Memorandum submitted by the Independent
Scientific Group on Cattle TB (L45)
INTRODUCTION
1. The Independent Scientific Group on Cattle
TB was established in February 1998 as part of the Government's
implementation of the Krebs Report. While Krebs' original recommendation
was that a group of independent experts be appointed specifically
to oversee the design and monitor the progress of a large-scale
field trial, Ministers asked the Group to take a holistic view
of Krebs' wider recommendations on TB in cattle and to advise
on the best ways to establish a substantive scientific base to
underpin future disease control policies. Ministers made it clear
that their ultimate objective was to control TB in a way which
ensured that cattle and badgers could co-exist harmoniously. The
Group's primary focus is (and must remain) on the science, but
it has also had to recognise that any future control policy for
cattle would need to be socially and economically acceptable as
well as environmentally sustainable.
2. In the past, TB control policies have
included the testing and slaughter of infected cattle and various
levels of badger removal. The effectiveness of these policies
is unknown and they have been criticised to varying degrees by
both farmers and conservationists.
3. Evidence for the involvement of badgers
is based principally on the observed incidence of infection of
M.bovis in badgers inhabiting affected areas, coupled with
the finding that early badger removal strategies appeared to result
in a decreased incidence of cattle TB. However, in the absence
of comparable detailed information from appropriately selected
experimental control areas, it has been suggested that variation
in disease incidence in cattle over time or in different regions
may reflect factors other than exposure to badgers (such as farm
management practices or climate). The evidence from Northern Ireland,
where no attempts have been made by Government to control badgers
and where changes over time in the incidence of bovine TB have
been similar to those in South West England, has been used to
support this view.
FORMULATING THE
QUESTIONS
4. The Group accepts that there is probably
no single solution to the problem of TB in cattle. For future
Government policy to have a firm scientific base, an objective
approach must be made not only to obtain definitive information
on the quantitative contribution of cattle, badgers and other
wildlife species to the incidence of TB in cattle, but also to
understand how the TB bacillus spreads within cattle, badger and
other wildlife populations, and the circumstances associated with
transmission within and between species.
5. There are insufficient data available
to assess the quantitative significance of factors such as husbandry,
farming techniques, and exposure to wildlife reservoirs on the
development and maintenance of TB in affected cattle herds. We
also need to identify the factors which determine prevalence of
infection in badgers and other wildlife populations, the incidence
of diseased animals and the maintenance of infection in social
groups.
6. In summation, there is a clear need better
to understand the epidemiology of TB in cattle and wildlife, which
will be essential if the overarching objective of ensuring the
co-existence of TB-free cattle with other wildlife is to be achieved.
QUESTIONS WHICH
MUST BE
ADDRESSED
7. Formulation and evaluation of policy
options demands basic information on:—
A. The origin of infection. What proportion
of TB cases in cattle is caused by:
— other wildlife species?
B. What risk factors predispose cattle herds
to TB outbreaks?
Various factors have been proposed as contributors
to herd breakdowns, but no rigorous analysis has ever been carried
out to identify and then assess the quantitative significance
of these factors.
Such risk factors might include:
— farm husbandry (eg livestock management,
stocking density, source and purchases of stock and cropping practices);
— TB incidents in nearby herds;
— climate;
— geographical features;
— exposure to infection in badgers
and other wildlife;
— different strains of M. bovis.
C. What is the pattern and epidemiology of
TB infection in badgers and other wildlife?
The pattern of TB in badgers and other wildlife
at both national and local level is not known, and nor are the
factors that influence the maintenance of infection within species
and how and why this may very between sites.
Factors affecting TB in badgers and other wildlife
might include:
— abundance and social structure;
— population dynamics;
— prevalence of infection;
— severity of disease;
— different strains of M. bovis.
D. What are the possible and most probable
routes of transmission of infection to and within cattle herds
and between species—urine, faeces, respiratory discharge?
HOW CAN
THESE QUESTIONS
BE ANSWERED?
8. The Group recognises that it can advise
Ministers on the development of a science base capable of underpinning
a sustainable long-term TB control policy only on the basis of
a multi-faceted approach. We believe this must include:
(ii) the use of an epidemiological questionnaire;
(iii) a road traffic accident survey;
(iv) analysis and re-analysis of existing
data; and
(v) other related research.
(i) The field trial
Trial Design
9. The Group is committed to the triplet
approach proposed by Krebs, providing, as it does, the opportunity
to collect accurate, quantitative data with the necessary scientific
controls. Ultimately, 30 100km2 areas (assembled into 10 triplets)
will be recruited into the field trial. This is the number of
areas considered to be necessary to detect differences between
proactive, reactive and survey-only treatments and to provide
the replication necessary to demonstrate the consistency of these
effects.
10. The number of years required to detect
treatment differences depends on the strength of the effects as
well as the underlying level of TB incidence in cattle. For example,
if breakdowns were occurring at a rate of 100 per year in the
survey-only areas, a reduction in TB incidence in cattle as low
as 20 per cent in either of the treatment areas would be detectable
in five years.
11. It is recognised that a field trial
cannot have the precision of a laboratory experiment. For example,
field operations are unlikely to prove 100 per cent efficient.
In addition, non-compliance with the trial through interference
with culling operations, denial of access for survey or culling
teams and illegal killing of badgers would all reduce the differences
between treatment areas. Such non-compliance in particular might
increase the amount of time needed to detect underlying differences
between treatment effects. However the trial has been designed
to take account of these sorts of variables, and has sufficient
power to detect relatively small differences in herd breakdown
rates, within a five year period. Unless very severe, non-compliance
would therefore be unlikely to affect the conclusions of the trail,
but merely to extend its duration.
Information to be gained
12. In order to answer the key questions,
the trial has been designed to provide the following essential
information:—
(a) What proportion of TB ourtbreaks in cattle
are caused by badgers?
By comparing the rate of herd breakdowns in proactive
treatment areas (where badgers will have been reduced to low numbers)
with those in survey-only areas where no removal will occur, the
trial will allow us to estimate the proportion of TB outbreaks
directly or indirectly attributable to the presence of badgers.
(b) How does the incidence of TB in cattle
relate to the underlying pattern of TB infection within the badger
population?
Data from proactive trial areas will provide
the first extensive information on TB infection across badger
populations. These data will include:
— the distribution of TB infection
in badger populations and how this is affected by variables such
as population density, social group size, disposition of badger
territories and past badger removal operations;
— whether the TB status of badgers
occupying farms is associated with the recent history of TB in
cattle, in particular on farms which have not experienced TB outbreaks;
— whether some TB strains are more
likely than others to pass between species; and
— whether TB incidence in other wildlife
hosts is influenced by the presence of infected badgers.
Such extensive data have not been available in
the past principally because there is no live test available which
can reliably distinguish infected from uninfected badgers. For
this reason the information can regrettably only be obtained at
present by post-mortem examination of badgers.
(c) Is the culling of badgers an effective
method for controlling TB in cattle?
Comparison across the three treatments will determine
the extent to which proactive and reactive culling affect TB incidence
in cattle. The potential merits of culling as a policy will be
further analysed by an economic assessment of the cost-effectiveness
of culling and a full environmental impact assessment to determine
the ecological consequences of badger removal.
Animal Welfare
13. If badger removal is to be a component
of future policy, it is important that it should be carried out
in a way which minimises suffering to badgers. The Group has therefore
been concerned with several aspects of culling which relate to
badger welfare. In particular the Group considered alternative
means of capturing badgers and recommended that only cage traps
be used in the trial.
14. An additional welfare concern is that
culling lactating female badgers may leave their cubs to starve
below ground. The Group recommended that a three month "closed
season" be imposed from 1 February to 30 April inclusive,
during which no culling would occur. The timing of this season—based
upon known or projected birth dates of badger cubs—will
reduce the culling of females with dependent cubs to a practical
minimum. The Group recognised that this measure might affect TB
control in cattle in reactive areas by delaying the response to
TB incidents confirmed during the late months of the year. Nevertheless,
the Group considered this closed season an acceptable compromise.
Auditing
15. In order to maintain high standards
in the implementation of the trial, the Group recommended that
various procedures, including field operations, data collection
and animal welfare be subject to both internal and external audit.
MAFF are in the process of drawing up contracts to take this forward.
Data Analysis
16. The emphasis of our work has been on
the planning of investigations, on implementation in the field
and on the development of procedures, so as to ensure that the
information collected is reliable and comparable across trial
areas. A very large amount of data will thus be collected and
its careful analysis and interpretation will be of key importance.
Detailed consideration has been given as to how to achieve this.
Issues likely to arise include:
(a) the most direct analysis will compare
herd breakdown rates under the three different trial regimes,
allowing for the number of farms, herds and animals in the different
trial areas. Careful assessment of the precision of the comparisons
and of the stability of the conclusions in time and across areas
will be made; and
(b) a more detailed analysis will exploit
the differences between farms within the trial areas, notably
allowing for evidence of local badger activity (before and after
culling) and of other relevant features.
17. The first interim analysis is planned
to take place after a total of some 100 herd breakdowns have been
confirmed within the trial. Further interim analyses will be conducted
about every six months thereafter.
Additional Work in Trial Areas
Environmental Impact
18. Evaluation of badger culling as a means
of TB control demands an investigation of its environmental impact.
The Group therefore advised that a full environmental impact assessment
be carried out as an integral part of the trial.
Molecular Epidemiology
19. The use of laboratory tests which identify
differences between strains of the bovine TB organism, offers
a means of determining whether or not cattle, badgers and other
wildlife species from the same localities are infected with the
same strains. Over the last few years molecular typing methods
which distinguish strains of TB have been developed and these
are being used to analyse TB isolates from cattle and badgers.
Research has been initiated to develop these techniques further
in order to provide a greater degree of resolution.
20. The results of molecular typing can
be used to infer possible transmission between the different species,
but epidemiological data on population densities, incidence of
infection, infectivity and potential routes of transmission are
required to draw firm conclusions on the primary source of infection.
The trial areas will not only provide an extensive source of TB
isolates from badgers, cattle and other wildlife for molecular
typing, but will also generate the epidemiological data required
to determine likely sources of infection.
Dynamics of Infection in Other Wildlife Species
21. M. bovis is capable of infecting
a wide range of animal species, including man. In the UK the badger
is a reservoir host that is endemically infected. Previous studies
have shown infection in other species, but it is unclear whether
this is a spill-over from cattle or badgers, or if it is self-maintaining
in those species. It is possible that these other species could
act as a source of infection for cattle and badgers.
22. It is therefore important to understand
the role of other potential hosts in maintaining the disease and
to make a quantitative assessment of the risk they pose. This
requires the collection of data from other wildlife sources in
order to establish the prevalence of infection, together with
its severity and molecular type, and the extent to which there
is potential for contact with cattle. It also requires analysis
of data collected to assess the risks to cattle associated with
other wildlife.
23. Specific research into this aspect is
being funded by MAFF and will be undertaken primarily in trial
areas.
(ii) The epidemiological questionnaire
24. It is central to our holistic approach
that we identify and quantify risk factors associated with a cattle
TB breakdown. For several years MAFF has adopted the practice
of collecting information from all farms experiencing a TB breakdown,
using a structured questionnaire. This questionnaire has now been
re-designed and extended in order to improve objectivity, and
to compare farms which have had TB breakdowns with farms where
no breakdowns have taken place in recent years. Data will be collected
on a variety of potential risk factors including herd composition,
cattle movements, farm enterprises, management practices, steps
taken to avoid contact between cattle and badgers, and the presence
of other potential wildlife sources of TB infection. These data
will be analysed to identify the factors which predispose farms
to TB breakdowns. The use of this questionnaire (designated TB99)
within the field trial is critical, and it provides a unique opportunity
simultaneously to analyse the effects of a range of farm practices
and the pattern of disease in badgers.
(iii) Road traffic accident survey
25. Trial data on badger TB prevalence must
be supplemented by information from areas outside the trial with
both high and low levels of TB in cattle. This in turn will allow
an extension of the analysis of the relationship between TB in
badger populations and cattle. Since no reliable live test is
available for use in badgers we propose that data from a road
traffic accident (RTA) survey be collected from the counties of
Cornwall, Devon, Gloucester, Hertfordshire, Worcestershire, Shropshire
and Dorset. These data would initially be used to estimate regional
prevalence and later as sample sizes increase to estimate local
prevalence. In areas that are enrolled into the field trial the
RTA prevalence data will be validated by comparison with TB prevalence
data from badgers culled in the trial.
(iv) Analysis of past bovine TB data
26. MAFF holds a large body of data collected
on farms at the time of past breakdowns, and this too needs to
be analysed for evidence of risk factors and improved understanding
of the dynamics of the disease in cattle.
(v) Other related research
27. The trial provides an opportunity for
further development and validation of a live test for TB in badgers
and also to validate observational techniques being developed
to measure badger population density and social group size. This
information is of critical importance since it may be used in
the future to predict risk of infection in local cattle populations.
28. The Group has also been asked by Ministers
to advise on other complementary research, including vaccines
and vaccination.
ECONOMIC ANALYSIS
29. A sustainable TB control policy must
be cost effective as well as being technically feasible, and environmentally
and socially acceptable. For this to be determined it is crucial
to have appropriate data to allow the balance between the costs
and benefits of any proposed disease control policy to be assessed.
The first element of the required information is the predicted
effect of alternative intervention measures—the relationship
between the TB control actions taken (whether involving badger
culling, changes in farm husbandry, vaccination, etc) and their
impact on the level of herd breakdowns, ecological effects, etc.
Much of this should become available with the results of the trial
and other studies the Group is fostering.
30. The other major elements of relevant
data are: the costs (public and private) of disease control; the
economic losses avoided by this control; and the savings on statutory
testing costs. These can be used in a cost-benefit analysis. Other
elements are less easily quantified but nevertheless need to be
considered in assessing policy options. These include the value
placed on badgers by society, concerns over human health, and
potential external trade effects.
PROGRESS
31. One year on from the Group's establishment,
considerable progress has been made. Prior to the start of field
work, objectives had to be clearly defined and standard operating
procedures put in place. Following Government approval in August
1998 for the trial, field work commenced immediately. Two triplets
(in Devon/Cornwall and Gloucestershire/Herefordshire) have so
far been enrolled, and one of them has been subjected to the proactive
treatment. The initial data from the Putford area of the Devon/Cornwall
triplet is now undergoing preliminary analysis. A report on the
current state of implementation will be made to Ministers shortly.
32. The Group appreciated from the outset
the logistical challenges that a trial of this kind would present.
The sheer physical scale of the field trial was outwith anything
in the Group's or the Government's experience and we, like MAFF,
realised we would be on a steep learning curve.
33. The two triplets we would ideally have
wished to complete by January 1999 are now close to completion.
Everyone involved has worked hard to meet this objective. In this
context, we would wish to record our thanks to MAFF, and in particular
to the managers and staff of the Ministry's Wildlife Unit for
their success in translating the Group's instructions and advice
into action on the ground. It is also worth noting the high rate
of co-operation in ensuring we complete our work in the shortest
possible time-frame.
34. Meanwhile, work is in hand to increase
the number of staff available for field operations and we expect
a further four triplet areas to be enrolled by the end of the
year.
35. The Group has been involved with MAFF
in taking forward the wider research programme recommended. A
number of research projects will be conducted within the trial
areas, and will requireco-ordination. Contracts are being finalised
for work to begin in the next financial year.
36. Finally, the Group wishes to record
its appreciation of the support given by Ministers to the trial.
COMMENT
37. The Group fully appreciates the impact
that cattle TB is having increasingly on individual farmers and
on farming communities, and also the concerns of environmentalists
that badgers are being killed. We also recognise the opposition
to the field trial that therefore exists in some quarters.
38. However, we remain convinced that the
trial provides the best scientific method for understanding better
the epidemiology of TB in cattle and wildlife reservoirs, and
for underpinning with sound science future policy options for
the control of cattle TB, which will ensure the co-existance of
healthy cattle and badgers.
9 February 1999
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