Bovine TB: the Gamma test explained

A few months back we discussed the skin TB test; this month it's time to look in detail at the Gamma blood test.

TB in cattle is usually a chronic infection and clinical signs of the disease often take a long time to develop. Infected cattle can become infectious before they show any obvious signs or develop lesions in their internal organs. All our main diagnostic tests (skin and Gamma) detect the immune response to bovine TB, rather than look for the bacterium itself.

What is the Gamma Test?

The Gamma test is a blood test which is used alongside the skin TB test in confirmed outbreaks; if you have visible lesions +/- positive cultures. The Gamma test tends to find infected cattle that the skin test may have missed - either by finding early cases that can't respond to the skin test yet or long standing cases which may have lesions but have given up responding to the skin test.

How does it work?

The Gamma test is carried out in the lab using freshly collected blood samples which are mixed with tuberculin - the same tuberculin we inject on day one of the skin test. The test measures the levels of ‘interferon gamma’ (IFNγ) which is released by the white blood cells of TB infected cattle as they're stimulated by the tuberculin.

Blood samples from TB-infected animals will release greater amounts of IFNγ in response to bovine tuberculin compared to avian tuberculin and a bigger response to the bovine mean it's a "Gamma Positive".

How accurate is the Gamma test?

No test is perfect and once again we need to look at the balance between sensitivity (how good is it at picking up all the positives?) and specificity (do we believe a positive result?)

Sensitivity: 90%

The gamma tests finds 9 out of 10 reactors on farm, but still leaves 1 in 10 reactors behind.

Specificity: 96.6%

This means the gamma test will find on average 3 or 4 false positives in every 100 cattle tested - but some studies show a big range in that average false positive rate.

This number of false positives is the reason the Gamma blood test is only ever used once TB is confirmed in a herd - what you gain in finding more of the truly infected animals has got to be balanced out with the economics of losing the false positives.

When is the Gamma Test used?

The Gamma test has been used alongside the skin test in specific TB breakdown herds. The aim of combining the skin and blood tests is two-fold: to shorten the duration of the TB breakdown and reduce the risk of leaving infected animals undetected in the herd by the time movement restrictions are lifted.

Any Gamma test-positive animals must be culled to increase the probability of correctly identifying all TB-infected cattle in the affected herd - these will qualify for compensation just like skin test reactors but don't count as reactors unless post mortem results find visible lesions (VL).

The Gamma test has been compulsory for every new TB breakdown herd with lesions +/- culture positive animals in:

• Low Risk Areas of England and Wales
• Scotland
• The Edge Area of England
• Repeat IR in Wales

What about us in the High Risk Area?

Since 1st April 2017, Gamma testing has also been compulsory for TB breakdowns with lesions and/or culture positive animals in the High Risk Area (HRA) of England where any of the following three criteria are met:

1. The outbreak was due to contact with infected cattle
2. The affected herd is in an area that's had at least 2 years of licensed badger population control
3. Repeat skin testing is not resolving the outbreak

A recent TB consultation means from 16th July there is a change in policy for how the Gamma test will be used in our area.

This means Gamma testing won't be used automatically in breakdown herds in badger cull areas. Instead Gamma testing will be targeted in herds that have a new TB breakdown if they've had a previous breakdown in the last 18 months. The aim here is to focus the Gamma on finding the missed reactors.

Which animals are tested?

All animals over 6 months old are blood tested by a bleeding team from APHA. Animals under 6 months old are not tested as their immune systems are still developing and you can get a bigger numbers of false positives in that age group.

When does the Gamma test happen?

Ideally the Gamma test is done as soon as possible after we find an animal with TB lesions, and usually before your first short interval skin test. This means any Gamma positives can be removed before we start the short interval skin TB tests, and gives you maximum chance of two clear skin tests.

For some herds the Gamma test is scheduled in at the same time as the skin test - this may slow down the skin test, but means the cattle are only handled once.

What results can you get from a Gamma Test?

There are 4 outcomes from a Gamma test:

1. Gamma test positive: this animal will be culled and checked for visible lesions at post-mortem
2. Gamma test negative
3. Gamma test reject: if the blood has clotted or there isn't enough blood in the tube or the blood arrived at the wrong temperature the lab won't test the sample and another sample will need to be taken
4. Gamma test resample: if a blood sample fails a quality control test or the results are inconclusive, a new sample will need to be collected

Gamma positives have to be culled but are not classed as " TB reactors" unless there are visible lesions at post-mortem. If there are no visible lesions on any Gamma positives, you will just need two clear skin TB tests to go clear.

If you are struggling with TB, please get in touch - we are here to help.

Next Month: are there any other new TB tests on the horizon?

Johne's Disease Update

A couple of technical changes to the way Johne's is managed for TSDG herds and for those of you enrolled with Johne's CHeCS programs.

Definitions can vary as to what counts as a Johne's positive animal - is it a clinical animal that's scouring? Or a consistently high antibody positive cow? Or an animal that has shown a single blip in a long line of low antibodies tests? Even when a cow goes clinical with Johne's and there is bubbly scour and weight loss, a quarter of these will still test antibody negative.

Johne's disease testing is all about managing risk rather than generating cull lists; protecting your calving yards and replacement heifers has got to be the priority regardless of external pressures or changes to rules.

NEW TSDG RULES:

For herds that test individual milk antibodies every quarter, milk recording organisations (MRO) use a 2 in 4 classification - a cow is classified as positive (red cow or J5) if she has 2 high results in the last 4 tests. TSDG interprets quarterly milk antibody results from MRO a bit differently - a cow is only "Johne's Positive" if the last 2 consecutive results are high or "2 in 2". This now spans missed tests because of a dry period or treatments.

TSDG herds must have no more than 2% of the herd test positive (2 in 2) - if the prevalence goes over 2% then you need to muck sample all of those positives you intend to keep until the next quarterly test to confirm if they're shedding or not using a faecal PCR test.

If the faeces tests negative but the antibody stays high the cow can stay but she has to have repeated negative muck tests every 6 months. If any faecal PCR ever comes back positive and the cow is confirmed to be shedding, that animal MUST be culled ASAP, before the next testing quarter and cannot contribute to the bulk tank.

If a "2 in 2 Positive" goes on to have a negative milk antibody test, she will revert back to a test negative status and won't need any more faecal PCR testing as long as her antibody remains under 30 (NMR) or 0.3 (CIS). If she ever goes over that threshold just once she will need to restart the faecal PCR testing.

WARNING - TEST POSITIVE ANIMALS THAT BECOME "TEST NEGATIVE" ARE STILL HIGHLY LIKELY TO BE INFECTED WITH JOHNE'S SO THEY SHOULD STILL CALVE SEPARATELY AND BE TREATED AS HIGH RISK ANIMALS.

Johne's Disease - changes to CHeCS

Planned changes were meant to be coming in from 1st July for CHeCS herds but have now been pushed back to 1st October 2021.

Previously a blood test Johne's positive animal could have confirmatory faecal testing done - this is now only allowed if the herd prevalence is under 2%.

All CHeCS labs now must now use the same confirmatory faecal ring testing kit.

Blood test positive animals and their offspring from CHeCS herds have never been able to be sold for breeding - and this now extends to offspring born in the 12 months prior to their dam testing positive.