You’ll probably remember the last time you had the flu, but what about that time you had measles — or was it chicken pox? Your blood knows: it keeps a record of every virus you’ve ever been infected with. A tiny drop of the stuff can now be tested to reveal a person’s viral history.
The test, called VirScan, reveals that adults around the world tend to have been infected by an average of 10 viruses over their lifetime. It could also be used to identify links between viral infections and mysterious diseases like chronic fatigue syndrome.
When a virus infects us, our immune cells respond by producing antibodies that neutralise it when they bind to specific proteins on its surface. These antibodies continue to be made long after the virus has been cleared from our body, ready to mount a quicker response should it return.
This means that their presence can act as a viral footprint — a clue that the viruses they target were once in the bloodstream. To test whether someone has been infected with a virus, expose a sample of their blood to a viral protein. If antibodies target it then the virus has infected the person in the past.
Stephen Elledge at Harvard University and his colleagues have pushed this idea further and developed a way to test a blood sample for every single family of human virus in one go.
Gotta catch ’em all
Elledge says that all he needs to carry out the test is a tiny amount of a person’s blood — less than a drop. It costs just $25, and could help doctors identify hidden infections. “A lot of people have hepatitis C, for example, without realising,” says Elledge. You could imagine routinely screening people in this way, he says.
To develop VirScan, Elledge and his colleagues used an international database to look up all viruses known to infect humans — around 1000 strains from 206 viral species. Using this information, they recreated the DNA in each virus that’s responsible for making its proteins, and put the DNA segments into individual bacteriophages — viruses that infect bacteria. Each bacteriophage then manufactures a particular viral protein on its surface.
When someone’s blood is mixed with the bacteriophage brew, any circulating antibodies latch on to the associated proteins on the bacteriophages. Sequencing these bacteriophages then reveals the person’s viral history.
David Matthews at Bristol University in the UK thinks the best use of VirScan might lie outside of diagnostics, considering we already have quick and easy tests for individual viruses. “Usually when you’ve got a set of symptoms, doctors have a pretty good idea of what you’ve got,” he says.
Moreover, the immune system takes a while to make antibodies, so you might not find a strong antibody response in the early stages of an infection. The test would also not be able to distinguish between antibodies made as a result of an infection and those triggered by a vaccine.
Instead, the technique might be useful in outbreaks of new viruses. Understanding how our immune system responds to other viral fragments might reveal clues as to which family the new virus belongs to, says Pamela Vallely at the University of Manchester, UK. “If we’d have had this test during the HIV outbreak in the 1980s, it would have given us a clue for where to be looking to find out more about the virus,” she says. “It’s a really exciting technique.”
As well as playing an investigative role in outbreaks, VirScan could also offer a way to investigate whether viruses are involved in disorders that aren’t well understood. For example, Elledge’s team will be collaborating with another group to test people with chronic fatigue syndrome, to see if they might have been infected with any of the same viruses.
“Multiple sclerosis is usually wheeled out as being linked to a virus,” adds Vallely. “You could check.”
Down on the farm
The team used the test to screen blood samples from 569 people from four countries — the US, South Africa, Thailand and Peru. As you might expect, adults appeared to have encountered more viruses than children. Each person had been infected with an average of 10 viruses over their lifetime.
Matthews thinks it would be worth extending the screen to animal populations. He envisages screening wild populations of animals thought to be linked to emerging diseases. “You could test the wild bat population to get a good idea of what viruses are out there,” he says.
At the same time, farm animals could be comprehensively screened. Farmers that are able to identify viruses affecting the health or yields of their herds might be able to halt the spread of those viruses, says Matthews.
“It is a fantastic piece of work and will be very, very useful,” he says.