Habitat loss and climate change increase the risk of new diseases

Although scientists have not determined how covid-19 emerged, the leading theory is zoonotic spillover (transmission from animals). The death toll from covid has given efforts to prevent future pandemics new urgency. A recent study in Nature on bats, which carry sars-CoV-2’s closest cousins, finds that the risk of such spillovers is rising—though changes in human activity could return it to safer levels.

If covid is indeed zoonotic, it probably jumped first from bats to a “bridge” animal and then to people. The authors focus on the Hendra virus. This is also excreted by bats and infects horses, which spread it to humans. Of the seven people known to have caught Hendra, four died. The paper studied Hendra spillovers from fruit bats in subtropical eastern Australia, which have risen since 2006. It found that habitat loss combined with climate-induced food shortages explained the increase.

Australian fruit bats feed on eucalyptus trees’ nectar. Following El Niño events, when temperatures in the equatorial Pacific Ocean rise, fewer trees flower. This leads bats to form smaller roosts and eat inferior food, weakening them and probably causing them to excrete more pathogens.

Before 2003, when spillovers were rare, these changes lasted only for brief spells of food scarcity. But since 1996 humans have cleared a third of the bats’ winter habitat. Instead of hunting for nectar, bats now spend long periods in roosts near humans. Horses are exposed to those feeding in trees on farms, causing spillovers.

Bats, the only flying mammal, are potent vectors for zoonosis. Another paper in 2022 found that global warming, by forcing animals to change habitats, is expected to double the rate of first encounters (and thus potential viral spread) between mammal species. The study calculated that 90% of first contacts, concentrated in tropical, mountainous parts of Asia and Africa, involve bats, because they travel long distances and interact with lots of species.

It may be too late to slow viral transmission between animals: the study showed that this will be even more common if climate change is moderate than if it is severe. In contrast, the impact of habitat loss appears reversible. The paper on Australia found that when eucalyptus trees bloomed again, bats flocked back. Regardless of the origins of covid, restoring bats’ habitats could help prevent the next pandemic.■

Sources: “Pathogen spillover driven by rapid changes in bat ecology”, by Peggy Eby et al., Nature, 2022; “Climate change increases cross-species viral transmission risk”, by Colin J. Carlson et al., Nature, 2022