lu et al. 2016; Carlson et al. 2018). China and the USA have led response activities for several epidemic and pandemic outbreaks impacting humans and animals which have required a One Health perspective. Three relevant case studies which are expanded upon below include: the West Africa Ebola outbreak, the SARS pandemic, and the emergence of amphibian chytridiomycosis.

The first outbreak of Ebola virus disease (EVD) outside of Central Africa demonstrated the importance of focusing on wildlife host and human ecological risk factors in advance of major disease outbreaks and the need for international collaboration in outbreak response (Fig. 2). Despite nearly 40 years of research since the first outbreak in 1976, including public investment of US$ 1.035 billion between 1997 and 2015 (Fitchett et al. 2016), national and international public health agencies were caught off guard by the 2014 outbreak in West Africa (Kamradt-Scott 2016) that resulted in 28,600 cases with more than 11,300 deaths (WHO Ebola Response Team et al. 2016). Evidence from humans and wildlife indicating the distribution of Ebola virus (species Zaire ebolavirus) in West Africa existed prior to this health crisis. Distribution and migration patterns of the hammer-headed fruit bat (Hypsignathus monstrosus), little collared fruit bat (Myoncycteris torquata), straw-colored fruit bat (Eidolon helvum) (Leroy et al. 2005), Franquet’s epauletted fruit bat (Epomops franqueti) (Olival and Hayman 2014) and the greater long-fingered bat (Miniopterus inflatus) (Kupferschmidt 2018), species implicated as reservoir hosts for Ebola virus, were known to extend into West Africa with opportunities for spread of the virus. Human serological exposure also indicated a wider geographical range for ebolaviruses including Guinea (Boiro et al. 1987), Liberia (Van der Waals et al. 1986) and Sierra Leone (Schoepp et al. 2014) well in advance of 2014. During this outbreak, China and the USA collaborated together for the first time in an international health emergency outside of their borders, which constituted China’s largest ever humanitarian mission in addressing a public health emergency of international concern (Huang 2017). Twenty-four of the Chinese public health experts who were deployed to Africa were graduates of, or residents in, the Chinese Field Epidemiology Training Program (CFETP) established by the US Centers for Disease Control and Prevention (CDC) (Centers for Disease Control 2016). Also, the Chinese government sent 115 military medical professionals to Sierra Leone to work along with US medical personnel to assist with infection prevention and control, clinical care and health promotion and training (Lu et al. 2016). Further investment from China and the USA in working together on response efforts will undoubtedly be mutually beneficial for pathogens of importation concern. Efforts toward unraveling the disease ecology of ebolaviruses—including a better understanding of the ecology of reservoir host(s), the role of secondary spillover hosts, as well as human behaviors surrounding exposure—is also needed and would benefit exponentially from a China–US collaborative effort.

Figure 2 One Health concepts impacting emerging infectious diseases. Climate Change: With the introduction of Zika virus into the Americas, changes in maximum occurrence of mosquito vectors in the USA, due to a changing climate, impact risk of Zika virus distribution. Human Ecology: High-risk human behaviors involving contact with farmed wild animals contributed to the emergence of SARS. Biodiversity: Alteration of wild animal reservoir host populations impacts spillover risk for zoonotic infectious diseases. Animal Host Ecology: Distribution of the bat reservoir hosts for Ebola virus (species Zaire ebolavirus) likely caused the first human outbreak in West Africa. Together with the impact of global trade and travel, these case examples of the interconnectedness of humans, animals and the environment demonstrate how human and animal ecology influence the global spread of disease. Full size image

The Severe Acute Respiratory Syndrome (SARS) pandemic caused by a novel zoonotic coronavirus (SARS-CoV) was the first pandemic of the twenty-first century and spread to more than 30 countries (Fig. 2). Initial isolation of SARS-related coronavirus (SARSr-CoV) from masked palm civets and the detection of SARS-CoV infection, in humans working at wet markets selling these animals in Guangdong Province, suggested that masked palm civets could serve as a source of human infection (Guan et al. 2003). Subsequently, SARSr-CoVs were detected in Chinese horseshoe bats (Rhinolophus sinicus) and provided strong evidence that bats are the natural reservoir of SARS-CoV (Ge et al. 2013; Li et al. 2005; Yang et al. 2015). Through long-term human and wildlife surveillance, investigators from China and the USA subsequently found that bats carry a diverse range of SARSr-CoVs (Ge et al. 2013; Li et al. 2005; Yang et al. 2015; Lau et al. 2005; Drexler et al. 2010; Yuan et al. 2010; He et al. 2014; Wu et al. 2016; Hu et al. 2017), extending as far as Yunnan Province, and some of them can directly infect humans without intermediate hosts (Wang et al. 2018). While it is unknown whether the SARS outbreak could have been preempted with this knowledge, the joint efforts of China and the USA to rapidly determine where, how and when the virus was spilling over and what human behaviors and populations were at greatest risk for infection may have reduced the severity of the outbreak and will help in mitigating future spillover events. The SARS outbreak was a prime example of the importance of contextualizing epidemiologically notable human behaviors in social, economic and cultural systems in order to decipher causality of an EID.

Pandemic diseases in wild animals (epizootics) can also result in devastating impacts to a country’s biodiversity and natural resources. For example, amphibian chytridiomycosis, caused by the novel pathogen Batrachochytrium dendrobatidis, is responsible for massive losses of biodiversity to an entire Class of organisms (Amphibians). The global pandemic lineage of the pathogen originated in Eastern Asia (Ostfeld and Keesing 2012) and disseminated through human trade and transportation (O’Hanlon et al. 2018) into the biodiverse areas of Australia and the Neotropics. The lack of demographic studies, combined with limited population estimates in the IUCN Red List prior to this pandemic, made it difficult to understand the scope of the disease. A better understanding of wildlife population dynamics before massive declines occur is essential to better understanding biodiversity’s impact on EIDs and what elements of biodiversity disease theory (Fig. 2) apply (Jones et al. 2008; Keesing et al. 2010; Murray and Daszak 2013; Morse 1995; Ostfeld and Keesing 2012; Civitello et al. 2015). Interestingly, another fungal pathogen, white-nose syndrome (WNS), which has decimated bat populations in the USA, may also have an Asian or Eurasian origin because it has been found on bats in Northern China (Hoyt et al. 2016). Recent studies suggest that the systemic effects of WNS may down-regulate anti-viral responses in bats persistently infected with coronaviruses and increase the potential of virus shedding (Davy et al. 2018). Thus, a pathogen predominantly infecting wildlife may additionally have cascading effects on spillover of other pathogens of significance for human health.