Subject: Maternal transmission of BSE in cattle

From: [email-masked] (J Ralph Blanchfield)
Date: Thu, 01 Aug 1996 17:44:02 GMT

Hello Everyone, 

In a statement today, the UK Ministry of Agriculture, Fisheries and
Food [MAFF] announced 

"There may be very low levels of maternal transmission of BSE in
cattle".

This possibility was foreshadowed in the current IFST Position
Statement on BSE, which included the following:-

"Until recently, there has been no evidence of maternal transmission
in cattle. A large-scale epidemiological study [Hoinville et al
(1995)] should have brought to light the occurrence of maternal
transmission if it occurred, but did not, which was strongly
suggestive of no significant occurrence. However, a computer-modelling
study (as yet unpublished) of the cases of BSE in cattle born after
the ruminant feed ban (BAB cattle), while indicating that the great
majority of cases were due to post-ban feeding with ruminant feed (see
below), also indicated a small but significant likelihood of maternal
transmission

"The definite answer on maternal transmission should be forthcoming
>from  the long-term experimental trials started in 1989, involving 315
calves born to BSE-infected dams and 315 controls born of healthy
dams, The trials are due to run until November 1996, when the youngest
of the calves reach seven years of age, at which time they will be
slaughtered, their brains examined histologically, the study unblinded
and the results analysed.  The analysis and results will be available
in early 1997.  It is only then that it will be possible to make a
scientific judgement on whether or not maternal transmission occurs.
To end the trial prematurely to obtain an earlier answer would greatly
reduce the validity of the answer.  Many of the animals having reached
their seventh birthdays have already been slaughtered and their brains
"stockpiled". It is now intended to complete the histopathology on
about 500 of these by August and issue a preliminary assessment,
pending completion of the trial and full assessment."
The MAFF announcement is as a result of that preliminary assessment.
It is unfortunate that MAFF, while evidentlyand understandably keen to
be seen to publish the preliminary conclusion as soon as possible,
appears to have adopted the unfortunate practice that has become all
too frequent, of announcing a conclusion from experiments  before the
scientific details have been published.in a peer-reviewed journal, for
independent assessment by other scientists. 

I anticipate that the epidemiologists and statisticians among us will
have a field day interpreting the results when they are published.

If a low level of maternal transmission occurs, it has, of course,
been occurring during the dramatic year-on-year decline of the numbers
of new cases in recent years, and likewise has no bearing on the
further decline forecast over the next few years. It may well,
however, prevent virtually complete eradication of BSE. 

But,, as the IFST Position Statement also said 

     "The recent proposals for carefully selective culling are
expected to lead to a speeding-up of that process, but at the expense
of slaughtering and incinerating thousands of healthy animals. If the
experimental trial referred to above were to turn out to show any
significant maternal transmission, however, selective culling would
take on an additional value".

Reference

Hoinville L J et al (1995)  "An investigation of risk factors for
cases of BSE born after the introduction of the feed ban", Veterinary
Record, 136, 312-318.

Regards

Ralph
******************************************************************
J Ralph Blanchfield
Food Science, Food Technology & Food Law Consultant
Chair, IFST Member Relations & Services Committee
Web Editor, Institute of Food Science & Technology
IFST Web address: http://www.easynet.co.uk/ifst/ 
******************************************************************

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From: Oz <[email-masked]>
Date: Thu, 1 Aug 1996 21:03:41 +0100

In article <[email-masked]>, J Ralph Blanchfield
<[email-masked]> writes
>however, prevent virtually complete eradication of BSE. 

Eh?? It is clear, that if it happens, it must be at a very low level.
Let's take it at 1%, and consider 200,000 animals affected over 5 years.

Next generation: 2000 (over 5 years)

Second generation 20  . One could cull the last calf at this point.

Third generation one or nil. One could cull the last calf at this point.

Fourth generation nil.

Allow say five years per cow generation. This is far from 'prevent
virtually complete eradication'.

A similar low level of horizontal transmission (if present) is currently
being dealt with by the cohort cull, but a generation too soon to make
sense. The observation on animals born within three days of the last
calving of an animal that subsequently succumbs to BSE would be more use
since I seem to remember that there was a small but significant
correlation in this case.

------------------------------- 
'Oz     "When I knew little, all was certain. The more I learnt,
        the less sure I was. Is this the uncertainty principle?"