LANCET
Volume 351, Number 9110 18 April 1998
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BSE: the final resting place
How to dispose of dangerous waste is a question that has vexed the human
race for hundreds of years. The answer has usually been to get it out of
sight--burn it or bury it. In Periclean Athens, victims of the plague
were incinerated in funeral pyres; in 14th century Venice, a law
stipulated that Black Death corpses should be buried to a minimum depth
of 5 feet; and now, as the 20th century draws to a close, we are
challenged by everything from industrial mercury to the smouldering
reactors of decommissioned atomic submarines.
The Irish Department of Agriculture will convene an expert panel on
April 27-29 to discuss the disposal of tissues from animals with bovine
spongiform encephalopathy (BSE). Proper disposal of tissues from
infected cattle has implications for both human and animal safety.
Safety for human beings is an issue because there is now unassailable if
still indirect evidence that BSE causes infections in man in the form of
"new variant" Creutzfeld-Jakob disease (nvCJD).1-3 Safety for animals is
also an issue because BSE-affected cattle could possibly transmit
disease to species other than cattle, including sheep, the species that
was almost surely the unwitting source of the BSE epidemic.
The first matter to consider is the distribution of infectivity in the
bodies of infected animals. The brain (and more generally, the central
nervous system) is the primary target in all transmissible spongiform
encephalopathies (TSE), and it contains by far the highest concentration
of the infectious agent. In naturally occuring disease, infectivity may
reach levels of up to about one million lethal doses per gram of brain
tissue, whether the disease be kuru, CJD, scrapie, or BSE. The
infectious agent in BSE-infected cattle has so far been found only in
brain, spinal cord, cervical and thoracic dorsal root ganglia,
trigeminal ganglia, distal ileum, and bone marrow.4 However, the much
more widespread distribution of low levels of infectivity in human
beings with kuru or CJD, and in sheep and goats with scrapie, suggests
that caution is advisable in prematurely dismissing as harmless other
tissues of BSE-infected cattle.
A second consideration relates to the routes by which TSE infection can
occur. Decades of accumulated data, both natural and experimental, have
shown clearly that the most efficient method of infection is by direct
penetration of the central nervous system; penetration of peripheral
sites is less likely to transmit disease. Infection can also occur by
the oral route, and the ingestion of as little as 1 g of BSE brain
tissue can transmit disease to other cattle.5 Infection by the
respiratory route does not occur (an important consideration with
respect to incineration), and venereal infection either does not occur
or is too rare to be detected.
How can tissue infectivity be destroyed before disposal? The agents that
cause TSE have been known almost since their discovery to have awesome
resistance to methods that quickly and easily inactivate most other
pathogens. Irradiation, chemicals, and heat are the three commonest
inactivating techniques. Irradiation has proved entirely ineffective,
and only a handful of a long catalogue of chemicals have produced more
than modest reduction in infectivity. The most active of these are
concentrated solutions of sodium hypochlorite (bleach) or sodium
hydroxide (lye). As for heat, even though the agent shares with most
other pathogens the feature of being more effectively damaged by wet
heat than by dry heat, boiling has little effect, and steam heat under
pressure (autoclaving) at temperatures of 121Ã’_Ã_ºC is not always
sterilising. To date, the most effective heat kill requires exposure of
infectious material to steam heat at 134Ã’_Ã_ºC for 1 h in a porous-load
autoclave.6 Exposure to dry heat even at temperatures of up to
360Ã’_Ã_ºC for
1 h may leave a small amount of residual infectivity.7 The standard
method of incineration, heating to about 1000Ã’_Ã_ºC for at least several
seconds, has been assumed to achieve total sterilisation, but needs
experimental verification in the light of suggestions that rendered
tissue waste might find some useful purpose as a source of heating fuel.
Thus, TSE agents are very resistant to virtually every imaginable method
of inactivation, and those methods found to be most effective may, in
one test or another, fail to sterilise. It seems that even when most
infectious particles succumb to an inactivating process, there may
remain a small subpopulation of particles that exhibit an extraordinary
capacity to withstand inactivation, and that, with appropriate testing,
will be found to retain the ability to transmit disease. Also, almost
all available inactivation data have come from research studies done
under carefully controlled laboratory conditions, and it is always
difficult to translate these conditions to the world of commerce. Even
when the data are applied in the commercial process, the repetitive
nature of the process requires vigilance in quality control and
inspection to ensure adherence to its regulations.
The final issue that must be addressed is the "lifespan" of the
infectious agent after disposal if it has been only incompletely
inactivated beforehand. Given the extraordinary resistance of the agent
to decontamination measures, the epidemiological and experimental
evidence indicating that TSE agents may endure in nature for a long time
should come as no surprise. The first real clue to this possibility came
from the Icelandic observation that healthy sheep contracted scrapie
when they grazed on pastures that had lain unused for 3 years after
having been grazed by scrapie-infected sheep.8
Support for this observation was obtained from an experiment in which
scrapie-infected brain material was mixed with soil, placed in a
container, and then allowed to "weather" in a semi-interred state for 3
years.9 A small amount of residual infectivity was detected in the
contaminated soil, and most of the infectivity remained in the topmost
layers of soil, where the tissue had originally been placed--in other
words, there had been no significant leaching of infectivity to deeper
soil layers.
It is therefore plausible for surface or subsurface disposal of
TSE-contaminated tissue or carcasses to result in long-lasting soil
infectivity. Uncovered landfills are a favourite feeding site for
seagulls, which could disperse the infectivity.10 Other animals might do
likewise, and if the landfill site were later used for herbivore
grazing, or tilled as arable land, the potential for disease
transmission might remain. A further question concerns the risk of
contamination of the surrounding water table, or even surface
waste-water channels, by effluents and discarded solid waste from
treatment plants.
A reasonable conclusion from existing data is that there is a potential
for human infection to result from environmental contamination by
BSE-infected tissue residues. The potential cannot be quantified because
of the huge number of uncertainties and assumptions that attend each
stage of the disposal process.
On the positive side, spongiform encephalopathy can be said to be not
easily transmissible. Although the level of infectivity to which
creatures are exposed is not known, it is probably very low, since sheep
that die from scrapie, cattle that die from BSE, and human beings who
die from nvCJD represent only a small proportion of their respective
exposed populations.
Whatever risk exists is therefore extremely small, but not zero, hence
all practical steps that might reduce the risk to the smallest
acceptable level must be considered. What is practical and what is
acceptable are concepts that will be hammered out on the anvil of
politics: scientific input, such as it is, already waits in the forge. A
fairly obvious recommendation, based on the science, would be that all
material that is actually or potentially contaminated by BSE, whether
whole carcasses, rendered solids, or waste effluents, should be exposed
to lye and thoroughly incinerated under strictly inspected conditions.
Another is that the residue is buried in landfills to a depth that would
minimise any subsequent animal or human exposure, in areas that would
not intersect with any potable water-table source. Certainly, it has
been, and will continue to be, necessary in many instances to accept
less than the ideal.
Paul Brown
Laboratory of Central Nervous System Studies, National Institute of
Neurological Disorders and Stroke, Bethesda, MD 20892, USA
1 Will RG, Ironside JW, Zeidler M, et al. A new variant of
Creutzfeldt-Jakob disease in the UK. Lancet 1996; 347: 921-25 [PubMed].
2 Bruce M, Will RG, Ironside JW, et al. Transmissions to mice indicate
that 'new variant' CJD is caused by the BSE agent. Nature 1997: 389:
498-501.
3 Collinge J, Sidle KCL, Heads J, Ironside J, Hill AF. Molecular
analysis of prion strain variation and the aetiology of 'new variant'
CJD. Nature 1996; 383: 685-90 [PubMed].
4 Wells GAH, Hawkins SAC, Green RB, et al. Preliminary observations on
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NJ: Humana Press, 1996: 105-18.
7 Brown P, Liberski PP, Wolff A, Gajdusek DC. Resistance of scrapie
infectivity to steam autoclaving after formaldehyde fixation and limited
survival after ashing at 360Ã’_Ã_°C: practical and theoretical
implications,
J Infect Dis 1990; 161: 467-72 [PubMed].
8 Palsson PA. Rida (scrapie) in Iceland and its epidemiology. In:
Prusiner SB, Hadlow WJ, eds. Slow transmissible diseases of the nervous
system, vol I. New York: Academic Press, 1979: 357-66.
9 Brown P, Gajdusek DC. Survival of scrapie virus after 3 years'
interment. Lancet 1991; 337; 269-70.
10 Scrimgoeur EM, Brown P, Monaghan P. Disposal of rendered specified
offal. Vet Rec 1996; 139: 219-20 [PubMed].