Spillover: How do animal diseases lead to pandemics?
What do HIV, Ebola virus, SARS CoV, Influenza and the novel Coronavirus have in common? All of these viruses originated in non-human animals.
Where do these viruses come from?
Human activity drives the emergence of new pathogenic (disease-causing) viruses. As we push back the boundaries of the last wild places on Earth – felling the bush for farms and plantations – viruses from wildlife interact with crops, farm animals and people.
Species that evolved separately are now mixing. Global markets allow the free trade of live animals (including their eggs, semen and meat), vegetables, flowers, bulbs and seeds – and viruses come along for the ride.
Humans are also warming the climate. This allows certain species to expand their geographical range into zones that were previously too cold to inhabit. As a result, many viruses are meeting new hosts for the first time.
How do they make the jump?
Virus spillover is a complex process and not fully understood. In nature, most viruses are confined to particular hosts because of specific protein “lock and key” interactions. These are needed for successful replication, movement within the host, and transmission between hosts. For a virus to infect a new host, some or all protein “keys” may need to be modified. This process depends on evolutionary potential and chance.
Simply put, if the virus is adaptable, it may succeed in replicating and proliferating in the new human host. Maybe it kills the person and the line of transmission comes to an end there—as happens with rabies. But if the virus is even more adaptable, it may acquire the ability to pass from one human host to another, perhaps by sexual contact (as with HIV), perhaps in bodily fluids such as blood (as with Ebola), perhaps in respiratory droplets launched by coughing or sneezing (as with influenza or SARS).
What makes a virus adaptable? The changeability of its genome, plus Darwinian natural selection. Those viruses with single-stranded RNA genomes, which replicate themselves inaccurately and therefore have highly changeable genomes, are among the most adaptable. Coronaviruses belong to that group.
For instance, a virus can jump from host A to host B, but it won’t replicate well or transmit between individuals unless multiple protein keys mutate either simultaneously, or consecutively. The low probability of this happening makes spillovers uncommon.
To better understand how spillovers occur, imagine a virus is a short story printed on a piece of paper. The story describes:
how to live in a specific cell type, inside a specific host
how to move to the cell next door
how to transmit to a new individual of the same species.
The short story also has instructions on how to make a virus photocopying machine. This machine, an enzyme called a polymerase, is supposed to churn out endless identical copies of the story. However, the polymerase occasionally makes mistakes.
It may miss a word, or add a new word or phrase to the story, subtly changing it. These changed virus stories are called “mutants”. Very occasionally, a mutant story will describe how the virus can live inside a totally new host species. If the mutant and this new host meet, a spillover can happen.
We can’t predict virus spillovers to humans, so developing vaccines preemptively isn’t an option. There has been ongoing discussions of a “universal flu vaccine” which would provide immunity against all influenza virus mutants. But so far this hasn’t been possible.
How do we prevent future spillovers?
The CDC estimates that nearly 75% of all emerging infections are zoonotic. Out of this 72% come from wildlife. Therefore, it is worthwhile to explore potential solutions to the problem of spillover, especially in light of the current COVID-19 pandemic, a disease which likely originated in bats.
Click here to read more about the Wuhan market where COVID-19 likely originated and whether it should be shut down.
Given the enormous number of viruses that exist, and our willingness to provide them global transport, future spillovers are inevitable. We can reduce the chances of this by practicing better virus surveillance in hospitals and on farms.
We should also recognize wildlife, not only for its intrinsic value, but as a potential source of disease-causing viruses. So let’s maintain a “social distance” and leave wildlife in the wild.