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Audit of the Sources of BSE Infectivity in Human Food
Project Code: M03018
Comer, P ; Huntly, P
This Report presents results from a risk assessment study commissioned by the Food Standards Agency as part of the contingency planning in the event that a BSE infection is found in sheep in the UK. This study has attempted to quantify the risk of exposure to the human population from BSE infectivity in lamb and sheep meat based on a set of assumed infectivity scenarios. A second study was also commissioned in parallel from the Imperial College Department of Infectious Disease Epidemiology with a broader scope of work.
The main steps in the assessment have been:
- To identify the numbers of sheep and lambs slaughtered and consumed in the UK.
- To consider the uses of sheep meat and the quantities of any tissue that may contain infectivity entering the human food chain.
- To assess the potential infectivity in a sheep with BSE and the way the infectivity would develop through the incubation period for the various tissues of interest.
- To develop a simple model that sums the infectivity consumed for each age group and tissue type, combining the infectivity, the amount of each tissue consumed, the relative infectivity, the age factor for each age group and any reduction factor.
The study is based on data for 2000 as this is the last complete year of data. Data for 2001 would have been significantly affected by the foot and mouth epidemic. About 16 million lambs and sheep were slaughtered in 2000 of which about 13 million would have been consumed in the UK the remainder being exported. There is only limited data on the age of sheep at slaughter, and four age groups have been defined; lambs less than 6 months at slaughter (50%), lambs 6 months to 1 year at slaughter (33%), hoggets (1 to 2 years old)(0.1%) and cull ewes (16%).
It is not known whether or not BSE infectivity is currently present in the UK sheep flock and if it were what the incidence might be. A programme to look for evidence of BSE in sheep diagnosed with scrapie has so far found no cases. However, the experiment is still ongoing and relatively few brains have been tested (about 180). This result does not exclude the possibility of a low level of BSE in sheep. With this uncertainty, the risk assessment is based on the premise that BSE is present in the UK flock using a set of four infectivity scenarios, defined in terms of the percentage of scrapie cases that are BSE; Low (0.01%), Medium (0.1%), High (1%) and Maximum (10%). The prevalence of scrapie in the UK breeding flock is assumed to be about 0.1%.
There are a number of ongoing experimental studies on BSE in sheep. These have shown that the time course and pattern of infection are different to those for BSE in cattle and more in line with scrapie. An important factor is that infectivity is found early in the incubation period in certain tissues, in particular the lymph nodes and the intestines. Lymph nodes are widely distributed throughout a lamb carcase. This suggests that, if a BSE infection is present in sheep, then there could be infectivity present in lambs at less than 1 year old.
For this assessment it has been assumed that:
1. The infectivity of CNS tissue in a sheep with BSE as an oral dose to humans will be the same as the infectivity of the CNS tissue of a cow with BSE.
2. The relative infectivities of different tissues in a sheep with BSE will be similar to those for scrapie as reported by Hadlow (1982).
The exposure estimates reported here are intended to give an indication of the scale of the risk potential for an assumed prevalence of BSE in sheep. There is significant uncertainty in the estimates and they should not be regarded as absolute measures of risk. They can be used for assessing the benefit of possible risk reduction measures and some are reported below.
The overall exposure of the UK population to any BSE infectivity present in sheep has been estimated to be 4 human oral ID50 units per year for the Medium infectivity scenario (0.1% scrapie cases as BSE) with a 95 percentile range from 0.06 to 210. The results for the other scenarios are simply a factor of 10 up or down. This is the total infectivity consumed by the whole UK population. The exposure to any one individual would therefore be low. However, the risk levels are not so low that they could be judged to be clearly insignificant.
The results show that about 50% (range: 35% - 68%) of the exposure is due to animals olderthan 1 year (mainly cull ewes) and 50% (range: 30% - 63%) from older lambs (aged 6 months to one year). 85% (range: 74% - 90%) of the exposure is due to the infectivity in lymph nodes and 11% (range: 5 - 22%) is due to intestines used as natural sausage casings. These values are dependant on the assumptions made on development of infectivity through the incubation period and on the effect of processing on infectivity in casings.
Risk Reduction Measures
The results for the distribution of the exposure to infectivity suggest four risk reduction measures for consideration:
RRM-1: No animals older than 1 year allowed in food for human consumption.
RRM -2: All lymph nodes removed from carcase as far as possible. It is assumed that 95% of the lymph tissue would be removed
RRM-3: Use of intestines banned a) from animals older than 1 year, and b) from all animals.
RRM-4: All offals (intestine, liver, thymus, stomach) from animals older than 1 year banned.
This analysis shows that the greatest risk reduction from an individual measure is achieved by RRM-2, removal of carcase lymph nodes from all ages, which gives an 80% reduction; RRM-1, no animals older than 1 year used for human food, gives a 55% reduction. By combining RRM-1 and RRM-2 the exposure is reduced by over 90%. A similar risk reduction is obtained by banning the use of all offals from animals over 1 year old (RRM-4) rather than banning the complete animals (RRM-1). The greatest risk reduction (97%) is obtained by combing RRM3b, banning use of intestines from animals of all ages, with RRM-1 and RRM-2. Again, a similar reduction in exposure is obtained by substituting RRM-4 (banning the use of all offals from animals over 1 year old) for RRM-1.
The sensitivity of the results have been tested to variations in the assumptions made about 1) the increase in infectivity in lymph tissue and intestines over the incubation period, and 2) the casings processing reduction factor.
Infectivity in Lymph Nodes and Intestine
Infectivity assumed to reach 100% of clinical value at 6 months rather than 50%
- Total exposure to infectivity increased by factor of 2, with higher risk from lambs of 6 month to 1 year.
- RRM-1 less effective, but RRM-2 remains an effective risk reduction measure
Casings Processing Reduction Factor
The exposure risk from lymph nodes and intestine has been estimated for values of the casings processing reduction factor varying from 500 to 10.
- At a processing reduction factor of 10 the exposure from intestine is just greater than that from lymph nodes and they are equal at a value of 13.
- The exposure from lymph nodes is significantly greater than that from intestine for all values greater than 20.
- RRM-3, banning the use of intestine, is relatively ineffective in reducing the risk of exposure to infectivity at all reasonable values of the processing reduction factor.
At the present time it is not possible to predict the risk of exposure from BSE in sheep with any certainty. In order to improve our understanding about the potential risk it is recommended that:
1. The present programme to screen sheep for TSEs in general and BSE in particular should be expanded so that there would be confidence that a low incidence of BSE would be identified.
2. Current research studies on the pathogenesis of BSE in sheep should be expanded to include titrations to report the level of infectivity in selected tissues.
3. Additional data are collected on the total quantity and distribution of potentially infective tissue in the carcass (e.g. lymph nodes, major nerves); cuts of meat and the quantities of potentially infective tissue contained therein (e.g. chops, joints, saddle); and consumption trends in relation to age of animal and cuts of meat across different societal groups.
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