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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.'
There are alternative tests for bovine TB but currently only the skin test is permitted under EU regulations. Of great interest is the report published in 2011: 'An Inter-Laboratory Validation of a Real Time PCR Assay to Measure Host Excretion of Bacterial Pathogens, Particularly of Mycobacteriumbovis'. Whilst this relates specifically to badgers it could also be used for cattle. It ca be read in full at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0027369
Advances in the diagnosis of Mycobacterium bovis infection in wildlife hosts may benefit the development of sustainable approaches to the management of bovine tuberculosis in cattle. In the present study, three laboratories from two different countries participated in a validation trial to evaluate the reliability and reproducibility of a real time PCR assay in the detection and quantification of M. bovis from environmental samples. The sample panels consisted of negative badger faeces spiked with a dilution series of M. bovis BCG Pasteur and of field samples of faeces from badgers of unknown infection status taken from badger latrines in areas with high and low incidence of bovine TB (bTB) in cattle. Samples were tested with a previously optimised methodology. The experimental design involved rigorous testing which highlighted a number of potential pitfalls in the analysis of environmental samples using real time PCR. Despite minor variation between operators and laboratories, the validation study demonstrated good concordance between the three laboratories: on the spiked panels, the test showed high levels of agreement in terms of positive/negative detection, with high specificity (100%) and high sensitivity (97%) at levels of 105 cells g21 and above. Quantitative analysis of the data revealed low variability in
recovery of BCG cells between laboratories and operators. On the field samples, the test showed high reproducibility both in terms of positive/negative detection and in the number of cells detected, despite low numbers of samples identified as positive by any laboratory. Use of a parallel PCR inhibition control assay revealed negligible PCR-interfering chemicals co- extracted with the DNA. This is the first example of a multi-laboratory validation of a real time PCR assay for the detection of
mycobacteria in environmental samples. Field studies are now required to determine how best to apply the assay for population-level bTB surveillance in wildlife.
Have anyone heard reports of reduced productivity amongst bovines subject to repeat TB testing, as a result of the tuberculin rather than the handling?
Many will probably be surprised to read the following press release from Defra (dated 2/5/13) - surely this information should have been made available years ago - but no, it wasn't so many farmers never knew what they could be buying in .... is it a case of shutting the stable door after the horse has bolted!
TB risk information about cattle is to be shared at the point of sale so farmers will know the animal’s TB testing history. This is being done to help them avoid bringing TB into their herd.
Under recommendations made by the industry-led Risk-Based Trading Group (RBTG) in England, crucial TB risk information about cattle will be shared at the point of sale so farmers will know the animal’s TB testing history before purchase. Defra and industry will work together to develop buyer and seller guidance on TB risk to help farmers make full use of this new information.
The RBTG has also recommended the development of a new national database for farmers, vets and auctioneers that will use more detailed TB risk information to help farmers assess the TB risk of cattle they are looking to buy.
Professor Bill Reilly, Chairman of the TB Risk-Based Trading Group, said:
We believe that the recommendations contained in our report are practical solutions that will be of real value to help reduce the spread of this terrible disease. We encourage industry and Government to work together to implement our recommendations.
The Risk-Based Trading Group was set up by Defra in July 2012 to make recommendations to Government on helping farmers in England make informed decisions on the TB risk of cattle they want to buy.
The Bovine TB Risk-Based Trading Group’s report to Defra: www.gov.uk/government/publications/bovine-tb-risk-based-trading-empowering-farmers-to-manage-tb-trading-risks
Farmers Weekly have today (www.fwi.co.uk/articles/05/02/2013/137488/tb-skin-test-questioned-after-false-results.htm#.URD1fq2kidE) reported on the following case. Interestingly several farmers have told us similar stories.
The accuracy of the bovine TB skin test has been questioned after a cow that tested negative for five years was found to be riddled with the disease. The cow was part of the Gelli Aur College Farm in Carmarthenshire, which has been under TB restrictions since 2007 and lost 300 cattle to the disease.
Interesting, back in 2009 here is a report about bTB at Geli Aur - you can draw your own conclusions!
TB devastates Welsh College's dairy performance
Robert Davies
Monday 19 January 2009 06:11
Bovine TB has had a devastating impact on milk production and profitability at Gelli Aur College in Carmarthenshire.
More than 250 cattle have been culled over the past 18 months, including 100 following a single test and farm manager John Owen admits the outbreak has also sapped staff morale.
He says it does not help when nothing is being done about possible wildlife reservoirs of infection, including a large number of badger setts and wild deer. But he is reluctant to take up the suggestion that the Gamma Interferon test should be used to pinpoint all infected cattle on the farm.
"We do not want to lose a large number of animals in one go as we want to maintain cash flow, have cows for teaching students and hang on to our quality staff."
The latest figures show that TB has hit the low input spring calving herd most severely. In December it numbered only 86 cows, 42% fewer than in the same month in 2007 and well below the planned total of 180.
The culling rate was 74% in the year to July 2008 and herd milk production declined by 7% to 660,000 litres. Overall the herd lost £42,161, or 6.4p/litre, excluding compensation for cattle culled because of bTB.
Six months later the £133,690 rolling herd margin over purchased feeds was 19% lower than in December 2007, though a higher average milk price had increased rolling margin/litre from 18.41p to 20.86p.
Mr Owen believes the average £1250 a head valuation of culled spring calving stock just about covers the cost of replacements, though sourcing clean cattle is proving difficult.
Farm manager John Owen said the level of infection in the fifth-lactation animal only came to light when it was slaughtered.
"We culled her at the end of her productive life and at slaughter she was found to be riddled with TB," said Mr Owen. "The level of infection in that cow suggests she had been infected for years."
He is concerned that the homebred cow may have been at the root of successive test failures.
"The cow kept passing the test, there was nothing to suggest that she was infected," he said. "It is a concern that she may have been spreading the disease to other animals within the herd."
The farm has just recorded its first clear test and will have its next test later this month. "We are keeping our fingers crossed that we will get the all-clear," said Mr Owen.
He admitted that six years of movement restrictions had affected the farm business significantly and had pushed the stocking rate to its upper limit.

Summary of Cambridge report (Defra funded research) which focuses on cattle-to-cattle transmission can be viewed at: www.fwi.co.uk/Articles/19/10/2012/135812/Scientists-reveal-the-hidden-burden-of-bovine-TB.htm Interesting extracts below.
Research suggests that around one in five British herds that have been cleared of movement restrictions after testing for bovine tuberculosis may still be harbouring the infection.
The new research reveals that TB testing misses many animals harbouring the disease and shows that large herds are particularly vulnerable to rapid transmission.
The study, by scientists at Cambridge University, points to a high level of hidden infection in herds - up to 21% - that have been passed as TB-free.
It also strongly suggests that larger herds suffer not just a higher incidence of the disease, but also a faster rate of spread between cattle.
"Many aspects of bovine TB, and its transmission, remain a mystery - and it's long been known that the protocol and testing methods used to diagnose the disease are far from perfect," said James Wood, Alborada professor of equine and farm animal science at Cambridge University.
"One of our models suggests that up to 21% of herds may be harbouring infection after being cleared from movement restriction
Since the 1950s, diagnostic testing has been based on reactivity to tuberculin, administered as an injection into the animal's skin, which responds with a lesion if the animal is positive.
But many scientists consider the current testing method used to diagnose bovine TB, the tuberculin skin test (TST), as unreliable
The models provide a first estimate of the quantity of infection missed by cattle testing and the contribution of this "hidden burden" of infection to the persistence of bovine TB within herds.
They also estimate a high rate of re-introduction of infection into herds, particularly in high incidence areas.
Andrew Conlan, a researcher in Cambridge's disease dynamics group, who specialises in mathematical modelling, said that around 38% of herds that are cleared of bovine TB experience a recurrent incident within 24 months, suggesting that infection "may be persisting in herds".
"The intensity of surveillance in the UK means that animals rarely progress to be sick, with the only evidence of disease being a positive result to testing, but this is known to miss some cases," added Dr Conlan.
This article in Farmers Weekly says that Defra will make more farmers' lives a misery by dragging the two and three year testing areas into annual testing, starting Jan 2013.

'Estimating the Hidden Burden of Bovine Tuberculosis in Great Britain'
Abstract from the Cambridge paper referred to in the above post.
The number of cattle herds placed under movement restrictions in Great Britain (GB) due to the suspected presence of bovine tuberculosis (bTB) has progressively increased over the past 25 years despite an intensive and costly test-and-slaughter control program. Around 38% of herds that clear movement restrictions experience a recurrent incident (breakdown) within 24 months, suggesting that infection may be persisting within herds. Reactivity to tuberculin, the basis of diagnostic testing, is dependent on the time from infection. Thus, testing efficiency varies between outbreaks, depending on weight of transmission and cannot be directly estimated. In this paper, we use Approximate Bayesian Computation (ABC) to parameterize two within-herd transmission models within a rigorous inferential framework. Previous within-herd models of bTB have relied on ad-hoc methods of parameterization and used a single model structure (SORI) where animals are assumed to become detectable by testing before they become infectious. We study such a conventional within-herd model of bTB and an alternative model, motivated by recent animal challenge studies, where there is no period of epidemiological latency before animals become infectious (SOR). Under both models we estimate that cattle-to-cattle transmission rates are non-linearly density dependent. The basic reproductive ratio for our conventional within-herd model, estimated for scenarios with no statutory controls, increases from 1.5 (0.26–4.9; 95% CI) in a herd of 30 cattle up to 4.9 (0.99–14.0) in a herd of 400. Under this model we estimate that 50% (33–67) of recurrent breakdowns in Britain can be attributed to infection missed by tuberculin testing. However this figure falls to 24% (11–42) of recurrent breakdowns under our alternative model. Under both models the estimated extrinsic force of infection increases with the burden of missed infection. Hence, improved herd-level testing is unlikely to reduce recurrence unless this extrinsic infectious pressure is simultaneously addressed.
Email dated 18/10/12 from Prof PT.
As I have said before - I think about 25,000 cattle test positive (by the skin test) each year in the UK. Assuming the test is close to 100% specific (actually about 99.7% specific) and 80% sensitive (DEFRA would agree on this). That means that there are around 6000 cattle which have bTB which test negative EVERY year. And they will not be the same 6000 each year as not every herd is tested each year, so the true number of infected bTB cattle at any one time not being detected will be somewhat higher. The main danger with these false negatives is in two area. First in herds where the rest of the herd is also negative so they can be traded and spread bTB around the country. Secondly in those herds under restrictions because at least one animal had a positive test. There will also be false negatives, not removed from the herd which continue to infect other animals... this will mean that the disease may remain on the farm, even if the herd then has a clean test - not really surprising but it is good to see academics confirming this. Whole herd slaughter would in fact deal with this as you correctly point out, but then that would mean killing a lot of cattle (25% of herds in the SW) and of course is totally unacceptable.
So I agree much of the problem is probably caused by the 6000 and not having a herd slaughter policy....you need a much more sensitive test to have a policy designed at the individual animal....
But I think we all know the present policy is not working and has not worked for a long time.
We now have more Defra-funded) research (www.cam.ac.uk/research/news/scientists-build-a-clearer-picture-of-the-spread-of-bovine-tuberculosis/) confirming just how unreliable the skin test is - the skin test is the backbone of the existing test/cull system currently used to 'eradicate' bTB.
Scientists at Cambridge have used mathematical tools to develop models for estimating the efficiency of cattle-based controls for bovine tuberculosis (bTB).
The models built by the team represent an advance over previous models as they are informed directly by extensive data on reported incidence and spread of the disease, rather than expert opinion. Importantly, they provide a first estimate of the quantity of infection missed by cattle testing and the contribution of this hidden burden of infection to the persistence of bTB within herds.
Applied to recent data, the models suggest that around one in five of British herds that have been cleared of restrictions, following testing for bTB, may harbour the infection. It also points to a higher incidence, and faster spread, of the disease in large herds. The research focuses on the cattle-to-cattle transmission of bTB within herds.
“Many aspects of bTB, and its transmission, remain a mystery – and it’s long been known that the protocol and testing methods used to diagnose the disease are far from perfect. One of our models suggests that up to 21 per cent of herds may be harbouring infection after being cleared from movement restrictions,” said James Wood, Alborada Professor of Equine and Farm Animal Science at Cambridge University.
“However, our models also estimate a high rate of re-introduction of infection into herds, particularly in high incidence areas. This rate of re-infection is high enough that even if improvements in testing eliminated the hidden burden of infection, rates of recurrence would not be reduced.”
An article by farmers Jolyon and Alex Higgs in Farmers Weekly today will probably strike a chord with many cattle owners having to suffer the zero tolerance policy in Wales. They refer to the unhelpful and threatening letters received from the AHVLA.
The article is re-printed below.
On Gower we rear bucket calves from dairy herds, and during the past year have carried out 37 follow-up TB tests on calves, whose original herd has gone down with TB. This meant at least 10 separate testing days.
We are happy to carry out these tests, but feel that the Animal Health and Veterinary Laboratories send out notices that are unnecessarily threatening.
Last October, a full clear test was carried out on time. However, we still received a notice prohibiting the movement of bovine animals because the computer had not been updated.
At the end of the month, another restriction was imposed for failing to test a calf that was ineligible to be tested as it had been tested in the previous 60 days.
In March, a further notice was served on us for failing to test a dead calf.
In the past six weeks, a new computer system has been set up linking AHVLA with our vets. We are now receiving 'Zero Tolerance Advisory Letters', one month before a TB test is due, if a test date has not been booked with the vet.
As the AHVLA is not infallible, reminders sent out to farmers – who have not yet done anything wrong – could perhaps be worded less severely.
Finally, we recently asked to postpone a test, due on October, on four calves – two of which have been on our holding over a year – until our full herd test, which is due on 22 October. This was not allowed.
Excellent letter from a farmer published in the Independent that sums up the situation very well (www.independent.co.uk/voices/letters-badger-culling-is-emotive-but-debate-over-scientific-evidence-is-ignoring-established- facts-8182022.html)
www.independent.co.uk/voices/letters-badger-culling-is-emotive-but-debate-over-scientific-evidence-is-ignoring- established- facts-8182022.html
'We've allowed the facts about bovine TB to be buried by the furore surrounding the proposed badger cull and negative reporting on the part of farming bodies has painted a picture of an entire cattle industry brought to its knees by the effects of the disease. But this is misleading. Defra reports on its website that "11.5 per cent of herds were restricted in 2011". They could equally report the positive side, which is that 88.5 per cent of herds were not restricted in 2011. Only a small proportion of the national herd is affected by bovine TB.
The impact on an infected herd is not caused by the disease itself. The heartache is caused by the "test and cull" policy deployed by our government in response to an EU directive which demands eradication of bTB and simultaneously bans the use of cattle vaccine, thereby making it impossible to achieve the goal they set.
But rather than tackle the EU to allow cattle vaccination, ministers have sanctioned a mass badger-shoot to placate a minority of vociferous farmers who seem hellbent on decimating the badger population. The fact that a badger cull spells disaster in PR terms for the entire farming industry must have eluded them all'.
G E Purser (A badger-friendly farmer), Clapton-on-the-Hill, Gloucestershire
From the Alpaca website at http://www.alpacatb.com where alpaca owners are having to fund their own research, we have the initial results of the pilot study re PCR testing alpacas. The skin test is even less reliable for camelids.
The pilot project demonstrated that a two-stage PCR test on camelid clinical samples has the potential to detect Mycobacterium bovis with reasonable sensitivity. Samples were taken post-mortem from 21 alpacas with gross lesions of tuberculosis, most of the alpacas selected had severe lesions. Fifteen faeces samples were positive and ten nasal swabs were positive. Samples that were negative in the PCR tests were from alpacas with less severe pathology
We also saw research into PCR testing for badgers at (http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0027369) is the first example of a multi-laboratory validation of a real time PCR assay for the detection of mycobacteria in environmental samples ...
www.idexx.com/view/xhtml/en_us/livestock-poultry/ruminant/bovine-tuberculosis.jsf Details of the test the subject of the last posting is as follows:
Control of bTB is complicated by the lack of sensitive tests. Although tuberculosis tests based on cell-mediated responses (skin and gamma interferon) can detect animals in the early stages of infection, they still fail to detect up to 20% of truly infected animals. Using antibody testing for bTB control programs can significantly improve the detection of bTB-infected herds. In bTB-negative areas, antibody testing can be used on other sample streams as an easy, cost-effective surveillance tool.
The IDEXX M. bovis Ab Test is intended for the detection of Mycobacterium bovis (M. bovis) antibody in cattle serum and plasma samples. Adding IDEXX M.bovis Ab test to bTB control programs will increase detection by identifying infections other tests miss. This test is a robust and reproducible tool that offers a fast, 2-hour protocol for objective, quantitative results. IDEXX easy-to-use M.bovis ELISA format requires no specialized training or handling requirements and can be repeated as often as necessary, allowing more rapid bTB control.
'IDEXX announced that the World Organization for Animal Health (OIE) has added the IDEXX M. bovis Ab Test to its registry of tests. OIE develops and maintains international standards for diagnostic tests and vaccines, and this marks the first time the organization has certified an antibody test for the detection of bovine tuberculosis (TB). Based on a stringent independent review, the IDEXX M. bovis Ab Test was validated and certified by the OIE as fit for the purposes defined in the kit insert.'
'Released in 2011, the IDEXX M. bovis test offers a "fast, three-hour protocol that delivers objective, quantitative results compatible with typical laboratory data management systems," IDEXX said. Its "easy-to-use" ELISA format requires no specialized training or handling requirements and can be automated for high-volume use. The test also can utilize serum or plasma samples already collected for other disease surveillance purposes.'
“The IDEXX M. bovis test is already used in several countries, including Chile and Ireland,” said John Lawrence, Senior Research Scientist, IDEXX Livestock and Poultry Diagnostics. “We’re also participating in trials to gain specific country-level approvals, as well as undergoing research studies in France, the United Kingdom and New Zealand.”
More info from idexx.com/mbovis.
'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.
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.
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.
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.

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.
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.'
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.
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!
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?
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.'

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.
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.

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