Gamma Interferon blood testing
The gamma interferon (g-IFN) blood test, developed in Australia in the late 1980s, is a laboratory based blood test for the diagnosis of bovine TB in cattle. It is approved for use (under EU legislation) alongside the tuberculin skin test (as an ancillary test), to improve the sensitivity of the testing regime.
The g-IFN test has good sensitivity, appears to detect infected animals earlier than the skin test and can be repeated as often as necessary without the need to wait 60 days between tests. Additionally, the two tests detect marginally different groups of animal with some animals only disclosing to one or other diagnostic test. Therefore, the g-IFN test is used in many countries in combination with the skin test, to improve the detection of infected cattle in herds with persistent bovine TB problems or with a high prevalence of disease, and so speed up the resolution of confirmed TB breakdowns by identifying as many infected cattle as possible at the earliest opportunity.
Following ad-hoc use in GB from 2002, and the results of a field trial that ended in 2005, a range of scenarios for using the test in GB was identified based on disease control benefits and value for money. In October 2006 a new g-IFN test policy was rolled out across GB – since then the test has been applied in certain specified circumstances.
Criteria for Gamma Interferon Testing in England
On a mandatory basis:
- On skin test negative animals in all confirmed new TB incidents in areas of low TB incidence
On a discretionary basis:
- On skin test negative animals in herds in high risk areas with persistent confirmed infection that fail to resolve through repeated short-interval tuberculin skin tests and that have in place certain herd biosecurity controls – decisions made on a case by case basis
- On skin test negative animals in severe TB incidents, to inform decisions around whole or partial herd slaughter – decisions made on a case-by-case basis.
Additionally, the test is used occasionally to enhance specificity in the following limited circumstances:
- Non-specific reactor procedure for unconfirmed breakdowns in 2, 3, or 4 yearly tested herds
- Suspected fraudulent reactors
Review of the gamma interferon testing policy
The g-IFN policy was reviewed during 2008, with the review report being published in July 2009. The review considered the impacts of the g-IFN test policy and also compared the GB policy with those in place in other countries. It concluded that no significant changes should be made to the GB policy.
- Report: Gamma Interferon diagnostic blood test for bovine tuberculosis: A Review of the GB Gamma Interferon testing policy for tuberculosis in cattle
- Annex A.1: Analytical workstream report Project SB 4008
- Annex A.2: Annual Gamma Testing Report – 23 October 2006 to 31 October 2007
- Annex A.3: Annual Gamma Testing Report – 1 November 2007 to 31 October 2008
- Annex C (i): Defra analysis of all costs per testing scenario (Annex (Ci)
However, in England, as part of a package of measures to tackle bovine TB, we have increased use of the test to enhance TB controls by applying it to confirmed breakdown herds in the two-yearly testing buffer areas (i.e. those parts of the country between high and low risk TB areas).
Scientific research has shown that the average specificity (accurate identification of uninfected animals) of the gamma test is 97% – which is only slightly lower than the 99% plus for the skin test. A trial set up to establish this aspect of its performance under GB conditions published its results in 2006.
The average sensitivity (accurate identification of infected animals) of the gamma test is 87% – comparable to or marginally better than the skin test. Because the gamma and skin tests detect slightly different sub-groups of infected cattle, by combining the two tests a higher overall sensitivity can be achieved.
It is a common misconception that, as 82% of gamma interferon test positive animals do not show evidence of TB in the slaughterhouse or laboratory, they were “false positives”. A positive result to the blood test indicates the presence of replicating M. bovis organisms – the cause of bovine TB. There is evidence that cattle that test positive are likely to be in the early stages of disease, when the signs of infection are often too small to be seen by the naked eye during abattoir post-mortem examination. Although samples are taken from the parts of the animal most likely to be infected, it is very difficult to isolate the organisms from tissue samples without visible lesions. Therefore, failure to detect lesions of tuberculosis by post-mortem examination at the slaughterhouse, or to culture M. bovis in the laboratory, does not imply that a test reactor is not infected with bovine TB, especially when the test is used in known infected herds. The actual risk of the gamma test identifying a false positive animal is 3 in 100.
Further, the proportion of test positive animals without disease confirmed at slaughter represent a potential risk to other animals. Work in the Republic of Ireland shows they are between 7 and 9 times more likely than a blood test negative animal to become skin test reactors at subsequent skin tests if not removed.