Disease-carrying animals thrive on our farmed land and in our cities

By Michael Le Page

We are changing the world in a way that favours animals such as bats – the source of the new coronavirus – that carry more diseases. That is the conclusion of an analysis looking at what changes are occurring in ecosystems as people move in.

“Some species are doing better and they are disproportionately likely to be those that transmit diseases to people,” says Rory Gibb at University College London.

His team took advantage of a global project looking at how ecosystems change in disturbed areas, such as land cleared for farming, compared with undisturbed areas nearby. Nearly 7000 studies of this kind have now been done worldwide.

The researchers combined these findings with data on what diseases animals carry, and whether these diseases can infect people. They found that small, fast-lived animals such as rodents, songbirds and bats tend to become more abundant in areas where people have moved in – and that these animals carry more diseases compared with larger, longer-lived species that have declined or disappeared.

It isn’t clear why they harbour more diseases. One idea is that fast-lived animals invest more in reproducing at the cost of their immune defences, making them more vulnerable to pathogens, says Gibb.

The finding suggests that the way we are changing landscapes is increasing the risk of diseases jumping species. However, this risk also depends on other factors such as how likely people are to be exposed and how vulnerable they are to a particular disease, says Gibb, which the study didn’t look at.

One implication is that the disease risk could be reduced if ecosystems are restored by, say, reintroducing lost predators. “The way we manage landscapes is important,” says Gibb.

For instance, schistosomiasis is caused by a parasite spread by snails. Reintroducing a river prawn that preys on snails has reduced its incidence in Senegal.

Not all animal diseases that can infect people have pandemic potential, like coronaviruses. Many, such as Lyme disease, are caused by pathogens that can infect people but don’t usually spread from person to person.

Journal reference: Nature, DOI: 10.1038/s41586-020-2562-8