A stop solution (PRV gI
Antibody Test, IDEXX Laboratories) was dispensed into
each well, plates were read at 405 nm (Model AL310, Bio-
Tek Instruments, Inc., Winooski, VT, USA) and the results
given as optical density (OD). For each plate, sample results
were adjusted by subtracting the OD of the negative
control. ELISA values more than three standard deviation
units above background were considered positive.