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The Skin Test

 Added by  Sally
 3 Apr 2010, 7:55 PM
There is an interesting statement in 'Testing for Bovine Tuberculosis in California' by John H Kirk. The report is describing the skin test and warns 'be aware that in non-infected herds it is considered normal to have 1-5% false positives or reactions at the site not due to Bovine Tuberculosis, on a whole herd test. This response may be due to previous exposure to Avian Tuberculosis or the Johne’s Disease bacteria, another Mycobacterium disease.'
We had a cow which had been inconclusive on two occasions but the vet agreed he would leave her as she was heavily in calf. He believed in calf animals gave false positive reactions to the skin test. He said he could tell from the feel of the lump and he said he was sure she would be clear the next time. On the following seven tests she had no reaction whatsover. She is still with us and never had any health problems.
On 23 December we responded (see below) to DEFRA's reply as set out in the previous posting. We are currently awaiting a response.
We have now read the papers you referred to. The only manuscript from which there is actual data from which the specificity of the SCITT test can be calculated is the Leslie et al paper from 1975. All the other manuscripts refer either directly (or more usual indirectly, via the Monaghan paper – and the Monaghan paper is not original data but a review paper) to this data. In none of these references does ANYONE actually calculate the specificity of the skin test. With the exception of the Leslie paper, they all refer to other work, which usually refers back to the Leslie paper. The Leslie paper does have data from which the specificity could be calculated, but they did not actually do this. Only the Leslie paper gives raw data – all others refer to other articles. Please could you therefore indicate how the specificity was actually calculated and where the data came from. We understand that the international standard for tuberculin (used in the SCITT test) was adopted in 1986 (copy of relevant OIE manual attached). So if you are using the Leslie data for test specificity can you also confirm that the formulation of the tuberculin in the skin test has not changed since 1975. If it has, then using the Leslie data is surely invalid as the test has actually changed?

On 10 December we emailed DEFRA under the Freedom of Information Act asking; 'Defra has published the figure of 99.9% as the specificity of the skin test. Please provide the basis used for this figure.'
The following response was received on 16 December under ref: RFI 3706
REQUEST FOR INFORMATION: Single intradermal comparative cervical tuberculin (SICCT) test
Thank you for your query about the specificity of the single intradermal comparative cervical tuberculin (SICCT) test currently in use in Great Britain. We received your request on the 10th December 2010. Your request has been considered under the Freedom of Information Act 2000.
You may find some background information about the test interesting. The SICCT test is an effective test for bovine TB when correctly performed. National campaigns involving systematic skin testing of cattle herds and slaughter of test reactors have eradicated bovine TB, or reduced it to very low levels, in the majority of industrialised countries where cattle is the only maintenance host of infection.
The SICCT test applied to cattle in bovine TB-free herds in GB is estimated to have a specificity of 99.9% at standard interpretation, which is equivalent to a 0.1% probability of false positives or a one in 1,000 chance that a non-infected animal will be wrongly classified as a reactor. In other words, the SICCT is a moderately sensitive and highly specific screening test for TB in cattle.
Nevertheless, it is important to appreciate that the sensitivity and specificity values quoted above are indicative working averages. The actual performance of a bovine TB screening test in a particular herd under field conditions will depend on a range of variables, such as the degree of adherence to the correct testing procedure, the within-herd prevalence of cattle sensitised to other non-tuberculous environmental mycobacteria and any factors that may alter the delayed-type hypersensitivity response of an individual animal to tuberculin (e.g. nutritional status, pregnancy, stress levels, concurrent infections, etc.). Additionally, for every diagnostic test with a readout given in a quantitative or ordinal scale, there is a trade-off between sensitivity and specificity so that different interpretations (cut-offs) of the tuberculin skin test can be used under different disease incidence scenarios. The sensitivity of the SICCT test can be enhanced in herds with post-mortem or cultural evidence of TB infection by application of the so-called severe interpretation, but this is at the expense of lowering its specificity.
The basis of the 99.9% figure for the specificity of the skin test is obtained from estimates published in scientific review papers. A non-exhaustive list of references to the sensitivity and specificity values we are working to include the following papers:
• Monaghan, M.L. et al. (1994), Veterinary Microbiology 40, 111–124.
• De la Rua-Domenech R. et al (2006), Research in Veterinary Science 81, 190–210.
• De la Rua-Domenech R. (2006), Government Veterinary Journal 16(1), 65-71.
• Vordermeier M. et al. (2008), Government Veterinary Journal, 19(1), 38-43.
• Lesslie, W. and C.N. Hebert, Comparison of the Specificity of Human and Bovine Tuberculin Ppd for Testing Cattle. 3. National Trial in Great Britain. Veterinary Record, 1975. 96(No.15): p. 338-341.

The following extract raises some interesting points. It is from Select Committee on Environment, Food and Rural Affairs Written Evidence, Memorandum submitted by Dr Helen Fullerton, Farming and Livestock Concern UK back in May 2004
5.1 Studies have shown it is difficult for an infected animal to transmit to others. In one trial when groups of two reactor steers were confined with one attested steer in 10 loose houses for 12 months, only four of the challenged animals were found at slaughter to be infected[7]The other six had presumably acquired immunity. Yet Defra deny that cattle can develop TB immunity. The Independent Scientific Group (ISG) did indeed raise the question, commenting: "it is unclear whether or not cattle can resolve infection with M.bovis, and if so, whether such animals are detected by TB testing procedures"[8]The answer may lie in the high level of false positives diagnosed by the TB skin test. Some 8-12% of UK reactors are shown at post mortem not to have been infected.[9] In the Anglesey outbreak September 2003, 20 reactors were diagnosed in a herd of 160 sucklers, of which five heifers and a bullock were confirmed, and 13 sucklers and a bull were false positives, unnecessarily slaughtered. The much-touted interferon- (IFN) test is at present unacceptable because it diagnoses even more false positives than the skin test
5.2 False positives are mainly ascribed to environmental mycobacteria. Huge numbers of these inhabit soil, water, herbage and the digestive tracts of herbivores, and occasionally a species has been found to sensitise cattle to tuberculin. But I suggest that false positives are identifying animals that have mounted a cell mediated immune response, conferring an immunity which neither the skin test nor the IFN test can distinguish from infection.
6.1 False negatives are cattle that do not respond to the skin test, but are identified by lung lesions as carriers of infections when they eventually come to be slaughtered. They are "silent carriers" who, although healthy themselves, can infect the susceptible or pass it on by maternal transmission. It is thought that about 10% of those tested are false negatives, but this is probably an underestimate. The cause of the non-responsiveness is a suppression of the circulating Th1 lymphocytes and hence of the inflammatory reaction at the injection site, a condition known as anergy. There is also a reduction in the interferon- which these cells produce, so that the IFN- test in this respect is no better than the skin test. Finally the defensive nitric oxide produced by macrophages is reduced. This means (a) that the TB bacillus persists, protected from nitric oxide's lethal free radical derivatives, and (b) since there is no immune over-reaction that destroys host tissue, the cow can live for years in good health, a long term and undetectable reservoir of infection.
6.2 Suppressive agents giving rise to chronic—as opposed to temporary—anergy, include malnutrition, secondary infections, intestinal parasites, zinc deficiency, corticosteroids and stress[10]Recent research explains the suppressive role of zinc deficiency. Adequate levels of serum zinc activate a molecular cascade that propagates to the nucleus, stimulating DNA sequences specific for lymphocyte proliferation[11]Hence where zinc is deficient, the Th1 lymphocytes do not multiply to induce an immune reaction.
6.3 MAFF have argued that silent carriers are a minute fraction of the cattle population as evidenced by the small number diagnosed at the abattoir. This assertion was disproved by McIlroy et al. who carefully dissected the lungs of reactors which had tested negative the year before and found that the lesions were often minute, and would have been missed in routine inspection[12]
6.4 I propose that irrespective of any contribution by infected badgers, silent carriers are the prime cause for the persistence of the hot spots in SW England, as also in Ireland, and contribute to the spread to other areas. Since there are no badgers on Anglesey, is there a silent carrier, imported from the mainland? Although soils are nutrient deficient, the farmer's husbandry avoids stress. But note it was only the young stock, five heifers and a bullock that were confirmed positives. Was susceptibility triggered by stress eg at weaning?
6.5 It is obviously urgent to remove the silent carriers. I suggest that if trace element deficiencies in the cattle are corrected, the suppressive effects of zinc and selenium deficiencies on lymphocyte proliferation, and of cobalt (B12) deficiency on parasite overload would be lifted and the silent carrier identified by the skin test. We should be warned that unless this is done, a reservoir of infection will persist in the old and in new hot spots, far more potent than any from the badger. Carriers eat from the same mangers, dropping saliva and mucous, breathe out aerosols in the same cubicle house air as their companions, putting at risk the susceptible. And despite the proposed mandatory pre- and post movement testing, the silent carrier will escape detection and carry the disease to other herds.
7 Costello E et al. Vet J 1998; 155:245-250.
8 Independent Scientific Group. 2nd Report MAFF Pubs 1999.
9 Monaghan M L et al. Vet Microbiol 1994; 40:111-124.
10 Lagrange P H and Hurtrel B in Mycobacterium Tuberculosis Eds. M Bendinelli and H Friedman. Plenum Press 1998 pp 171-193.
11 Hirshfinkel M et al. PNAS 2001; 98:11749-11754.
12 McIlroy SG et al. Vet Rec 1986; 118:718-721.
Keith (Guest)
Apparently a third of all herd breakdowns are not confirmed as actually having bTB! This is a significant number that warrants further investigation. It is not good enough for the powers that be to conveniently state that the lesions were too small to see or that the tissue culture does not always pick up the disease.

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