It is almost certain that the Institute of Animal Health complex at Pirbright was the source of the foot-and-mouth disease outbreak, but the route by which the virus was released may never be conclusively determined, scientists said yesterday.

Several possible scenarios are being considered by the Health and Safety Executive (HSE) and other experts, but there is no “smoking gun” that identifies any as the most probable cause of the infection.

The chances of the virus escaping by each route is very low, and once it did get out, the chances that it would reach and infect susceptible livestock would be low.

Tony Wilsmore, the director of the Veterinary Epidemiology and Economics Research Unit at the University of Reading, said: “For both to happen you are multiplying two probabilities that are less than one, and when you do that, you get a lot less. If you multiply 0.1 by 0.1, you get 0.01.”

When the full genetic code of the virus is sequenced, it may pinpoint whether the source was the institute or the commerical Merial vaccine laboratory, but even that is uncertain.

The foot-and-mouth virus is composed of about 8,300 “letters” of RNA, a cousin of DNA. It is possible but not certain that the strain used in vaccine production has acquired a mutation in one of these, that would set it apart from the institute’s reference strains.

Mark Woolhouse, Professor of Infectious Disease Epidemiology at the University of Edinburgh, who worked on the 2001 foot-and-mouth epidemic, said: “It is important that we do establish what happened here, or it will be very difficult to rebuild confidence in these laboratories. But it is not immmediately obvious what has happened. The truth is that we may never know.”

Airborne release

Until the HSE report was issued, this was the scenario that many experts had considered most likely. The virus can spread on the wind and a leak could have been carried for several miles given the right conditions. Laboratories with category four biosecurity status, such as the Pirbright complex, however, have safety mechanisms that should prevent pathogens from escaping in the air. The HSE found no evidence that any of these had failed.

Facilities must be isolated by an air lock, and air passing out is cleaned by two high-efficiency particulate arrestance filters. Category four labs are also maintained at negative pressure, so that if there is a leak of any sort, air will move into the lab from outside and not vice-versa.

The HSE confirmed that the pressure and filtration systems at the lab were adequate, and also noted that local wind conditions allowed only very small windows at which there could have been any risk. It ruled that there was only a “negligible combined likelihood” of airborne release.

Liquid waste

Experiments at the institute and vaccine manufacture by Merial would have used solvents and other liquid reagents. These would have been contaminated with virus and would have needed treating before disposal.

Merial, in particular, would have had to dispose of large amounts of fluid waste from the production of 10,000 litres of vaccine between July 14 and July 25. The institute conducted only small experiments over that period, each using less than 10 millilitres of virus, so presented less of a risk.

Decontamination can be done with heat or chemicals. The institute’s animal isolation unit relies on thermal decontamination of effluent, and a chemical system covers the rest of the site. If either failed, fluids contaminated with foot-and-mouth could have been flushed out of the laboratory through an effluent pipe.

This ought not to have posed much risk under normal circumstances. The effluent would have flowed into the sewage system, and would have come into contact with neither animals susceptible to foot-and-mouth nor with people, vehicles or wild animals that might have spread it to farms.

There are two concerns here. One is that the HSE reported “doubts about the integrity of the drainage system, including pipework that leads to the final effluent treatment plant” at the Merial site. A leak could have allowed contaminated fluids to accumulate on the ground, from which the virus could have been picked up on workers’ shoes or a passing vehicle.

A similar problem may have arisen because of flooding. The HSE considered that there was a negligible chance that the virus reached farms directly through floodwater: the distance is too great and the Normandy farm is uphill from the Pirbright plant.

It is possible, though, that standing water containing the virus contaminated shoes or tyres, which then carried it to the farms. Professor Woolhouse said: “It would have to have been a double failure: both the decontamination and drainage systems would have to have been compromised. Even so, out of all the scenarios, this has to be one of the most plausible. The others seem even more remote.”

Human transmission

The HSE report considered this to be a “real possibility”, despite extensive safety measures. Scientists, however, thought the risk low. Workers must enter the laboratory through an air lock and change into sterile gowns that fit tightly at the wrists and cover the shoes. They must also wrap over the chest, hair is covered and masks are worn.

All this protective clothing must be removed when leaving the laboratory. It is sterilised in a machine called an autoclave, which uses pressurised steam heated well above 100C to kill any germs. After changing out of their gowns, workers must then shower before leaving the secure area.

Even if one of these steps was not conducted properly, it is still unlikely that a worker could have carried the virus to the infected farms. “The normal procedure is that anyone who has been into these facilities shouldn’t go onto a livestock farm for five days,” Professor Woolhouse said. One possibility is that a contaminated worker walked somewhere near a farm. An allotment adjacent to the first infected farm, which is said to be used by some laboratory staff, was under investigation yesterday.

Professor Woolhouse said: “We need to think about whether the spirit as well as the letter is being observed.”

If the foot-and-mouth virus did contaminate a person’s clothing or body there are two ways in which it could have reached the infected animals.

“The most likely route is that someone walked on something that the animals ate,” Professor Woolhouse said. “That is the rationale for closing footpaths.”

Keith Plumb, a biosafety expert from the Institute of Chemical Engineers, said that the second possibility was that virus spread by contaminated boots could have been picked up by a fox or rodent and carried to the farm.

Sabotage

Deliberate release is being considered as an option by the HSE, partly because all the other possibilities are so remote. In every other case, several events that are all unlikely would have had to have happened at once: a decontamination failure, followed by a drainage failure, then movement of a contaminated person on to a farm.

Professor Wilsmore said: “If you’ve got somebody who wants to spread it, that’s a different story.

“Until we got this report, I thought that airborne spread was the likeliest cause. But when you start to think that mechanical spread – by so-called fomites such as straw, manure, a car wheel or boots – is unlikely, then you start to think. . . I’m sure they will be looking very hard at anybody who has a motive to spread the disease.”

The main case against sabotage is that there is no positive evidence that it has taken place.

Solid waste

Though solid waste is not explicitly discussed in the HSE report, it remains a possibility. Used pieces of equipment such as vials and disposable gloves must be treated before they leave the lab, again by thermal or chemical methods, and there is a chance that this was not done properly.

Dr Plumb said: “Most of this is decontaminated by autoclave, but autoclaves have failed in the past. It certainly can’t be ruled out, particularly as anyone handling this waste would have assumed it had been decontaminated and wasn’t a risk.”

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