Zoonotic Diseases and the Next Big One

In the age of COVID-19, David Quammen’s 2012 book Spillover is beginning to be viewed as almost prophetic. The book is about the, er, spillover of diseases from animals to humans. The technical term for such inter-species diseases transfer is “zoonotic”. Such occurrences shouldn’t be surprising, he says:

“Just as a lion might occasionally depart from its normal behaviour – to kill a cow instead of a wildebeest, a human instead of a zebra – so can a pathogen shift to a new target.”

It’s just how nature operates. There are no rules: whatever works will happen.

 

The significance of zoonotic v/s non-zoonotic diseases in humans is this:

“Zoonotic pathogens can hide (in other species).”

Ergo, even if you eliminate it in all humans, it’s still out there (in other species). And so it can always come back, either as-is or with some mutation. This explains why we could eliminate smallpox: it is not a zoonotic pathogen. In other words, it had nowhere else (no other species) to hide. (Coronavirus, sadly, is zoonotic…)

 

Quammen (and many epidemiologists) have been saying that such disease outbreaks are not “happening” to us. Rather:

“They represent the unintended results of things we are doing.”

And such incidents are only going to increase for a simple reason:

“It’s not that they target us especially. It’s just that we are so obtrusively, abundantly available.”

 

The path to such outbreaks isn’t easy to understand, as we’ll see in later blogs. It can take so many routes. And no, this is not a recent phenomenon. But the concept is roughly as follows. There are “reservoir hosts”, species in which the pathogen resides in large quantities, without causing any significant harm to the host. Such arrangements have evolved over long periods of time. When ecological disturbances impact the reservoir hosts, the pathogen is “forced” to find a new host, and thus new diseases emerge in new species:

“Shake a tree, and things fall out.”

 

Of course, just because a virus enters a new species doesn’t mean it’ll succeed there. Usually, quantity (how much of the virus) matters. Which brings us to the topic of an “amplifier”, an intermediary species:

“An amplifier host is a creature in which a virus or other pathogen replicates – and from which it spews – with extraordinary abundance.”

 

As you can see, many things have to line up for a spillover. But epidemiologists have been warning for long about the “Next Big One”. But as we’ll see in blogs to follow, that’s like those warnings of a big terrorist attack. Without knowing who or how or where or when, it’s not clear what can be done. Chillingly, Quammen had written this line in his book… in 2012:

“Will the Next Big One come out of a rainforest or a market in southern China?”