In NI, surveillance for TB is carried out through skin testing, supplementary blood testing and abattoir inspections. While clinical cases of bovine TB do sometimes occur, they are very rare. Testing schedules are designed to match the level of risk of infection in a herd. It is important that infection is detected as early as possible to limit onward transmission of infection.
Herd level surveillance
| WHY IS HERD SURVEILLANCE AND TESTING NEEDED?
Herd level testing can identify most TB reactors in a herd. They can then be isolated and removed, so that the spread of the bacteria can be limited, meaning that fewer additional reactors will result and the outbreak is limited, freeing the herd to trade animals again. Herd level testing is also a requirement for trade with the EU, affecting intra-Community movements of cattle for breeding, production or slaughter. |
All bovine herds, including all calves aged 6 weeks and over, are tested for TB at least once per year. Cattle are injected with two types of tuberculin on day one of the test, and the skin reactions to these injections are measured, compared and evaluated on day 4.
When TB is disclosed or suspected in a herd, a more rigorous testing schedule is imposed. If no testing has been carried out recently, then the herd will be tested as soon as possible (at a Restricted Herd Test) to attempt to identify other reactors at an early stage so that they can be removed, reducing the risk of spread to healthy animals.
In general, two clear full herd tests are needed (Restricted Herd Tests (RH1 and RH2)), at least 60 days apart, to give reassurance that the herd does not have TB. The final test before derestriction (RH2) will only be set if there were no skin reactors at the RH1 test. Once reactors are disclosed, calves, including those under 6 weeks of age, may be tested.
Samples taken from reactor animals at slaughter are tested through microscopic examination and bacteriological culture (the latter of which takes up to 6 weeks). When TB is confirmed, either through laboratory testing, or when there is more than one reactor detected, or when a significant number of cases have been detected in an abattoir, future skin tests will be assessed using ‘severe interpretation’. Using a more stringent test interpretation increases the likelihood of detecting infected animals.
The skin TB test is estimated to miss one in four infected animals. Repeat herd testing is carried out to help detect infected animals previously undetected and any newly infected animals in the herd (for example, recently introduced animals that are infected or wildlife infected animals).
At least one Check Herd Test (CHT) will be carried out six months after herd restrictions have been lifted, to provide increased assurance that there is no residual infection in the herd. In the case of confirmed outbreaks, a second CHT is required after a further six months.
TB in neighbouring herds
| WHY IS MY HERD BEING TESTED WHEN MY NEIGHBOUR’S HERD HAS TB?
There is a risk that TB can spread to neighbouring herds, so it is worth checking all herds that are contiguous to a breakdown or that have significant connections with a breakdown herd. Neighbouring herds, whether grazed or housed, can sometimes be the source of an infection. |
Due to the risk of spread of TB across boundaries to contiguous herds, either through cattle to cattle transmission or because of a common wildlife source, Lateral Check Tests (LCTs) will be scheduled for high-risk herds following a mapping exercise.
Animal level surveillance
| WHY ARE CARCASES CHECKED FOR SIGNS OF TB?
Post-mortem inspection for TB is carried out on all cattle intended for human consumption because TB is a disease of public and animal health significance. (Meat from reactor cattle can enter the food chain, provided that lesions are not found in multiple parts of the carcase.) Post-mortem inspection can also be a useful way to detect TB between routine herd tests. |
Post-mortem examination for gross signs of TB is carried out on all cattle going for human consumption. When TB is disclosed, it is described as finding a ‘Lesion at Routine Slaughter’ and an immediate herd test is carried out if the herd has not been tested recently.
Inconclusive test results
| WHY DO INCONCLUSIVE TEST RESULTS MATTER?
An Inconclusive reactor has shown a reaction to tuberculin; however, the reaction is not strong enough for it to be deemed to be a reactor. It presents a risk of TB spread to the rest of the herd, so it’s important to isolate these animals effectively and have them retested promptly when the test is due. |
Cattle with inconclusive test results are much more likely to be infected compared to cattle that have negative tests. Following identification of an inconclusive animal, the Officially TB-free status of the herd is suspended. A negative result on retesting the herd will lead to derestriction of the herd. If the inconclusive animal has a further inconclusive test result or a positive reaction, the herd will be restricted as a TB breakdown herd.
Tracing tests
| WHY DOES MY HERD NEED TESTED AS A RESULT OF TRACING?
There is a risk that TB infection could be present in a traced herd and be the source of infection for other herds, therefore checking the herd’s TB status is important in limiting transmission pathways. |
When TB is disclosed in a herd, either because of a Lesion at Routine Slaughter finding or following a confirmed test, then tracing is carried out to identify herds which have moved reactor animals into the TB herd during a defined period. Any of these herds identified because of backward tracing will be subject to a herd level Backward Check Test.
Forward Check Tests may be carried out to identify herds into which risk cattle have moved from the reactor herd.
Overdue tests
| WHY DO MY CATTLE HAVE TO BE TESTED AT THE SAME TIME EVERY YEAR?
Timely testing is crucial in the efforts to control TB. It’s important that the status of every herd in NI is established at least every year, so that progress can be monitored, and the legal requirements are met that allow commercial trade. |
If a herd test becomes overdue, then movements of cattle may be restricted from the herd, except to slaughter. However, if the test is not completed within 1 month of the due date, then inward movements may also be restricted. If an annual test goes overdue for 3 months, then the herd will need to have 2 clear tests (60 days apart) to obtain a TB-free status again.
Tests are expected to be carried out within required timeframes – if the timing targets are not met, then the officially TB free (OTF) status will be removed and replaced with a Suspended (OTS) status, which may be downgraded to a Withdrawn (OTW) status. An individual animal that has not been tested for 15 months will be placed under restriction, except for a movement to slaughter.