UNIVERSITY PARK, Pa. — Since its first documentation in 1976 there have been over three dozen outbreaks of Ebola virus disease in Central and West Africa, the largest of which resulted in the death of over 11,000 people between 2013 and 2016. A severe and often fatal disease, Ebola causes fever, weakness and bleeding, and spreads through contact with the bodily fluids of someone who is infected.
Researchers at Penn State recently published two papers that looked at factors that contribute to how these outbreaks begin and how severe they become. The first paper, which reported on environmental and human factors preceding Ebola spillover events, appeared in a recent issue of the journal Biology Letters alongside a brief video seminar on the paper on Cassyni . The second paper examined the post-spillover links between human movement and the spread of Ebola and appeared in the journal Scientific Reports .
Kelsee Baranowski, a postdoctoral researcher in biology in the Penn State Eberly College of Science and the Center for Infectious Disease Dynamics, and an author on both papers, spoke about the research and how the results could inform public health interventions to prevent pathogen emergence or slow the spread of Ebola and other infectious diseases.
Baranowski : Ebola virus has a high fatality rate in humans; most people who get infected die from it — though survival rates increase with care. A spillover event typically starts outbreaks in humans. This is when the virus initially jumps to humans from wildlife. Transmission occurs through contact with sick gorillas, chimpanzees or antelopes, which can happen during hunting and related activities. Sometimes fruit bats are implicated in spillovers but we often don't know exactly how the first person in an outbreak was infected. Once one person is sick, the virus can transmit from them to other people and then spread to other communities. In Central Africa, outbreaks are happening more frequently and are appearing in areas where Ebola was not previously reported.
Baranowski : A spillover is when a virus is transmitted from one species to another. Scientists think fruit bats naturally carry Ebola and spread it to other animals through their feces or possibly saliva on discarded fruit. Other animals — like gorillas, chimpanzees or antelope — get infected when they contact something contaminated with Ebola virus, but we're still figuring out exactly how this happens in the wild. Humans can get infected when they contact infected animals.
A spillover event is often the spark that begins a human outbreak. Sometimes only a few people get sick, but other times it leads to major outbreaks. Large outbreaks are the result of person-to-person transmission, not from multiple people contacting infected animals. Outbreaks can also start from a resurgent infection, which is when viral persistence in a survivor causes them to become ill and infectious again. Viral genetic sequences now indicate some Central African outbreaks started this way rather than from spillovers.
Baranowski : In our research, we looked at weather patterns and vegetation health before spillover events. We didn't consistently find singular environmental trigger that caused Ebola spillover events. Instead, there seem to be multiple environmental conditions that lead up to spillovers.
When we grouped spillovers by similar environmental conditions, we noticed a few patterns. Many spillover events that arose from human contact with infected wildlife, specifically gorillas, chimpanzees or antelope, grouped together. Other spillovers with unknown causes grouped together. The groups largely reflected either how people got infected with Ebola or the type of ecosystem they were reported in.
We found weak evidence that locations had more growth in human populations in the two years before a spillover was reported, compared to years when no spillovers were reported. And surprisingly, deforestation didn't play a big role. While only a couple locations had notable forest loss, most of that happened years before the spillover occurred. We found little to no support that deforestation or human population factors strongly impact Ebola spillovers.
Baranowski : To investigate that, we scoured used bookstores and estate sales for paper maps of roads and rivers in Central and West Africa that were printed between 1960 and 2020 from a single Michelin series. Roads and rivers are proxies for human movement and serve as transit infrastructure that we could now measure longitudinally with these maps. We digitized the paper maps and compared features of the transit networks around each outbreak location at the time when each outbreak occurred against case numbers in the early transmission period of each outbreak.
We used this approach because many of these outbreaks occurred at times and in locations where data derived from more technologically dependent measures of movement were not available. Even when these kinds of data do exist, many are privately owned and cannot be accessed freely and quickly, as would be necessary during an outbreak. In contrast, road and river data are relatively widely available and can be rapidly analyzed during an outbreak.
We found a strong positive correlation between the total road and river lengths in each outbreak location and the number of Ebola cases reported early in each outbreak. Transit infrastructure enables movement and connectivity and can assist the spread of communicable pathogens. But the same infrastructure can also assist in disease surveillance, disease prevention, and outbreak management efforts. Simple measurements can be very informative in guiding outbreak prevention and response in real time.
The paper maps are now part of the collection in the Donald W. Hamer Center for Maps and Geospatial Information at Penn State University Libraries, and the digital maps are publicly available in our GitHub repository associated with the paper.
Baranowski : Yes! Human movement helps spread many communicable diseases, not just Ebola. This method could help inform the surveillance, prevention and management of other transmissible diseases. Even more broadly, characteristics of movement infrastructure can provide insight about movement patterns that can be applied to many scenarios beyond disease outbreaks, including natural disaster preparedness and response, for example.
Baranowski : We're currently expanding our work to include Marburg virus, a close relative of Ebola virus. We know more about Marburg's reservoir host, the Egyptian fruit bat, and most spillover events occur after humans contact infected feces from the reservoir host. Studying the environmental anomalies preceding those spillovers will helps us further contextualize the factors leading up to Ebola spillovers.
We're also looking at environmental data from the 1980s to see what the environmental conditions looked like during a long period when no Ebola outbreaks were reported anywhere. Although, it’s worth noting that just because a spillover wasn’t reported, it doesn’t necessarily mean one didn’t happen. It’s possible that small, self-limiting outbreaks are undetected.
In addition to Baranowski, the research team for the Biology Letters paper included Nita Bharti, associate professor of biology at Penn State and the Center for Infectious Disease Dynamics in the Penn State Huck Institutes of the Life Sciences .
In addition to Baranowski and Bharti, the research team for the paper in Scientific Reports included Alexandria Gonzalez, research assistant, and Behnam Nikparvar, postdoctoral researcher, both in biology at Penn State and the Center for Infectious Disease Dynamics; Heather D. Ross, map specialist in the Donald W. Hamer Center for Maps and Geospatial Information in the University Libraries; M. Jeremiah Matson at the University of Utah; Stephanie N. Seifert at Washington State University; and Vincent Munster at the National Institute of Allergy and Infectious Diseases.
Biology Letters
Observational study
Not applicable
Multiple environmental conditions precede Ebola spillovers in Central Africa
28-Jan-2026
The authors declare that they have no competing interests.