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Exceptional private use of non-validated tests for TB on cattle in England

APHA can consider requests from private vets for the exceptional private use of non-validated tests for TB on cattle in England under certain conditions.

Background

Article 13 of The Tuberculosis (England) Order 2014 states that "a person must not perform a test for tuberculosis on a bovine animal except with the written consent of the Secretary of State, and a person to whom any such consent is given must, as soon as that person knows the result of the test, report it immediately to the Secretary of State (i.e. APHA)."

The comparative intradermal tuberculin skin test and the supplementary interferon-γ (IFN- γ) blood test are the only ante mortem tests currently approved for the statutory cattle TB testing programme in Great Britain. 

Where the herd does not qualify for mandatory IFN-γ testing under the current policy and APHA does not think it appropriate to use this test on a discretionary basis at the government’s expense, the Private Veterinary Surgeon (PVS) can still consider its voluntary use at the herd owner’s expense. For further information on applying to APHA for IFN-γ testing, see the private IFN-γ testing page.

In the more advanced stages of infection (as more disseminated TB pathology develops) cattle tend to generate detectable levels of circulating specific antibodies against Mycobacterium bovis (M. bovis). This feature can be exploited by antibody or serological tests.

Antibody assays are not currently approved for statutory TB testing of bovine animals in GB.  However, an antibody test for TB in cattle is available for discretionary use by APHA in exceptional circumstances in multiple-reactor herds with lesion and/or positive culture cattle where, despite repeated short interval skin testing and the use of the IFN-γ test, ongoing cattle-to-cattle transmission due to anergic cattle is suspected.

The current antibody test for TB in cattle available at APHA is the IDEXX M. bovis antibody ELISA test (IDEXX Laboratories, Maine, USA). This test achieved Office International des Epizooties (OIE) approval and was added to the Registry of Tests for the detection of Tuberculosis in Cattle in June 2012.

The bovine IDEXX ELISA is an antibody detection assay that uses a mixture of recombinant antigens (MPB83 and MPB70) of M. bovis and can provide test results within two hours. OIE data suggest an overall moderate test sensitivity of 65% and a specificity of 98% for this test.

The highest sensitivity of the test is also dependent on a prior tuberculin skin test, which triggers an anamnestic boost to specific antibody levels in infected cattle. APHA recommend that blood samples are taken within 10 to 30 days of a prior skin test.

Supplementary IDEXX testing carried out by the APHA in GB, with the agreement of Defra, Welsh Government or Scottish Government, requires the prior consent and a written agreement from the owner to allow the removal of any seropositive animals with statutory compensation.

A good understanding of the performance of any diagnostic test is needed before it can be officially approved for veterinary use, preferably with validation to the level required by the OIE. In England, consideration can be given to the private use of new tests for TB in cattle under development even if the test is not listed by the OIE.

Examples of non-validated tests for TB that may be proposed for exceptional private use in cattle herds with chronic TB breakdowns are the Enferplex TB serological test produced by Enfer Laboratories Ireland and approved in GB for use in camelids, the Polymerase Chain Reaction (PCR) testing of faecal samples, or the Actiphage assay developed by Nottingham University and PBD Biotech.

Private use of non-validated tests for TB, and/or the IDEXX ELISA where its use is not proposed by APHA, may be permitted in England subject to agreement by Defra, to supplement, not replace, the current statutory TB testing regime in herds with chronic TB breakdowns. This would be in exceptional circumstances in multiple-reactor herds with lesion and/or positive culture cattle where repeated short interval skin testing and, where appropriate, the use of the IFN-γ test has not resolved the incident.

Conditions for private use of non-validated tests on cattle in England

Before the private use of non-validated tests or the IDEXX ELISA in breakdown herds, the herd (or management group) in question must have undergone IFN-γ testing by APHA if this is required under the current Defra policy. This also includes IFN-γ testing of the herd where it meets the current criteria for discretionary parallel IFN-γ testing under current Defra policy and under the Enhanced Management of Persistent Breakdowns (EMPB) procedures.

A precondition to approval for the private use of a non-validated test or private use of the IDEXX ELISA is that the PVS must inform the farmer(s) in writing that:

  • the test is non-validated and not officially approved by Defra or, in the case of the IDEXX ELISA, that it is recognised by the OIE (but not under EU legislation) as a validated supplementary test for TB in cattle
  • the test results may be unreliable because the results are difficult to interpret, meaning the test may not offer any certainty about the presence or absence of infection.  The current known performance of the test and links to published articles should be provided  
  • APHA is under no obligation to remove (and pay compensation for) any test-positive animals (unless they become reactors to the skin test or IFN-γ positive) and therefore the removal and slaughter costs are the responsibility of the animal owner
  • any animal that has a positive result to a non-validated test will be voluntarily restricted by the farmer to the premises for its lifetime. Movements to slaughter will be permitted.
  • any animal that has a positive result to the IDEXX ELISA will need to be privately slaughtered before APHA can restore the Official Tuberculosis Free (OTF) status of the herd and lift the movement restrictions.

Before approval for the private use of a non-validated test or the IDEXX ELISA can be given, the farmer must indicate their informed consent prior to the test being carried out, by responding in writing to the letter from the PVS.

This consent, which must include the farmer’s agreement to voluntarily restrict any test-positive animals to the premises for their lifetime, must be copied to APHA prior to final approval and before the test is deployed.

The test results must be reported and shared with APHA by the PVS after every testing event, including the planned management of tested animals into, e.g. different epidemiological groups.

Process for arranging non-validated tests or the IDEXX ELISA test

Communication and pro-active engagement with APHA before private use of non-validated tests or the IDEXX ELISA is essential. The PVS must, in the first instance, approach the APHA Case Vet for the persistent breakdown herd to seek prior permission in advance, as required by The Tuberculosis (England) Order 2014, including a clear plan regarding:

  • what animals will be tested
  • what type of samples will be taken (e.g. blood, milk, faeces, bulk milk)
  • what test(s) will be used and when
  • the current known performance of the test(s), such as published articles and/or access to proof-of-concept data
  • how use of this test will contribute evidence needed for validation of the test(s), including collaboration with the manufacturer
  • whether a Home Office licence is required and, if so, has been obtained
  • what will be done with the results
  • how APHA will be kept informed as required

APHA expects the PVS to engage with the test manufacturer so use of the test(s) can contribute to the evidence needed for the validation of the test(s).

APHA TB diagnostic experts and the APHA Case Vet may be able to offer advice on the use of the test(s) where APHA has been involved from the outset.

Provided the farmer has given written authority for their information to be shared, the APHA Case Vet will discuss the application with the PVS including the previous test history of the herd and its eligibility for IFN-γ testing.

The PVS must furnish evidence to APHA that a Home Office licence has been obtained, or that one is not needed e.g. blood samples taken at the time of IFN-γ testing or other disease screening may require a licence for scientific research unless it was agreed with the Royal College of Veterinary Surgeons (RCVS) that it formed a legitimate part of a disease control intervention that was covered by the Veterinary Surgeons Act.

Test results

The test results must be reported and shared with APHA by the PVS after every testing event, including details of the planned management of tested animals into, e.g. different epidemiological groups.

The PVS should be aware that under The Tuberculosis (England) Order 2014 it is a legal requirement to report suspicion of TB in cattle.  This legislation also provides that if an APHA vet considers any cow to be a TB suspect animal, they can serve a notice to restrict the use of the cow’s milk. 

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