Preliminary tests on two British sheep with apparent symptoms of foot and mouth disease have come back negative, but the UK faces an anxious 96-hour wait for confirmation.
A new outbreak of the virus would be devastating for British farmers, following a catastrophic epidemic in 2001 which saw six million animals slaughtered, and cost Britain an estimated £4 billion.
The sheep come from a farm in Hawnby, Yorkshire, which has had no previous cases of foot and mouth disease (FMD). However, its livestock were slaughtered last year as a precaution after cases appeared on a neighbouring farm.
There have been no cases of FMD in the UK since September 2001 and the farm had just re-stocked. The veterinary inspection required after re-stocking uncovered mouth lesions in the two sheep.
Initial tests on swabs from the lesions, using an antibody-based assay for the virus, are negative. But Chris Bostock, head of the Institute for Animal Health at Pirbright in Surrey, says that the test cannot detect tiny amounts of the virus.
The swabs have now been used to infect bovine thyroid cells in culture, to allow any virus present to replicate. Medium from those cells will then be used to infect a second culture. If, after 96 hours, none of the cells show signs of infection, the sheep will be declared free of FMD.
The virus is notorious for persisting in the environment and for being extremely infectious. Farms where herds were destroyed merely on suspicion of the virus, like the one in Hawnby, must use pressure hoses, detergents and disinfectants to eliminate any lingering virus, then wait three months before re-stocking.
It is unlikely that any virus can persist three months in the environment once animals are gone, says Bostock. It can persist without causing symptoms in carrier animals, although it is not clear whether such animals transmit the infection.
Re-stocked livestock are being tested for antibodies to FMD that will reveal any history of exposure to the virus. So far 6000 have been slaughtered, even though they may no longer be infected.
If the Yorkshire sheep have FMD, says Bostock, its RNA sequence should reveal whether it is a leftover from the 2001 epidemic, or another, newly imported infection. If so, the IAH has a collection of FMD strains from all over the world that may help track where it came from.
New Scientist
27 Feb 2002