A MODIFIED SERUM NEUTRALIZATION TEST FOR THE DETECTION OF ANTIBODY TOPORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS IN SWINE SERA

Various conditions were evaluated and modified to improve the sensitiv ity of the serum neutralization (SN) test for detecting antibody in pi gs infected with porcine reproductive and respiratory syndrome virus ( PRRSV). Higher SN titers were consistently obtained by the addition of 20% fresh swine serum to the virus diluent and by the use of a permis sive cell clone (MARC-145) derived from the MA-104 cell line. Test ser a used to assess the SN test were obtained from 2 groups of 3-week-old pigs infected intranasally with PRRSV (MN-1b). Using the modified met hod, SN antibody was first detected 9-11 days postinoculation (PI), wi th a peak evident at 11-21 days PI. The antibody subsequently declined , and a second peak was observed between 41 and 45 days PI. The first antibody peak was not observed and the SN antibody was only detectable between 32 and 41 days PI when the test was done with 20% heated swin e serum or without supplemental swine serum. The SN antibody during 2- 3 weeks PI was found to be sensitive to 2-mercaptoethanol or anti-swin e IgM treatment. The SN antibody titers were high when homologous PRRS V isolate was used in the test but were markedly low for heterologous PRRSV isolates. No difference in antibody titers was observed when hom ologous and heterologous PRRSV isolates were tested by indirect fluore scent antibody assay. These results indicate that the modified SN meth od is useful in detecting earlier and higher PRRSV antibody and that i t can differentiate among PRRSV isolates.