Q: Why only “very close”?

A: The identification made over the weekend is preliminary, and will have been made by partially sequencing the virus’s genome, looking first at the genes that most commonly vary between strains. Tests are also likely to have been performed on its antigenic properties — the type of immune response it stimulates in cells. The full sequence of its genome was being completed yesterday, and will confirm the strain precisely.

Q: Why only perhaps? Aren’t genetic signatures useful for tracing the precise origins of diseases?

A: They are, but in this case their utility depends on whether the strain had mutated before it escaped. Foot-and-mouth disease is caused by a virus made not of DNA but of its cousin, RNA, which means it mutates quite rapidly. However it can only do this inside cells: on its own, it does not replicate, and thus cannot mutate.

Vaccines are grown using cell cultures, usually of hamster cells, but whether the virus used by Merial was cultured for long enough to set it apart genetically from IAH reference strains remains uncertain. This is a question scientists will seek to answer once the full genome sequence is available.

Q: Shouldn’t it be impossible for a virus to get out of a facility like Pirbright?

A: It should certainly be very difficult. The IAH has category four biosecurity status, the highest level. Buildings have air locks, and staff must wear special sterile clothing and shower before entering and exiting. Rooms where viruses are exposed are kept at negative pressure, so that if there is a leak air flows in rather than out, and air that is sucked out to maintain this pressure is filtered. Waste is also treated with heat and chemicals to kill any viruses that might escape.

Q: If it is a vaccine strain, is it more or less likely to cause further infections?

A: It is mainly good news. Vaccines use less virulent forms of the FMD virus, and should be less likely to spread. There is a drawback, however. Less virulent strains are less likely to cause symptoms in infected animals, which may mean it takes longer to identify any other farms that have been affected.

Of 97 cattle culled so far, only two have tested positive for the virus, which suggests a less harmful strain

Q: What do epidemiological models suggest about future spread:

A: They are encouraging. If the index case is the only one infected in the first place, models suggest the risk of further cases is low.

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