A dairy worker in the US suffered from an itchy, bleeding eye. A young woman in Australia fell ill after an overseas holiday and was hospitalized. Another man in Mexico, already sick and bedridden, was seriously injured and died.
Each recent case was caused by a different type of influenza virus. In both cases, it is an animal virus that does not normally occur in humans. Why should stories like this worry us?
When incidents like this make the news (and they often do for flu viruses), reporters write to virologists and ask: How concerned are you?
The correct answer depends on many things, including our personality: some people are natural optimists, others tend to be pessimists, and how the virologist approaches the story.
But our expertise gives us insight into what to look for in the news about new viruses. The next time you read about a new virus in the news, here are some questions that can help you decide how much you should worry.
How far is it?
This is often the first question. It is very difficult for a virus to adapt to grow well in a new host species. Influenza viruses—mostly avian viruses but notorious for recurring human pandemics—dominated in just a few decades.
Transmission of viruses from non-animal hosts to humans is a stepwise process.
Have people been exposed to a new virus and developed an immune response, but have no symptoms of infection? If a person has a “warehouse” infection (whether it causes serious illness or not), is there any sign that the virus has adapted enough to spread to other people? If the virus is now circulating in humans, is it still spreading enough to become extinct?
How much do we know?
Surveillance is a job that requires resources and collaboration, but it is important to understand and control the virus. What are we looking for?
Testing people for their immune response to the virus (serology) shows who has been infected before. Sequencing the viral genome (from an infected person or the environment) tells us where the virus is, but also allows us to work out how it spreads and mutates.
We can do this because viruses mutate quickly. Mapping out the differences between genetic sequences allows us to build a family tree (“phylogenetic tree”), which can be used to reconstruct how the virus arrived at a specific location.
Are we looking at one large virus or individual viruses? A family tree can show this. When we look at changes in a virus’s genome, we can look for signs that it’s adapting to a new form—assuming we understand enough to infect the virus.
What do we deal with?
The better we understand viruses, the better we can predict what they will do next. For well-studied viruses, such as influenza viruses, we know of several genetic changes that indicate adaptation to new host species.
What else can we look for? We are more concerned about viruses jumping between the same terrain because it is easier for viruses to do so. Influenza in dairy cows is more contagious than bird flu.
We can see the possible routes of spread – respiratory viruses are likely to spread faster than sexually transmitted viruses. We can also try to predict the consequences of infections-viruses that cause serious diseases, but in terms of transmission, we are worried about more serious conditions that can cause the virus to spread undetected to people.
However, viruses are complex things and it is very difficult to predict what they will do in practice.
The current outbreak of influenza A virus H5N1 in cattle is a clear example of this. The influenza A virus infects cows and is spread through milk. Although H5N1 is known to cause severe illness, some animals seem to carry the virus without becoming seriously ill.
Experimental virology, where animals and cell cultures are infected and studied under controlled conditions in a safe laboratory, can be critical to understanding what viruses can really do.
Could it be worse?
It is difficult for viruses to adapt to humans, so anything that gives the virus more opportunities to infect is a concern. A persistent virus poses a greater risk than a one-time case.
We are more concerned about viruses in animals that are close to humans. The spread of H5N1 in North American cattle is more concerning than the spread of H5N1 in South American elephant seals. We worry about viruses taking shortcuts to adapt. This may be the case for the flu virus.
