How can TB be detected in live animals?
In Northern Ireland, live animal surveillance is undertaken using:
- The TB skin test – Herds and animals are tested using the Comparative Intradermal Tuberculin test. This means animals are injected with 2 different types of tuberculin (avian and bovine) and the results at the two sites are compared.
When the test is being used to certify animals for export the avian response is ignored and the test is interpreted based on just the response to bovine tuberculin. This means that occasionally an animal which has not been classified as a reactor or inconclusive may still not be eligible for direct export.
- Clinical examination of animals – If any animal shows signs that are suspected to be due to TB then legally this must be reported to DAERA.
- Interferon Gamma (IFN-g) testing – This is generally referred to as ‘the TB blood test’ and is used as a supplementary test to the skin test.
How do I prepare for a TB test? What do I need to know?
- All cattle should be properly tagged for the test and handling facilities must be fit for purpose.
- As herd keeper you or someone acting on their behalf must be present during the test. Adequate help must be provided to collect, pen, and secure the animals for identification and testing.
- Herd records must be up to date (birth, deaths, and movements). All births and deaths should be notified to DAERA before the test starts.
- All animals 6 weeks of age and over must be presented for skin testing.
- In the case of the blood test, you will be advised which cattle need to be blood tested.
- If, in exceptional circumstances, you need to postpone the test you must advise the testing vet immediately.
- In some circumstances medication can interfere with the test. Once the test starts, no routine medication should be given. This includes dosing and vaccinations.
- However, if an animal is sick or lame and its welfare is likely to be compromised unless medication is administered, please consult with your veterinary practitioner, and follow their advice. If medication is given you should advise the local DAERA office as soon as possible as well as the vet conducting the test.
- Any herd treatments (for example dosing or vaccination) should be carried out after the test has been completed on the day the test is read.
- Veterinary Medicine Records should be made available to the testing vet so that they can enter or provide details of the tuberculin used to carry out the test.
- If you have any difficulty getting your test arranged please get in touch with your testing vet and DAERA as soon as possible and before the test becomes overdue.
How is the TB skin test carried out?
The TB skin test is the common name for the Comparative Intradermal Tuberculin test.
It works by detecting the animal’s immune response against the disease and is regarded as the single best indicator of infection with M. bovis in live animals. In Northern Ireland, all herds are skin 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.
One type of tuberculin is injected under the outer layer of the skin of the neck at each of the two sites. One tuberculin is made from killed M. bovis (bovine tuberculin) and the other from killed Mycobacterium avium (avian tuberculin), the bacterium that causes TB in birds.
Avian TB does not cause disease in cattle but at the skin test it can cross react with bovine tuberculin if cattle have been exposed to it, so the avian tuberculin is injected as a control.
On Day 4 of the test, the skin reactions to the two types of tuberculin are measured and compared.
How are the skin test results interpreted?
Routine skin tests (e.g. annual herd tests) are interpreted under “standard interpretation”. This means:
- When the bovine site reaction exceeds the avian site reaction by more than 4 mm, the animal is a reactor under standard interpretation. When the test is being read all reactors will have a DNA tag applied.
- If the bovine site reaction measures 1-4 mm more than the avian site reaction, the animal is an inconclusive (“doubtful”) under standard interpretation.
- If the bovine reaction is no larger than the avian reaction the test result is negative.
Where infection is confirmed in a herd, more stringent interpretation criteria are used. This is often referred to as “severe” interpretation.
The DAERA Patch Vet who is managing a breakdown will assess the level of risk and may also decide to apply a stricter interpretation or to remove additional skin test negative high-risk animals as ‘Negative in Contacts’ (NICs).
Do all infected animals test positive on the skin test?
Unfortunately not. The ability of a test to correctly identify an infected animal is referred to as the test “sensitivity”. The higher the sensitivity of the test, the lower the chance it will miss infected animals.
Numerous studies have attempted to quantify the sensitivity of the skin test:
- Studies in GB have suggested that test sensitivity is between 51% and 100% with an average figure of approximately 80% at standard interpretation.
- A study carried out in NI in 2019 estimated the sensitivity of the skin test at standard interpretation to be approximately 88%.
Therefore, as a rule of thumb it is likely that approximately 20% of TB-infected cattle may test clear on any one skin test using standard interpretation.
Using severe interpretation significantly increases the sensitivity of the skin test and therefore reduces the chance of infected animals escaping detection.
Repeating the skin test also reduces the risk of leaving infected animals undetected in the herd.
Do all skin test “reactors” have TB? Could my reactor be a false positive?
The ability of a test to correctly identify an animal that is free from infection as negative is referred to as the specificity. The higher the specificity, the lower the probability of “false positives”.
The skin test has an excellent test specificity of 99.98% under standard interpretation. This means that it is very rare that a non-infected animal will be wrongly classified as diseased. The absence of visible “lesions” at post-mortem does not mean that an animal was not infected.
What about the ‘TB Blood Test’? What are the pros and cons?
- The interferon gamma (IFN-g) test is a blood test which has been approved since 2002 in the EU. It is most often used in conjunction with the TB Skin Test to improve the chances of detecting infected animals in herds where TB has already been confirmed.
- Animals which have been exposed to M. bovis will often respond to the IFN-g test before they will respond to the TB skin test. This means it can be useful in detecting infected animals at an earlier stage of the disease. The IFN -g test may also identify some TB infected animals which do not respond to the skin test.
- Currently, uptake of the IFNG test is voluntary although it is compulsory for farmers to give up any IFN-g test positives that are detected, even when they are skin test negative.
- The IFN-g test has a higher sensitivity than the skin test. This means it has less false negatives and will miss less TB infected animals. For this reason it can often be more useful in clearing a herd of infection than just using the skin test alone.
- Unfortunately, it has a lower specificity, and therefore more “false positives” compared to the skin test. We estimate that approximately 3-4% of animals tested will be false positives. This means 3 or 4 out of every 100 negative animals are likely to test positive.In Northern Ireland due to the lower specificity, IFN-g testing is mainly used in particular high risk breakdown situations.
- This test is not suitable for use in calves under 6 months of age as the results in young calves may be unreliable.
- There are some logistical limitations as samples must be received at the lab within tight time limits.
Why do we still use the TB Skin test? Are there other options?
The “skin test” is widely used throughout the world. It works by detecting the animal’s immune response against the disease and is still regarded as the “single best indicator of infection with M. bovis in live animals”. Although some improvements have been made over the years (for example using better tuberculins), the skin test we use in NI has remained largely unchanged for decades. This test is still the cornerstone of our TB programme.
In order to maintain trade, countries can only use tests that are approved by the EU and WOAH (World Organisation for Animal Health (WOAH). Currently only the skin test and the IFNG blood test are approved for use in EU Approved TB Eradication Programmes and are suitable for use in NI. DAERA will continue to monitor the science and in the future other tests may gain approval and be considered for use.
The IFNG blood test has too low a specificity for widespread use on uninfected herds as it would generate an unacceptable (and unaffordable) number of false positives in these herds. It is however very useful in helping to clear infection from infected herds.
How can TB infection be diagnosed or confirmed after slaughter? What are “lesions”?
Over time TB infection results in the development of granulomas known as “lesions” in the carcase. If lesions grow large enough to be seen with the naked eye they are called “visible” lesions.
TB lesions are typically seen in the lymph nodes of the head and chest and in the lungs, but they may also occur in the gut and at other sites.
Every bovine animal slaughtered in Northern Ireland for human consumption undergoes routine meat inspection to check the carcase and organs for signs of disease, including bovine TB.
Reactor animals undergo a more detailed post-mortem examination to search for TB-like lesions. Your Patch Vet will be able to tell you if your reactor/s had any visible lesions or you can also get postmortem information via NIFAIS on Line (NoL).
At the post-mortem examination, samples may be taken and sent to the Agri-Food Biosciences Institute (AFBI) at Stormont. Staff at the laboratory will examine the samples under the microscope, which is called a ‘histology test’.
They may also try to grow the bacteria to see if it contains M. bovis which is called the ‘bacteriology’ or ‘culture’ test.
Histology results usually take about two weeks to be completed and it usually takes at least eight weeks for DAERA to receive culture results, since M. bovis grows very slowly in laboratory conditions. Information on laboratory results is available from the Patch Vet dealing with your breakdown.
What is an LRS?
Sometimes animals that are not TB reactors will show ‘TB-like lesions’ at routine meat inspection. Such animals are termed ‘Lesioned at Routine Slaughter’ or ‘LRS’. Samples from LRS animals are always sent to the lab for further investigation.
LRS animals will trigger a herd restriction and if disease cannot be ruled out by the lab, a TB Breakdown.
Why was TB infection found in an animal that I sent to slaughter when my last herd test was clear?
There are 3 possible reasons for this:
- The animal may have become infected after the last test.
- The animal may have been infected at the time of the last test, but the infection was too early to be detected by the skin test.
- The animal may have been a false negative as not all infected animals will test positive. (See Section on skin test sensitivity)
How likely are my reactors to have “visible lesions” detected at slaughter?
Recent figures show that approximately half of all reactors are visibly lesioned. The remainder are NVL (i.e they showed No visible lesions).
If an animal has “no visible lesions” how can it have TB?
If no lesions are found at post-mortem examination, this does not mean that the animal was not infected with TB. The skin test has a very low level of false positives.
There are several reasons why so many reactors have no lesions reported:
- Not all TB lesions can be seen due to their location or very small size. A TB lesion in an animal may be smaller than the size of your thumbnail or may be in a site where it can be very difficult to detect.
- Infected animals are often detected by the test before visible lesions have developed.
- The post-mortem is carried out “on the line” at the abattoir so there is only a very limited time frame for carrying out the examination. Meat inspection is carried out mainly for public health reasons and is not designed to be a forensic postmortem.