Dairy companies must provide bulk tank milk samples for monthly testing, and every dairy supplying milk for commercial processing has a sample from their bulk tank milk tested each fortnight.
Tanker operators take the samples at the point of collection – farmers don't have to do anything extra.
Many factors influence our ability to detect antibodies produced by infected cows. Factors include the number of infected cows contributing to the milk supply, the stage of infection, average milk production per cow and vat volume.
For this reason, it is important that dairies are screened at different stages of the lactation cycle to give us the best chance of detecting infected dairies.
The ELISA (enzyme-linked immunosorbent assay) looks for antibodies that the cow has made to fight off M. bovis.
A bulk tank milk 'non-detect' test result does not prove absence of infection. Nor does a bulk tank milk 'detect' test result prove presence of infection.
M. bovis infection status can only be determined by on-farm testing of the herd.
Farms with a 'detect' result will be placed under a Notice of Direction (NOD) restricting cattle movements while on-farm blood sampling, and testing of the herd is carried out to determine its disease status.
More than 97% of farms with a 'detect' result are found to be clear of infection after on-farm investigation is completed.