Analyzing the 100 largest zoonotic disease outbreaks, investigators found the most significant drivers to be water contamination, sewage management, weather conditions, and change in vector abundance.
What do the 100 largest zoonotic disease outbreaks all have in common?
A recent study published in Philosophical Transactions of the Royal Society B reviewed a global dataset of 4463 bacterial and viral zoonotic outbreaks to compare the characteristics of the 100 largest.
As many of 70% of emerging human diseases are zoonotic, a figure that appears to be growing over time. The investigators attributed this to the unprecedented mixing of humans, wild species, and domesticated animals, caused by globalization and land conversion. These developments triggered changes in vector abundance, human population density, unusual weather and climate, and water contamination.
Modern zoonotic outbreaks are contained more quickly, with most zoonotic outbreaks limited to fewer than 100 cases. However, there is still inadequate research of the socioeconomic causes of zoonotic diseases. The availability of hosts may be the largest risk factor for outbreaks, indicating population density may be a major impetus of spreading zoonotic disease. Additionally, latitude has been correlated with disease diversity, with tropical countries showing much greater disease diversity than high latitude countries.
Many factors, like international travel and trade, are thought to be drivers of zoonotic outbreaks, but few of these theories have been quantitatively tested on a global scale.
The investigators identified significant zoonotic pathogens using the GIDEON Guide to Outbreaks. To score the outbreaks, they used zoonotic outbreak literature to compile a list of 48 potential driving factors. The drivers represented a variety of ecological, environmental, and socioeconomic phenomena. Drivers were scored in relation to the zoonotic outbreaks as either no reported relation (0) or as a contributing factor by at least one source (1). Analyses did not include drivers reported in less than 3% of outbreaks.
Of the 100 zoonotic outbreaks considered, water contamination was indicated in 40%, sewage management in 31%, weather conditions in 29%, and change in vector abundance in 21%. These 4 drivers were always statistically significant, but notably, they were more commonly found in large outbreaks than in the controls.
Large zoonotic outbreaks typically had more driving factors than the comparative background outbreaks; the average number of drivers for large outbreaks was 3.19, and 1.91 for small outbreaks. The pathogens of large zoonotic outbreaks were more likely to be viral-borne and vector-borne.
What the investigators called “failures of societal and medical resources,” such as war, were 4 times as likely to drive large outbreaks as background outbreaks. Large outbreaks were significantly more likely to occur in areas with disproportionately high population density. Antibiotic resistance was also 3 times as likely in large outbreaks. Among the background control cases, food contamination was the most common driver. The controls were also more likely to be caused by bacteria than vectors.
The investigators expect that zoonotic disease outbreaks will only increase, creating a vital need to understand and prepare for factors affecting outbreak severity and variation.