As a member of a multidisciplinary team of University of Georgia researchers, environmental economist Susana Ferreira wants to get to the root causes of zoonotic disease spillovers, but her focus will be different than that of her colleagues.
Ferreira, a professor of agricultural and applied economics in the College of Agricultural and Environmental Sciences is part of a team led by Patrick Stephens of the Odum School of Ecology’s Center for the Ecology of Infectious Diseases (CEID). The team received a $2.4 million, five-year grant from the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, to investigate and model the effects of environmental, spatial and anthropogenic factors of zoonotic spillover, focusing on Ebola and related filoviruses.
Zoonotic spillover is the transmission of a pathogen from a vertebrate animal to a human. According to PubMed, disease spillover is common — more than two-thirds of human viruses are zoonotic.
By gaining a better understanding of how these factors influenced outbreaks in the past, the researchers hope to provide information that policymakers can use to predict and prepare for those of the future.
In any spillover episode, there is close interaction between the environment, public health and economics, Ferreira said.
“When we are examining how a pathogen transmits from an animal to a human, we have to answer the question, ‘How does the human put themselves in that position?’ The decisions being made about land use change and habitat conversion are interesting to me because, in many cases, the reasons are economic,” she said. “There are not many studies that focus on the big picture and how human encroachment can be driven by agricultural expansion or resource extractions that put people in the contact zone for these diseases.”
More than half of all infectious disease outbreaks worldwide are zoonotic, involving the spillover of pathogens from animals to humans. Ebola and other diseases caused by filoviruses are among the most deadly and costly of such zoonoses.
The 2014-16 West Africa Ebola epidemic was the largest known human Ebola outbreak, resulting in more than 11,000 deaths and an economic and social burden of more than $50 billion. In 2002-03, Ebola killed more than 5,000 wild gorillas, a species of critical conservation concern.
Seeking scientific consensus
Despite the severe economic toll and threat to human and animal life of zoonotic diseases, there is a lack of scientific consensus about drivers and dynamics of transmission at ecological boundaries where spillover occurs.
“In order to move from wild animal hosts to humans (or other animals), there are a number of challenges that a pathogen has to overcome,” said Stephens. “It must come into contact with humans, and it must adapt to living and reproducing in a new species. We refer to this process as crossing the ecological boundary that separates two populations. Understanding the circumstances that make it more likely that this will happen is absolutely critical if we are to anticipate future disease spillover events such as the one that led to the current COVID-19 pandemic,” he said.
The researchers will investigate how the characteristics of these ecological boundaries affect the dynamics of zoonotic transmission. They will examine the wild areas in which pathogens already exist, the areas of human encroachment in which a pathogen may spread, and the contact zone between the two where species interact and spillover physically occurs.
These boundaries are often the result of human activity motivated by socioeconomic factors, such as when people move into a wild area to develop land for farming or housing. This project will be one of the first to study the combined effects of ecological drivers with the effects of socioeconomic factors on transmission risk.
“Ebola tends to flare in regions that are politically unstable and conflict-ridden. Certainly, conflict is going to weaken the health infrastructure, displace populations and disrupt livelihoods. That is one of the things we want to look at in this study, examining the broad socioeconomic factors that may increase risk of zoonotic disease spillover,” Ferreira said.
Building accurate statistical models
The first major goal of the research is to build more accurate statistical models of spillover, using data from past events to understand the factors that influenced them.
“For example, preliminary analyses showed that past Ebola spillover events in the Democratic Republic of Congo have occurred in areas near roads and where rapid ongoing forest destruction is occurring,” said Stephens. “This may represent areas where humans are encroaching on wild areas where there previously was little contact.”
Ferreira said examining the socioeconomic conditions in areas where disease outbreaks occur can help determine what economic activities increase the risk of disease spillover.
“We must compare all the economic costs of land conversion— including costs from biodiversity loss and costs associated with the increase risk of zoonotic disease spillover — to the benefits of the land conversion in increasing the supply of food, wood or other natural resources,” Ferreira said. “We are looking at the first link in the chain — when someone makes the decision to use land in an intensive way by clearing forests, encroaching in the contact zone. This may be driven by big corporations, but also by small landholders or by those who sell meat in a wet market. There is an external costs in terms of an increase in risk of spillover from their economic decision — whether it is made by a poor farmer or hunter-gatherer or a corporation.”
The researchers will use insights such as these to develop new mathematical models of zoonotic spillover at ecological boundaries. They will use these models, and newly developed databases of historical filovirus outbreaks, to build risk maps and other tools that decision-makers can use to predict and prepare for future outbreaks.
“Ecologists have developed theoretical and statistical models that can help predict spillover, but they typically do not incorporate socioeconomic factors,” Ferreira said. “Part of my role will be helping include those economic factors, for example income and education levels in the area, the status of health care systems, the prevalence of conflict, the type of land use and land-use change patterns, into their models.”
The insights gathered will be applicable to a number of other zoonotic disease systems as well, including West Nile virus and avian influenza. In addition to the insights and tools developed, this project will provide training opportunities for a number of postdoctoral and graduate student researchers through 2025.
“This is, above all, a development challenge. We will be helping to understand the causes of outbreaks while also learning lessons that will help to improve access to health services and education in the areas where disease outbreaks are likely to occur,” Ferreira said.
In addition to Stephens and Ferreira, researchers leading the work include John Drake and J.P. Schmidt in the Odum School of Ecology and Nicole Gottdenker in the Department of Pathology at the College of Veterinary Medicine.
This project was developed out of CEID’s Spillover Working Group, with preliminary research supported by a UGA Presidential Interdisciplinary Seed Grant.