TB Breakdown Test Types

The Disclosure Tests

This is not a specific type of test but is the term used for any test at which a reactor animal is first discovered in your herd. It is often an AHT (Annual Herd Test) but could be a risk test, e.g., LCT (Lateral Check Test) set due to a neighbouring TB breakdown, or an individual animal test, e.g., a CTT (Check Test Trace) on an animal moved out of a herd that has subsequently become a TB breakdown. Sometimes the disclosure happens when you slaughter an animal that is found to have TB like lesions. These are called Lesions at Routine Slaughter (LRS).

Once reactors have been found on your farm, you will need to test all your cattle, excluding calves under 6 weeks old at the time of the test (unless there is evidence of infection in groups of young calves).

RHT (Restricted Herd Test)

When a breakdown begins with an animal positive at an individual animal test or a Lesion at Routine Slaughter (LRS), an immediate stabilising test (RHT) may be set if the herd has not recently competed a full herd test. The purpose of an RHT is to:

• establish the extent of infection in the rest of the herd,
• remove infected animals as early as possible, and
• prevent further spread of infection to healthy animals.

Herd-keepers are advised to complete an RHT at the earliest opportunity to establish the disease status of the herd. RHTs are extremely valuable surveillance tests for the Department and affected herd-keepers. In 2023 approximately 30% of RHT tests revealed at least one reactor animal. Early disclosure and removal of these infected cattle may prevent further disease spread within the herd.

RH1 (Restricted Herd Test 1)

An RH1 is set after any herd test that has skin positive reactors. An RH1 is also set after any RHT.

RH2 (Restricted Herd Test 2)

In an OTW herd, an RH2 is set after an RH1 with no skin positive reactors.

CHT/CH1/CH2 (Check Herd Test)

One or two Check Herd Tests will be arranged for your herd 5 to 6 months after movement restrictions are lifted, to check that no infection remains and to make sure that any original source of infection has not infected more animals in your herd. One herd test (CHT) is required after an OTS breakdown; two tests (CH1 and CH2) 5 to 6 months apart are required after an OTW breakdown.

Lesions at Routine Slaughter

All animals slaughtered for human consumption are subject to a post-mortem examination. This examination looks for visible signs of TB infection. When visible signs are seen, the animal is said to have had a “Lesion at Routine Slaughter” or LRS. The lesion is sent from the abattoir to the laboratory (AFBI) for:

• Histology - an examination under the microscope
• Bacteriology - an attempt to grow TB bacteria (Mycobacterium Bovis) from the lesion.

The subsequent treatment of the herd, including the number of herd tests required, depends on the laboratory results as shown in the table below:

Table A

Histology	Bacteriology	Tests required
Negative	Positive for M. Bovis	RHT*, RH1 and RH2 required at Severe interpretation
Negative; no alternative diagnosis	Negative	RHT* and RH1 required
Inconclusive	Positive for M. Bovis	RHT*, RH1 and RH2 required at Severe interpretation
Inconclusive	Negative	RHT* and RH1 required
Alternative diagnosis of parasitic lesion	Positive for M. Bovis	RHT*, RH1 and RH2 required at Severe interpretation
Alternative diagnosis of parasitic lesion	Negative for M. Bovis	No herd test required**
Alternative diagnosis of timber tongue or tumour	No bacteriology test required	No herd test required
Positive	Positive for M Bovis	RHT*, RH1 and RH2 required at Severe interpretation
Positive	Negative	RHT*, RH1 and RH2 required at Severe interpretation

*See RHT section above.

**An alternative diagnosis other than timber tongue is a very uncommon occurrence. Herd restrictions remain in place until bacteriology test completed, RHT and RH1 may already have been completed in this timeframe.

Inconclusive Tb Reactors

When an Inconclusive (IC) animal is identified in an OTF herd, the herd and any associated herd/s becomes OTS. This is because the inconclusive result casts a doubt over the overall TB situation in your herd/s. Until the status reverts back to OTF you will not be able to move cattle out of your herd or associated herds other than for direct slaughter:

Table B

Test result	Further action	Tests needed to lift restrictions
Inconclusive result	Isolate and do not move IC animal. No live animal movements permitted until IC resolved, other than for Direct Slaughter Restrictions will also apply to associated herds.	Re-test individual ICs after a minimum of 42 days.

Re-Test of an Inconclusive animal at RI1 test (herd is OTS due to IC in a herd)

Table C

Test result	Further action	Tests needed to lift restrictions
Negative	None.	Animal and Herd restrictions withdrawn.
Not negative, i.e., inconclusive again or positive.	Animal taken as a reactor and slaughtered.Herd and associated herd/s restricted as a TB breakdown.	As for Table A Above.

Backward Check Test (BCT)

When TB is found in a herd by the disclosure of a skin test reactor or a ‘Lesion at Routine Slaughter’ (LRS), an important part of the veterinary investigation is to identify where the infection may have come from. The animal may have picked up TB in the final herd or it may have become infected in a previous herd and brought disease into the final herd.

Routinely, where TB is confirmed and/or the final herd has its Officially Tuberculosis Free status withdrawn (OTW), the previous herds of each reactor/LRS animal are identified. If a full herd test has not been completed at least 42 days after the reactor/LRS animal left a previous herd, DAERA will apply a herd movement restriction to that herd (and any associated herds) until the herd(s) complete a Backward Check Test (BCT) with negative results.

The due date of the BCT is set to ensure that the test takes place at least 42 days after the reactor/LRS animal left that herd. BCTs are also set for associated herds. All cattle 42 days of age or more must be tested.

Veterinary risk assessment may suggest that an animal was highly likely to have been infected in a previous herd before it entered the final herd. In this scenario, the previous herd may also be considered a TB breakdown herd and will be subject to standard breakdown herd movement restrictions and testing. Most animals that have left a herd within the previous 4 months before disclosure and fulfil the criteria will trigger a breakdown on the previous herd.