Public summary

Note of SEAC Working Group on disposal of carcasses of cattle culled for Foot and Mouth Disease (FMD) control (24 May 2001)

Animals Over 5 Years Buried Prior to Environment Agency's Advice of 26/03/01

There is considerable uncertainty as to exactly where and how many cattle over 5 years in age have been buried and as a consequence of control measures applied to limit the FMD epidemic. Current data suggest that there are at least 55 burial sites in the UK that contain cattle aged over 5 years in age. The total number of cattle buried in these sites was estimated at 6700, but this was certainly an underestimate. It was likely that up to 10,000 cattle aged over 5 years could have been buried between the outbreak of FMD and the advice from SEAC (and the revised advice from the Environment Agency) in late March. If a prevalence of 0.4% of late stage BSE carcasses was assumed, then around 40 such carcasses could be included amongst the total. Work was continuing to improve the estimates of the number of animals buried.

It was noted that, especially for on-farm burial, burial sites not only included carcasses but might included a wide variety of other items which a farmer might wish to dispose of in the same pit. This would be an added practical complication when considering the question of disinterring carcasses.

The nature of the burial sites varied widely, including some where it was known that cattle over 5 years in age had been buried, some where it was known that no cattle in this age-group were present and some where it was not known whether or not such cattle were present. Protection of water courses was the key concern. It was not possible to put together any kind of generic risk assessment to cover all sites and the Environment Agency were carrying out specific risk assessments on all affected sites. Their conclusions could range from instances where the assessment was that there was no significant problem to sites where pollution was occurring and where immediate action would be needed. For sites in which there was assessed to be a potential problem exhumation was not the only option. Other approaches such as the use of barriers between the burial site and the water course or the removal of authorisation from a private water supply might be applied.

In discussion it was noted that any risk analysis should take the form of a risk comparison for different actions (and might include no action). There would be risks in all different courses of action. The risks of accidents involved in disinterring carcasses, for example, might well be equal to or even higher than the risks to human health from leaving the carcasses in situ. The relative risks of all different courses of action needed to be considered.

The Working Group went on to consider a recent scientific paper suggesting that much of the infective material would have liquefied and leaked from the carcasses within around two months. This would mean that exhumation of the carcasses may leave most of any infectivity behind. In relation to this the Working Group noted that FMD carcasses would have been buried between 20 February and the end of March. Any infective material within the carcasses could therefore already have liquefied in the ensuing 1-3 months.

Little research had been undertaken of the fate in soil of the infective agent. Unknown amounts would or could already have eluted away into the soil or become bound to soil particles or have been decomposed by bacteria. Knowledge was lacking on how decomposed infective material might be partitioned between these three routes or on how long it might persist in the ground. The research had not been done although a MAFF project in this area was about to begin. Long term research to answer these questions would be required and there was no easy way to obtain answers in the timescales that were critical to addressing the current issues.

The Working Group considered that it was most likely that decomposition of any infected carcasses would result in the release of low levels of infectivity over a long period of time. It was not known whether the delivery of a single dose of infectious material presented the same risk as delivery of the same amount of infectivity sub-divided into multiple small doses delivered over an extended time scale; limited evidence suggested that multiple dosing delivered over an extended time scale might present a lower risk.

The Working Group identified a specific risk from cattle ingesting the infective agent with soil. Cattle ingest a considerable amount of soil with the grass they eat and if the agent binds to surface soil this would represent a potential risk.

In relation to this point and more generally the Working Group noted that exhumation might present a risk in itself in that it might result in contaminated soil being brought to the surface.

There was an additional question with respect to the treatment of the leachate from burial sites. At some sites this material was being pumped from the burial sites and either being put into landfill or being pumped out to sea as sewage. It was noted that for large volumes of material, it would not be practical to treat the material to destroy BSE infectivity either by raising the pH (a pH of 12-13 would need to be achieved for at least 1 hour) or by heating.

The Working Group advised that there were no known methods specifically designed to measure the presence of the infective agent in the environment. Existing methods for detecting PrPSc were fairly sensitive to the way samples were handled and there was a danger of false-negative results. In addition, it was likely that any infective agent present in environmental samples would be too diluted to be detectable.

The Working Group discussed whether they should consider the wider risks from other TSEs such as scrapie and whether they should consider risks to animal health as well as to human health. It was agreed that at this meeting they should focus on BSE risks to human health only, although SEAC flagged up the need to consider the wider risks to animal health and in particular the potential risks to sheep from scrapie linked to the burial of carcasses as part of the FMD control measures.

The Working Group concluded:

  • That there was a need for site specific risk assessments.
  • In considering the risks it should be noted that the number of infected cattle involved in any one site would usually be small. A proportion of fallen cattle (animals found dead on farm) had been buried on-farm for a number of years. A quick calculation assuming 20% of fallen stock animals had been buried on-farm and that there was a 1% occurrence of BSE in these animals, suggested that approximately 150 BSE-infected animals were being buried each year. This should stop shortly when MAFF start collecting fallen cattle over 30 months for its BSE testing programme. It should also be noted that far more BSE infected cattle had been buried at the height of the BSE epidemic.
  • The key risks were related not to the carcasses directly but to the release of infectivity in leachate either in its liquid form or in the ground. Most of the infectivity would probably already have leached from the carcasses given the timescales involved. Again the risks from this material should be considered on a site-by-site basis.
  • Any consideration of risk should involve a risk comparison. Disinterring carcasses may in itself create significant risks.
  • It was considered that SEAC's conclusions on carcass burial from 1991 and 1996 were not directly relevant to the issues being discussed on this occasion.
  • The number of older animals buried on any one site was clearly of central importance to the site-by-site assessments.

Disposal of Ash from Cattle Carcasses Burned in Pyres

There were at least 400 separate locations where pyres had been burned. Some on-farm pyres had been very large involving more than 1000 carcasses. At least 200,000 animals had been burned in this way of which at least 50,000 were cattle born before 1996.

Ash has been buried on-farm wherever possible but there are a number of cases where for a variety of reasons, e.g. the results of a local risk assessment, this was not done.

Some work had been done since the last time SEAC met to look at the amount of protein remaining in pyre ash to compare with the assumption made in the risk assessment that 90% of infectivity was destroyed. The results of these experiments produced figures of 99% for two of the sites and a figure closer to 80% for the third. Two factors needed to be noted here: firstly, that there might be different levels of destruction in different pyre types. Not enough data were available to determine whether this was the case; and secondly that the way in which samples were taken was very important. Some parts of an individual pyre might burn better than others and the presence of any infectivity might be very patchy. This meant that significantly more infectivity might be present in ash from some parts of the pyre than in others.

A pessimistic estimate suggested that up to 60 BSE infected animals within 3 months of clinical disease could have been burned on pyres. It should be possible to estimate of the number of animals over 5 years of age in any one pyre. It was considered possible to rank the BSE risk of individual pyres as high, medium or low given a knowledge of their location, composition, and the geographical distribution of BSE.

The Working Group considered that the BSE risk with respect to ash from individual pyres should also be assessed on a site-by-site basis. Again the comparative risks of different courses of action needed to be considered. Leaving the ash in situ was not a risk-free option.

The use of mobile incinerators was described as a possible means of re-burning the ash prior to disposal although it was acknowledged that very little was known about how these machines would function in practice when used for burning ash from pyres as they had currently only been used for burning whole carcasses.

The Working Group noted that SEAC had already advised on the risks from both large and small incinerators. If mobile incinerators achieved the same temperature levels and profiles then this would indicate that they were achieving an appropriate effect with respect to destruction of infectivity.

The Working Group also noted that there was no risk-free option. Leaving the ash in situ could pose more of a risk than disposing of it in other ways. It had been assumed at the meeting on 30 March that the ash would be buried and this had been viewed at the time as likely to be associated with low risk. Even if the ash could not be buried in situ the Working Group considered that it could be transported to an alternative site and buried there.

The Environment Agency said that in some instances a suitable alternative site could be found on farm for the ash from smaller pyres. The problem came when there was a need to take the material off the farm for burial. There were problems both with using landfill sites for ash from more than one pyre and finding the multiple sites for the ash from large pyres because of the quantities of material involved. In addition burial in certain sites might present risks other than the TSE risks, for example as a result of high levels of potassium in the ash. The objective was to identify a range of risks and options so that disposal could be distributed across a variety of different disposal methods. This would help to deal with the logistics of disposing of the ash across a reasonable timescale.

The Working Group noted that SEAC's preferred option was high temperature incineration. However other options might also represent a low risk, particularly if incineration of all this material was not practical and given that leaving the material in situ or storage prior to incineration were not risk-free options. SEAC had, for example, already accepted that the burial of the ash from pyres represented a low risk with respect to disposal. Although landfill or burial was considered to be less safe than incineration the BSE risks from putting this material into landfill were seen as likely to be extremely small although it was acknowledged that the absolute level of risk was unknown. It was agreed that the ash should, in any case, not be left in situ, where it represented a higher risk than alternative options.

It was suggested that the different disposal methods could be placed in a hierarchy with respect to the level of risk they represented. From lowest to highest risk these were:

  • high temperature incineration
  • incineration in mobile destructor
  • burial on farm
  • burial in licensed landfill
  • storage prior to incineration
  • leaving in situ/ doing nothing.

This did not necessarily represent a preference order for what should be done with the ash but instead represented an order in which disposal options might be considered. A range of methods might be considered with the amount of destruction of infectivity balanced against other considerations.

It was calculated that, on average, one metric ton of ash would contain of the order of 0.01 human oral infective units or 10-8 infective units per gram.

The Working Group reached the following conclusions with respect to ash:

  • There were no risk-free options. Although high-temperature incineration was the ideal option, SEAC had already accepted that burial on-farm and in landfill represented a low risk with respect to TSEs for disposal of pyre ash. Leaving the ash in situ was likely to be associated with a higher risk than disposal through these alternative routes.
  • They noted that their previous recommendation, that the burial of FMD ash represented a low risk, still stood. Landfill was considered to be a low risk option for disposal as long as the risks of transporting the material were not high. This recommendation was consistent with SEAC's previous advice with respect to disposal of ash.
  • They endorsed the proposal for the risk hierarchy and that a range of different disposal methods should be applied depending on the individual TSE risk for a particular site, the results of individual site specific risk assessments, and the options available for that site with respect to disposal. This would probably result in a mixture of incineration and burial being used for disposal of the ash.
  • They noted that there was a need, when considering the issue of disposal, to look at contingent risks and to compare the risks of various courses of action.
  • They noted that site specific risk assessments should be used to make these judgements on a site-by-site basis.

Letter to SEAC from a public health physician

A letter from a public health physician asking about worker safety was discussed. Most of the issues raised had been covered under the previous item. It was considered that inhaling the infective agent could present potential risks as there was evidence of conjunctival transmission, and the lining of the lung might take up infectivity in the same way. There was no reason to believe that the possibility of infectivity being taken up through this route this would differ significantly from the oral route. The Working Group reiterated their continued support of protective clothing in workers dealing with this material.

Re-use for food use of lorries and containers used to transport cattle carcasses arising in the cull from FMD control

Current regulations allow the re-use of lorries used for transporting FMD carcasses to be reused for transporting food afterwards. Many of the lorries involved are specifically dedicated for the transportation of specified risk material (SRM), of the others the most common use would be for transporting grain, much of which would be going for animal feed.

Current regulations allow lorries which have been used for transporting SRM to be used for transporting food once they have been thoroughly cleansed and disinfected. Carcasses of animals culled for FMD control clearly represented a smaller TSE risk than SRM and any change to the recommendations with respect to the transportation of such carcasses would therefore impact on SEAC's previous advice with respect to the transportation of SRM. This would require a change in the law.

The Working Group noted the importance of thorough cleaning as well as disinfecting containers before reuse. Cleaning or washing and the removal of any grease or other matter sticking to the walls of the container was more important than disinfecting with respect to the removal of the prion protein.

A rough estimate was that up to 8 carcasses with pre-clinical BSE infection might be carried in the lorries being used to transport FMD carcasses of which only one or two would be transported in lorries which would afterwards be used for transporting food. All the lorries were fully lined with a plastic liner with sawdust on top. Any SRM leakage should not therefore have come into direct contact with the metal surfaces of the lorries. If anything the assumptions made with respect to the numbers of animals involved was considered to be on the high side.

The Working Group underlined that the lorries should be thoroughly cleaned and decontaminated before being reused for food. There would be value in SEAC reviewing the question of washing and disinfecting at a later meeting and making some specific recommendation on the most appropriate washing and disinfecting procedures to achieve adequate cleansing.

The Working Group also noted the need to be consistent with similar situations with respect to SRM. Abattoirs were quoted as one example where SRM routinely comes into contact with equipment used for food preparation. The concerns about cleansing these environments and the risks with respect to TSE contamination would be significantly greater than the risks from lorries used to transport FMD carcasses which were later used to transport food. Arguably, it would be inappropriate to have more stringent requirements for the cleaning of the lorries.

It was agreed that:

  • SEAC's current advice should stand with respect to lorries used to transport carcasses of animals culled for FMD control.
  • That thorough disinfecting and washing was an acceptable way to clean these lorries for future food use, and that thorough cleaning through washing was more important than disinfecting for the removal of TSE activity.

Any Other Business

It was agreed that a note of the meeting would go to the SEAC meeting in June.

SEAC Secretariat
May 2001