Interferon-gamma blood test What is it and why do we use it?

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1 Interferon-gamma blood test What is it and why do we use it? April 2018

2 Interferon gamma (IFNγ) test What is it? How does it work? How accurate is it? Why do we use it? When do we use it?

3 IFNγ test Supplementary blood test used in addition to the tuberculin skin test Approved for use in cattle under EU legislation and by the OIE (World Organisation for Animal Health) It is the only blood test approved in the UK and the EU to supplement the skin test for TB in cattle

4 Why do diagnostic tests for bovine TB need to be approved? Diagnostic tests need to be fully validated as per OIE guidelines, so that we can interpret the results that they generate Validation involves multiple steps to assess the diagnostic performance characteristics of the test The test must also be shown to be repeatable and standardised

5 Tests for bovine TB need to be approved under EU legislation for trade purposes and to establish the official disease status of herds IFNγ and IDEXX antibody tests are the only blood tests currently listed by the OIE as validated for TB in cattle

6 How does the IFNγ test work? The tuberculin skin test and IFNγ test both measure the host s immune response to the TB bacterium (Mycobacterium bovis), rather than detect the bacterium itself

7 How does it work? The IFNγ test is a comparative test like the skin test It measures the animal s immune response to avian and bovine tuberculins (same as used in the skin test) Tuberculins are mixtures of proteins extracted from cultures of M. bovis bacteria grown in liquid media in the laboratory, and then killed by heat

8 How does it work? Instead of injecting tuberculins into the skin, the animal s blood is stimulated with avian and bovine tuberculins in the laboratory Blood sample White blood cell from an animal infected with TB White blood cell releases the cytokine IFNγ (chemical messenger) IFNγ Levels of IFNγ measured Stimulated by avian and bovine tuberculin

9 How does it work? Blood samples from TB-infected animals will release greater amounts of IFNγ in response to bovine tuberculin compared to avian tuberculin Being a laboratory-based test, it is subject to strict quality controls, its readout is more objective and its protocol is easier to standardise compared to the skin test Samples need to be transported to the lab quickly in temperaturecontrolled packaging systems as the white blood cells must be kept alive

10 How accurate is it? No diagnostic test is perfect! There is a always a trade off between sensitivity and specificity Find all infected animals Don t remove uninfected animals

11 How accurate is it? 90% Sensitivity The probability that a test will correctly identify an infected animal as positive The higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result)

12 How accurate is it? The IFNγ test is 90% sensitive and will miss, on average, 10% of infected animals in a herd 20 truly infected animals 2 missed by the IFNγ test

13 How does this compare with the skin test? The IFNγ test is more sensitive than the skin test The skin test is 80% sensitive at standard interpretation and will miss, on average, 20% of infected animals 20 truly infected animals 4 missed by the skin test

14 How accurate is it? 96.5% (minimum) Specificity The probability that a test will correctly identify an animal that is free from infection as negative The higher the specificity, the lower the probability of incorrectly classifying an uninfected animal as infected (a false positive result)

15 How does it compare to the skin test? With the IFNγ test you expect to get, on average, 3-4 false positives per 100 disease-free animals tested (false positive rate of 3.5%) This means that the IFNγ test is less specific than the skin test The skin test has a higher specificity of 99.98%, which means you only get one false positive per 5,000 disease-free animals tested (false positive rate of 0.02%)

16 So why use IFNγ if there is more chance of false positives? Due to a higher probability of false positive results, the IFNγ test is not used for testing of officially TB-free herds In most cases it is only applied to TB breakdown herds with lesion and/or culture positive animals In these herds the risk of taking out low numbers of false positives is outweighed by the need to identify and remove all infected cattle

17 So why use IFNγ if there is more chance of false positives? In 2017, 98,529 samples were IFNγ tested in England The overall proportion of test-positive results was 5.2% (5094 animals slaughtered) The IFNγ test is used in a targeted, proportionate way in TB breakdown herds to maximise the sensitivity of testing The IFNγ test policy must balance costs and benefits in the context of the strategy for achieving officially TB free status for England

18 What are the benefits? The skin and IFNγ tests are used together to maximise the sensitivity of testing and increase the chances of finding all infected animals in the herd

19 What are the benefits? The IFNγ test can identify cattle at an earlier stage of infection than the skin test (experimental cattle infection shows 3-4 weeks vs 7 weeks) Studies in GB, NI & RoI consistently show that IFNγpositive but skin testnegative cattle have a higher risk of becoming skin test reactors and develop visible lesions if not removed (compared with IFNγ and skin testnegative animals in the same herds)

20 What are the benefits? Using the IFNγ test at the beginning of a new TB breakdown can shorten the duration of movement restrictions by increasing detection of infected animals It can also reduce the likelihood of residual cattle infection in herds that regain Officially TB Free status after a breakdown 58% of infected herds in the High Risk Area have had a breakdown in the last 3 years

21 What are the benefits? 448,971 animals from TB breakdown herds IFNγ tested from 2006 to April 2017 Over 33,000 IFNγ-positive animals identified that otherwise would have been missed

22 What about no visible lesion (NVL) IFNγ and skintest positive animals? If a skin test or IFNγ positive animal does not have visible lesions at slaughter, it does not mean that it wasn t infected

23 What about no visible lesion (NVL) IFNγ and skin-test positive animals? Post mortem meat inspection is a relatively insensitive method of confirming TB infection Test positive (but NVL) animals may have been in the early stages of infection when TB lesions were too small to see with the naked eye or not yet developed in the organs Small lesions can be missed at the slaughterhouse Can get genuine false positives (3.5%), i.e. the animal was not infected but tested positive

24 What about further IFNγ positive results after a skin test? Residual infection, or the reintroduction of infection can cause more test-positives downstream The skin & IFNγ tests identify slightly different populations of infected individuals Overall in a confirmed infected group, while most infected animals will be identified by both tests, there will always be animals picked up by one test but not the other Infected individuals in a herd will not all be at the same stage of infection. So while removing those that have reacted to the skin or IFNγ test, there may still be others that have only just been infected

25 When do we use the IFNγ test? Edge & Low Risk Areas Mandatory for new TB breakdowns with lesion and/or culture positive animals On 1 st January 2018, Cheshire, Derbyshire, East Sussex, Oxfordshire and Warwickshire became fully incorporated into the Edge Breakdown herds with lesion and/or culture positive animals in the former High Risk Area parts of those counties are now eligible for IFNγ testing

26 Why? The overall incidence of TB in the Edge Area is still increasing, therefore cattle measures need to be strengthened. IFNγ testing Increases the overall sensitivity of testing in TB breakdown herds Helps identify infected animals in the herd earlier Potentially reduces the time under movement restrictions Mops up infection that the skin test leaves behind Potentially helps reduce recurrence rates due to infected animals remaining in the herd and causing repeat breakdowns (or infecting other herds and badgers)

27 Increase in IFNγ use as a result of policy changes since 2013 Annual number of supplementary IFNγ blood tests carried out in England, by risk area

28 When do we use the IFNγ test? High Risk Area (HRA) From April 2017, mandatory IFNγ testing in TB breakdown herds with lesion and/or culture positive animals that satisfy any of the following criteria: 1. New breakdowns in badger culling areas where at least two seasons of effective culls have been completed 2. Where there is clear evidence that repeated skin testing of the herd has failed to clear the infection (persistent/chronic breakdown herds) 3. Where APHA s investigation concludes that the most likely transmission route for the affected herd is infected cattle (e.g. brought-in animals, contact with neighbouring cattle)

29 IFNγ test policy in badger culling areas Implemented from 1 st April 2017 New TB breakdown herds with lesion and/or culture positive animals in eligible cull areas Cull area Year culling started Year eligible for mandatory IFNγ testing policy Area-1 Gloucestershire Area-2 Somerset 2013 Area-3 Dorset April 2017 Area-4 Cornwall Area-5 Cornwall Area-6 Devon Area-7 Devon Area-8 Dorset January 2018 Area-9 Gloucestershire Area-10 Herefordshire

30 Why not wait four years in badger culling areas? Areas that have undergone badger culling have a reduced risk of TB from badgers, and so the TB risk from cattle to cattle transmission becomes even more important Residual cattle infection is an important cause of recurrent herd breakdowns in the HRA Need to remove infection from cattle herds as quickly as possible to prevent spill-back into the remaining badgers Two badger culling seasons strikes a balance between allowing culling enough time to reduce the wildlife reservoir while not unnecessarily delaying the benefits of gamma testing

31 What about environmental contamination with M. bovis? Sunlight, rain dilution, temperature and other microorganisms (e.g. antibiotic producing fungi) all affect the survival time of M. bovis in the environment IFNγ test is deployed 15 months after the onset of badger culling. A review of published literature suggests that M. bovis does not survive in soil beyond a few months under different environmental conditions A study found no viable M. bovis bacteria in the bedding of a sealed badger sett six months following vacation of the sett (Gallagher 2000)* Viability of infection in badger carcases has been examined, with no detectable M. bovis bacteria found after six weeks following burial (Gallagher 2000)* * J Gallagher, Clifton-Hadley RS (2000). Tuberculosis in badgers; a review of the disease and its significance for other animals. Res Vet Sci Dec;69(3):

32 M. bovis survival times in soil Conditions Length of survival Method Reference 20-40ºC in Australia Autumn/winter in Michigan, USA Winter/spring in Michigan, USA Spring/summer in Michigan, USA 4ºC in lab (moist dark) 4ºC in lab (moist dark) 22ºC in lab (moist dark) up to 4 weeks Culture Duffield B, Young D (1985). Survival of Mycobacterium bovis in defined environmental conditions. Vet. Microbiology,10: days max. 88 days max. 11 days max. Culture Culture Culture Fine A, Bolin C, Gardiner J, Kaneene J (2011) A study of the persistence of Mycobacterium bovis in the environment under natural weather conditions in Michigan, USA. Veterinary Medicine International Volume 2011 (2011), Article ID , 12 pages. up to 150 days up to 150 days Culture qpcr Barbier E, Rochelet M, Gal L, Boschiroli ML, Hartmann A. (2017) Impact of temperature and soil type on Mycobacterium bovis survival in the environment. PLoS One Apr 27;12(4):e up to 90 days Culture 22ºC in lab (moist dark) up to 120 days qpcr

33 IFNγ testing in badger culling areas early results Badger cull area Total number of samples tested Number of positive IFN-γ test results Proportion of samples with a positive IFN-γ test result (%) Dorset Gloucestershire Somerset All areas The overall proportion of samples with an IFN-γ positive test result was 6.7% (data from first 9 months of the policy - April to December 2017 inclusive)

34 Area-1 Gloucestershire Submission and month (2017) Herd November September Total number of samples Number of positive IFN-γ test results Proportion of samples with a positive IFN-γ test result (%) October November November December

35 Area-2 Somerset Submission and month (2017) Herd Total number of samples Number of positive IFN-γ test results Proportion of samples with a positive IFN-γ test result (%) September November November November November November October November September October November December December

36 Area-3 Dorset Submission and month (2017) Herd Total number of samples Number of positive IFN-γ test results Proportion of IFN-γ test positive samples (%) June July July July July August July August July August October (2 nd round ) November (2 nd round) July August November (2 nd round) November (2 nd round) August August October (2 nd round) November (2 nd round) August August August August August October October

37 IFNγ testing in badger culling areas early results For the first 9 months worth of data, 6.7% of samples from TB breakdown herds in the three badger culling areas tested positive, in line with what Defra expected The percentage of IFNγ test positive samples in Area-3 Dorset has been lower than in Area-1 Glos. and Area-2 Somerset Some herds have yielded higher than average IFNγ % positivity, whilst others have been below average Outliers can occur as no ante-mortem test for TB in cattle is perfectly sensitive or specific, and the herds that undergo IFNγ testing are exposed to different levels of M. bovis infection

38 When do we use the IFNγ test? Discretionary use by APHA Severe ( explosive ) TB breakdowns with high numbers of reactors that don t automatically qualify for mandatory testing To inform decisions about partial/complete herd slaughter (in extreme situations)

39 De-coupling of skin and IFNγ tests In England, IFNγ testing in TB breakdown herds is undertaken as soon as practicably possible for disease control reasons: Increases the chances of finding infected animals. The IFNγ test is more sensitive than the skin test and identifies a different population of infected animals IFNγ test can identify cattle at an earlier stage of infection than the skin test infected animals are removed earlier (less time for spread of infection to other cattle and wildlife) Potentially reduces the duration of the breakdown and the chances of residual infection remaining when movement restrictions are lifted

40 De-coupling of skin and IFNγ tests To achieve this, the default position is to 'de-couple' the IFNγ test from the skin test and complete it as a standalone test as soon as practicably possible, giving the farmer a reasonable period of notice By de-coupling, we are essentially removing infected animals that were missed by the previous skin test earlier, thus maximising the probability that the herd goes clear at its next skin test On some occasions it may not be practical to decouple e.g. if the IFNγ test is to be undertaken very close to the next skin test More information on decoupling on the TB Hub

41 Animals under 6 months of age are excluded (their developing immune system interferes with the test) TB breakdown herds eligible for IFNγ testing will initially undergo one round of blood testing The IFNγ test is repeated up to three times as long as standard interpretation skin test reactors and/or animals with visible lesions (including slaughterhouse cases) continue to be found after application of the blood test i.e. if there is still evidence of residual infection in the herd which was not picked up by previous skin and IFNγ testing

42 Private IFNγ testing (England only) Private vets can submit samples for IFNγ test at keeper s cost with prior approval from APHA Limited to specific scenarios outside of the governmentfunded testing programme where owners seek additional assurances as to the TB-free status of animals over and above statutory testing e.g. to supplement pre/post movement testing, screen animals joining high value herds, test animals following a negative routine or tracing skin test More information at:

43 Private IFNγ testing (England only) Cattle not eligible for private testing: Test reactors & other cattle awaiting slaughter for TB control purposes Cattle from TB breakdown herds undergoing government-funded IFNγ testing Cattle from herds under restrictions for overdue skin testing If a positive result is received, the animal is (with few exceptions) compulsorily slaughtered with compensation paid, the herd will be placed under movement restrictions and normal breakdown procedures followed

44 Private IFNγ testing (England only) Private IFNγ test cost (Oct 2017 to March 2018) Cost per test ( ) Single test 5+ tests 10+ tests High specificity test High sensitivity test

45 Antibody (serological) tests for bovine TB Blood tests that measure the antibody immune response of the animal to M. bovis (generally detect relatively later compared to IFNγ response) Lower sensitivity than the IFNγ test and affected by the skin test - animals need to be skin tested before taking the blood sample, to boost the serum antibody levels detected by such tests Antibody tests are not currently approved for statutory TB testing in GB or the EU

46 Antibody (serological) tests for bovine TB Data from GB show that antibody tests are less sensitive overall compared to the skin & IFNγ tests, but they can be useful for identifying small numbers of infected cattle that are skin and IFNγ test negative (~3-5% of cattle from fully confirmed TB breakdown herds in a Defra study)

47 Antibody (serological) tests for bovine TB IDEXX ELISA test is OIE registered and used by APHA in exceptional cases Requires prior informed consent from the keeper Positive animals are slaughtered and compensation is paid Used by APHA in exceptional cases as a targeted thirdline test in difficult TB breakdown herds with lesion and/or culture positive animals, where skin & IFNγ tests are not clearing the herd of infection, and where new infection is not being introduced

48 Non-validated diagnostic tests for bovine TB Tests not officially approved for use in cattle due to insufficient data on their performance characteristics Results may be unreliable and the test may not offer any certainty about the presence or absence of infection 1. Private vet needs prior permission from APHA 2. Must follow Defra s protocol for exceptional use of non-validated diagnostic tests in persistent TB breakdown herds 3. Owner/private vet decision to voluntarily slaughter or retain test-positive animals, no compensation paid 4. Test-positive animals retained by the herd owner are restricted to the holding for life

49 Take-home messages IFNγ test is more sensitive than the skin test. It can identify TB infected cattle that are earlier in the course of infection and those missed by the skin test It is less specific than the skin test and so it is used in TB breakdown herds with lesion and/or culture positive animals. It cannot be used for surveillance testing or as a replacement for the skin test IFNγ test positive animals do not necessarily show visible lesions at post mortem. This is because they are often in the early stages of infection when TB lesions are too small to be seen by the naked eye and sometimes lesions are missed at the slaughterhouse

50 Further information More information on bovine TB, including further background on the IFNγ test is available on the TB hub Biosecurity visits may be requested from the TB Advisory Service