Herd-level assessment of a kinetics ELISA in New York State dairy herds

Testing for Mycobacterium avium subsp. paratuberculosis (Map) is typically done on a group of cows (e.g. in control programs), which changes the interpretation of the herd test results. In New York State, a kinetics ELISA (KELA) with multiple thresholds is used and the results of individual cows are reported to the farmers. The implication of the KELA results for the whole herd are not reported yet. The objective of the current study was to use probability theory to interpret KELA results at the herd-level. The likelihood of being a non-infected or an infected herd was determined based on the proportion of cows in the four KELA categories. The herd-level interpretation of the test results was validated on real herd data. When 10, 30, 50, 100 or 150 cows are tested the odds of being non-infected are higher than the odds of having a TP of 1 % when there are at most 0, 2, 3, 4 and 7 KELA reactors respectively. When in a sample of 10 cows a single cow tests positive (65 < KELA < 135), the sample most likely (58 %) contains a truly infected cow and the probability increases to 95 % when 2 cows of the 10 test positive. A sample with one cow with a KELA over 135 is very likely (> 90 %) to contain at least one truly infected cow. Herd-profiles for the probability of infection can be developed based on the distribution of cows in the KELA categories. The probability for a herd to be infected is mostly dependent on the proportion of cows in the highest KELA category.