Biology / Ecology / Health / Microbiology / Science

Spillover: Avenues for zoonotic disease

It’s hard to avoid the news about the novel coronavirus at the moment, but I thought I’d tackle a related but more general question: why did I sigh “of course” when a Chinese market was pinpointed as the site for the first coronavirus transmissions to humans? Okay, you might not have known that I did that, but I did.

The issue with these markets is that — though this one is referred to as a fish market — they carry all kinds of animals, living and dead. A quick Google shows people talking about this particular market having marmots, snakes, chickens, rats, civets and dead hedgehogs available for sale, making this an absolutely prime place for disease to spread. It needn’t have been the case for any specific virus we might be worried about: it’s a risk in general, ripe for zoonoses (diseases found in animals that then cause disease in humans).

These markets pack livestock together in close quarters, allowing all kinds of potential transmission options: contamination of the animals’ food (or dead animals ready for human consumption) by the faeces of other animals, respiratory infections being passed along due to the close circumstances, via the bloodstream from cuts and scrapes… It’s a melting pot, and of course every species carries its own diseases. These diseases might not make the animals visibly sick; they may not make the animals sick at all, if the animals have had a long time to adapt to the pathogen. But the pathogen might be new to the unsuspecting chicken it infects, and then it can have dire results.

Worse, this also provides an opportunity for different strains of viruses and bacteria to meet, mingle, and exchange DNA. Influenzavirus can be transmitted from a bird to a pig, where it can find another influenzavirus with a great selection of genes to spread through humans. It’s only one more step from that point to reach a human and then explode through the population.

In reality, these markets are just one way we humans bring about our own problems, of course. Authors like David Quammen have been making it clear for a while now: everywhere humans are pushing on the historical boundaries, venturing further and further into territory previously left to the wildlife — cutting down forests, destroying habitat, and flushing animals out and into contact with humans. Of course this is exposing us to new pathogens that formerly stayed safely sequestered in remote populations.

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