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Select Committee on Environment, Food and Rural Affairs Written Evidence


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 Spread14.8
Bought in7.2
Residual8.5
Mechanical0.3
Wildlife33.7
Not certain35.4


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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