August 6, 2007

experts respond to Foot and Mouth outbreak

Dr Matthew Keeling, Department of Biological Sciences, University of Warwick, said:

“The Surrey case of foot-and-mouth disease was positively identified on the evening of Friday 3rd August; in the following 60 hours no further cases have been confirmed. By this point in the 2001 epidemic, 2 cases had been positively identified and a further 3 were suspected. Therefore, early indications are that this epidemic is far more contained than in 2001.

“The source of infection is still unconfirmed, and at the moment it is too early to say with confidence whether any further farms have been infected from this source. This uncertainty highlights the need for vigilance by the farming community both within the Protection and Surveillance zones and beyond.

“If only the Wanborough farm has been infected by the (unidentified) source, then preliminary modelling results suggest that the risk of further cases — infected by the Wanborough farm — is relatively low, as is the risk of spread to other regions. (However, it should be stressed that these models are based upon average patterns of disease spread and therefore are not reliable predictions of the true behaviour at the scale of individual farms.) In addition, the index case is in a region of relatively low animal density, which should allow any subsequent spread, if it occurs, to be quickly controlled.

“If further cases have been caused by the (unidentified) source, then control options may have to be reassessed as there is an increased risk that a larger scale outbreak could occur.”

Keith Plumb, Pharma Subject Group, Institution of Chemical Engineers, said:

“Foot and Mouth Disease Virus (FMDV) is covered by the governments Specific Animal Pathogenic Organisms regulations and it a Category 4 organism because it is so virulent. DEFRA provide specific requirements for facilities that are licensed to handle Category 4 organism. The biosecurity measures required include three basic areas of protection, contained buildings, personal protective clothing and waste handling systems.

“Contained buildings require special entry systems such as air locks, changing rooms and showers. However, it is not possible to make a building completely air tight so the regulations require that the areas where the virus is exposed is held under 75 Pa negative pressure relative to atmospheric pressure. This is a fairly low pressure and lower than is used in most pharmaceutical applications. To achieve this negative pressure air must be continually extracted from the building. This regulations require that this air is passed through two High Efficiency Particle Absolute (HEPA) filters before being exhausted to atmosphere.

“There are a number of potential problems with the HEPA filters. Firstly they must be correctly installed to ensure that there is no leakage past the filter elements. Secondly they must be regularly checked and maintained. On start up of the system there will be a high flow of air through the filters as the building is pulled down to the required pressure, this high flow could damage the filters. Finally HEPA filters are designed to work with particles in the region of 300 nanometers and bigger but the FMDV is in the order of 25 nanometres. Small particles such as viruses tend form agglomerates when they are suspended in air and these will be filtered out but there is some evidence that FMDV can pass HEPA filters. It is worth noting that a recent EU document shows the Pirbright government laboratories having 25 filtration systems.

“Gowning procedures are very important in biosecurity. If protective clothing is not properly put on and removed it is possible to get contamination on the body and the undergarments. The DEFRA requirements do not call for a full strip change so it is possible that poor gowning has been the cause of the out break. It is a requirement that all gowns and gloves must be autoclaved to disinfect them before they leave the buildings and it unlikely that this procedure was overlooked. Viruses are easier to disinfect by this root than bacteria and so failure of the autoclave is also unlikely to be the source of the out break.

“The final security measure is waste disposal. Liquid wastes must be heat or chemically treated. Again viruses are easy to disinfect by this treatment and so this equipment should be easy to maintain in effective order. The DEFRA requirements allow the use of “Dunk Tanks”, these are large double ended baths of decontaminant. It is possible that these could have been misused and allowed waste material to leave the facilities without being properly disinfected.”

Dr Paul Denny, Centre for Infectious Diseases, Durham University, said:

“Given the strict code of practice under which the virus must be grown it is difficult to understand how an outbreak could accidentally occur unless procedures at Pirbright were not followed correctly or if the infrastructure is inappropriate or poorly maintained. Either way this event raises serious questions concerning bio-security at Pirbright.”