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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.'
Bovine tuberculosis testing
The Ecologist asks, 'Are the shortcoming of existing cattle TB tests soon to be exposed by a simple, clever blood test that has been waiting in the wings? The development may shine light on practical compromises in the extended, failing fight against TB in England - at huge tax-payers expense'.
Biologist TOM LANGTON looks at a growing dilemma in the world of bovine TB cattle testing.
Bovine tuberculosis testing
The Ecologist asks, 'Are the shortcoming of existing cattle TB tests soon to be exposed by a simple, clever blood test that has been waiting in the wings? The development may shine light on practical compromises in the extended, failing fight against TB in England - at huge tax-payers expense'.
Biologist TOM LANGTON looks at a growing dilemma in the world of bovine TB cattle testing.
As bovine TB is said to be spreading across the UK (and more badger cull areas have been agreed), it is no secret that the new government, like the previous is in turmoil over its response.
Chief Scientist Ian Boyd and now Chief Vet Nigel Gibbons are both leaving their roles.
Not specifically because spread of a disease that they have been unable to stop, but their time is up and they can slip away from the deteriorating situation. Interestingly both made it plain at the March 2017 London bTB conference that industry behaviour is holding progress back.
Read the full article. A history of the disease in the UK and alarming facts about existing controls. but will the new blood test bring the required results? What about vaccination - again this option seems to have disappeared from the agenda.
An East Anglian bio-tech firm has pioneered a new test for bovine tuberculosis (TB), which it claims could revolutionise the speed and accuracy of disease diagnosis.
PBD Biotech, based in Thurston, near Bury St Edmunds, has secured £200,000 of investment to commercialise its diagnostic kit, which detects the presence of disease-causing mycobacteria in blood and milk samples.
Founder and chief executive Dr Berwyn Clarke said the current testing method – which can take days to assess an immune response from the animal plus six weeks of laboratory work to confirm inconclusive results – is only 75pc accurate.
By contrast, he said his test could provide definitive, accurate results within six hours, which could save time money for farmers, and reduce animal stress.
He is in the process of securing the necessary approvals from animal health regulators, but said the first commercial kits could be ready as soon as October.
The new TB test could potentially bring major benefits to cattle farmers – but first it must win regulatory approval.
Shipdham dairy farmer Ken Proctor is a Norfolk representative on the Bovine TB Eradication Advisory Group for England (TBEAG).
After attending a meeting at the APHA (Animal and Plant Health Agency) laboratories in Weybridge in Surrey, he said: “It is one of several tests the APHA is watching with great interest. The problem at the moment is that it needs OIE (the World Organisation for Animal Health) approval.
Validation data presented today by researchers at the 4th Congress of the European Association of Veterinary Laboratory Diagnosticians (EAVLD) shows that Thermo Fisher Scientific’s VetMAX M. tuberculosis Complex PCR kit is a reliable tool to confirm the presence of mycobacteria belonging to the M. tuberculosis complex. Development of the new real-time PCR kit extends the company’s portfolio of tests for the detection of bovine tuberculosis (bTB). When used in combination with the company’s existing bTB portfolio, consisting of reagents for skin tests and IFNγ release assays (Bovigam), the newest solution provides veterinarians with an effective combination of tests for their bTB screening programs.
Dr. Jean Louis Moyen, Director of the Regional Analysis and Research Laboratory of Dordogne, France, who was among the scientists to present data at the conference, said: “We have tested positive and negative field lymph node samples from cattle, wild boar and badger with the VetMAX M. tuberculosis Complex Real-Time PCR kit in our laboratory as part of the validation study. The test showed excellent diagnostic sensitivity and specificity with 87 out of 88 infected lymph nodes correctly identified and no false positive results in the 284 samples tested. Implementation of the test in the lab was really easy.”
Enhanced Detection
The new PCR test detects all seven strains belonging to the Mycobacterium tuberculosis complex (i.e. M. tuberculosis, M. bovis, M. africanum, M. microti, M. canetti, M. caprae, and M. pinnipedii) and does not detect 42 other tested related pathogens. It includes a ready-to-use master mix and uses the Xeno internal positive control in a single well, duplex PCR approach.
“In the context of increasing TB prevalence, the field needs diagnostic tools that are easy to handle and provide sensitive, reliable and fast results to help ensure the efficacy of surveillance and control programs,” said Martin Guillet, global head and general manager of AgriBusiness at Thermo Fisher. “The results we present at the EAVLD show that this kit meets these expectations and is in line with our mission of enabling our customers to make the world healthier, cleaner and safer.”
The PCR-based kit is designed to reduce costs for the farmer and government-funded programs by delivering results typically in three hours instead of several weeks with bacterial culture, thus limiting the spread of infection while also reducing labor in the lab.
“The eradication of bTB is complex and the flexible application of testing schemes will help avoid unnecessary culling and lengthy farm closures, as well as help eliminate the occurrence of bovine TB worldwide,” Guillet said. “We feel this can shorten the overall length of a TB program in a country.”
The VetMAX M. tuberculosis Complex PCR kit is currently in development with registrations pending.
The European Commission has told anti-badger cull campaigners it backs investment in better tests for TB in cattle to help eradicate the disease.
Dominic Dyer, CEO of the Badger Trust and joint founder of a new Europe-wide badger protection group, said talks held with EU Commissioner and Acting Head of the Commission's Directorate for Heath and Food Safety Bernard Van-Goethem, should have brought the introduction of the gamma interferon blood test closer.
Mr Dyer, who reported back from the meeting in Brussels this week, said: “When pressed on the issue of gamma interferon testing for cattle, Bernard Van-Goethem confirmed that the European Food Safety Authority had found this test method when used in conjunction with the TB skin test, could result in the identification and removal of more TB infected cattle from herds.
“He confirmed that the Commission was happy to see EU funds being used for greater use of gamma interferon testing, although it is yet to be approved for TB free disease free status within the EU or the World Organisation for Animal Health.”
New bovine TB test being trialled
A DORSET vet is part of a team pioneering a new bovine TB test which could transform how cattle are assessed.
Alastair Hayton, of Synergy Farm Health, which is based at Evershot, said the new blood and or milk test – which has just been given the thumbs up by Defra to validate – will be significantly cheaper than the current interferon test, and is hoped will significantly aid the detection of bovine TB, especially in endemic TB herds, when used in conjunction with the current tests. The expectation is that it should help herds return to TB free status more quickly, as well as aiding in other areas such as pre or post purchase or movement testing.
The fact that there is the potential that milk could be tested to determine if an animal has the disease opens up a far easier method for farmers and vets to test cows.
Mr Hayton said: “We are pretty confident that the test can deliver what we need it to do, previous work suggests that it has the potential and we are hoping that the new form of the test will perform even better.”
The team includes experts from Scotland and Ireland. The test works by measuring antibodies to bovine TB, unlike the two tests which are currently used, the skin and interferon tests, which measure the cattle’s cellular response. The tuberculin skin test, used across Europe, works by injecting a small amount of tuberculin into the skin of an animal. If an animal is infected, the immune system will react and cause swelling a few days later.
Mr Hayton said: “The immune system can be described as having two arms. The first arm relates to cell mediated immune responses and the second arm relates to antibody related responses. It would make sense when detecting disease to use methods that will look for both areas rather than focusing purely on cell mediated responses, even if this side of the immune response to TB is considered to be the most important. This being particularly the case in bovine TB where we recognise that cell mediated responses can wane in the face of disease such that when chronically or severely infected cattle are tested with the skin test or interferon test they can fail to respond to the current tests.”
According to research by the University of Cambridge, the TB test currently used in Britain could miss as many as one in five TB infected cattle.
Mr Hayton said: “It is still in the late development stage but we have been told by Defra we can move towards validating it which is great news and hopefully we can move quickly to getting the data we need for it to be accepted at the EU and UK level.
“This is a big thing for the industry. You don’t get new bovine TB tests every day. We’ve had great support from the NFU in helping us to get where we are. We are looking at being on the cusp of going out and sampling cows and we will be looking for help from farmers in the next stage, as we will need permission to use the test on their animals and to have access to the follow up data.”
The new bovine TB testing contracts are being allocated to vets. Whilst TB testing remains such a lucrative part of veterinary business is it any surprise that vaccination is not being brought in for cattle?
Over a fifth of infected cattle found at slaughter? The skin test is not adequate. When will cattle vaccination be an option? Yet again badger culling has taken attention away from the most viable solution.
The Express carried an exclusive story yesterday (http://www.express.co.uk/news/uk/429440/EXCLUSIVE-Government-Chief-Vet-admits-he-can-t-define-what-s-a-humane-kill-of-badgers) proclaiming Government Chief Vet admits he can't define what's a 'humane' kill of badger. This beggars belief when the government has been stressing that the reason for the two ongoing badger culling trials are to test for how humane free shooting is.
The Government's Chief Veterinary Officer admitted there is “no definitive criteria” for measuring how humane the current pilot operations are.
Express Online has obtained a copy of a letter written from chief vet Nigel Gibbens in which he has admitted that ministers will have no hard and fast rules on what constitutes a humane kill when they come to decide whether the pilot badger cull has been a success.
The Department for Food and Rural Affairs (Defra) has previously insisted any decision to roll the pilots in Somerset and Gloucestershire out nationwide will significantly depend on whether killings have been humane.
A letter sent by Mr Gibbens to the Humane Society International UK on Thursday in which he tried to outline how the cull will be assessed said:
“Ministers will have access to advice from individuals with expertise in these areas [animal welfare and veterinary pathology].
“The independent panel includes individuals with such expertise, who will assess the results of the monitoring and report to Ministers.
“There are, however, no definitive criteria for determining humaneness in this context.”
Also disturbing from reading the excellent article, 'What I have learned about the badger cull', by a county councillor in Somerset, Mike Rigby, 'the marksmen are responsible for selecting which carcasses to forward for assessment. They’re hardly going to send in botched jobs for analysis are they?'
Green Party spokesman on animal issues Caroline Allen, who is a vet, demanded an immediate halt to the cull.
She said: “They don’t know how they are going to measure humaneness, so they are not going to be meeting a key objective of the cull.
“I think as the Government’s chief vet you have to take the welfare of the animals very seriously, and it seems he has neglected to do that.”
Genetic Predisposition to Pass the Standard SICCT Test for Bovine Tuberculosis in British Cattle (full report, published March 2013, can be accessed at www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0058245)
Bovine tuberculosis (bTB) imposes an important financial burden on the British cattle industry, yet despite intense efforts to control its spread, incidence is currently rising. Surveillance for bTB is based on a skin test that measures an immunological response to tuberculin. Cattle that fail the test are classified as “reactors” and slaughtered. Recent studies have identified genetic markers associated with the reaction of cattle to the tuberculin test. At marker INRA111 a relatively common ‘22’ genotype occurs significantly more frequently in non-reactor cattle. Here we test the possibility that the putative protective ‘22’ genotype does not confer resistance but instead causes cattle that carry it to react less strongly to the prescribed test, and hence avoid slaughter, potentially even though they are infected. We show that, after controlling for age and breed, ‘22’ cattle react less strongly to the immunological challenge and may therefore be less likely to be classified as a reactor. These results highlight the potential discrepancy between infection and test status and imply that the effectiveness of the test-and-slaughter policy may be being compromised by selection for cattle that are genetically predisposed to react less strongly to tuberculin.
Bovine TB Research Shows Promising Results
USDA scientists with the Agricultural Research Service are striving to improve tests and vaccination methods to overcome obstacles that prevent the eradication of bovine tuberculosis in cattle.
At the National Animal Disease Center in Ames, Iowa - scientists are developing better tests to help producers identify and remove TB-infected cattle from herds and keep healthy animals.
Veterinary Medical Officer Ray Waters says the tuberculin cattle skin test has helped eradication efforts - but has drawbacks - like a 72-hour waiting period for results.
Interferon-gamma release tests require live white blood cells that must be processed quickly.
Waters says traditional serum tests would be more convenient and less expensive.
Scientists have demonstrated that improved antigens - substances that cause the immune system to produce antibodies against foreign bacteria - are crucial in developing effective serum tests.
The findings were instrumental in the recent development of a new serum TB test.
Microbiologist Tyler Thacker - also collaborating on this effort at USDA - has developed another type of test based on polymerase chain reaction analysis of DNA.
This new test detects the causative agent of bovine TB in fresh tissues.
USDA says the test is quicker, accurate and helps distinguish between the causative agent of bovine TB and environmental mycobacteria that can cause false-positive results.
Info from: www.kmjnow.com/09/10/13/Bovine-TB-Research-Shows-Promising-Resul/landing.html?blockID=711987&feedID=806
Email from MG 5/6/13 draws out attention to the vervatum discussion that arose to the following extract from the EFRA report - '(Para 77) 'The mandatory skin test has a sensitivity of between 60-80% - i.e. it can miss up to 40% of infections. Johne’s disease, liver fluke, pregnancy and even the diligence of the tester can affect the result of the skin test'.
Q243 Chair: How accurate do you think the current tuberculosis skin test is?
Carl Padgett: We can go from the studies that Defra have the figures for. If we look at sensitivity, spotting those animals that are affected, there is a range of 60% to 80%. The more you undertake the tests-on lots more animals, on a full herd, for example-the sensitivity goes up because the more it is repeated, the more chance there is of finding the infected animal, so sensitivity increases. It is highly specific so you do not get many false positives with that particular diagnostic test.
Q244 Chair: The Commission said yesterday that, in their view, they thought it was 99% effective. You are saying 80% effective.
Carl Padgett: Specificity is 99.9% or greater. That means that, if you took 100 negative animals, 99 of them would come back as truly negative and one false positive might occur, to agree with a 99% figure.
Q245 Chair: Can the result be affected by another disease in the animal or the fact that the animal was pregnant? Would that have an implication for the skin test?
Carl Padgett: A number of infections have been looked at. There is certainly speculation about the role of bovine viral diarrhoea (BVD, which alters the immune status of the herd involved. So, are we getting the same reaction to the skin test when it is effectively an immune reaction that we are testing for? There is some work from the University of Liverpool recently that indicated the role of liver fluke in reducing the sensitivity of the test.
Q246 Chair: Do you think that non-reactive cattle are a source of transmission of TB?
Carl Padgett: With the characteristics of the test we have at the moment, it is highly likely that some herds have infected animals left at the end of the testing procedure, yes. Indeed, some work from Cambridge at the moment is looking at that. They published something saying along of the lines of one in four or five cleared herds may still have infected animals in them.
Q247 Chair: Do you think the skin test should be used in conjunction with other tests, such as the ancillary gamma interferon diagnostic test?
Carl Padgett: I think we have to try to deploy it as best we can within the armoury of other diagnostic tests, yes. One thing we need to remember about the skin test is that it has served worldwide, although in slightly different patterns and protocols for use, as a very good diagnostic and eradication tool. If we look at lowinstance areas of the country at the moment, it serves well there in the eradication of infection and the little foci of infection that appear. So it is a very good test that can actually eradicate the disease when we are dealing with the infection in focused elements of individual herds.
Q248 Barry Gardiner: It must be an awful thing to have to tell a farmer that one of their herd has reacted as positive. It can really devastate that farmer’s livelihood, can it not?
Neil Blake: Yes, it can. It is a difficult conversation to have every time it happens. Unfortunately, in the last 10 years or so, it is happening with increasing regularity. Yes, it is always a difficult conversation and I suspect Farm Crisis Network, for example, are a lot busier nowadays than they used to be.
Q249 Barry Gardiner: That means that the pressure placed on vets not to report, particularly where the results are borderline, is a real pressure. It is a human pressure.
Neil Blake: With every disease on a farm you are working with farmers to try to alleviate the problems associated with that disease. TB is a statutorily controlled programme. You apply the test diligently and we are required to notify where we discover disease. That is what we endeavour to do. So, irrespective of the difficult conversations we have to have, we are pretty well trained in terms of delivering that bad news.
Q250 Barry Gardiner: You will be aware of the practice on Anglesey last year where they were suspended from testing pending the completion of retraining of the staff, because the audit of their testing had shown that they were not being as rigorous as they ought to have been.
Carl Padgett: I am not aware of the specifics of the case as such, but I understand the generality. Yes, quality assurance of TB testing is a major issue and we have to have measures and procedures in place that ensure the test is being undertaken to the best of the veterinary surgeon’s ability to give the best results. We have a population of veterinary surgeons who are no different from any other human beings. Some people unfortunately, as with any system, may cut the odd corner or two, or feel that they have the most expeditious route of delivering the test. We therefore have to have an independent quality control mechanism, developed more robustly and delivered to ensure that veterinary surgeons are testing to the best of their ability. My own belief is that most of them are.
Q251 Barry Gardiner: This is a very sensitive area, and I am trying to ask these questions sensitively because I respect the job that these guys have to do and it is a tough one. Wales has adopted an auditing programme. Should we have an audit of tests in England to ensure there is not an underreporting of TB reactors by vets?
Carl Padgett: We should have an auditing system in place, and indeed we are involved with helping AHVLA develop one that makes sure testing is done properly, rather than for the avoidance of underreporting.
The Environment, Food and Rural Affairs Committee has published its report today on progress to develop a vaccine solution for bovine TB. In it it is scathing about the imperfect test. Bear in mind this test forms the backbone of existing policy ....
'We currently rely on a skin test that could miss one in four infected cows. Liver fluke, Johne’s disease and even pregnancy may impact on the result of the skin test. While the skin test has served us well, if other more sensitive tests exist, they should be employed alongside it. We accept that the gamma interferon blood test is expensive and not without limitations, but a test that catches infection earlier when animals are less likely to have transmitted the disease is a valuable tool and one that should be deployed as widely as possible. The Government should explore whether it is possible to improve the performance of the test and bring down its cost.'
'Farmers must be able to reassure themselves that the livestock they bring onto their farms are free of TB. We remain convinced that, alongside enhanced bio-security and movement control, improvements to the testing regime can deliver real benefits.'
''In 2012 alone more than 8 million tests were carried out on cattle in Great Britain and 37,754 cattle were slaughtered. According to Defra, there are approximately 10 million cattle in the UK: ‘a little over two million adult dairy cows in the UK, a little under two million adult beef cows, and about six million younger animals’. In 2012 it is estimated that 1% of the dairy herd was slaughtered because of bovine TB.''
(Para77) 'The mandatory skin test has a sensitivity of between 60-80% - i.e. it can miss up to 40% of infections. Johne’s disease, liver fluke, pregnancy and even the diligence of the tester can affect the result of the skin test'.

It is inevitable that increased bTB testing (necessitating handling cattle for two days each time), brings greater risks to human health than any risk of catching bTB.
We have been given details of the seven incident reports received by the Health and Safety Executive (HSE) involving injury as a result of bTB testing over the last few years. It is stressed that there may well be more as the search undertaken relied only on the use of the term 'TB'. In addition the HSE tends to deal only with employees - the self employed, we are told, are responsible for their own health and safety. Brief details are set out below.
There was no incident report for the Pembrokeshire farmer who died during TB testing earlier this year - because this case is still under investigation.
08/05/2009 a vet, whilst undertaking TB testing on a herd was pushing cow into crush from behind when another cow behind her kicked out causing lacerations.
19/05/2009 a vet - steer escaped during a TB test, charged and kicked right chest and shoulder.
19/06/2009 a vet - right arm, fracture to radius whilst reading TB test. He entered the pen where some cattle were being held and without warning one of the cattle kicked her right arm throwing her across the pen - she landed on her back.
11/04/2010 animal health officer, during TB testing a cow his arm was trapped between the animal and crush when he was trying stik a needle into the animal. - notes say possible broken are 'due to return to hospital to have an X Ray to determine whether or not it is broken'.
13/06/2011 farm worker kicked in abdomen during TB test.
'06/08/12 down as a major injury - he went to hospital. Farmer -'we were trying to put the cattle in the cattle crush ti do testing and one of the young heifers kicked out which made contact with the injured person on his back and he fell down. The heifer then tried to move away, on doing so it stood on the injured person's ankle.
31/01/2013 vet was undertaking TB test some cattle attempted to break out past the vet (herding them from the handling yard to shed where testing was taking place) who attempted to stop them and turn them around but he was knocked/fell to the ground and dislocated a shoulder.
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 ...

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