Thought this might interest the prion-watchers among us. It's a
ProMED exchange on the possibility of transmissibility of the BSE
infectious particle via water systems, and the risk of actual
Note that the risk of infection is defined in the first posting in
terms of LD50 (the dose at which 50% of test subjects die), and the
subsequent discussion around whether that toxicological model *or* a
low-dose/immediate response model is the best way to assess risk.
This is an important difference in methodological approach for those
of us in sustag or organic ag: at what threshold do xenotoxins or
infectious agents cause effects? I hope to share with you some
research-based stuff on that in the late summer regarding pesticide
SANETters have asked me what ProMED is. It is the Program for
Monitoring Emerging Diseases, sponsored by the Federation of American
Scientists (FAS). It is "a prototype for the communications system
that will be needed to monitor emerging infectious diseases globally.
Establishing a direct partnership among scientists and doctors in all
parts of the world is a central goal of the ProMED initiative.
Sharing information on a timely basis, and discussing emerging
disease concerns, furthers that goal." Ag-related issues come up on a
fairly ongoing basis, which I pass along to you from time to time.
BSE, WATER RISK ASSESSMENT - UK
A ProMED-mail post
Date: Sun, 17 May 1998 08:39:25 GMT
From: J Ralph Blanchfield <firstname.lastname@example.org>
Source: Bovine Spongiform Encephalopathy <BSE-L@RZ.UNI-KARLSRUHE.DE>
A paper by Gale P, Young R, Stanfield G and Oakes D, "Development of a
risk assessment for BSE in the aquatic environment" is published in
the Journal of Applied Microbiology, 1998, Vol 84, 467-477.
The summary notes the resistance of BSE prions to degradation, and
states that levels of disinfection used for drinking water treatment
would have little effect. The summary continues:
"This paper presents the assumptions which were used to model the
risks from a rendering plant disposing of cull cattle carcasses in the
catchment of a chalk aquifer which is used for drinking water
abstraction. The risk assessment approach focused on identifying the
hydrogeological and physical barriers which would contribute to
preventing BSE infectivity gaining entry to the aquifer. These
barriers include inactivation of BSE agent by the rendering process,
removal from the effluent by treatment at the plant, filtration and
adsorption in the clay and chalk, and dilution in the ground water.
The importance in environmental risk assessment of the cow-to-man
species barrier is considered. Two key conclusions about the
environmental behaviour of the BSE agent are that prion proteins are
'sticky' and bind to particulates, and that the millions of BSE prion
molecules comprising a human oral LD50 are subject to some degree of
dispersion and hence dilution in the environment. Assuming that the
rendering plant processes 2,000 cull cattle carcasses per week, the
risks to drinking water consumers were estimated to be remote. Indeed,
even using worst cases assumptions an individual would have to consume
2 litres per day of tap water for 45 million years to have a 50%
chance of infection through drinking water drawn from the aquifer".
Intrigued by the reference in the summary to "sticky" I think others
might be interested to see what they say about that in the body of the
paper (which, AFAIK, is not accessible on-line). It is, of course,
only one of the many factors dealt with as part of their approach to
the risk assessment.
Under the heading " 5.1 BSE will be bound to particulates and
suspended solids in the aquatic environment" they write
"Infectivity is associated with biological membranes and infectious
prion particles are insoluble (Prusiner 1991). This is responsible for
many of the difficulties which have been encountered during attempts
to purify and characterize the infectious agent in scrapie. Indeed,
infectivity is sometimes described as 'sticky'. The reason for this
'stickiness' and the insolubility of PrPsc are explained by the
biochemistry and biophysical properties of the PrPsc molecule and, in
particular, its hydrophobicity. The Prpsc molecule is amphipathic
(Meyer at al 1986). This means that both hydrophilic (water-loving)
and hydrophobic (water-hating) regions exist on each and every PrPsc
molecule. Its amphipathic properties account for both its binding to
cellular membranes and its aggregation into rods (Meyer et al 1986).
In view of their hydrophobic moieties, molecules such as prions cannot
exist fee in water. Indeed, through the maximization of hydrophobic
interactions, amphipathic molecules attach with great tenacity to
other molecules to the exclusion of water. It is therefore believed
that any BSE agent in the aquatic environment will be bound to solids
and particles. In whole cattle carcasses and rendered cattle
carcasses, those solids and particles to which PrPbse attaches will be
proteinaceous and carbohydrate in nature. Through decay of those
components in the aquatic environment, the bound proteins will be
released and hence dispersed (molecular dispersion) immediately
attaching to other particles. Thus prions will be subject to
progressive dispersion and dilution while remaining bound to
particulates at all times."
The references that they cite in that extract are
Prusiner S B (1991) "Molecular biology of prion diseases", Science,
Vol 252, 1515-1522.
Meyer R K et al, "Separation and properties of cellular and scrapie
proteins", Proc. National Academy of Sciences, USA, Vol 83, 2310-2314.
[A question was raised in the resulting discussion concerning the
Re: >>Assuming that the rendering plant processes 2,000 cull cattle
carcasses per week, the risks to drinking water consumers were
estimated to be remote. Indeed, even using worst cases assumptions an
individual would have to consume 2 litres per day of tap water for 45
million years to have a 50% chance of infection through drinking water
drawn from the aquifer".<< >
>I never believe such simple calculations
with unknown factors and one should not forget that even one prion can
eventually infect an individual and that such nonlethal infections can
become lethal if there are amplification mechanisms like cannibalism
in medicine and agriculture.<
The authors did not forget or overlook that. On the contrary, one of
the sections of their review is headed. in bold type "6.3 Low dose
extrapolation suggests that even a single BSE prion molecule has a
small but finite probability of initiating infection".
It is _not_ a "simple calculation". In my two posts related to that
paper I merely quoted from the short summary and picked up the point
about "stickiness". These short extracts were quoted from ten pages of
small print in which they clearly set out their assumptions and the
basis for them, and the working of their calculations, including the
those that you ask about. To comment meaningfully, one really needs
first to study the whole paper. [It's in the paper. - Mod.MHJ]
J Ralph Blanchfield, MBE
Food Science, Food Technology & Food Law Consultant
Chair, IFST External Affairs
Web Editor, Institute of Food Science & Technology
IFST Web address <http://www.easynet.co.uk/ifst/>
Date: Mon, 18 May 1998 12:58:29 -0400
From: Robert A. LaBudde <email@example.com>
Source: Bovine Spongiform Encephalopathy <BSE-L@RZ.UNI-KARLSRUHE.DE>
All proteins are of course composed of both hydrophilic and
hydrophobic regions, the insolubility indicating not much hydrophilic
region is exposed. Hydrophobic regions, not being "happy" surrounded
by water, would logically co-aggregate and precipitate, or attach to
other hydrophobic molecules in the milieu, such as oils or organics.
This is why the removal of organic solvent from rendering might have
played a role in the BSE problem. Traditional methods of protein
deactivation normally depend on leveraging the hydrophilic portion of
the molecule. When water is absent from the structure it is much more
difficult to cause heat or chemical damage.
Michele Gale-Sinex, communications manager
Center for Integrated Ag Systems
UW-Madison College of Ag and Life Sciences
Voice: (608) 262-8018 FAX: (608) 265-3020
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