The impact of milk handling procedures on Ostertagia ostertagi antibody ELISA test results

The impact of various milk handling stressors were analyzed using a commercially available enzyme-linked immunosorbent assay (ELISA) test measuring Ostertagia ostertagi antibodies in milk from dairy cattle (Svanovir®). An indirect ELISA has the ability to determine the amount of milk production losses related to intestinal parasitism. The ELISA test recommends fresh defatted milk, however, milk collected from Dairy Herd Improvement (DHI) programs in North America undergo many stressors, including, heating, freezing and are not defatted. Normalized optical density ratios (ODRs) were compared between fresh defatted milk and milk subjected to one or more stressors with a linear mixed model accounting for differences in variation between the fresh and the frozen samples. Concordance correlation coefficients were also analyzed for comparisons to other similar studies.After accounting for random cow and container effects, the treatment factors interacted with each other (p < 0.001). Biologically interesting contrasts were created to explain the interaction. The estimated difference in ODR between the milk samples handled according to recommendations of the manufacturers of Svanovir® and the whole milk samples that were subjected to the most extreme treatment (heated, frozen, thawed, and re-frozen for 4 weeks) was 0.062 (p < 0.001). This difference represented less than 5% of the range, and was thus considered biologically negligible. Frozen whole milk processed by DHI programs, the most likely method of collecting on-farm samples in North America, will likely yield reliable results for the indirect ELISA tests, particularly, Svanovir®.