A team of inspectors will report to UK government ministers today on their investigation into the most likely source of the outbreak of foot-and-mouth disease on a farm in Surrey, south of London.
The private Merial Animal Health company, which manufacturers foot-and-mouth vaccine, and the government-run Institute for Animal Health, which share a site at Pirbright, three miles from the farm at the centre of the outbreak, have both said that their biosafety procedures have not been breached. But if Pirbright is confirmed as the source, ministers and farmers will be anxious to know how - despite the highest "category 4" level of biosecurity - a leak could have happened.
The investigation involves at least five inspectors from the health and safety executive, including three from the specialist biological agents unit, as well as representatives of the environment agency, the health protection agency and the department for the environment, food and rural affairs.
Inspectors have been looking for any evidence of a breach in the infrastructure or procedures at the labs. "It is difficult to understand how an outbreak could accidentally occur unless there is something wrong with the procedures, or they haven't been followed or unless there's a problem with the facility itself," said Paul Denny at the Centre for Infectious Diseases at Durham University, England.
One priority will be to establish the sequence of events relating to recent handling of the virus. The inspectors will interview staff and study lab records to track what went into or out of the laboratories.
They will also want to know what experiments were done, by whom and when. They will pay particular attention to any incidents reported to the labs' disease security officers who monitor "near misses" that might lead to a breach of biosecurity.
The inspectors will also want to check the physical soundness of the facilities and equipment. One safety feature is that the labs are maintained at negative pressure, so air rushes in rather than out through any air leak, meaning dangerous organisms stay inside.
The pressure difference is maintained by pumping air out of the lab through a double filter designed to catch particles. Any damage to the filters could cause a leak. The government labs alone at Pirbright have 25 of these filtration systems.