Michael Dulaney explains some key concepts from his book Sentinels

‘Nearly two-thirds of all human pathogens, including Covid-19, have their origins here. The term is a technical one, and refers, essentially, to a pathogen present in animals that jumps across to infect humans. These are not just viruses — anything from fungi, bacteria, protozoa, worms, and other parasites can make the leap. All human influenzas are zoonoses. So are HIV, Ebola, chickenpox, tuberculosis, rabies, Lyme disease, Rift Valley fever, and many others. They can be as catastrophic as a global pandemic, or as harmless as a medical curio.

What I found, as I interviewed more experts and observed their work, is that zoonoses reveal our intimate connection to the rest of nature. Even in our urbanised, highly technological present, they are a link to the wild places, to the many seen and unseen animals that live alongside us, even in the most artificial environments. They involve species as diverse as frogs, racoons, horses, pigs, lemmings, dolphins, and swans. The transmission route between us and them is rarely direct, often passing between another species or several species along the way.’

‘Spillover’ is the term used to describe this jump between species. The US Centers for Disease Control and Prevention estimates that three out of every four new infectious diseases, and nearly all pandemics, emerge from animals in this way. Researchers have counted around 200 new infectious diseases that have broken out more than 12,000 times over the past three decades. 

On average, one new infectious disease jumps to humans every eight months. The vast majority will never pose a grave risk to human health, but those that do come from all manner of sources. We know that new diseases are now making the jump at a faster rate than ever before. And the cause for this great acceleration is us.

The scientific evidence, by now, is clear: human impacts on the environment are almost always ultimately responsible for spillover. The specific causes might involve anything from logging, land clearing for agriculture, road building, industrial farming, the consumption and trade of wild animals, mineral extraction, suburban sprawl, chemical pollution, nutrient runoff to the oceans, or any other combination of technical or social processes. The new diseases are spread and amplified by income inequality, the exploitation of human labour, and the global trade of commodities. All of these, you might notice, are a feature of capitalism, an economic system that takes the whole of biological life and turns it into a machine for extracting maximum value — the very same force driving mass extinctions and the climate crisis.’

‘Sentinels in this book come in many forms. In epidemiology, sentinels are often understood narrowly: animals that provide an early warning of disease before humans are affected. Those species, the ‘canaries in the coal mine’, appear in these chapters. Others are more like witnesses to the new planetary condition. They warn of social ruptures, ecological breakdowns, and political and economic decisions that make disease inevitable. 

In that sense, sentinels do not merely signal what is coming; they reveal what is already underway.’