The majority of emerging human diseases are anticipated to occur from zoonotic events – Ebola hemorrhagic fever and salmonellosis are examples of infectious diseases that began when pathogens jumped from wild animals into humans. Most modern outbreaks are quickly brought under control and result in fewer than 100 cases. However, infectious diseases outbreaks that infect hundreds to thousands of people still occur. All zoonotic infectious diseases have drivers, and understanding the circumstances of how drivers for small-scaled versus large-scale outbreaks differ is an area of focus for infectious disease ecologists.
CEID research scientist Patrick Stephens and a number of his CEID colleagues recently published Characteristics of the 100 Largest Modern Zoonotic Disease Outbreaks in the Royal Society Journal Philosophical Transactions B. By studying 4,463 zoonotic infectious diseases reported by GUIDEON Guide to Outbreaks that occurred between 1975 and 2017, they identified the 100 largest in terms of human cases and compared them to a group of 200 randomly selected “control” infectious diseases. All outbreaks were classified as zoonotic by the CDC, the UK Health Ministry, and the Pan American Health Organization working groups.
Stephens and his team developed a list of forty-eight potential drivers of outbreaks according to ecological, environmental, and socio-economic factors, and they were used to score 300 infectious diseases. Their analysis determined large-scale outbreaks had 3.19 mean drivers, and small-scale “control” outbreaks had 1.91 mean drivers. However, out of the 48 predicted drivers, at least 20 of them occurred in less than 1% of large-scale outbreaks despite being previously mentioned in reviews of modern outbreak causes. The most common driver of large-scale zoonotic outbreaks was contaminated water, followed by abnormal changes in weather patterns and increases in disease vector abundance. Often these events went hand-in-hand with unusually high levels of rainfall resulting in unusually high numbers of disease vectors, such as mosquitos. Poor sewage management also played a role in large zoonotic outbreaks, and the authors indicated that fewer medical and societal resources could significantly play a role in large outbreaks. Furthermore, large outbreaks were statistically more likely to occur from viral zoonoses compared to bacterial zoonoses.
This research by Stephens and his colleagues is one of the first peer-reviewed studies to examine and quantify a variety of zoonotic disease outbreak drivers. By establishing a general understanding of zoonotic drivers, these researchers hope to gain future insight into the severity and complexity of future infectious disease outbreaks.
Stephens, P., Gottdenker, N., Schatz, A. M., Schmidt, J. P. & Drake, J. Characteristics of the 100 largest modern zoonotic disease outbreaks. Phil. Trans. R. Soc. B 376, 20200535 (2021). https://doi.org/10.1098/rstb.2020.0535