Science & technology | Zoonotic disease

Whence new plagues?

A prediction of the places from which new illnesses are likely to emerge

MOST new human viral infections come from other animals. Ebola fever, SARS and AIDS all started in this way. Animals are also the sources of influenza epidemics. Keeping an eye on birds and beasts, the viruses they carry, and which of those viruses are found in people is thus a prudent thing to do. And that is the self-appointed task of the EcoHealth Alliance, a charitable research organisation based in New York. This week some of the alliance’s scientists, led by Kevin Olival, published the results of their latest research in Nature. Among other things, they attempt to estimate what people do not know about these “zoonotic” viruses, as well as what they do.

Dr Olival’s study is restricted to mammals. It does not, therefore, pertain to things like the sources of avian flu. But within that limit it is as comprehensive as the data allow. It looks at all 586 species of virus known to have been found in at least one mammal. Those mammals amount to 754 species (humans included) from 15 orders—groups such as primates, bats, carnivores and even-toed ungulates (deer, cattle, sheep, antelopes, camels and so on). Of the viruses studied, 263 (ie, 45%) had been detected in humans and 188 of those were zoonotic in the sense that they had also been found at least once in another mammal species. This does not prove a virus passed from animal to human. It could have travelled the other way. But it is a starting-point for research.

The objectives Dr Olival and his colleagues set themselves were to build a model that predicted how many zoonotic viruses a particular animal species might be expected to carry, and then to compare that with the number already known to be carried by it. They did this by asking how closely related a species was to Homo sapiens (on the assumption that viruses will find the jump between related species easier), and how likely, given a species’ range, habitat and behaviour, it would be for it to interact with people. They also estimated, and attempted to correct for, how much effort had been put into looking for viruses in a given species. Sampling bias, for example, almost certainly explains why so many known viruses infect humans.

All this work yielded an estimate of the number of unknown zoonotic virus species out there in the world’s mammals. It also enabled the team to draw up “heat maps” showing places where the actual and predicted number of zoonotic viruses least resemble one another, and which therefore have the highest risk of springing a nasty surprise on the world.

The biggest threat comes from bats, which carry many more zoonotic viruses per species than other mammalian orders do. The places most at risk of an unknown zoonotic bat virus emerging are the Amazonian and Orinoco rainforests and the Caribbean coast of South America. Ungulates pose more of a threat to the east and centre of Africa, and carnivores to the east and south of that continent. Primates (the non-human variety) threaten equatorial regions of South America, Africa and Asia.

Having maps like these, rough and ready though they are, is important because they can help researchers choose the most fruitful places to conduct studies into zoonotic transmission. They do need to be used with care. The method Dr Olival and his team adopted does not distinguish zoonoses with epidemic potential from those that might infect a mere handful of humans. But the maps do increase the chance that the next SARS or AIDS might be spotted, almost before it has emerged, and many lives saved as a consequence.

This article appeared in the Science & technology section of the print edition under the headline "Unknown unknowns"

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