Bovine Tuberculosis (TB) testing

We have an EU Commission-approved TB Eradication Programme. Our Programme complies with Council Directive 64/432/EEC (as amended) on animal health problems affecting intra-community trade in bovine animals and swine. This enables us to secure approximately €5 million per year towards the costs of our Programme. Directive 64/432 requires a system of TB surveillance to be in place.

Details of the TB Programme

Our Eradication Programme includes a robust TB testing regime and is vital to safeguarding NI’s £1,000 million plus export-dependent livestock and livestock products industry. Over 90% of our herds are free to engage in such international trade at any time. Securing annual EU approval for the TB Eradication Programme and compliance with Directive 64/432/EEC will continue to be a priority, to ensure continued access to this important export trade.

In NI, live animal surveillance is undertaken using 4 diagnostic methods:

  • clinical examination of animals
  • the TB skin test – herds and animals (except for direct export) are tested using the Single Intradermal Comparative Cervical Tuberculin (SICCT) test
  • Single Intradermal Tuberculin test – for export certification, the SICCT is carried out, but the avian response is ignored
  • Interferon Gamma testing (a blood test) where considered epidemiologically necessary, as a supplementary test to the SICCT

 

The TB Skin Test

The TB skin test is the common name for the Single Intradermal Comparative Cervical Tuberculin (SICCT) test. This skin test is regarded as the definitive indicator of infection by the bacterium that causes TB in cattle - Mycobacterium bovis. It is the required test in the EU and has proved to be a reliable tool worldwide. In Northern Ireland, all herds are tested annually, as a minimum requirement, but some are tested more frequently if they are considered at increased risk.

 

On Day 1 of the test, two sites are clipped on the neck of the animal. The skin thicknesses at both sites are measured and recorded. Two types of tuberculin, one made from killed Mycobacterium bovis and the other from killed Mycobacterium avium, are injected under the outer layer of the skin of the neck (i.e. into the dermis) at the ‘bovine site’ and the ‘avian site’ respectively. On Day 4 of the test, the skin reactions to the two types of tuberculin are measured and compared. When the bovine site reaction exceeds the avian site reaction by more than 4 mm, the animal is declared a reactor under standard interpretation. When the bovine site reaction measures 1-4 mm more than the avian site reaction, the animal is declared an inconclusive under standard interpretation.

TB is a difficult disease to diagnose and no diagnostic test for it is perfect. The skin test may leave an infected animal behind or, more rarely, remove an animal that is not infected. However, the skin test is the best test available for screening live cattle.

Sensitivity is the ability of a test to correctly identify an infected animal as positive, i.e. the higher the sensitivity of the test, the lower the probability of incorrectly classifying an infected animal as uninfected (a false negative result).

Studies have shown that the skin test has a sensitivity of 50-60% at standard interpretation. Repeating the skin test increases the likelihood of detecting the infected animals in a herd. This is why an infected herd usually requires at least two consecutive negative skin tests before restrictions are lifted.

Using severe interpretation increases the sensitivity of the skin test.

Specificity is the ability of a test to correctly identify an animal that is free from infection as negative, i.e. the higher the specificity, the lower the probability of classifying an uninfected animal as infected (a false positive result).

The skin test has an excellent test specificity of 99.98%. This means that it is very rare that a non-infected animal will be wrongly classified as diseased.

 

Isolation

Animals which have inconclusive or positive (reactor) results should be isolated from the rest of the herd. This decreases the risk of further transmission of infection from infected to non-infected cattle.

 

What TB statuses may be applied to cattle herds?

OTF

OTF means Officially Tuberculosis Free and is used in EU legislation (Directive 64/432) to describe those cattle herds that may undertake intra-community trade. A herd is OTF if:

  • all animals undergo a TB test with negative results annually, and,
  • there are no clinical signs or suspicion of TB infection in the herd.

OTS

OTS means Officially Tuberculosis free status Suspended. The main reasons for OTS are:

  • there is a total of 1 reactor or 1-5 LRS (Lesion at Routine Slaughter) animals during the course of the breakdown, and, no animal has had 2 positive results from the four possible tests: skin, post mortem, histology, and bacteriology

or

  • there is an animal with an IC (inconclusive) result at an OTF skin test

or

  • an animal from your herd has been found to be a reactor/LRS in another herd and your herd has been set a Backward Check Test (BCT)

or

  • testing is overdue (see Overdue TB Tests section).

OTW

OTW means Officially Tuberculosis free status Withdrawn. The main reasons for OTW are:

  • there is a total of more than 1 reactor or more than 5 LRSs during the course of a TB breakdown, or
  • any animal has had 2 positive results from the four possible tests (skin, post mortem, histology and bacteriology), or
  • any animal has had a bacteriology positive result for M bovis, or
  • an animal shows clinical signs of TB, or
  • an animal from your herd has been found to be a reactor/LRS in another herd and your herd has been set an RHT or RH1 herd test, or
  • testing is overdue (see Overdue TB Tests section).

 

Why was TB found in an animal after my last herd test was clear?

This may happen for several reasons including:

The TB skin test assesses the disease status of the herd on the day it is completed. Cattle in your herd may have become infected since the previous herd test was completed. This may happen through cattle moving into your herd, contact across fences with your neighbours’ cattle, or contact with infected wildlife, e.g. badgers. Evidence of disease can develop quickly in an animal, as soon as 3-4 weeks after exposure to infection; although it can also take considerably longer.
Although the tuberculin test is the best test currently available, like all diagnostic tests, it is not perfect. At standard interpretation, the TB skin test has a test sensitivity of 50-60%, i.e. it will miss some infected animals. Infected animals that are negative to the TB skin test may show evidence of disease later, e.g. at the next skin test, or when slaughtered.
TB infection can remain in an animal for a long time but cause no clinical symptoms or response to the skin test. Such animals can be difficult to detect in a herd. If they start shedding TB bacteria they can infect other animals, whilst remaining undetected themselves.


 
How are TB Skin Test results interpreted?

Interpretation of the TB test looks at the reaction at each injection site and compares the reaction at the two sites. Interpretation can be altered depending on the level of TB in the area or farm or other disease factors. Guidelines have been developed over many years from experimental work in laboratories and millions of actual tests on farms. Using the readings obtained at the TB test, the vet performing the test classifies each animal as negative, inconclusive or positive (reactor) to give the herd keeper an early indication of which animals to isolate. However, this on-farm interpretation may be altered by the DAERA Patch Veterinary Officer (Patch VO) responsible for the area, as they have more detailed information available when interpreting the test.

Where infection is confirmed* in a herd or when there have been more than 1 animal skin positive, or more than 5 LRSs in a herd, a stricter assessment of all skin test results will be carried out. This is called severe interpretation. The test history of the herd is also examined, along with grazing history and groupings within the herd. This information may lead the Patch VO to remove further high risk animals from the herd; particularly animals that have previously been inconclusive.

*TB is confirmed in a herd when 2 tests (from skin test, post mortem, histology, and bacteriology) are positive in one animal, or an animal has a bacteriology positive result for M bovis.


What testing will my herd need if I have a reactor?

My herd has OTS status:

If TB is not confirmed* at post mortem or laboratory examination, and there is only 1 skin positive animal in your herd, it may be possible to remove restrictions after only one clear herd test - an RH1 (as long as all other requirements have been fulfilled). Note that, if a positive TB result was first discovered at an individual test or at slaughter, rather than at a herd test, an initial RHT herd test may also be required.

After TB restrictions have been lifted, one further Check Herd Test (CHT) 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 cause of infection has not resulted in the infection of more animals in your herd.

 

My herd has OTW status: 

If TB has been confirmed* in your herd, or when more than 1 animal has been positive in your herd at the skin test, or more than 5 animals have been LRS at the abattoir, it may be possible to lift restrictions after two clear full herd tests in a row (RH1 and RH2), involving every animal in the herd. These herd tests will usually be carried out a minimum of 60 days apart. We may apply a more severe interpretation than in routine tests to make sure your herd becomes free of infection as soon as possible. We may remove further high risk animals from the herd; particularly animals that have previously been inconclusive. Note that if a positive TB result was first discovered at an individual test or at slaughter, rather than at a herd test, an initial RHT herd test may also be required.

*TB is confirmed in a herd when 2 tests (from skin test, post mortem, histology, and bacteriology) are positive in one animal, or an animal has a bacteriology positive result for M bovis.

After TB restrictions have been lifted, 2 further Check Herd Tests (CH1 & CH2) will be arranged for your herd - CH1 will be 5 to 6 months after movement restrictions are lifted, and CH2 will be 5 to 6 months after CH1. This is to check that no infection remains and to make sure that any original cause of infection has not resulted in the infection of more animals in your herd.

 

TB Breakdown Test Types

The disclosure test

This is not a specific type of test, but is the 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.

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. Its purpose is:

  • to establish the extent of infection in a herd, and
  • to remove infection as early as possible.

Occasionally, an RHT may be a part test, carried out on the highest risk groups in a 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 is required after an OTS breakdown; two tests (5 to 6 months apart) are required after an OTW breakdown.

 

Which animals will require testing after a reactor is found?

In general, once we have found reactors on your farm, we will need to test all your cattle, including calves less than 6 weeks old. Occasionally, an RHT will not require all animals to be tested.

TB testing regime in an unrestricted herd (OTF) when a

  • Reactor, or
  • Lesion at Routine Slaughter (LRS)

is disclosed:

 

(A) When a Positive Animal (Reactor) is identified at a Disclosure Test:

Table A

Test result

TB confirmed at post mortem, histology, or bacteriology New Herd Status Tests needed to lift restrictions

1 reactor or up to 5 LRSs

No OTS RHT if necessary* plus one clear RH1 herd test. The RH1 must be at least 60 days after the last reactor was isolated/removed (and at least 42 days after a clear RHT). Tests usually at Standard interpretation.

1 reactor or up to 5 LRSs

Yes OTW RHT if necessary* plus 2 consecutive clear herd tests - RH1 and RH2. An RH1 at least 60 days after the last reactor was isolated/removed (and at least 42 days after a clear RHT). Plus an RH2 at least 120 days after the last reactor was isolated/removed and at least 42 days after the first clear RH1. All tests at Severe interpretation.

More than 1 reactor or more than 5 LRSs

Yes or No OTW RHT if necessary* plus 2 consecutive clear herd tests - RH1 and RH2. An RH1 at least 60 days after the last reactor was isolated/removed (and at least 42 days after a clear RHT). Plus an RH2 at least 120 days after the last reactor was isolated/removed and at least 42 days after the first clear RH1. All tests at Severe interpretation.

* Note: If the first reactor is identified at an individual animal test, and there has been no herd test within the previous 2 months, an immediate stabilising herd test will also be required. This test is called an RHT.

 

(B) When a Lesion at Routine Slaughter is identified:

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 B

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,
e.g. tumour, timber
tongue, parasitic, or non-TB abscess

Positive for M Bovis RHT*, RH1 and RH2 required at Severe interpretation

Alternative diagnosis,
e.g. tumour, timber
tongue, parasitic, or non-TB abscess

Negative 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

*An immediate Restricted Herd Test (RHT) is set to establish if there are other TB reactors in the herd. It is optional only if the herd was tested within the 60 days previous to the LRS.

 

Interferon Gamma test (IFNG)

This is a blood test approved since 2002 in the EU for use in conjunction with the TB Skin Test. Animals which have been exposed to M bovis can respond to IFNG before they will respond to the TB skin test. The IFNG will also sometimes identify TB-infected animals which do not respond to the skin test.

The IFNG has a higher sensitivity (it will miss less TB-infected animals) but lower specificity (it may wrongly classify more non-infected animals as diseased) compared to the skin test. Due to the lower specificity, IFNG is not currently used outside of TB breakdowns in NI.

Veterinary Service Animal Health Group has been using IFNG testing alongside the skin test in particular high risk breakdown situations since July 2004.

Use of IFNG is voluntary and it is not compulsory for farmers to give up any IFNG positives that are detected, unless they are also skin test positive.

 

TB testing regime in an unrestricted herd (OTF) when:

Inconclusive is disclosed:

When an Inconclusive (IC) animal is identified in an OTF herd, the herd becomes OTS:

Table C

Test result

OTW breakdown in past 3 years? Further action Tests needed to lift restrictions

Inconclusive result

No

Isolate and do not move IC animal

Negative animals with no restrictions can move, within UK only

Re-test individual ICs after a minimum of 42 days

inconclusive result

Yes

 

Isolate and do not move IC animal

No live animal movements permitted until IC resolved

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 D

Test result

Further action

Tests needed to lift restrictions

negative

None

Restrictions on animal (and herd) withdrawn

not negative, i.e. inconclusive again or positive

Animal taken as a reactor and slaughtered

Herd restricted as a TB breakdown herd

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. Any calf less than 42 days of age that has not remained in its natal herd (the herd in which it was born) since birth must be tested also.

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.

 

Are there restrictions imposed due to overdue TB herd tests?

The EU requires a certain level of TB testing depending on the level of infection in the region. Any herds that do not follow this testing requirement are placed under movement restriction, so that all other herds can be allowed to trade freely.

To avoid restrictions, simply ensure that your test is completed on time.
Each herd has a test set in the future with a “due by” date.

If your TB herd test is not completed by the due by date, DAERA will apply restrictions automatically, and you will be unable to move animals out of the herd except to slaughter. Animals can however still move into the herd.

Should your test be delayed by more than 1 month past the due by date, DAERA will apply restrictions that prevent buying in, selling, and slaughtering animals until the test is completed and received at your Divisional Veterinary Office. No animals may move into your herd, except possibly one bull (with DAERA permission).

If an Annual Herd Test remains outstanding for 3 months beyond the original due date, the herd will require two clear tests (60 days apart) to re-establish its TB status. You will be required to pay for the second of these herd tests.

If your test remains outstanding for 4 months beyond the original due date, enforcement action is initiated. This may include prosecution.

 

Why do I have to do a PNT test?

If an animal has not been TB tested in the last 15 months, it will automatically be restricted and require a PNT test (Private test, Not Tested for 15 months). The restriction on the animal is TBN (TB Not tested for 15 months).

 

Are there restrictions imposed on single animals that have missed annual TB herd tests?

Yes. Any individual animal that has not been TB tested in the previous 15 months will be restricted and require a TB test (PNT test). It will not be permitted to move off farm except directly to slaughter in NI. Only the individual animal will be restricted, not the whole herd. An individual animal may have missed an Annual Herd Test because it has been sold from one herd to another and may end up not tested for 15 months because of this.

 

How do I arrange to have these animals tested and will there be a charge?

The PNT test can be completed as part of the next scheduled herd test, or alternatively, herdkeepers may prefer to have the animal tested privately at their own expense. In any case, the animal cannot leave the herd live except to immediate direct slaughter in NI.

 

How will I know when the animal I am purchasing was last tested?

DAERA cannot disclose the testing history of an animal without the permission of the owner. However, you can ask the seller directly when the animal last had a TB test. Some information is available at marts via APHIS on-line. At some livestock markets, prospective buyers are able to see the date of the last TB test for an animal on the display board over the ring at the time of sale.

 

Why do I have to do an LCT test?

A Lateral Check Test (LCT) is required when a herd is at risk to disease due to its location, usually due to another herd immediately next door. LCTs are usually carried out on herds that graze land adjacent to that of a breakdown herd (and its associated herds). Such herds may be a source of infection or may be at risk of becoming infected. LCTs are set in order to detect and reduce local spread of TB. Before any LCT is set, DAERA checks maps of the area and considers field usage, seasonal factors, boundary fences, wildlife and other factors.

LCTs continue until the risk no longer exists either because the original risk is removed, i.e. the neighbour’s herd clears or suitable precautions are put in place. They are usually required about three times per year whilst risk continues.

 

How can I find out when my test is due?

By checking APHIS on-line, by asking the vet to whom your test is allocated or by phoning your local Divisional Veterinary Office.

 

How early can I test?

A herd test can be done in the month preceding the due date.

 

How soon will my herd be restricted after the due date?

A herd is restricted if a test is not completed and processed within 7 days after the due date.

 

Who is doing my test?

A herd test can be done by a DAERA vet or a private vet. Most AHTs (Annual Herd Tests) are done by private vets. You will have nominated your preferred private practice. DAERA vets carry out most high risk tests such as Reactor Herd Tests following disease disclosure.

 

Do I have to test calves at the TB test?

It depends on what type of test is required in your herd. Usually calves under 6 weeks of age are not tested in AHTs. However, bought-in calves have to be tested in all herd tests. Calves under 6 weeks of age, which are in their birth herd, have to be tested at Reactor Herd Tests (RHT, RH1, RH2) during TB breakdowns.

 

Can the TB and Br test be carried out together?

Yes. This is coordinated when your private vet sends DAERA the list of their tests for the following week. We will then try to synchronise the TB and brucellosis tests if they are both due.

 

Can I buy, sell or go to a meat plant in the middle of a test?

The TB test requires two visits three days apart. Between the 2 visits, you can buy in but you cannot sell. This is because we do not know the test result and do not want animals that may show signs of reaction moved off farm. If you intend to send animals to a meat plant in the middle of a test it is essential that you seek permission from a Patch Vet in the Divisional Veterinary Office in advance of the commencement of day 1 of the test.  If permission is granted by the Patch Vet, you must  identify and provide the tag numbers of the animals involved to the testing vet on Day 1 of the test. These animals must be tested on Day 1. This is so that if there is a delay in slaughter, the test can be completed.

 

What is the TB test cycle for different tests?

AHTs (Annual Herd Tests) are set for the same due date each year.
LCTs (Lateral Check Tests) are generally required every 4 months, which means that they are set with a due date 5 months ahead to allow for completion of the test. This pattern may be altered because of grazing arrangements, housing, etc, which may have an impact on risk.
CHTs (Check Herd Tests) are carried out 5-6 months after a herd breakdown is resolved.

 

Why do I have to do an LCT (Lateral Check Test) on my whole herd when I only keep a few animals on the ground that is beside the breakdown?

The risk to your herd from a breakdown herd is assessed by the DAERA vet for your area (the Patch VO). You can phone and discuss the situation with him/her. TB may have spread from animals which grazed beside the breakdown to other parts of your herd - these will be missed if the whole herd is not tested. Occasionally a part herd test may be considered rather than a full herd test.

 

What is required from me to carry out any TB test?

  • all cattle should be properly tagged for the test
  • the herd keeper or someone acting on their behalf must be present during the test and adequate help must be provided to collect, pen and secure the animals for identification and testing
  • the up to date herd register must be available for inspection. All births and deaths should be notified to DAERA before the test starts
  • no animals should be withheld from the test
  • if, in exceptional circumstances, a test has to be postponed, the herd keeper must advise the testing vet immediately
  • once the test starts, no medication should be given. In some circumstances medication can interfere with the test. In an emergency, please consult with the local DAERA office. Any herd treatments should be carried out after the test has been completed on the day the test is read 
  • Veterinary Medicine Record Book

 

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