Evolution of a Pandemic

While my research as a scientist focuses on plant evolution, students overall are much more interested in the evolution of diseases. Not surprisingly, coronaviruses became our case study for this spring.

How will SARS-2 evolve in humans?

The virus causing COVID-19 (which has the unwieldy official name of 'SARS coronavirus-2', or SARS-CoV-2) might adapt to its new human hosts. While we do not know the precise origin of the virus, the evidence suggests that it recently moved to humans from bats, perhaps with another species as host along the way. While a virus may be able to infect both bats and humans, it will likely be slightly better able to transmit in one species than the other. As a result, when a virus shifts hosts, mutations that allow it to more successfully infect the new host will lead it to spread faster and become more common in the viral population. This could be happening with SARS-CoV-2, but the evidence is not clear yet.

Tree showing evolutionary relationships between different coronaviruses. Coronaviruses that infect humans are shown in bold. Horizontal branches indicate evolutionary changes to the virus. Virus genome data from NCBI, analyzed using MEGA X, following Zhang et al 2020.

Caption: Tree showing evolutionary relationships between different coronaviruses. Coronaviruses that infect humans are shown in bold. Horizontal branches indicate evolutionary changes to the virus. Virus genome data from NCBI, analyzed using MEGA X, following Zhang et al 2020

 

SARS-CoV-2 might also evolve to be less deadly. A perfect virus would reproduce and move successfully from host to host. Imagine for a moment that you are a virus and your host dies before you have colonized a new host. Bad luck: your lineage is extinct. A perfect virus would make many copies of itself, transmit itself to many new hosts, but would keep its host alive and active. Mutations that make a very deadly virus less deadly could lead to a longer host lifespan and more transmission to new hosts, and so might become more common.

Myxoma virus infecting rabbits in Australia shows how this can happen. In the 1800s, Australia was sadly lacking in proper British animals to hunt and eat, so colonists introduced rabbits. The rabbits quickly spread, causing large amounts of ecological and economic damage. The Australian government released the myxoma virus to kill the rabbits in the 1950s. At first, the virus killed over 99% of the rabbits. But after a few years, the virus had evolved to become less deadly. Strains of the virus that kept the host alive longer were able to infect more new hosts than strains that killed the host quickly.

SARS-CoV-2 might be evolving to be less deadly, and there are reports that this is happening, but the evidence so far isn't conclusive. However, several factors might make this less likely for SARS-CoV-2 than for Myxoma. First, we know that the disease caused by the virus does not kill most infected individuals, and many cases are quite mild or produce no symptoms at all. This means that a mutation that made the virus less deadly would not increase its fitness in most cases. Second, the evidence suggests that individuals are most likely to infect others just before they develop symptoms (Cheng et al 2020). If most transmission occurs early, mutations that make the virus less deadly would not necessarily increase in frequency.

Could Human Behavior Affect Viral Evolution?

Our behavior might affect the way that the virus evolves. When cholera reached South America in the 1990s, it evolved in different ways in different regions. Cholera transmits in feces, either through unwashed hands or through contaminated drinking water. Where people lacked access to clean water, cholera evolved to become more deadly: a mutation that led to a bacterium that created many copies of itself, even if that killed its host, could become common because many new hosts could be infected due to unclean drinking water. In regions where clean drinking water was available, the disease evolved to be less deadly. In these areas, if hosts were not alive to be in contact with potential new hosts, the disease could not spread, so mutations that led to milder illness became more common (Ewald et al 1998).

We might be able to shape the evolution of the coronavirus. Let's imagine that everyone is very friendly, with lots of enthusiastic singing and shouting indoors without masks. This would lead to a high probability of transmission for the virus, even if the virus severely sickened the host a week later. On the other hand, if we as individuals and society do everything possible to reduce transmissionwashing our hands, wearing masks, meeting outdoors rather than insidewe might shift the balance. Under those conditions, a virus that sickened the host quickly might have not find a new host, while a virus that had mutations that made it less deadly would have more opportunities to spread. Taking care of each other may lead the virus to evolve to be less deadly.

About the author: Eric Baack joined the Biology faculty at Luther College in 2007. He earned his Ph.D. in population biology at the University of California, Davis. His research looks at how new plant species arise through chromosome doubling, and what happens to crop genes when they move into wild relatives in sunflowers. Recent research collaborations with students have looked bacteria and other pathogens in the springs, streams, and rivers of NE Iowa. He teaches introductory biology, biostatistics, ecology of the SW, and evolutionary biology.

Releasing myxoma infected rabbits in Australia. Photo credit: CSIRO, Australia.