Viruses are thought to spread by entering a cell, replicating, and then moving on to infect new cells. But a new study published in Science reveals that some viruses spread much faster than previously thought, and it may be possible to stop the spread of disease by slowing them down.

Using live video microscopy, the scientists discovered that the vaccinia virus was spreading four times more quickly than thought possible, based on the rate at which it replicates. Videos of virus-infected cells revealed that the virus spreads by surfing from cell to cell, using a mechanism that allows it to bounce past cells that are already infected and reach uninfected cells as quickly as possible.

Early after vaccinia infects a cell, it expresses two viral proteins on the cell surface, which marks the cell as infected. When further virus particles reach the infected cell, these proteins cause the host cell to push out snake-like projections called "actin tails," which drive the virus particles away towards other cells that they can infect. The particles thus bounce from one cell surface to another until they land on an uninfected cell.

In the study, the researchers prevented the virus from making the proteins needed to make the actin tails in the early stages of infecting a cell and showed that this slowed the spread of the virus dramatically.

The researchers believe that other viruses also employ rapid spreading mechanisms. For instance, herpes simplex virus (HSV-1), which causes cold sores, spreads at a faster rate than should be possible given its replication rate. Thus, this phenomenon discovered with vaccinia may be a common feature a viruses. The discovery may ultimately enable scientists to create new antiviral drugs that target this spreading mechanism.

"Shortly after infection vaccinia expresses two virus proteins on the cell surface that mark the cell as infected. This effectively says to additional virus particles trying to infect the cell 'I'm infected already, there is no point coming here, you need to go elsewhere.' And remarkably the virus particles are physically repelled until they find an uninfected cell. Thus the virus can spread quickly to distant uninfected cells without needing to replicate in each cell on the way," said co-author Geoffrey L. Smith.

"This fundamentally changes how we think about virus dissemination and similar strategies may very well be exploited by many viruses," he added .

: Virginie Doceul et al., 'Repulsion of Superinfecting Virions: A Mechanism for Rapid Virus Spread', Science, January 2010, doi:10.1126/science.1183173