APPENDIX 6
RETYPED FROM ORIGINAL FACSIMILE COPY
CENTRAL VETERINARY LABORATORY
New Haw, Addlestone, Surrey KT15 3NB United Kingdom
Telephone 01932 341111 Facsimile 01932 347006 Telex 262318 VetWey G
CVL
Bob Baldwin MP
Federal Member for Paterson
P.O. Box 246
Maitland
NSW
Australia 2320
FAX: 00 61 49 343 607 10th September 1996
Dear Mr Baldwin
Heat inactivation of infectious bursal disease and
Newcastle disease viruses
As I promised I have read through the data you faxed
to me and also my own notes and documents relevant to the heat inactivation work we did
for General Foods Poultry New Zealand in 1988/89.
Having revisited the work I remain assured that it was carried out
correctly and of the validity of the findings. The problems I had with the Australian
authorities' interpretation of my data outlined in my correspondence with Peter Board in
1993 related to the extrapolation of the data to other temperatures not tested and the
point concerning the fact that probability of survival after heat treatment is dependent
on the starting infectivity level. This could confound stating specific times for
particular temperatures if not taken into account when defining acceptable treatment
processes. The other problem I had was that the levels of the probability of survival of
infectivity I gave as exampled, p = 0.1 and p = 0.001, seemed to be taken as a
recommendation of acceptable levels whereas that was not the intention as I was not asked
to suggest such levels.
As we discussed on the telephone I agree with your assessment that
probably the greatest threat to both poultry and the indigenous wild life in Australia
would come from the introduction of Newcastle disease and that restrictions should be
aimed at maintaining Australia's freedom from the virulent form of the causative virus.
I will do my best to answer the points raised in your letter of 5th
September 1996.
- The assumption that infectious bursal disease virus may be similar
to Newcastle disease virus.
Chemically, physically and biologically Newcastle disease [ND] virus
and infectious bursal disease [IBD] viruses are very different, but I think the
question here is whether or not the temperatures that may be considered acceptable for the
heat inactivation of IBD virus would be acceptable for ND virus. The truth is that IBD
virus is regarded as extremely resistant to heat and other sterilising agents. Indeed when
we did the work in 1988 we were surprised at just how resistant the virus was to the
temperatures we used! Comparisons of resistance between IBD virus and ND virus are
difficult to make as parallel experiments under identical conditions are not available. A
rough guide could be obtained from the heat resistance of ND virus in liquid whole egg
[see accompanying paper published by Richard Gough of this laboratory in 1973], in that
medium with the more resistant of the two strains tested at a temperature of 64.4C caused
a reduction of infectivity from about 109 to about 104.5 within 40 seconds for the first
phase of the inactivation curve and from about 104.5 to about 101 in 160 seconds for the
second phase. Contrast these figures with those we obtained with IBD virus at the much
higher temperature of 75C where there was a reduction of infectivity from 103.0 to 101.83
within 3 minutes and then a reduction from 101.83 to 100.17 after a further 17 minutes. Of
course the different media and other experimental factors will have an important influence
on these results, but it would seem these would have to be extremely different in severity
to have produced such markedly different heat resistance between the two viruses. It is
always preferable to have data from experiments run in parallel under identical
conditions, but in their absence the evidence available, both that quoted above and
published work, suggests that under comparable conditions IBD virus would prove to be
considerably more resistant to heat than ND virus.
- Lack of tests designed to detect ND virus
Ideally, in making assessments of the likelihood of diseases like ND
being imported with poultry products one would like to have as much specific information
as possible concerning the presence of the virus and its survival after processing in
conditions as close to the commercial situation as possible. Usually this is not possible.
My opinion is that extrapolation of the results in the laboratory to the field situation
is probably valid providing careful assessment of the exact process and conditions of
processing has been carried out to determine if any procedures or other parameters in that
process will affect such an extrapolation. In other words some assessment of the closeness
of the conditions in the laboratory and the commercial processing should be made and the
extent to which variations are likely to affect the desired outcome.
- Truth of the assertion that tests with ND virus have not been done
in chicken meat.
This problem has arisen several times in the past and I have looked
at the literature again - as far as I can tell no work has been done on the heat
inactivation of ND virus in chicken meat. There is good evidence that ND virus will be
present in muscles and other organs of infected chickens and this is particularly
important in vaccinated birds, which may have virulent virus present if infected despite
failing to show clinical signs.
The tests with ND virus done by my colleague Richard Gough were done
with homogenised whole egg "spiked" with a known quantity of virus. In the work
done with IBD virus we used homogenised bursae from infected chicks.
- Possible contamination with ND virus.
As I mentioned on the telephone contamination of meat after
processing would depend very much on the working practices of the processing plant, the
precautions imposed and the extent to which these are carried out. Without being able to
observe these directly it is difficult to make any assessment. If such contamination were
to take place it would seem most likely to me that this would occur as a result of contact
with faeces or body fluids from infected chickens or carcases, or, of course, with
something already contaminated with infective material.
- Transfer of NDV on packaging
This is similar to the last point and would involve contamination
with infective material. ND virus is reported to survive for very long periods on
contaminated materials.
- Assessment of difficulties posed by commercial realities and
handling of meat.
I do not feel equipped to comment on this area which is outside my
field of expertise. Shouldn't the importing country specify the procedures to be followed
and ensure that they are?
- Current situation of Newcastle disease world wide.
The virulent forms of ND virus continue to e present and may be
regarded as endemic in many S. American, African and Asian countries. In the European
Union the outbreaks occurring predominately in the Benelux countries and Germany during
1991-1995 appear to be tailing off now. Denmark, which had been free of NDV since 1972
reported 14 outbreaks in 1995 and 2 so far in 1996, all these were in backyard flocks. One
of the problems with Newcastle disease is that that OIE definition is sufficiently vague
that there is some ambiguity in the definition of an outbreak used by different countries
[at least those outside the EU]. For example, some countries in Asia and Africa use, as
mesogenic vaccines, viruses
that would be considered sufficiently virulent to impose the
stamping out policy in European Union countries. The evidence we have from viruses
submitted to the International Reference Laboratory is that there are probably several
different strains of virulent ND viruses circulating in the world at present. Any of these
strains could be destructive for the Australian poultry industry if introduced and allowed
to spread, obviously it would be hoped that should Newcastle disease ever occur in
Australia the primary outbreak would be quickly identified and the disease eradicated and
that the biosecurity and hygiene levels already in practice would be sufficient to
minimise the risk of spread. The risk to indigenous wildlife is not so clear cut. It is
generally assumed that all avian species are susceptible to infection with ND virus,
although to date this has been confirmed in about 250 species. Despite this assumption,
the virulent viruses have not been regularly isolated from wild birds. In waterfowl, for
example, nearly all isolations have been of low virulence and similar to the ND viruses
known to be present in Australia, even though waterfowl are, to some extent, resistant to
the worst effects of virulent ND viruses. However, the infection of cormorants in Canada
in recent years [1990-92] has shown that ND viruses may infect wildlife with serious
consequences. There is always the potential threat that introduction of the virus could
result in endemic disease and the perpetuation of the virus in a given species.
I hope these comments are what you required from me even if they are
not all you would have wanted. Please let me know if there are any further points or
clarification required.
Yours sincerely,
Dennis Alexander (Signed)
Direct TEL: 01932 357 466: Direct FAX: 01932 357 856;
Email: dalexander.fla@gtnet.gov.uk