Validation of the specific isotype assay to detect antibodies against foot-and-mouth disease virus in bovine milk

The specific isotype assay (SIA) detects IgG, against foot-and-mouth disease (FMD) virus in bovine milk. A strong correlation was demonstrated between milk antibody titres, and those in serum as measured by the liquid phase blocking ELISA. Thus the SIA would be useful on a herd basis to monitor the milk of vaccinated cattle to determine when re-immunisation is advisable. The SIA titration ELISA was then simplified to a single dilution test and optimised to differentiate the reactions in the milk of FMD-naive cows from those in animals which had been infected with FMD or vaccinated against the disease. For milk from immunised cattle, the pH of the sample was important and borderline positive specimens with a pH of 6.0 or below gave negative results. For milk from naive animals, the optical density (OD) registered in the SIA varied according to the time of year that samples were collected which, in turn, influenced the OD above which milks might be considered positive. Studies showed that the pH of milk could be maintained within the range suitable for the SIA by either storing for up to 1 week at 4 degrees C or by freezing at - 20 degrees C for an indefinite period. (C) 2000 Elsevier Science B.V. All rights reserved.