Skin test

SICCT (Single Intradermal Comparative Cervical Tuberculin), also known as the skin test, is the primary tool for routine whole herd testing and pre and post movement testing to minimise the spread of disease in the UK and Europe.

It is acknowledged as a good herd test. The test is effective 3-6 weeks post TB infection. The performance of the test wanes in the later stages of disease as disease load increases and the immune response is depressed (anergy).

The skin test elicits an immune response to bovine and avian tuberculin. Reactions measured on day 1 and at 72 (+/-four) hours are compared to give the test result.

• Cattle are tested at 42 days and older.
• Cattle may test positive, negative or IRs (inconclusive).
• IRs are re-tested at 60 days. Keeper can privately slaughter before re-test.
• One in five infected cattle may be missed (false negative), on average 80% sensitivity.
• One in 5,000 cattle may be incorrectly identified (false positive), 99.8% specificity.
• Severe interpretation increases the sensitivity of the test to correctly identify infected animals once disease in a herd is identified but increases the number of false positives which may be found.

Find out more at: TB Hub | Tuberculin skin testing – Bovine TB.

PCR testing

This test looks for the bacterium responsible for bTB, the test can aid swift disclosure of the disease within three weeks where microbiological culture is not required (6-24 weeks).

The PCR Test is used in:

• bTB post-mortem test samples from cattle, DCs (direct contacts)
• privately or compulsory slaughtered or dead IRs TB slaughterhouse cases in cattle and non-bovines
• non-bovine animals TB test reactors, DCs or clinical TB suspects, and where TB lesions are identified at post-mortem examination
• domestic pets.

Find out more at: Genome.GOV | PCR fact sheet.

Microbiological culture

This is the gold standard method for bTB diagnostic testing from tissue samples (of both bovine and non-bovine farmed animals). The bacteria M.Bovis is attempted to be grown from samples in carefully controlled laboratory conditions.

40% of skin test-positive cattle have suspected TB visible lesions at post-mortem inspection and/or successful isolation of M. Bovis on culture.

Environmental challenge

Other mycobacterium can be present in the environment on farm; cattle exposure is unharmful but its presence can mask reactions and lead to a false positive or false negative test results.

Gamma test

The interferon-gamma test (or ‘gamma’ test for short) is a blood test used in addition to the skin test to increase the likelihood of detecting bTB in cattle in bTB breakdown herds with visible lesions or +ve culture.

• One in ten infected cattle may be missed (false negative), on average 90% sensitivity.
• 3-4 animals in a 100 may be incorrectly identified (false positive), 96.5% specificity.
• Infected animals can be identified at an earlier stage of infection.
• Infected animals that do not respond to the skin test can be picked up.
• By removing infected animals missed through skin testing the risk of TB infection between cattle and to wildlife is reduced.
• APHA compulsorily use the gamma test in recurrent and chronic bTB breakdowns.
• APHA use gamma testing on a case-by-case basis to resolve persistent breakdowns (18 months or more).
• Suitable for cattle aged six months and over.
• Test positive cattle to government funded gamma testing are removed, compulsorily slaughtered and compensation paid.

Private vets can apply to APHA for keepers to access privately funded gamma testing subject to a strict criteria.

The test is approved for use by WOAH (World Organisation for Animal Health), the UK and the EU.

Find out more at:

• TB Hub | Interferon-gamma blood testing of cattle.
• TB Hub | Refinements to the interferon-gamma testing policy in the High Risk and Edge Area of England.

IDEXX antibody test

The IDEXX test has WOAH approval as a supplementary test for bTB but is unvalidated for routine use in the UK and Europe.

The blood test locates antibodies that are produced as part of the immune response following M.Bovis infection in an animal. The test is currently used by APHA (Animal & Plant Health Agency) as a third line of defence.

• Test sensitivity 65% (WOAH).
• Test specificity 98% (WOAH).
• Chronic and persistent TB breakdown herds on a case by case basis.
• Where a gamma test has taken place.
• Entire herd or specific groups of animals at risk.
• Can test youngstock under six months old.
• Where there is still a risk of undisclosed cattle-cattle infection.
• Approved for TB testing South American Camelids.
• APHA recommend Idexx is used following a skin test within 10-30 days to maximise sensitivity through a booster effect.
• Seropositive cattle to government funded IDEXX testing are removed, compulsorily slaughtered and compensation paid.

Private use of the IDEXX test by keepers in England in a bTB breakdown or to aid herd management following a gamma test requires written consent from the APHA to the private vet.

Compensation is not paid by Defra and costs of removal are borne by the keeper. Result must be statutorily shared with APHA.

Find out more at: National Library of Medicine | A review of the tuberculin tests, gamma-interferon assay and other ancillary diagnostic techniques.

Enferplex test

Enferplex is an additional WOAH approved antibody blood test unvalidated for routine statutory testing for TB in cattle in the UK.

• It measures antibody responses to 11 specific M/.Bovis antigens.
• 76% – 94% test sensitivity in confirmed infected cattle (high sensitivity interpretation)
• 30 days post skin test give the highest sensitivity rate
• Specificity of 98% – 99%.
• Reactions to two or more antigens is positive.

Find out more at: Nature.com | Diagnostic accuracy of the Enferplex Bovine Tuberculosis antibody test in cattle sera.

Novel and non-validated tests

Actiphage test

A bacteriophage, or phage, is a virus which infects a bacterial host by injecting its DNA into the bacterial cell, which is then forced to make new phage particles. Once the new phage are made, the viruses then rupture the bacterial cell. This releases the new phage into the environment, which in turn can infect more bacteria. A specific phage capable of infecting specific bacteria, in this case mycobacterium bovis, is used.

The activity of the phage is used as an indicator that bacteria are present in a blood or milk sample taken from an animal.

Test manufacturers estimated that Actiphage has a sensitivity of 95% and a specificity of 100% relative to the skin test. However further trials are needed to confirm that accuracy and validity of the test.

PBD Biotech, the creating company, has in recent years gained funding to take it through the process of gaining WOAH validation. The test can be used on a private basis provided the APHA has given approval.

Non-validated and/or non-statutory (non-Defra approved) tests are only considered by APHA in exceptional circumstances and subject to prior permission on a herd-by-herd basis. The protocols are applicable to cattle herds experiencing chronic/persistent TB breakdowns.

Lesion and/or culture positive cattle have been identified and repeated skin and interferon-gamma blood testing has not resolved.

A comparative (head-to-head) study of the use of the Enferplex and IDEXX antibody tests alongside the gamma-interferon test in OTF-W (officially bovine tuberculosis free status withdrawn) breakdowns in England and Wales by APHA is ongoing.

Find out more at: TB Hub | Skin testing.