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RISK DUE TO BSE INFECTIVITY FROM BURNING CATTLE Dr Richard Lawson The document
"Assessment of Risk due to BSE Infectivity from Burning Cattle" was
prepared for the Ministry of Agriculture, Fisheries and Food by "DNV
Consulting" on 28th February 2001. It can be obtained from MAFF Animal
Health (BSE and Scrapie) Page St, London SW1P 4PQ tel 02079046256 METHODOLOGY The DNV paper starts with
an estimate of infectivity in cattle, that is, the amount of BSE infective
tissue in a given number of the present herd. It finds that 7.2 per thousand
of dairy cattle and 1.7 per thousand of beef cattle would be expected to
carry the BSE infective agent. It estimates that 0.1 gram of brain tissue
from an infected cow could cause vCJD infection in a human. The conclusion is
that the brain and spinal cord of an infective carcass would give 700 ID50
infectious unit doses. ID50 is the dose necessary to cause BSE in 50% of cows
who ingested that dose. The brain and spinal cord of one contaminated cow,
spread uniformly through the human population could cause 350 new cases of
vCJD in humans. The paper then looks at
the event tree for burning cattle. The main routes are direct inhalation of
smoke particles, consumption of unprocessed crops, and consumption of water
supply from ground and surface water. They then assume that a proportion of
the infectivity is destroyed in the fire. They accept that the fire will not
be hot enough to destroy the BSE infective agent (which is very heat stable)
and that the fire will draw up light ash particles into the plume for later
deposition as fall-out. They assert, "with a
hot fire the particles will tend to be lifted away from the immediate area
and people close to the fire should not be exposed." This is quite
clearly not the case. We have seen pictures of smoke from the fires dragging
along the ground and those exposed to the smoke will get a high dose. This is
the key point: they base their assumptions on incinerators, which give wide
dispersal. There can be no justification for extrapolating from incinerators
to pyres in terms of fallout plumes. The conclusion of DNV is
that from a fire in which 100 cattle are burned, the infective ID50 dose from
100 cattle released and returned to the population is 0.00017 to 0.00071,
that is, that a dose thousands of times too small to cause a single case of
vCJD in an exposed human. Summated from the 233,678 cattle burned to date,
the total dose to exposed population would be 0.397 ID50. If the total number
burned at the end of the Maff slaughter firebreak process were to rise to two
million, there would be one case of vCJD created according to the DNV method. Using the DNV
assumptions,
The number of dairy cattle needed to cause this single
case will be less, due to the higher infective load in this type of cattle. The DNV paper concludes that "the individual risk of
exposure would be low." An alternative analysis is, unfortunately, not so
reassuring. The DNV paper frankly acknowledges the extent to which
assumptions have been used throughout. In the annexe containing the input data, the fraction of
infectivity entrained in the smoke, a factor of 0.1 and the fraction of ash
spilled before burial at a value of 0.01 is, with refreshing candour for a government
document, sourced as a "Guess". More seriously, the estimates for fractions of
particulates falling on a populated area, unprocessed crops etc., eight
parameters in all, are sourced as from an incinerator. Incinerators operate
at very high temperatures, and are not at all comparable to pyres. The smoke
plume from an incinerator is completely different as it is expelled at high
temperature from a tall chimney. The use of incinerator data in this case
invalidates the assumptions and conclusions of the MAFF paper. Furthermore, the assumptions are spread out for the
population as a whole and there is no analysis for targeted, at risk groups.
It is normal in epidemiology to consider a group with high exposure such as
people living close to pyres and the workers and managers who are tending the
fire itself. Using the data supplied in the DNV document, 1. Take a pyre of size 100 cattle. 2. There is a 0.17 chance that one of them will be
infected. 3. Each infected animal holds 350 lethal doses of BSE/vCJD 4. Therefore 59.5 lethal doses go into the fire.
(350x0.17) 5. CNV assume that one tenth of this infective load will
escape destruction in the fire 6. Therefore 5.95 lethal doses are released into the smoke
from 100 cattle. 7. If the fire reduction is only 50%, nearly 30 lethal
doses are released form the fire.
1. Of these, 233,678 are cattle - nearly all beef cattle. 2. Using the low figure of 5.95 per 100, the total release
is 13,904 (233,678 / 100 x 5.95) 3. Using the high figure of 30 doses per 100, the total
release to date is 70,103 (233,678 /100 x30) 4. The total number of lethal doses released if the
cull-only policy will continue to rise in proportion to the number of cattle
slaughtered. The majority of these doses will be concentrated on a relatively
small population around the pyres. 5. Clearly not all of these doses will be ingested by
humans. Most will fall out onto pastureland, (where they may re-infect the
next cattle that graze there), and also onto roads, and roofs. The doses will be distributed in concentrations that
diminish with distance from the fires. Concentrations will be greater in calm
weather conditions, and less in windy conditions. Some of the doses will be ingested by humans breathing the
smoke from the pyres. The Maff document assumes a widespread distribution,
similar to that resulting from an incinerator with a tall chimney. This
assumption is completely invalid. Smoke from the pyres drags along the
ground, giving a high deposition and air concentration of particles. DNV consulting neglected to examine the exposure to high
risk, target populations, of whom the most obvious are workers tending the fires,
and residents whose homes lie on the vicinity of the pyres, and who may be
exposed to the smoke day and night. It is therefore possible that anyone living downwind of a
pyre, or a worker tending the pyre may have a significant risk of contracting
new variant CJD. It would be possible accurately to estimate the number of
new cases of BSE and vCJD this release would cause through modelling of the
smoke plume. It is imperative that field trials be carried out to find
empirical evidence of BSE/vCJD infectivity in the smoke, and in fallout areas
around the fires. RECOMMENDATIONS 1. It is recommended that people exposed to the smoke
should be issued with respirators capable of trapping particles onto which
the BSE infective agent might be bound and that people living downwind of
fires whose homes are enveloped in smoke should be either evacuated or be
advised to stay indoors with the doors and windows closed. 2. It is recommended that there should be immediate
sampling of the smoke from the fires by scientists independent of MAFF and
the Government. The smoke samples should be tested the presence of viable BSE
particles. Vegetation and soil samples should be checked also. 4. At the same time, an analysis should be made for the
presence of viable Foot & Mouth disease picorna virus in the smoke. 5. Pyres should not be used at all because of their risks
of spreading FMD and the risk from the carcinogens and infective agents. The
alternative, burial, is also environmentally disastrous since it will
contaminate the ground and surface waters with high BOD and disinfectant, but
it is the lesser of the two evils. 6. In the light of the Purdey hypothesis, especially
stringent precautions should be applied in those areas where there is high
manganese level in the soil. 7. Due to the environmental effects of the disposal
problem, it is recommended that the slaughter policy should be abandoned, and
vaccination firebreaks should be used instead. Dr Richard Lawson |
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