23 Nov Short Interval Testing Regimes

APHA have recently released the OV Briefing note 18/23 detailing the testing regimes for bovine TB breakdown herds in England, available at http://apha.defra.gov.uk/documents/ov/Briefing-Note-1823.pdf

This note explains why the follow-up testing regimes after reactors are disclosed on a test appear to vary from farm to farm and from test to test.  What the different regimes try to address is the differing positive predictive value of the skin test which will vary depending on the level of infection in the population being sampled, something which should be dropping in a herd as they move through the breakdown process. 

The number of follow-up skin tests a reactor will trigger depends at what stage in the breakdown process the herd is, i.e. was the test the initial disclosing test or a follow up test, and also on whether animals removed as reactors have lesions at PME and whether any samples taken are either PCR or culture positive.  To summarise, not all skin test positive reactors will automatically trigger two further short interval tests, some may only see one further short interval test being carried out on that herd.  As explained above this is because not all reactors to the skin test are equal in terms of the positive predictive value of the test in that particular situation, hence greater emphasis on postmortem and culture/PCR results at subsequent rounds of testing.

We have suggested that the decision trees in the briefing note also be published on the TB Hub so that farmers can access this information. 

If there are further areas of policy that appear mystifying, or decisions that do not seem to make sense around testing regimes and policy, please do always let us know.  We can request information that may be helpful in explaining decisions that are made which should help TB testers to better inform clients on likely next steps or outcomes in their herd.