Memorandum submitted by the British Cattle
Veterinary Association
BOVINE TB
1. The BCVA is a specialist division of
the British Veterinary Association comprising 1500 members of
whom over 1000 are practising veterinary surgeons working with
cattle in farm animal veterinary practice. In this respect a large
number of our members come into direct contact with TB control
policies as they affect their client's farms. BCVA are represented
on the DEFRA TB Forum and are members of various stakeholder groups
affecting the industry. We are very grateful for the opportunity
to submit comments to the EFRA Committee Review.
2. BCVA's policy with respect to the control
of bovine TB is an holistic one. A strategy developed to combat
an infectious disease must take into account all reservoirs of
infection in order to be successful. In the case of bovine TB
reservoirs exist not only in cattle, but also in wildlife species
such as the badger and deer. Any control strategy must take account
of such reservoirs if true control is to be achieved.
3. The following comments are firstly grouped
into the three specific areas addressed in the Inquiry remit,
namely; vaccination, the Irish experience and trace elements.
Any additional comments then follow.
VACCINATION FOR
BOVINE TB
4. BCVA support the research and development
of potential vaccines for the control of bovine tuberculosis in
both the badger and cattle and consider it one of the most important
potential elements of a long-term strategy for the control of
bovine tuberculosis.
5. BCVA do not consider the development
of a suitable vaccination strategy is likely within the medium
term (ie next 10 years). This conclusion is based on reports and
views from scientists in the field, and in particular the report
of the ISG's vaccine scoping sub-committee. The practical difficulties
of developing a candidate vaccine, devising a delivery system
and subsequently identifying vaccinates from non-vaccinates if
used in livestock are immense.
6. BCVA is concerned that no progress is
being made in the control of TB within hotspot areas and that
the new strategies being developed, whilst required and supported
by BCVA, are primarily being aimed at the prevention of spread
of disease from these areas rather than control within.
7. BCG vaccine has been shown to have some
efficacy in the reduction of pathology, disease, bacterial burden
and haematogenous spread of TB. As a result, the use of BCG in
badgers may be of benefit in the short term as a method of control
by reducing the overall load of infection. We are disappointed
that this strategy has not been investigated in greater depth.
8. BCVA would anticipate that the delivery
of a vaccine to badgers would have to be via trapping and injection.
Oral delivery strategies carry the risk of inadvertent environmental
contamination of BCG via faecal shedding of viable bacteria by
vaccinated badgers. This would result in the accidental vaccination
of cattle and the potential for false positives reactors being
found at herd testing.
9. BCVA recognise that a badger vaccination
strategy carries considerable practical difficulties, particularly
if repeat vaccinations are required for example. Such a strategy,
however, would create the opportunity for other work to be undertaken
concurrently. The use of blood tests to identify infected setts
would be a good example of this, thereby facilitating the identification
and possible removal of reservoirs of infection outside cattle.
Such a combined strategy of vaccinating of "healthy"
badgers and removing infected badgers may be of benefit in alleviating
the pressure of infection within the cattle population.
10. BCVA encourage field trials of such
a strategy to be undertaken as soon as possible to investigate
its worth.
11. BCVA concur with the Godfrey report
that immediate action must be taken in relation to wildlife reservoirs.
Defra cannot afford to wait for the outcome of the Krebs's trials
whilst this disease continues unchecked in hotspot areas despite
cattle controls. The damage caused by inactivity whilst waiting
for the RBCT results could be mitigated by adopting a sound medical
approach; enhance the immunity of disease free badgers by vaccination,
identify infected badgers for removal and generally reduce the
overall infectious load.
THE IRISH
EXPERIENCE
12. BCVA are fortunate to have strong links
with Irish researchers into TB and consequently have been privy
into unpublished material into TB control. Much of this data is
still unpublished and confidential in nature and consequently
the following summary of our understanding of the Irish experience
has been tailored to honour this understanding.
13. We are very grateful to Professors More
and Doherty for their help in providing information and it is
in particular a summary document by Professor Simon More reviewing
the current status of Irish research that this submission is based
on albeit in an extremely abbreviated format.
THE INFECTED
HERD
Source of infection
14. Analysis of epidemiological investigation
reports completed in association with confirmed herd breakdowns
have produced some potentially significant findings:
— A range of sources are considered
important.
— It is often impossible for field
Veterinary Investigators to attribute the cause of the outbreak
to a single source.
— The relative importance of these
sources has changed over time.
Table 1
ATTRIBUTED CAUSE OF CONFIRMED TB BREAKDOWN
IN EIRE
| O'Keeffe and Higgins 2003
1994-1998 n=1481
| |
| Lateral Spread | 14.8 |
|
| Bought in | 7.2 |
|
| Residual | 8.5 |
|
| Mechanical | 0.3 |
|
| Wildlife | 33.7 |
|
| Not certain | 35.4 |
|
| |
|
Cattle-to-Cattle transmission
Direct
15. Prof More has summarised the work performed on the
importance of cattle-to-cattle transmission both within Eire and
also the US, UK and the Netherlands. He reports that based on
the light of available evidence and assuming early detection via
testing programmes some workers concluded that cattle to cattle
transmission was rarely the source of infection (Griffin and Dolan
1994).
16. Further work by Griffin (2003) via modelling estimated
that in Ireland 60% of reactors could be attributed to an external
source, 38% are false positives and only 3% are due to within
herd transmission. (These calculations are based on a positive
Tb skin test and not on confirmed infection).
Indirect
17. The role of slurry spreading and the survivability
of Tb in waste material have indicated that the spread of slurry
has the potential to spread disease.
BCVA comment
18. These findings would suggest that the poor sensitivity
of the Tb single intradermal test is not a major factor for the
persistence of high levels of Tb within the herd following breakdown
and that more effort needs to be directed in preventing the introduction
of disease from external sources, be this from cattle (bought-in
stock) or wildlife.
TRANSMISSION FROM
WILDLIFE
19. The Irish have performed two major trials to attempt
to define significant evidence of the role of badgers in bovine
tuberculosis. These have been the East Offaly Badger research
project and the Four Area Badger Project.
East Offaly Badger research project (EOP) (1989-95)
20. At its most basic description, this trial compared
the relative level of bovine TB in a project area where badgers
were removed, to a control area where they were not. Results indicated
a significantly lower proportion of new confirmed Tb cases within
the badger removal area as compared to the control. This effect
has persisted with the rate of herd restrictions in the removal
area being one third of the present national average.
The Four-Area Badger Project (1997-2002)
21. This project was designed to replicate the above
EOP trial but with paired removal and reference areas in each
of four counties that were considered representative of main landscapes
in Ireland. Furthermore, in contrast to the EOP, the boundaries
of removal areas were designed wherever possible to incorporate
natural barriers eg rivers to reduce the effect of inward immigration
of badgers in the study period. Results have not been published
at the time of writing, however, personal communication to BCVA
members lead us to believe that similar results in terms of a
significant positive reduction in Tb incidence in removal areas
will be reported.
22. Criticism has been raised that the real effect of
badger removal has been distorted as a result of perturbation
effects in the control areas. However the Irish researchers have
found no evidence to suggest that this has occurred. Further work
is on going to assess this concern.
23. Of further note is that the average disease prevalence
within the captured badgers in the removal areas was 19.5% with
further work using detailed microbiological assessment increasing
the prevalence up to 50% infected.
BCVA Comment
24. BCVA find these conclusions compelling evidence that
there is an important link between the badger and cattle in the
epidemiology of bovine TB and that more emphasis needs to be placed
by Defra in breaking this link. Furthermore if the prevalence
of TB in the badger population is anywhere near this level in
the UK, (although previous work has suggested that this might
not be the case), then it would suggest the pointlessness of only
directing efforts to the removal of the bovine reservoir.
SPATIAL DISTRIBUTION
OF M. BOVIS
ISOLATES FROM
BADGERS AND
ASSOCIATION WITH
CATTLE BREAKDOWNS
25. Irish research would suggest that Tb infected badgers
are clustered within setts ie setts tend to be either free or
diseased. Despite this finding there is only limited evidence
that infected setts are geographically clustered. Little evidence
could be found when strain typing was employed between a clustering
of a given strain of TB in the badger and the bovine at a particular
locality but there was within a larger Geographical area. This
could be considered a result of long distance movement of badgers
within an area.
BCVA Comment
26. Defra should consider the trapping and identification
via ELISA testing of infected setts in TB hotspot areas. The current
sensitivity of the test is poor for individual animals, however
if TB is clustered within a sett, then testing a proportion of
the animals belonging to that sett gives a higher degree of certainty
to the infection status of that sett. This would help remove a
potential reservoir for cattle infection. Concurrent vaccination
of non-infected sets with intradermal BCG could also be considered
within this protocol (see para 10 & 11 above).
MINIMISING CONTACT
BETWEEN BADGERS
AND CATTLE
27. The Irish have identified areas where transmission
between badgers and cattle is likely. Similar work has been undertaken
in the UK. The likely benefit of any such control measures has
not been quantified.
RISK FACTORS
FOR TB
28. The Irish have identified within their research the
following risk factors for an increased incidence of disease:
— An increase in herd size though this may be artefactual
in that the TB intradermal test is not 100% specific.
— Region. As in the UK, Ireland has hotspots.
— Previous history of disease.
— Higher severity of the disease at breakdown ie
more reactors per breakdown.
HERD TESTING
29. The Irish have concluded that at present the sensitivity
of the gamma-interferon test is similar to the current single
intradermal test but that it detects a different population therefore
its in combination with the intradermal test increases the overall
sensitivity to around 97% and is therefore of use in herds with
a chronic problem. The poor specificity at present (95%) negates
its widespread use although some suggest that this aspect could
improve with newly developed antigens.
FACTORY (ABATTOIR)
SURVEILLANCE
30. The Irish consider that abattoir surveillance for
disease is critical to their whole program. In the years 1993-2001
between 27% and 46% of all herd breakdowns were detected using
this method. The Irish however are concerned about a potential
large variance of the efficiency of detection between plants and
are investigating potential causes and why such a high proportion
of reactors slip the net of testing and are only detected at the
abattoir.
PRE-MOVEMENT
TESTING
31. Pre-movement testing was mandatory until 1996 and
then became voluntary. Research on the benefits of pre-movement
testing concluded that there were two main advantages:
(i) Early identification of infected animals in the
exporting herd thereby reducing opportunities for within herd
disease transmission.
(ii) Reduction in spread of disease to new herds. In
1993 a total of 989 cows from 543 herds were identified as reactors
by this method. This presumably reduced the potential for spread
to a large number of buying herds.
32. No cost benefit analysis has been performed in relation
to pre-movement testing but it was noted that the benefits largely
accrued to the government whereas the costs were borne by the
farmers.
BCVA COMMENT REGARDING
IRISH WORK
33. The BCVA believe that there is much to be learnt
from the Irish experience of controlling bovine TB and will be
considering this subject further in the creation of our response
to the Defra consultation entitled "Preparing for a new GB
strategy on bovine TB".
34. BCVA supports the general intention to base policy
on sound science, but harbours some concern regarding the interpretation
of the term sound science. It is often the case that scientific
research tends to stimulate further scientific research, with
definitive answers being difficult to achieve. Both the Krebbs
trials in the UK and the field research in Ireland have already
courted criticism in their design prior to the publication of
any results. Organisations with vested interests will use any
potential flaws in scientific procedure to argue their case when
a result falls against them, and base their argument on the availability
of sound science. This results in stalemate.
35. The control of infectious disease in all species,
including humans, relies heavily upon the common sense interpretation
of the information available at any given time. Absolute scientific
justification for action is often a holy grail in such circumstances.
BCVA believe that such common sense decision making has been belittled
in the development of bovine TB strategy for some years, and that
this is largely to blame for the current situation in which the
industry finds itself. The BCVA submission to Defra regarding
the strategy for bovine TB will be based on such common sense
interpretation of the science to date, and we encourage the EFRAC's
inquiry team to judge both international and home-grown research
in a similar light.
TRACE ELEMENTS
36. The role of Vitamin E and selenium within the immune
response of the bovine has been the subject of considerable research.
Both agents have a role in the removal and production of toxic
lipid peroxides. Deficiencies impact on the effectiveness of the
immune system primarily via an effect on leucocyte function. Consequently
it is conceivable that deficiency of one or both of these agents
may impact on animal's susceptibility to disease.
37. BCVA assume that the EFRAC inquiry interest in this
subject has been stimulated by a submission enclosed in the EFRAC
Seventh Report of Session 2002-3 "Badgers and Bovine TB"
by Dr Helen Fullerton.
38. BCVA have concerns over this paper, believing that
it contains too many assumptions and flaws. Namely:
(i) That a failure of the test to detect false negatives
cases (diseased but not detected) is directly linked to a failure
of the delayed type hypersensitivity response. This may not always
be the case. Failure could result from operator error for example.
(ii) A failure of the delayed type hypersensivity could
be linked to suppression of Th1 lymphocytes which in turn could
be induced by
(a) zinc and selenium deficiency
(b) high levels of cortisone either as a result of
steroid administration or stress and
(c) parasite load which in turn could be linked to
cobalt and/or zinc deficiency.
39. BCVA have not seen any scientific evidence to support
these assumptions and indeed believe them to be completely erroneous.
Consultation regarding Dr Fullerton's claims with Professor Philip
Duffus of Bristol University, a leading veterinary immunologist,
corroborated this view noting that while the above factors may
affect immune response (eg selenium deficiency may impact on neutrophils
or lymphocyte response) there is no evidence to suggest that it
would have an impact on the delayed type hypersensitivity response
and thus the intradermal tuberculin test, features central to
Dr Fullerton's claims.
40. Furthermore Dr Fullerton maintains TB hotspot areas
overlap the areas of the country where soils often deficient of
cobalt, selenium, copper, iodine and zinc are to be found. If
an assumption is made that the effect of mineral status is via
an effect on the immune response, and thus a predisposition to
disease, it might also be reasonable to assume that other diseases
apart form TB may show a varying incidence in the UK dependent
on the background mineral profile. No such evidence is forthcoming.
41. BCVA have consulted Dr Neville Suttle, formerly of
the Moredun Institute, a leading authority on mineral and trace
element nutrition. Dr Suttle rejected the Dr Fullerton's premise
and stated that there was no evidence to suggest that a marginal
deficiency of any trace element would predispose to infectious
disease. This is supported by a review by Laven (2000) on the
current knowledge on Vitamin E, a vitamin intrinsically linked
with selenium and concluded that supplementation provided little
benefit in the control of infectious diseases other than mastitis.
BCVA see no further evidence to change this view.
42. BCVA do not dispute that immuno-suppression may be
a factor within the epidemiology of the disease. In fact papers
presented to BCVA have questioned and presented anecdotal evidence
of the potential role of BVD. This is an endemic virus within
the UK cattle population, over 90% of herds thought to be infected,
which has known immunosuppressive properties. BCVA would suggest
that if further work into the role of immuno-suppression was to
be performed BVD virus should receive a higher profile than mineral
status.
43. Finally in relation to Dr Fullerton's concern over
the Irish trial the EFRAC inquiry should understand that the setting
of reference ranges for virtually all minerals is based on incomplete
research. In fact different countries often have entirely varying
views on what constitutes deficiency within a particular mineral
category and within a particular production system. While this
may support Dr Fullerton if further evidence suggested that the
Irish had indeed set their reference ranges too high, the opposite
might equally be true. In summary this refute of Irish work is
again based on conjecture.
44. In summary BCVA would concur with Professor Bourne
and the ISG that the current evidence to support mineral deficiency
as an important factor in the epidemiology of TB is extremely
low and that this area should not be prioritised.
CONCLUSION
45. BCVA considers the bovine TB issue to be integral
to the success of the overall Animal Health and Welfare Strategy
being developed for the UK. BCVA is committed to the success of
such a strategy, and is willing to contribute to any discussion
during its development.
46. BCVA thank the EFRAC inquiry team for the opportunity
to contribute to this important inquiry. BCVA would be glad to
offer evidence in person to the team should that be required,
but in any event would be more than willing to provide further
information should it be requested.
British Cattle Veterinary Association
May 2004
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