Yes, bats carry disease. They also make us healthier

A new study of bats in the US shows that consuming at least 40 per cent of their body weight in insects each night, bats, these flying mammals, are essentially performing free pest control for farmers.

By Lara Williams

From ticks and mosquitoes to pangolins and raccoon dogs (two species accused of being the animal origin of Covid-19), the conversation around wildlife and health often hinges on the idea that animals can make us sick.

That’s not necessarily wrong. As humans encroach deeper into wild spaces and alter habitats, we’re creating opportunities for new zoonotic diseases, in which pathogens jump from animal to human, such as bird flu and Ebola. Meanwhile, climate change is bringing infections such as dengue to new areas.

But what is perhaps appreciated less – and not so well understood – is the unexpected value of biodiversity for our well-being.

A new study of bats in the US, for example, shows us exactly that. Consuming at least 40 per cent of their body weight in insects each night, these flying mammals are essentially performing free pest control for farmers.

Unfortunately, in 2006, bats in the US were struck by white-nose syndrome (WNS), a disease caused by an invasive fungal species. Appearing suddenly and unexpectedly, the fungus has gradually spread, decimating local bat populations along the way.

Though tragic, the epidemic has created something akin to a randomized control trial, allowing Eyal Frank, an assistant professor at the University of Chicago, to study the economic impact by comparing affected and unaffected counties pre- and post-WNS.

What Frank found in the peer-reviewed study that was published in science is that, after the onset of bat die-offs in affected counties, farmers increased their insecticide use by about 31 per cent on average.

This had an apparent financial impact, with total crop revenue declining in the years after WNS was detected. Frank estimates that the agricultural losses attributable to bat population decline is $26.9 billion for the affected counties across the 2006 to 2017 period.

But what’s more shocking is what happened to infant mortality rates after counties were struck by WNS. Death rates in babies rose by 8 per cent on average in affected counties as pesticide use increased. Wind and water erosion can carry agrichemicals away from farmland, potentially exposing people to chemical pollution through the air and drinking water.

Detections of insecticides in water samples across the US, including those not adjacent to farms, are higher between April and September – the agricultural production season — and even small doses can mean large concentrations for infants.

There’s a number of indications that point to a causal connection. Pre-WNS, there were no systemic differences between affected and unaffected counties – the parallel trends only start to diverge after the disease arrives. The fact that counties were exposed in different years allows us to rule out general farming trends or changes to agricultural policies.

Frank spent a long time testing the robustness of the results and checking other alternative explanations such as price shocks, health care, unemployment and weather – but none explain the rise in insecticide use and baby deaths in affected areas.

Thus, it’s possible to conclude that bats can have a real trickledown effect on human well-being. Similar patterns have been observed elsewhere: Frank also worked on a study that linked the rapid disappearance of vultures in India – caused by a livestock medication – with about 500,000 excess deaths from 2000 to 2005.

Without vultures to efficiently clean up carcasses, rotting animals ended up tainting water supplies and allowing feral dogs and rats to thrive. Orders of rabies vaccines rose after vulture populations declined.

Meanwhile, research in Wisconsin reveals benefits to humans from restoring ecosystems. Wolves, who were nearly eradicated by the mid-20th century, may help to reduce deer-vehicle collisions by scaring prey away from roads. As populations of Canis lupus returned to the state, road accidents involving deer fell while other types of car crashes didn’t.

These natural experiments are a rare glimpse into the tangible benefits keystone species – meaning a species with a disproportionately large impact on its environment – bring us.

But the connections hold on a global scale too. A large-scale analysis of nearly 1,000 scientific studies published in Nature found that biodiversity loss is the biggest driver of infectious disease outbreaks, followed by climate change and invasive species.

It’s not hard to imagine the ways in which biodiversity loss could be hugely detrimental to our health, especially when combined with other global changes: While the climate crisis creates the conditions for virus-spreading mosquitoes to thrive, imagine how much worse the problem would be without healthy populations of birds, bats, frogs, fish and spiders to eat them.

But science is still catching up on providing empirical evidence on the social costs of biodiversity loss, Frank told me. Part of the issue is that, unlike climate science, biodiversity doesn’t benefit from decades, if not centuries, of high-quality global measurements. With ecosystems around the world functioning in complex and unique ways, biodiversity is impossible to sum up in a single comparative metric.

There is an approach becoming more popular among governments, research institutions and international organizations called One Health, which recognizes that human, animal, plant and ecosystem health are interdependent.

But we can’t properly use that approach without answering questions with rigorous science. As the authors of the Nature study point out, there are few inquiries on interventions to remediate the effect of things like biodiversity loss on disease. Can ecosystem restoration, for example, be used as a lever to manage health outcomes?

The WNS epidemic has been devastating for several species of bats in the US, but it’s also taught us something really important: Protecting, monitoring and studying nature is vital for healthy humans.