How Viruses Hop from Wild Animals to Humans

There are strains of viruses that can jump from animals to humans. Sara Sawyer, a researcher at the BioFrontiers Institute of the University of Colorado, Boulder, spoke to Nature about what a virus needs to do in order to jump between species.

How do animals transmit diseases to humans?

Humans can't be exposed to most animal viruses because we ingest them all the time. Viruses have to trick their way into our cells so they can use them as a factory for making more of themselves. There are a number of tricks animal viruses need to perform.

All aspects of our immune system must be evaded first. They need to reproduce in our cells. It is rare for a virus that has been evolving under the pressures of an animal host to be able to do all those things.

Is it possible for animal Viruses to replicate in human Cells?

It was almost always. The viruses interact with hundreds of the host's genes. Humans and animals have different versions of the same proteins. If an animal virus can replicate weakly in a human cell, that is a worry, because it means that the virus can evolve.

Errors that are made as a result of viruses copying their genes are fodder for evolution. A replicating animal virus might be better at replicating itself in a human cell, or better at evading human defences. Natural selection will work in favor of the viruses that work best in humans. There are some animal viruses that can't be replicated in humans, and others that can.

Can you tell me if an animal virus is close to humans?

To understand which viruses are a hair's breadth away from being able to thrive in humans, we have to work out how many obstacles the viruses need to overcome.

We can see if the virus can get into human cells. One way to do that is to take a cell line from the animal's host and replace it with the human one. If the virus replicates at normal levels, it can use the humanreceptor. The human version is not ideal if we only get half of the expected replication. If we don't see any replication, that means the virus can't use thereceptor yet.

It's possible to repeat that process with everyprotein we know is important for every task the virus must perform. It allows us to identify what is stopping the virus from replicating in human cells and differentiate the viruses that need to overcome only one or two obstacles from those that are a long way off.

There are some viruses that are close to jumping into people.

There are certain viruses that are of interest. The primate environment is so similar to ours that we see primate viruses jumping into humans often.

The simian arteriviruses are a family of Viruses. The simian immunodeficiencyviruses that gave rise to HIV and the AIDS epidemic are very similar to these viruses. Our research suggests that the simian arteriviruses are capable of jumping into humans, like HIV.

What did you discover about simian arteriviruses?

The simian arteriviruses are endemic to Africa. These viruses can cause haemorrhagic fever and death in captivity. The simian haemorrhagic fever virus (SHFV) was used as a representative of the virus family, and CD 163 was identified as thereceptor it uses to enter cells. The human version of the virus was functional.

We wanted to know if the other machinery could be used to replicate the human cell. Not only did we find human cell lines in which it could do that, but also in one case, it produced an amazing number of copies of itself. We showed that, in at least one cell type, SHFV seems to resist the interferon response. If the worst happened, our adaptive immune system would be the last thing on our list. Like HIV, SHFV is a group of Viruses that Humans have no immunity to.

What should be done about this?

There are arteriviruses that have never been observed in humans. We could run blood tests on people in Africa where arteriviruses are endemic to see if they have immunity to them. There are no tests for these Viruses.

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