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Private cattle TB blood interferon-gamma testing

APHA is now offering the option to private vets of additional private interferon-gamma (IFNG) blood TB testing of cattle in England under certain conditions.

Background

The introduction of private IFNG testing is one of the objectives in Defra’s current strategy for achieving Officially TB-free status for England (published April 2014).

This is to help access to the IFNG blood test for cattle that are currently excluded, providing additional TB test options to owners to check and monitor the health of their cattle.

This facility will allow private veterinarians, with prior APHA approval, to submit blood samples to the APHA laboratory at the owner’s expense and in a limited number of scenarios outside of the government-funded testing programme, where owners seek additional assurances as to the TB-free status of animals over and above those afforded by statutory testing.

Examples include:

  • Supplementary pre- or post-movement testing of pedigree animals that are not subject to, or have passed, a compulsory skin test
  • More sensitive TB screening of animals joining high-value herds, including pedigree bulls entering semen collection centres
  • As a marketing tool to add value to herd/animals intended for sale
  • Ad hoc testing following a negative routine or tracing skin test
  • Rapid retesting of inconclusive skin test reactors where no government-funded IFNG blood test is planned.

In addition, supplementary testing of non-reactor cattle in TB breakdown herds with OTF status suspended or withdrawn may also be considered for APHA approval. For example, animals that have passed a short interval skin test and do not qualify for a government-funded IFNG test.

Cattle NOT eligible for private IFNG testing include the following:

  • Cattle from herds under TB restriction and undergoing government IFNG testing
  • Skin test-positive (reactor) and other cattle awaiting slaughter for TB control purposes
  • Animals in Scotland, Wales or Northern Ireland at the time of testing
  • Herds under restrictions for overdue tuberculin skin tests
  • Cattle that are under six months of age.

Please also note that the IFNG blood test cannot be used on its own to maintain or regain OTF status for a particular cattle herd.

IFNG test information

The standard IFNG blood test is a comparative test, like the tuberculin skin test used in the UK and Ireland.

It uses bovine (PPDB) and avian (PPDA) tuberculin as antigens to ‘challenge’ live blood cells. Blood cells from infected cattle produce IFNG in response to this stimulation and the IFNG so produced is measured using the BOVIGAM™ ELISA test. A test-positive result is recorded where the IFNG response to PPDB is greater than the IFNG response to PPDA.

Each blood sample also has its own positive and negative quality controls to ensure the test results are valid.

Two types of private IFNG test will be available:

  • High Specificity Test for Officially TB Free herd cattle only, for movement, sales, inconclusive reactors and additional surveillance following negative routine skin testing. This extended IFNG test incorporates an additional specific peptide cocktail to the standard INFG test which must also be positive in order for a positive overall test result to be issued. The inclusion of this peptide cocktail increases the Specificity of the test (i.e. reduces the likelihood of false-positive results), but at the expense of a lower Sensitivity (i.e. the probability of detecting infected animals) compared to the basic IFNG test, as follows:
    - Sensitivity: 74.0% [95% CI: 70.4 – 83.0]
    - Specificity: 99.2% [95% CI: 98.6 – 99.8]
  • High Sensitivity Test for cattle in TB-restricted herds that do not qualify for a government-funded IFNG test.. Higher test sensitivity is required for infected herds coming to the end of restriction testing to increase the likelihood of identifying any residual infected skin test-negative cattle. This test is the standard IFNG test format outlined above, and carries the highest Sensitivity (probability of infection detection), but at the expense of a lower Specificity (higher probability of false-positives) as follows:
    - Sensitivity 90.0% [95% CI: 87.2 – 92.8]
    - Specificity 96.5% [95% CI: 95.3 – 97.7]

Private vets unsure of which test is applicable should contact their local APHA office, who will decide which format of the IFNG test should be used depending on the circumstances of the animals or herd to be privately tested. Where the High Specificity Test is applied, the Results Report will provide both the High Specificity test result and the interpretation of the test at High Sensitivity (which may identify additional positive animals). While this latter result will not be acted upon by APHA/Defra, it provides additional information that the customer may wish to consider.

Due to the nature of the IFNG test, approximately 5% of samples may not yield a valid result on the first submission and a new blood sample would then be required from that particular animal. Samples with a ‘Resample’ result issued against them will still be charged for. If a new sample is submitted following a ‘Resample’ result, it will be charged as a new submission.

The comparative tuberculin skin test has, on average, a Sensitivity of 81.0% [95%CI: 77.3 – 84.7] and Specificity of 99.98% [95%CI: 99.9 - 100].

Submission of blood samples to APHA

Private vets should contact the APHA IFNG testing laboratory at Newcastle to book their IFNG test, after obtaining prior authorisation from the regional APHA office. A heparinised blood sample is required for the IFNG test.

Samples must be couriered to the APHA testing laboratory in a suitable temperature-controlled delivery box to prevent exposure to extremes of temperature that could damage blood cell viability. Information for delivery box options can be obtained from the laboratory (see below).

Private tests will only be carried out on Thursdays – samples must therefore be taken on a Wednesday and arrive at the laboratory by either 16:00 on the day of sampling or by 09:00 on Thursday morning. Samples taken earlier or incorrectly transported will be unsuitable for testing. Rejected samples will not attract a laboratory testing fee.

Written approval must be obtained from APHA prior to the start of the testing – the forms for IFNG test application and for the submission of samples can be found on submission forms page.

IFNG test cost (from October 2017 to March 2018)


Cost per test for:

Single test

5+ tests

10+ tests

High Specificity Test

£22.20

£18.00

£17.00

High Sensitivity Test

£16.20

£13.00

£12.00

Test results

All results will be sent directly to the submitting veterinary practice and copied to the appropriate regional APHA Office. Farmers should be made aware that animals giving a positive result are likely to be compulsory slaughtered with statutory compensation and the herd put under official restrictions. Some low-risk animals giving a positive result may be retested at government discretion and expense.

Further information

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