6 May 2011
By Pallab Ghosh
Science correspondent, BBC News
The mass cull of farm animals to control the spread of foot-and-mouth disease may be unnecessary if there is a new outbreak, scientists suggest. A new analysis of disease transmission suggests that future outbreaks might be controlled by early detection and killing only affected animals.
The scientists said their findings did not suggest the mass slaughter policy during the 2001 UK outbreak was wrong.
The research, by a UK team, is reported in
the journal Science.
Until now, vets had assumed animals could be infectious while they carried the virus that causes foot-and-mouth, which may be for between four and eight days.
However, by exposing calves to infected cattle and closely monitoring them, researchers from the Institute for Animal Health in Surrey and Edinburgh University discovered that the period of infection was less than two days.
Perhaps more importantly, the researchers also discovered that animals were not infectious until they showed symptoms of the disease.
Scientists had previously thought animals were infectious for hours or even days before manifesting any symptoms.
These results suggest that any future outbreak could be brought under control by closely monitoring animals and slaughtering them as soon as they become ill.
This approach is in contrast to the policy adopted to bring the 2001 epidemic under control.
Too drasticTen years ago, the UK Government slaughtered not only all the animals on infected farms but also all animals on adjacent farms, regardless of whether infection had been reported there.
The policy of "contiguous culling" was adopted following scientific advice that this was the only way of controlling the epidemic.
It led to the slaughter of 6.5 million cattle, sheep and pigs, some of which were burned on open air pyres.
Many vets and farmers felt at the time - and still argue - that the policy was far too drastic, and that a more targeted approach would have been sufficient to bring the outbreak under control.
Dr Bryan Charleston, of the Institute for Animal Health, was among those asked to undertake the new study in response to public concern about the use of contiguous culling.
The research, he says, "doesn't say that this policy was wrong" - but it does suggest that mass culling could be avoided in the future.
Writing in Science, his team says: "These results imply that controversial pre-emptive control measures may be unnecessary.
"Instead, efforts should be directed at early identification of infection and rapid intervention."
Dr Charleston also told BBC News it would be worth developing simple test kits to detect herds that are infected before the onset of clinical signs, and also to detect herds that are not infected "so they would not need to be culled".
Targeted approachSo if mass culling could be avoided in the future, why was it not "wrong" in 2001, as Dr Charleston argues?
Professor Neil Ferguson, of Imperial College London, was among those advising government on how to control the epidemic a decade ago.
He said the more targeted approach suggested by the new research would have been impossible by the time government had realised that there was a major outbreak.
"The biggest problem in 2001 was that by the time we realised what was happening, there were something between 30 and 50 infected farms," he said.
"It took a huge amount of effort to deal with that, and so very intensive surveillance of infected areas proved impossible from the outset."
But government agencies are now much better placed to detect new infections much earlier.
That being the case, the implications of this paper are that in future, vets will be able to nip foot-and-mouth outbreaks in the bud.
Indeed, this is what happened in 2007 when there was a small outbreak as a result of a leak of foot-and-mouth virus from a laboratory in Surrey.
That outbreak was confined to a small area, and so vets were able to monitor closely and test herds that were in close proximity to infected animals.
Those that were found to have the virus were culled; those that tested negative were not.
This approach was sufficient to bring the 2007 outbreak under control. But such a scheme could not be applied to a larger outbreak, according to Professor Ferguson.
He said: "When the outbreak is very small, it becomes more feasible to pick up any signs of infection on a farm as soon as possible; and this research suggests that might be very effective at stopping onward transmission.
"But in 2001, really rapid diagnosis proved to be challenging.
"If you have a lot of animals on a lot of farms, it's hard to inspect them all every day. So although the general conclusion is that rapid diagnosis might have a big effect in practice, it might be hard to achieve (once the outbreak exceeds a certain size)."
The research also suggests that vets should not be wary of using vaccination to control any future outbreak, as they were in 2001.
Then, there was concern that vaccination would lead to animals becoming infected at a very low level without displaying symptoms, and that these animals could in turn have infected animals in other farms.
The new research, however, suggests that this kind of subclinical infection is not a worry.
It indicates that if an animal does not show symptoms, it is not infectious; so vaccinating in the face of an outbreak might be more effective than scientists previously thought.
http://www.bbc.co.uk/news/science-environment-13299666
