Committee Reports > List of Committees > Dáil > 1986 > Committee on Public Expenditure > Report - Review of Public Expenditure for the Eradication of Bovine Tuberculosis

APPENDIX VIII

THE RELIABILITY OF THE COMPARATIVE TUBERCULIN TEST

(Department of Agriculture Submission)

The official position of the Department of Veterinary Service regarding the reliability of the intradermal comparative tuberculin test (S.I.C.T.), and also in regard to the feasibility of using a blood test for the diagnosis of bovine tuberculosis was stated in a letter which was published in the Irish Farmer’s Journal in October, 1984 (copy appended).

It is hoped that the following information will clarify what is in reality an academic difference of opinion regarding the reliability of the S.I.C.T. The test when carried out and interpreted in accordance with the Department’s instructions has a specificity of about 99.9% and a sensitivity of about 98% i.e. an overall reliability of about 99%. Statements that the S.I.C.T. has a sensitivity of only 80-85% ignore the reality that the 10-15% of diseased animals which are inconclusive to the test are either immediately removed as reactors or retested after an interval of 60 days at which time a final decision made regarding their disease status. The sensitivity of the test is further increased by the removal as reactors of which on clinical or epidemiological grounds the veterinary surgeon carrying out the test believes or suspects may be infected. It cannot be stated too often that inconclusive animals never pass a test.

The Sensitivity/Specificity Balance

The two indices of the reliability of a biological are specificity and sensitivity. A test with 100% speciwould ensure that no disease-free animals were classified reactors. A test with 100% sensitivity would ensure that diseased animals were reactors to the test. Therefore reliable test would have 100% specificity and 100% sensitive Unfortunately no such test exists. However, this drawbaovercome to a large extent by the inclusion of an inconclusion category in the interpretation keys for both of the tuberculin tests recognised by the EEC (Directive 80/219) for the purposes It is logical that it would be easier for a veterinary surgeon to understand the complexities of disease eradication.

MANAGEMENT

It has been an impediment to B.T.E. to have a civil service structure of management. No reflection is implied on individuals in the Civil Service, but collective management is not helpful

MOVEMENT

There is too much movement of farm animals between farmers in Ireland This factor has a major bearing on the spread of disease. This movement of animals, some of them carriers of disease, needs close monitoring.

Archaic bookkeeping in the scheme has never coped with this problem.

Ten tears ago computerised control of movement should have been introduced. Because of the management structures this change to computers has been too slow.

EAR TAGS

Irish animals move so much (3 to 4 times in two years) therefore proper identification of animals is imperative. The cattle ear tag that was used for the first twenty years of B.T.E. is not tamper proof. This potential made a mockery of certification. it is not excusable that we still do not have a satisfactory ear tag. This was not a problem in other countries with static animal populations.

SUCCESS

Of course there has been relative success in bringing down the incidence of Bovine T.B. from as high as 30% of cows to about 2%.

There has been spectacular success with Brucellosis eradication.

Management has always been too slow to take account of changing circumstances.

Personnel Management has left a lot to be desired. Veterinary Surgeons in practice have never been properly motivated to be more involved in T.B. disease management.

Epidemiology has never played a significant part in B.T.E. It is vita

The present intensive programme of B.T.E. was proposed more than twelve years ago by the I.V.U.

Faster testing will break the cycle in T.B. Veterinary Surgeons now have targets to aim at in carrying out B.T.E. work.

Veterinary clinicians should be involved in the management of B.T.E. at local country level.

Farmers, A.C.O.T. and particularly farm wives should have a structured involvement in B.T.E.

Without team work the final stages of earadication will be tedious.

REFERENCES

Collins, J.D. (1985). Tuberculosis in animals. Irish of Medical Science, 154, Suppl

Crilly, J. (1984). Unpublished study.

Lesslie, I.W., Hebert, C.N., Frenichs, G.N. (1976). Practised application of bovine tuberculin PPD in testing cattle in Great Britain. Vet. Rec. 98, 170-172.

O’Reilly, L.M. and MacClancy, B.N. (1975). A comparison of the accuracy of a human and a bovine tuberculin PPD for testing cattle with a comparative cervical test. Irish Vet. J. 29, 63-70.

O’Reilly, L.M. (1985). Studies in depopulated Mycobacterium bovis infected herds to determine the sensitivity of the single intradermal comparative tuberculin test. Paper presented to the Scientific Committee ‘Tuberculosis in Animals’ of the International Union Against Tuberculosis, Paris; 1 October, 1985.

of declaring, maintaining and restoring officially tuberculosis free herd status. The two EEC approved tests are the single intradermal and the single intradermal comparative tests.

In practice it has been found that when there is inconclusive category in the interpretation key, a high sensitivity is only achieved at the cost of a low . The scientific literature indicates that if all animals inconclusive and positive on the standard EEC interpretation were removed as reactors, the specificity of the comparative test would be as low as 99.1% (Lesslie etal., 1976). This means that with exacting careful testing there would be an overkill of 9,000 disease-free cattle per million disease-free cattle tested. In 1985 when there were approximately 11 million tests carried out, there would have been about 122,000 reactors i.e. a vast overkill or wastage of about 99,000 disease-free cattle not to mention the needless restriction of perhaps in excess of 30,000 diseases free herds.

In the known infected herds, maintaining the maximum test sensitivity and the early removal of diseased animals are the main priorities. The scientific literature indicates that when inconclusive and positive animals on the standard EEC interpretation are removed as reactors, the comparative test in the Irish environment has an overall sensitivity, on the basis of skin measurements alone, of at least 95% (O’Reilly and MacClancy, 1975; Crilly, 1984) and in heavily infected herds a sensitivity of 90-98% (Collins, 1985; O’Reilly, 1985). The sensitivity of the comparative test is further increased when a more severe interpretation standard is applied and/or when further reactors are removed on clinical or epidemiological grounds.

LOUIS M. O’REILLY MS, MVB, MRCVS

Senior Research Officer

Tuberculosis Section

3 June, 1986.