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Bulletin No.42 - TB - Defra Policy

AN OPEN LETTER FROM DEFRA TB POLICY TO BAS/BLS MEMBERS AND ALL KEEPERS OF SOUTH AMERICAN CAMELIDS IN ENGLAND
 
Below is an open letter from DEFRA explaining how they intend to move forward now that the results of the research that BAS, BCL and BLS sponsored with the AHVLA on the effectiveness of several blood tests for bTB has been considered. The DEFRA position has been reached following consultation with the 3 Societies and the TB Research and Support Group. As you will read, DEFRA is going to use a combination of the blood tests to aid in the rapid identification of camelids that are highly likely to have bTB in situations where a herd has had a proven case of TB (proven by post mortem and tissue culture).
 
In addition DEFRA is proposing that private testing of camelids using the blood tests (prior to movement or export say) could be authorised in the future and once agreed with the devolved governments of Scotland and Wales. We have not been given costs of carrying out private testing.
 
Those of you that wish to look at the VLA research report can find it on the BLS website at
 
http://www.britishllamasociety.org/webfiles/SAC.pdf

LIZ BUTLER
Vice Chairman BLS & Health & Welfare Representative
Nutfield Park Farm, South Nutfield, REDHILL, Surrey Tel: 01737-823375



Shaun Daniel BAS Liz Butler BLS
24 September 2012

Dear Shaun and Liz

AN OPEN LETTER FROM DEFRA TB POLICY TO BAS/BLS MEMBERS AND ALL KEEPERS OF SOUTH AMERICAN CAMELIDS IN ENGLAND

We are aware that some of your members may have become confused and anxious as a result of comments circulating within your sectors regarding the research commissioned by BAS, BLS and British Camelids Ltd into blood tests for the diagnosis of tuberculosis (TB) in camelids. We hope, therefore, that you can consider circulating the following to all your members with the aim of reassuring them about the facts behind these important issues.

What is the status of the research undertaken in 2011-2012 for BAS, BLS and British Camelids Ltd?
The study in question was completed earlier this year. It was led by the TB immunology team in the Animal Health and Veterinary Laboratories Agency (AHVLA) at Weybridge, a research group of international repute. This work assessed the key performance characteristics of a new cell-based interferon-gamma blood test and four existing antibody blood tests (Chembio’s STAT-PAK and DPP, the IDEXX’ ELISA and the ENFERplex) for the diagnosis of TB in live South American camelids. The research has been independently peer-reviewed and will shortly be published in a respected scientific journal
1.
1 Clinical and Vaccine Immunology, journal of the American Society for Microbiology. The paper was accepted on 12th August 2012 and is now in press.
The diagnostic sensitivities (i.e. the proportion of truly infected animals correctly classified as positive reactors to these tests) were determined using alpacas with typical TB lesions identified at post-mortem examination. These animals had been culled from culture confirmed TB breakdown herds and, although visibly infected at post-mortem, they had given a negative result on one (or more) tuberculin skin tests.
The diagnostic specificities (i.e. the proportion of animals free from infection correctly classified as negative) of the blood tests were estimated by sampling presumed TB-free alpacas from volunteer herds in GB with no history of
M. bovis infections and situated in areas of low bovine TB incidence. A small number of serum samples from alpacas in TB-free herds in the US were also included in the study.
More details about diagnostic sensitivities and specificities for individual blood tests and combined tests can be found in the study report, which we understand is available on the BLS website.

What is Defra planning to do with the results of that study?
The AHVLA study highlighted that the TB antibody tests have a moderate sensitivity (around 70%), with a very high specificity (approximately 97%). This means that the probability of false positive results is very low (3 for every 100 animals tested in truly TB-free herds). The moderate sensitivity of the antibody tests, which in any case is higher than that of the skin test, can be enhanced in TB-breakdown situations by combining antibody tests using the same blood sample and removing all animals that react to any of the tests. Defra considers that these blood tests have been sufficiently validated in South American camelids under GB field conditions to substantially improve the detection of TB-infected animals that would escape detection by skin testing alone. Defra, therefore, intends in TB breakdown situations to deploy antibody testing in conjunction with the skin test in all cases of culture-confirmed
M. bovis TB infection in South American camelids. This will also extend to high-risk TB tracings from such herds.
Conversely, outside a TB breakdown situation (i.e. in holdings that are presumed to be TB-free) we can further reduce the already low probability of false-positive test results by combining two antibody tests and only regard as suspect those animals that prove positive to both tests. Moreover, in the future, where private TB testing prior to movement and/or export of camelids might identify ‘seropositive’ animals (i.e. a blood sample indicates the presence of antibodies consistent with
M. bovis infection), AHVLA’s response should be to put a herd under precautionary movement restrictions pending a blood re-test of those animals. The exact interval of the re-test has yet to be agreed, although our preference would be for 30 days. Only if the second blood test confirmed the initial positive result would AHVLA officially declare a TB breakdown on the affected premises and recommend the destruction of the persistently positive animals for post-mortem examination and culture.

How are results from camelids that have tested positive to a blood test but exhibited no visible lesions during post-mortem examination to be interpreted?
It is quite normal for animals (not just camelids) from known TB-infected herds to react to a highly specific diagnostic test for TB, but then show no evidence of TB at post mortem examination and laboratory culture. For instance, less than half of all the cattle slaughtered every year in GB as reactors to the tuberculin skin test have visible lesions of TB detected at post mortem. Furthermore, as a TB outbreak is brought under control, the proportion of test-positive but visible lesion/culture-negative animals inevitably tends to increase.
TB is usually a chronic, slowly progressing infectious disease and, unfortunately,
M. bovis is a notoriously difficult organism to grow in the laboratory from animal tissues and other material. Therefore, post mortem examination and laboratory culture are not very sensitive methods and cannot be regarded as the “gold standard” for diagnosis of TB infections. Moreover, animals removed from premises on which M. bovis was positively identified are, by definition, likely to have been exposed to the bacterium and quite possibly infected with TB. Therefore, such animals must not be considered ‘false positives’ and the failure to detect gross lesions in a TB-test positive animal from an infected herd does not mean that the animal was uninfected. A more plausible explanation would be that the test detected the infection at a very early stage, before visible lesions had time to develop and, crucially, before the test-positive animals became more infectious and capable of spreading the disease.

What about the skin test?
Broadly speaking, there are two types of tuberculin skin tests used for TB screening of animals, the comparative intradermal test (with avian and bovine tuberculins as for normal TB screening of cattle in the UK and Ireland) and the single intradermal test (with bovine tuberculin only). At the moment AHVLA will normally use the more specific comparative skin test in camelids, but in certain circumstances (e.g. in particularly severe TB breakdowns) it may use the single intradermal test in an effort to maximise the probability of detecting as many infected animals in a herd as possible.
There has been some misunderstanding about the purpose of the research. It was to develop new diagnostic tools to address the well documented low sensitivity of the skin test in camelids
2, and not to validate the single or comparative tuberculin skin tests or different combinations of the skin and blood tests.
2 Alvarez J, Bezos J, de Juan L, Vordermeier M, et al. (2011). Diagnosis of tuberculosis in camelids: old problems, current solutions and future challenges. Transboundary and Emerging Diseases, 59(1): 1-10.
The AHVLA research has shown that the sensitivity of the antibody tests may be dependent upon a so-called ‘anamnestic’ response following the injection of tuberculin. This phenomenon is not unique to camelids and has been observed when antibody tests for TB are used in other species. That is why in camelid TB breakdowns AHVLA will try to take blood samples for StatPak testing 10 to 30 days after a skin test. There is no evidence to show that this injection of tuberculin leads to a loss of specificity (more false positive reactions) in the subsequent antibody tests.

Can I test my animals for TB privately?
We have still to reach an agreement with the Scottish and Welsh Devolved Administrations on how to handle private TB testing of camelids, but for example in Wales this cannot be done without written agreement of Welsh Ministers. That statutory prohibition does not apply in England and, in principle, the tuberculin skin test is already available for private TB testing of camelids, cattle and other farmed animals subject to authorisation from the local AHVLA office and notification to them of all results without delay. In addition to that, pre-export skin testing of camelids is also of course a private matter between animal keepers and their own veterinary surgeons.
Generally, in order to reduce the risk of spreading disease it is good practice to screen camelids for TB before and/or after they are moved between different premises and Defra recommends that keepers seriously consider this option in consultation with their private veterinary surgeons.
However, as already mentioned, the skin test is insufficiently sensitive for reliable TB screening of individual camelids. A negative pre- or post-movement test result using only the skin test could be quite meaningless, particularly if the animal originated from a herd situated in an endemic TB area and/or with a history of confirmed TB infection.
Therefore, Defra is developing plans with AHVLA and the Devolved Administrations of GB to make two of the validated antibody tests available to camelid owners and their vets for private TB screening, either on their own or following a private skin test. Defra's strong policy preference is for the private TB blood testing of camelids to be carried out only by an AHVLA laboratory or in laboratories properly accredited by AHVLA to perform such tests. We are minded not to accept results of private TB tests from labs that do not meet these criteria and we may wish to reserve the right to re-test any such privately tested camelids.
Notwithstanding all the above, your members should note that where a group of camelids is under some form of movement restriction due to suspected (or confirmed) TB and undergoing skin or blood testing under the direction of AHVLA, additional private testing by the owner cannot be carried out to reassess (negate) any positive results of such tests. This is because re-testing of positive animals with tests that are only moderately sensitive would increase the risk of false-negative test results.

We hope this helps clarify some of the other statements circulating within the camelid community.





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