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

APHA can consider requests from private vets for the exceptional private use of non-validated or non-Defra approved 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 England. 

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 with Mycobacterium bovis (M. bovis), as more disseminated TB pathology develops, cattle tend to generate circulating antibodies against the bacterium’s antigens. These antibodies can be measured in serum (and potentially milk) samples by certain types of tests.

Antibody assays are not currently approved for official, regular TB testing of bovine animals in England. However, the IDEXX ELISA is available for discretionary use by APHA as a third-line test in exceptional circumstances in herds with chronic/persistent TB breakdowns that have not been resolved despite multiple rounds of short interval skin testing and the use of the IFN-γ parallel test.

The IDEXX M. bovis antibody ELISA (IDEXX Laboratories, Maine, USA) is the diagnostic kit used by APHA when government-funded TB antibody testing of cattle is considered necessary. This test is currently performed at APHA Starcross. It is one of the two antibody tests approved by the Office International des Epizooties (OIE) to supplement the skin test for the detection of TB-infected cattle and it was added to the OIE’s Register of Validated Diagnostic Kits in June 2012.

The bovine IDEXX ELISA is a relatively simple and inexpensive antibody detection assay that uses a combination of two M. bovis recombinant antigens (MPB83 and MPB70) and can provide test results within two hours. The OIE’s register datasheet indicates that this test has a moderate test sensitivity of 65% and a specificity of 98% in cattle.

Achieving the maximum sensitivity of this test is dependent on a prior tuberculin skin test, which triggers an anamnestic boost (memory response) to specific antibody levels in M. bovis-infected cattle. APHA requires that blood samples for government-funded IDEXX testing are taken in the 10 to 30 days following a skin test.

Supplementary IDEXX testing carried out by APHA in England requires prior consent and a written agreement from the owner to allow the removal of any TB seropositive animals, with statutory compensation payments.

There is a further antibody test for TB in cattle that was validated to OIE standards more recently, known as the Enferplex TB test (Enfer Diagnostics). This test achieved OIE approval and was added to the OIE’s Registry of Validated Diagnostic Kits for the detection of TB in cattle in May 2019. This test is currently not approved by Defra, Scottish and Welsh Governments for official government-funded supplementary antibody testing of cattle, although it can be used on a private basis subject to prior permission from APHA as stated below.

The Enferplex TB test is a ‘multiplex’ enzyme immunoassay designed to detect the presence of antibodies to an array of 11 M. bovis antigens. Any antibodies present in serum (and potentially milk) samples are detected simultaneously by a multiplex immunoassay. Information in the OIE data sheet indicates a high test sensitivity of 71.4% (non-boosted) and 94.2% (boosted by a prior intradermal injection of tuberculin) and a specificity of 99.7% for this test using a high specificity setting.

As for the IDEXX test, the highest sensitivity of the Enferplex TB test is also dependent on a prior tuberculin skin test, which triggers the anamnestic (memory) antibody response in infected cattle. APHA strongly recommends that blood samples for Enferplex testing of cattle are taken within 10 to 30 days following a skin test.

A good understanding of the performance of any diagnostic test is needed before it can be officially approved for routine veterinary use, usually following validation to the standards set out by the OIE. In England, APHA will entertain (and may authorise) applications from private veterinary surgeons for the private use of two types of unofficial tests for TB in individual cattle herds:

  1. OIE-validated kits such as the IDEXX ELISA and the Enferplex TB antibody test
  2. Novel, semi-experimental assays that have not been validated (or may be undergoing validation) in cattle, such as Polymerase Chain Reaction (PCR) on faecal samples and the Actiphage blood assay developed by Nottingham University and PBD Biotech.

The private use of these tests is on an exceptional basis and subject to certain conditions, in order to supplement (not replace) the current statutory TB testing regime. This means that these unofficial/non-validated tests can only be deployed at the herd owner’s expense to identify additional infected animals in herds sustaining chronic TB breakdowns with lesion and/or positive culture cattle that have not been resolved through repeated short interval skin testing and, where appropriate, IFN-γ parallel testing. Private unofficial/non-validated TB tests cannot be used on skin test reactors or IFN-γ test positive animals awaiting slaughter, in order to negate or question positive results to the official TB tests.

Conditions for private use of non-validated and/or non-Defra approved tests on cattle in England

Before the private use of non-validated or non-Defra approved tests or the IDEXX ELISA in breakdown herds, the herd (or the relevant management group(s) thereof) in question must have undergone mandatory IFN-γ testing by APHA if the herd is eligible 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 or non-Defra approved test is that the PVS must inform and obtain informed consent in writing from the farmer(s) about the following points:

  • the test(s) in question is/are not validated and/or not approved by Defra for routine use in cattle
  • the test results may be unreliable and difficult to interpret, meaning the test may not offer any certainty about the animals’ true infection status
  • APHA is under no obligation to remove (nor is Defra liable to pay compensation for) any test-positive animals. Therefore, the removal and slaughter costs will be the responsibility of the herd owner, unless any of those animals becomes positive to a subsequent government-funded mandatory TB test, or APHA considers it necessary to remove any of those animas as a direct contact
  • any animal that gives a positive result to a private non-validated test, or to an OIE-validated test not yet approved by Defra for routine use, will be restricted to the premises for its lifetime. Such animal(s) can be moved under licence to slaughter, directly, via a slaughter gathering, to a veterinary research centre for post-mortem examination, or to an Approved Finishing Unit (AFU)
  • Additionally, in the specific case of any positives to a private IDEXX ELISA or Enferplex TB test, it will be necessary for the herd owner to privately slaughter such animals before APHA can restore the Officially TB Free (OTF) status of the affected herd. Refer to the section ‘Fate of Privately tested animals with positive results’ below for more details.

Before approval for the private use of a non-validated or non-Defra approved 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.

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 or non-Defra approved tests or the IDEXX ELISA test

Communication and pro-active engagement with APHA before private use of non-validated or non-Defra approved tests or the IDEXX ELISA is essential. The PVS must seek prior permission in advance, as required by The Tuberculosis (England) Order 2014, and should submit the completed Application for Authorisation of private use of a Non-validated or Non-Defra Approved Test for TB in Cattle in England by email to TB.Advice@apha.gov.uk for attention of the APHA case/duty veterinarian and the TB Leadership Team for Delivery (TB LTD). This will include 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.

The application form is available on the submission forms page. The completed form must be submitted to APHA with a minimum of 10 working days' notice before the intended date of sampling.

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 veterinarian 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. 

Fate of Privately tested animals with positive results

Animals testing positive to a non-validated (or non-Defra approved) test will not be able to be sold, other than to slaughter, in line with the agreement that enabled testing. Culling of non-validated (and non-Defra approved) test-positive animals will be a voluntary decision by the owner and the costs of such slaughter will be borne by the owner. Compensation will not be paid for animals culled following a positive result with the non-validated (or non-Defra approved) test.

Additionally, TB breakdown herds that have completed the normal APHA testing protocols to lift the movement restrictions, but contain one or more positive animals identified through private IDEXX ELISA or Enferplex antibody testing, will not be able to regain OTF status and move cattle freely without an APHA licence until one of the two following conditions are satisfied:

  • All the IDEXX or Enferplex positive animals have been privately culled or sent to an AFU at the herd owner’s expense and APHA’s approval to carry out private non-validated TB testing has been withdrawn; or
  • Six-months after the date on which the herd would have been eligible to regain its OTF status APHA will conduct a comparative skin check test of all the cattle in the herd over 42 days of age (including all the remaining IDEXX/Enferplex TB positive animals) at the government’s expense. This skin test will be simultaneously supplemented with a private IFN-gamma parallel test of the remaining IDEXX/Enferplex positive animals, funded by the owner. If any of those animals yields a positive reaction to bovine tuberculin on the skin test (irrespective of the avian reaction), or gives a positive result on the private IFN-γ test (or another animal in the herd fails the comparative skin test at standard interpretation), APHA will remove them with compensation and the TB herd breakdown/restrictions will continue.  By contrast, if all the IDEXX/Enferplex test positive animals and the other cattle in the herd pass this testing regime, APHA will restore the OTF status of the affected herd.

Any animal positive to a privately funded IFN-γ test where its use has been agreed in advance with APHA, and the test conducted at an APHA or similar approved laboratory, will be compulsorily slaughtered with statutory compensation, subjected to post-mortem inspection and, if necessary, sampled for culture.

Milk from dairy cows that tested positive to a non-validated/non-Defra approved test (i.e. any test other than the skin and IFN-γ tests cited in the current EU legislation) can enter the human food chain.  Milk from these animals will still be subjected to pasteurisation whilst the herd is under TB restrictions.

Once the TB breakdown herd has regained its OTF status, APHA’s temporary approval to carry out private non-validated TB testing will automatically lapse and the PVS will have to submit a new application to APHA if necessary.

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