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Predicting pandemics is a vital part of pre-emptive healthcare. But making assumptions about human viral outbreaks is fraught with difficulty due to the unpredictable nature of illness (and humans themselves of course).
However, human pandemics can be predicted by studying the pathogens carried by animals. Viral zoonoses, such as HIV, pandemic influenza and Ebola, are particularly concerning, given their capacity for devastation. In a paper published in Nature, Kevin J. Olival and his colleagues have created a comprehensive overview of past, present and future virus-sharing between humans and other mammals.
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A zoonotic disease is one that can be transmitted from animals to people, or a disease that commonly exists in animals but can cross over to the human population. A total of 586 animal–virus associations were analysed as part of the research, which can affect a total of 754 species of mammals. The data found that bats, primates and rodents carry the highest proportions of viruses, with bats carrying a significantly greater number than any other species. Apart from these recognised viruses, the team also undertook research to try to find 'missing' zoonoses – those which have yet to be discovered.
By predicting the total number of viruses that could be attributed to each vector group and subtracting this from the number of already known viruses, it was possible to find 'missing' viruses that could soon come to light.
The areas with the greatest potential for future viral outbreaks were seen to come from bats in northern South America and carnivores in East Africa. The map predicts 17 zoonotic infections in bats and 10 in rodents and primates. By focusing on these animal vectors, this research into future viral outbreaks could also help guide treatment plans. As the climate changes and habitats are destroyed, animals can move into urbanised areas, or humans can encroach upon their land, leading to a greater chance of contact.
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It is worth noting that since this data is based on extrapolating current and past trends, none of the key outcomes can be predicted with better than 30 per cent accuracy – far from a definitive prediction in zoonotic disease epidemiology.
Furthermore, due to the predictive nature of this research, there is a possibility that the authors’ analysis of zoonotic viruses may never be encountered humans - some diseases do not cross over between humans and people, at least not in epidemic proportions. A counter-argument is that these unknown diseases from the animal population could indeed cross over to human hosts, such as the Ebola virus.
The 2014-2016 outbreak of Ebola was thought to have arisen from the fruit bat of the Pteropodidae family, a natural host for the virus. Ebola is introduced to the human population through close contact with the blood, secretions, organs or other bodily fluids of infected animals. These can be from chimpanzees, gorillas, fruit bats, monkeys, forest antelope and porcupines found ill or dead or in the rainforest.
Ultimately, modes of transmission are a key factor when assessing the relationship between vector outbreaks. Once a virus adapts to move from human to human, the potential for pandemic worsens. This is inherently difficult to predict; diseases adapt at their own pace and mutations can fundamentally change our understanding of a virus.
The pandemic predictions don't give us a date these viruses could emerge. Without a time-frame, these predictions could be seen as a call for better disease surveillance, using knowledge of potential zoonoses in tandem with epidemiological metadata and rigorous data-sharing – to make healthcare agencies as prepared as possible.
James Lloyd-Smith from the University of California, Los Angeles, has said in response to the report that: "Although most pandemics are zoonoses, most zoonoses do not cause pandemics."
Instead, the pandemic predictions offer a glimpse into what could be, rather than what will be, but it is still an expression of grave potential.
This article was originally published by WIRED UK
