This is one of the TB cattle control enhancements introduced by Defra in April 2017 following a public consultation.
Once TB is found in a cattle herd, movement restrictions are applied and the herd is subject to a series of short interval tests (SITs). Previously, some SITs were scheduled at least 60 days after the previous herd test where reactors were identified. In other cases, they were scheduled at least 60 days after any reactors in the previous test had been removed i.e. a minimum of 60 days after the last possible day the reactors could have infected other animals on the farm.
TB reactors must be isolated immediately from the rest of the herd pending their removal, to reduce the risk of spreading bovine TB. However, the effectiveness of on-farm isolation for reactors can vary markedly between different farms, and this further reinforces the need to schedule the next SIT from the date of removal of the reactors rather than the date of detection.
The minimum interval of 60 days between two successive skin tests also prevents the phenomenon known as desensitisation, whereby an animal’s skin reactivity to tuberculin (the proteins used in the skin test) is depressed for some time afterwards. If another injection is administered too soon after the first, without sufficient time for the animal’s immune system to recover, it can result in failure to identify infected cattle as reactors. It is important to note that repeated skin testing in chronically TB-infected herds does not appear to have a significant desensitising effect on cattle, as long as the 60 day interval between tests is observed.
The control measure
To ensure a consistent approach and manage the disease risk appropriately, when reactors are identified in a herd, the next SIT must take place at least 60 days after removal (rather than detection) of all the reactors. This was already being applied by the Animal & Plant Health Agency (APHA) to some TB breakdown herds, however implementation of this measure ensures that it is applied consistently across the whole of England.
Interferon-gamma blood testing is sometimes used alongside skin testing in TB breakdown herds. When gamma testing takes place between skin tests, the date of the next scheduled SIT will not be affected even if positive animals are identified by the blood test, however additional skin testing may be required.
APHA acknowledges that effective implementation of this measure relies on timely removal of reactors from TB breakdown herds. Reactor removal times are tightly controlled and closely monitored by APHA to ensure that as far as possible, reactors are removed within 10 working days of disclosure, however on occasion delays can happen.
Cattle keepers also have the option to request delayed reactor removal in limited situations (e.g. to allow heavily in-calf reactor animals to calve). Each request is considered individually and decisions are informed by a veterinary risk assessment. If the risk of spread of bovine TB is considered to be high, then the request is refused. It is important to note that in cases of delayed reactor removal, regardless of the reason, subsequent testing of the herd must take place a minimum of 60 days after the removal of the last reactor. Consequently, there would be a delay with scheduling the next test and an increase in the length of time the herd is under movement restrictions.
Benefits of the measure
Bovine TB is a slow, progressive disease and once an animal is infected it takes time for it to develop an immune response that is detectable by the skin test. The minimum interval of 60 days between skin tests is designed to allow development of this immune response and so maximises the potential to detect any TB-infected animals that may remain in the herd.
From a disease control perspective it makes good sense to start the 60-day interval between skin tests from the last possible day that reactors could have infected other animals on the farm. This reduces the risk of an infected animal being missed at the next skin test and spreading disease to other cattle in the herd and so increases the likelihood of the herd regaining its officially TB free (OTF) status at an earlier date.
In all cases a herd’s SIT is scheduled at least 60 days after reactors identified in the previous test have been removed. This replaces the historic practice whereby SITs in some herds were scheduled 60 days after the date of the last test while reactors were still on the farm.
To ensure a consistent and rigorous approach across the whole of England. Scheduling SITs at least 60 days after reactors have been removed maximises the potential to detect infected animals at the next skin test.
When skin and gamma tests are carried out together in TB breakdown herds and blood test positive animals are identified (gamma reactors), these animals are taken into account when scheduling the next SIT. The next test is scheduled for at least 60 days after all reactors (i.e. skin test and gamma test reactors) have been removed. If gamma reactors are found between SITs, they are removed as usual and the next SIT already scheduled will stand. Depending on the post mortem and any culture results of the gamma reactors, additional testing may be required.
Prior to April 2016, TB breakdown herds in England only had their disclosing test re-interpreted at severe interpretation if one or more reactors showed visible lesions of TB at post mortem. This sometimes resulted in additional reactors being disclosed upon re-interpretation of the test, and a subsequent delay in scheduling the SIT whilst awaiting removal of the additional reactors. From April 2016, all TB breakdown herds in England (apart from herds in the Low Risk Area that meet certain criteria) have their disclosing test re-interpreted at severe interpretation regardless of the post mortem results of the reactors. This has greatly reduced the number of cases where additional reactors are disclosed at the next test. There are still situations in TB breakdowns where re-interpretation of test results is necessary, however it is uncommon.