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The Skin Test



 Added by  Sally
 3 Apr 2010, 7:55 PM


http://209.85.229.132/search?q=cache:ONP-IvlPiIwJ:www.vetmed.ucdavis.edu/vetext/INF-DA/TestingforTB.pdf+MMTR+bovine+TB+test&cd=3&hl=en&ct=clnk&client=safari
 
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.'
becky
http://cvi.asm.org/content/19/4/620.abstract
 
'Improved Skin Test for Differential Diagnosis of Bovine Tuberculosis by the Addition of Rv3020c-Derived Peptides' by Gareth J. Jonesa, Adam Whelana, Derek Cliffordb, Mick Coada and H. Martin Vordermeiera.
 
ABSTRACT
 
A peptide cocktail derived from the mycobacterial antigens ESAT-6, CFP-10, and Rv3615c allowed differentiation between Mycobacterium bovis-infected and M. bovis bacillus Calmette-Guérin (BCG)-vaccinated cattle when used as a skin test reagent for a “DIVA” test (i.e., a test capable of differentiating infected and uninfected vaccinated animals). Addition of the antigen Rv3020c improves the diagnostic sensitivity without compromising specificity in the face of BCG or Johne's disease vaccination.
 
lewistresinwen
I have heard that a dairy farmer in South West Wales has on 4 seperate occasions lost a number of cows that were inconclusive reactors.None of these animals showed any sign of TB on post mortem.He has now refused to have any more of his cows slaughtered and is mounting a legal challenge against his latest order to cull cows.
 
Sally
According to the Holyhead and Anglesey Mail (www.theonlinemail.co.uk/bangor-and-anglesey-news/local-bangor-and-anglesey-news/2012/04/18/anglesey-vets-pracice-banned-from- testing-for-bovine-tb-66580-30786655/) an Anglesey veterinary practice has been banned from testing for Bovine TB after it was found out it wasn’t conducting the process properly. The practice is the Gaerwen based vets Bennett-Williams which sent out letters to farmers and farming unions explaining the disruption to TB testing.
 
Cattle farmers who use the practice for TB testing have been told by Department of Environment, Food and Rural Affairs (DEFRA) to make alternative testing arrangements after an audit of TB testing procedures identified the problems.
 
To regain the right to deal with TB, vets at the practice will need to be re-trained.
 
An angry farmer told the Mail: “There are deadlines and legal requirements about testing that we as farmers need to meet.
“If this is not done right then it could put the whole TB control plan in jeopardy. We could believe that our cattle are healthy and then find that they have to be destroyed, which would put you out of business.”
 
A spokesman for the Animal, Health and Veterinary Laboratories Agency (AHVLA) part of government’s DEFRA department said: “Following an audit of TB testing procedures, which identified a failure to follow standard operating procedures, AHVLA has suspended a private veterinary surgeon working in North Wales from operating as a panel 1a (bovine TB testing) official veterinarian, pending re-training.
 
“Owners of cattle served by the suspended practice have been notified and advised to make alternative testing arrangements.”
 
The Mail apparently contacted the Bennett-Williams practise about the issue and was told they were too busy to comment.

 
Sally
An interesting piece on the Bovine TB Blog - entitled That EU elephant again and posted on February 12, 2012 (http://bovinetb.blogspot.com/). It reports on the costs of bTB testing - some £20 million a year and the existing contract with veterinary practices, the involvement of the EU and the usual confusion and recently announced measures to tighten up on controls. Well worth a read. What a mess.
 
becky
http://archive.defra.gov.uk/foodfarm/farmanimal/diseases/atoz/tb/documents/tb-facts.pdf
 
Some very interesting admissions from Defra in this document - see extracts below:
 
'No diagnostic test, including the tuberculin skin test, is 100% accurate, but the current skin test is effective (and is the primary diagnostic test required under EU legislation). On the one hand, the comparative skin test used in the UK and Ireland can be expected to vary around approximately 80% detection rate of all the infected cattle in a herd at any one test (at standard interpretation, range 52-100%). On the other hand, reactions to the tuberculin test can sometimes be caused by exposure to other mycobacteria which do not cause bovine TB.'
 
'Studies evaluating the sensitivity of the test suggest that its sensitivity lies between 52% and 100%.'
 
'Do animals with fluke show a stronger reaction to the skin test and result in false positive reactions? ... A review of the veterinary literature on this topic provides somewhat conflicting evidence. On one hand, the cattle TB pathogenesis study conducted in GB (Defra project SE3013) reported that skin test reactors and contacts with antibodies to liver fluke (Fasciola hepatica) were less likely to show with evidence of M. bovis infection at post-mortem examination. The effect was most significant in dairy reactors.... In Ireland, Flynn et al. (2007) established an experimental model of co-infection of F. hepatica and M. bovis BCG to examine the impact of liver fluke infestation on correct diagnosis of TB in cattle. They found that the sensitivity of skin and gamma interferon tests was compromised in co-infected animals and that F. hepatica infection altered macrophage function. Their results raise the question of whether F. hepatica infection can
affect the predictive capacity of tests for the diagnosis of bovine TB and possibly also influence
susceptibility to bovine TB and other bacterial diseases. In summary, this is a hypothesis that merits
further investigation.'
 
 
'Is TB testing compromised by the presence of Johne‟s disease?... Yes. Exposure to Johne‟s disease can cause cross reactivity when using the skin and gamma interferon tests for bovine TB.'
 
'If the skin test for bovine TB can be compromised by other mycobacteria (e.g. avium, microti),is the gamma interferon test compromised in the same way? ... Yes. In cattle, false positive reactions to the gamma interferon test can sometimes be caused by exposure to mycobacteria other than M. bovis.'
 
becky
Interesting presentation papers from the 2009 M Bovis conference in Wellington, New Zealand: 'How does the ‘field’ SICTT performance of Tuberculin PPDs compare? - from different manufacturers? - with different potencies? What if only the bovine site reading is considered – the SIT?'
 
Some of the conclusions:
 
A potency change in tuberculin would have repercussions for the TB eradication programme
 
Importance of having the injection sites in the same plane relative to the shoulder blade was confirmed
 
Tuberculin X failed to detect 20% of reactors with visible lesions and 9% of the visible lesion animals passed the SIT – the use of tuberculin of this potency could have serious repercussions and damage the reputation of the exporting country
 
Sensitivity and specificity is highly dependent on potency and the relative potency of both avian and bovine PPDs in the case of the SICTT
 
The World Organisation for Animal Health (OIE) and/or the EU in setting standards might consider if checks on tuberculin potency should be performed independently of the checks performed by the manufacturer so as to ensure quality of surveillance data and security of disease freedom certification based on the use of a variety of commercially available tuberculin PPDs.
 
Sally
We have been sent a link to www.fwi.co.uk/Articles/06/01/2010/119362/Strategy-to-eradicate-Johne39s-disease-needed-now.htm
 
The article (from Jan 2010) reports on Dairy UK's conference on Johne's disease. According to vet Keith Cutler if this disease is ignored 'it will rise unnoticed until it is almost impossible to do anything to control it'.
 
He also sai that it is rare you will see animals in severe stages of the disease becuase it's known that 70% of animals culled for 'other reasons' are culture positive for Mycobacterium paratuberculosis, the organism that causes Johne's disease.
 
Results from a Defra study released at the conference estimated that up to 42.5% of UK dairy herds were infected with this bacteria when sampled in 2006. This figure could have risen since then. Testing is problematic as they lack sensitivity with many false negatives.
 
In Denmark the disease is in 80-85% of cattle!
 
Sally
We have been sent this link http://www.agriview.com/news/dairy/patton-finds-teachable-moments-in-johne-s-program/article_b7a127c0-1b75-11e1-a128-001cc4c002e0.htm, which mentions Johne's disease.
 
Of particular interest is; 'The US Department of Agriculture has specific conditions we have to meet in order to vaccinate herds for Johne's, because it can lead to an increase in false positive tests for bovine tuberculosis, so our field staff used to do vaccinations, ...'
 
... and when you consider when you consider that (from same article) 'Johne's disease is a chronic, contagious bacterial infection that infects an estimated two-thirds of the nation's dairy herds', (relating to USA) just how many reactors/IRs are due to Johanes - which is caused by a Mycobacteria too?
 
becky
Interesting paper from back in 2009 relating to Northern ireland. The Control of Bovine Tuberculosis in Northern IrelandREPORT BY THE COMPTROLLER AND AUDITOR GENERAL NIA 92/08-09, 18 March 2009 available at http://www.bovinetb.info/docs/Bovine_Final.pdf
 
Of particular interest:
 
Re testing carried out by private vets or in-house staff:
 
'However, within the same ‘at risk’ type of bTB test, PVPs detected markedly fewer reactors than in-house staff who were almost twice as likely to classify a herd as a breakdown herd. '
 
... and on fraud:
 
'It is a matter of concern that two herd owners successfully prosecuted for fraud received a total of £6,400 compensation for subsequent bTB outbreaks.'

 
becky
According to This is Devon (www.thisisdevon.co.uk/President-aims-answer-bTB-cull-questions/story-13236670-detail/story.html) a farmer in the Westcountry is in the process of suing his own vet after one of his animals tested positive following the bTB skin test. The vet apparently injected the tuberculin into the animal's shoulder, for some reason, rather than on the neck, where it should have been done. The cow was in a crush and was not causing a problem.
 
The animal was subsequently found to be clear, and the farmer remonstrated to the AHVLA about the situation, who told him that careful siting of the test on the animal was critical: "because the sensitivity of the test varies in different areas."
 
As he said: "Vets are paid to do the tests – and should do them properly. Farmers should be aware of this. They could be 'going down' with TB when in reality they are clear."
 
The experiences of this Westcountry milk producer could be frighteningly common.
 
becky
http://blog.lancashirebadgergroup.org.uk/wp-content/uploads/2011/07/Animal-Health-Official-Veterinarian-Newsletter-March-2011.pd... On page 10 of this 2008 document, is an example of a Friesian cow with chronic mastitis. It had passed the TB skin test 11 times since 2003. In February three calves suckling from her tested positive to TB - one of these was classed as a reactor when TB tested and had lesions at post-mortem. The cow thought once again passed the skin test! Consent was given for the cow to have a blood test. She was positive to both the gIFN test and a Rapid Antibody test, so was slaughtered. The carcass was condemned - it had so many lesions.
 
becky
We have been sent details of this paper from 2008 which indicates some of the failings f the existing skin test. (http://veterinaryrecord.bmj.com/content/163/12/357.abstract):
 
Outbreak of bovine tuberculosis featuring anergy to the skin test, udder lesions and milkborne disease in young calves
M. G. Houlihan, MVB, MSc, MRCVS1, F. W. Dixon, BVM&S, MRCVS2 and N. A. Page, BVSc, MRCVS2
 
Abstract.
 
A severe outbreak of bovine tuberculosis in a 1300-head, multisite dairy herd in Great Britain had several unusual features, including anergy to the tuberculin skin test, milkborne disease in calves and a farm cat, and a risk of human infection. The outbreak was controlled by culling 221 cattle over 15 months, by using the γ-interferon (γ-IFN) test and by the examination of milk samples. The γ-IFN test detected infected animals that were not detected by the skin test.
 
Sally
... and from another fed up farmer (email 3.9.11) who is appealing against a test that was not undertaken properly.
 
He says that from what he has read it is wholly wrong (and not in the law or the Order) that negative tests can be ignored (and imputed as positive). Nor is it provided that two subsequent clear tests should be imposed where there is no actual sign of infection. It is wrong in logic that all bumps are treated by default as tuberculin reactions. It is unsound that a reaction to M.avium can be treated by DEFRA as a reaction to M bovis for the purposes of extending movement restrictions. Moreover, it is unsafe in law that unless all bumps are recorded that subsequent bumps are per se caused by a reaction to an injection at a SCTT test. It is also not in the law that where a culture is negative the AHVLA can proceed as if it were positive. It requires reasonable belief that there is an infection! The vets are misinterpreting or ‘gold plating’ the legislation!
 
He is appealing against the movement restrictions imposed as the one reactor identified was wrongly identified as such. The test was not carried out properly and a formal complaint has been made.
 
Sally
So here (www.thisisdevon.co.uk/President-aims-answer-bTB-cull-questions/story-13236670-detail/story.html) we have an indication that yet another farmer is not happy with the procedure regarding testing.
 
The article says:
 
'Meanwhile arguments about the cattle-testing methods have gone quiet – though the experiences of one Westcountry milk producer could be frighteningly common.
 
He is in the process of suing his own vet after registering a positive to the TB test for the first time ever, the vet having made the test on the animal's shoulder, for some reason, rather than on the neck, where it should have been done. The cow evidently was in a crush and was not causing a problem.
 
The animal was subsequently found to be clear, and the farmer remonstrated to the AHVLA about the situation, who told him that careful siting of the test on the animal was critical: "because the sensitivity of the test varies in different areas."
 
As he said: "Vets are paid to do the tests – and should do them properly. Farmers should be aware of this. They could be 'going down' with TB when in reality they are clear."'
 
becky
Email from Prof PT 16/8/11.
 
The skin test is a measure of the immunological response to infection. ie. a positive response tells you the animal has been exposed and mounted an immune response. That is not the same as being infected. However, the skin test as used in cattle is highly specific. The false positive rate is less than half of 1 percent of healthy animals. Indeed DEFRA do claim to find evidence of leisions or positive bacteria culture from most animals who skin test positive, so the test is probably quite reliable when it is positive....But the presence of lesions or even bacteria in the lesions does not mean the animal could not mount a successful immune response and clear the infection....
 
There is one theory that suggests that the present skin test is actually selecting for cattle who do not mount an immune response - all the cattle that response immunologically are culled, leaving the cattle that do not mount such a response infected and in the national herd...just a thought...
 
becky
Email from MR 16/08/11
The stock Defra excuse for slaughtering every animal failing a BTB skin test is caution (culling 'just in case' – however unreliable the test the animal just might be infected.
 
However, when humans were routinely given BCG they were tested using essentially the same skin test. The reactors were declared immune and not vaccinated. Many people are telling us about their memory of this in childhood, but none seem to have any recollection of any immediate or ongoing investigation for symptoms of TB in the reactors.
 
Surely if cattle are killed as a precaution, humans in the same circumstances would be examined for signs of actual TB and then kept under observation in the future, yet this does not seem to happen?
 
Is there an explanation for this apparent inconsistency?
 
becky
Media responses from Defra have regurgitated its usual responses regarding test reliability and has not even replied properly to some of the questions, including the one on costs. I find it hard to believe they can still claim the test was done properly when the court case confirmed it was not.
 
As usual Defra has failed to say that the test only indicates the animal has been exposed to the bacteria and (if we look at the human parallels) most would not be likely to succumb to the disease and are killed 'just in case'. It is a shame that more farmers do not challenge the testing as this would force change. Of course, the costs and stress preclude this for most farmers. The Jacksons were exemplary because Boxster is a champion and they are very fond of him but have obviously been put through hell and significant costs over the last 18 months - .
 
ww.fwi.co.uk/Articles/2011/08/16/128421/Animal-Health-defends-TB-test.htm
 
Sally
Email from PT 17/8/11
The judgement with BOXSTER essentially has set the precedent that any TB test that is not done according to the book can be challenged. So if any farmer has any evidence that a TB test on a "positive" animal was not done exactly right then they should challenge it. This means that farmers should ensure that every animal undertaking a skin test should have their skin measured by callipers every time. This vet got caught, but I know what he did is widespread practice. Vets are going to have to be very careful and work very slowly and methodically.... or they run the risk of a (positive) test being disregarded or even worse!
 
Sally
BOXY the champion 5 year old bull, after a 17-month wrangle over a bungled test for bovine TB, was formally declared free of the disease in August 2011. The result vindicates the Jackson family's decision to fight a slaughter order issued in April 2010 by the Leeds division of Animal Health, the veterinary service of the Department for Environment Food & Rural Affairs. The Jacksons successfully challenged the order on the grounds that the blood test was not carried out properly. Defra has spent a lot of taxpayers' money defending the mistake and could be paying out considerably more. In April this year, a High Court judge ruled that Defra had broken its own rules by mixing two blood samples and its diagnosis was invalid.
 
Defra argued in court that its breach of procedure was trivial and irrelevant. And it continued to hope and believe it would be proved right in the long run. But in June the bull passed a repeat of the skin test (which he had initially passed). Despite this the vets insisted on another blood test as well as a second skin test. The Jacksons objected for several reasons - not least because it meant trusting Defra to be objective in the privacy of its own laboratories. The results of the latest skin test was negative. "I am over the moon," said Mr Jackson. "The irony is that a second blood test is all I asked for in the first place."
 
He wanted a second opinion on the British Blonde bull because of the value of the animal and because he knew there had been some difficulty getting blood samples from it.
 
The wrangle has meant that Boxster has been out of action for nearly 18 months of the prime of his life. The cost of the sons and daughters he has not produced is one consequence of Defra's mistake.
 
Dermott Thomas, a spokesman for the Ipswich solicitors who handled the case, Barker Gotelee, said that an agreement had been reached with Defra on legal costs. Compensation was a possibility to be discussed.
 
Defra has confirmed: "This bull can be regarded as officially TB-free." Its statement defended its TB policy and made no apology for its behaviour in this case. But it said it would settle the Jacksons' legal costs within 10 days.
 
One effect of the case has been to make it more difficult for Defra to perform blood tests. It emerged in evidence that technicians often used two partial blood samples to make up one full one. But that is a breach of guidelines and the judge ruled that farmers were entitled to expect best practice.
 
 
Sally
Email from Prof PT 16/8/11
The skin test is a measure of the immunological response to infection. ie. a positive response tells you the animal has been exposed and mounted an immune response. That is not the same as being infected. However, the skin test as used in cattle is highly specific. The false positive rate is less than half of 1 percent of healthy animals. Indeed DEFRA do claim to find evidence of leisions or positive bacteria culture from most animals who skin test positive, so the test is probably quite reliable when it is positive....But the presence of lesions or even bacteria in the lesions does not mean the animal could not mount a successful immune response and clear the infection....
 
There is one theory that suggests that the present skin test is actually selecting for cattle who do not mount an immune response - all the cattle that response immunologically are culled, leaving the cattle that do not mount such a response infected and in the national herd...just a thought...
 
Sally
Email from M Ritchie 16/8/11
The stock Defra excuse for slaughtering every animal failing a BTB skin test is caution (culling 'just in case' – however unreliable the test the animal just might be infected.
 
However, when humans were routinely given BCG they were tested using essentially the same skin test. The reactors were declared immune and not vaccinated. Many people are telling us about their memory of this in childhood, but none seem to have any recollection of any immediate or ongoing investigation for symptoms of TB in the reactors.
 
Surely if cattle are killed as a precaution, humans in the same circumstances would be examined for signs of actual TB and then kept under observation in the future, yet this does not seem to happen?
 
Is there an explanation for this apparent inconsistency?
 
Loveyourbody008 (Guest)
The tuberculosis skin test is a test used to determine if someone has developed an immune response to the bacterium that causes tuberculosis (TB). This response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccine against TB (which is not performed in the U.S.). The World Health Organization estimates that 2 billion people worldwide have latent TB, while around 3 million people worldwide die of TB each year. The tuberculosis skin test is also known as the tuberculin test or PPD test.
 
The tuberculin skin test is based on the fact that infection with M. tuberculosis bacterium produces a delayed-type hypersensitivity skin reaction to certain components of the bacterium. The components of the organism are contained in extracts of culture filtrates and are the core elements of the classic tuberculin PPD (also known as purified protein derivative). This PPD material is used for skin testing for tuberculosis. Reaction in the skin to tuberculin PPD begins when specialized immune cells, called T cells, which have been sensitized by prior infection, are recruited by the immune system to the skin site where they release chemical messengers called lymphokines. These lymphokines induce induration (a hard, raised area with clearly defined margins at and around the injection site) through local vasodilation (expansion of the diameter of blood vessels) leading to fluid deposition known as edema, fibrin deposition, and recruitment of other types of inflammatory cells to the area.
 
An incubation period of two to 12 weeks is usually necessary after exposure to the TB bacteria in order for the PPD test to be positive. Anyone can have a TB test, and it can be given to infants, pregnant women, or HIV-infected people with no danger. It is only contraindicated in people who have had a severe reaction to a previous tuberculin skin test.
 
 
 
[url=http://www.allmedicals.com/s/active-tuberculosis]tuberculosis-latent[/url]
 
[url=http://www.allmedicals.com/s/active-tuberculosis]tuberculosis-resistant[/url]
 
[url=http://www.allmedicals.com/s/active-tuberculosis]pulmonary-tuberculosis[/url]
 
Sally
Email from MG dated 8/5/11
What ever the outcome of local Vet Wyn Lewis's disciplinary hearing (www.bovinetb.co.uk/article.php?article_id=88), the problems with the Bovine tb test will still be with us. Not only is the test expensive, disruptive and time consuming for farmers it isn't very good at detecting all the cattle it should. Defra state that the test is 99.9% accurate however nearly a million tests were carried out in Dyfed last year. This means almost 1000 cattle were falsely diagnosed. and killed. This was 20% of the total cattle killed due to Bovine tb in Dyfed last year.
 
Even worse, according to Defra, the test misses 20% of cattle with TB which means 1 in 5 are left in the herd to infect other cattle. When I told Elin Jones that the test inaccuracy was a major cause of reinfection in cattle, her only reply was 'well its the only test I have.' This is not good enough. Defra and WAG have spent tens of millions of pounds on the issue over many years and still no satisfactory test.
 
Cattle vaccine is available now but guess what,? we cannot use it because the Btb test cannot tell the difference between an infected cow and one that has been vaccinated. Until we have an accurate, simple to administer test, we will be unable to control bovine tb.
 
Farmer
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.
 
Sally
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?
 

 

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