Field sample evaluation of the “adapted for oral fluids” PRRS X3 ELISA protocol compared to the commercial PRRS Oral Fluids ELISA

The commercial PRRS Oral Fluids ELISA is a highly sensitive assay, particularly when detecting PRRSV exposure through antibody monitoring in large swine populations. Prior to the launch of this commercial assay, a modified protocol was developed for oral fluids using the existing PRRSV antibody serum test (PRRS X3). Although the modified test also has excellent specificity and sensitivity, the protocol requires overnight incubation and titration of a non-standard conjugate. This study evaluated the performance of these two assays, using identical samples submitted to 3 different laboratories.

A field study was performed to compare test results from oral fluid field samples using the USDA licensed IDEXX PRRS OF Test (OF) and a PRRS X3 modified protocol (ON) for oral fluid samples. Two hundred and ninety four expected antibody negative oral fluid samples from 34 field locations of a single production system were tested in two laboratories (A and C), and a subset (264) were tested at a third laboratory (B). Samples were tested within 1-5 days of collection in laboratories A and C, and after 1 freeze-thaw cycle at laboratory B. Laboratories A and B performed the ON protocol, while laboratory C performed testing using the commercial IDEXX PRRS OF Test. All three laboratories used the published cut off of S/P ≥ 0.40, and all three centrifuged samples prior to testing. For Laboratory A, S/P values between 0.20 and 0.40 were considered suspect and therefore additionally defined.

Negative (S/P < 0.4) results were obtained in 292, 264 and 272 of the oral fluid samples from Laboratories A, B and C, respectively. An additional 5.4% of Laboratory C S/P results ranged between 0.4 and 0.7.

After follow-up sampling and re-testing, 3 of the 34 expected negative field locations were determined to be seroconverting. These 3 locations represented 48 samples of which the OF Test detected 10 positive, whereas the ON protocol in Laboratory A detected only 2 positive and 2 suspects, at S/P ~0.2.

Outside of the one confirmed seroconverting nursery, there were 9 other nursery sites represented. Age at sampling and transition diet make up could not be confirmed, therefore detection of residual maternal antibodies (several nursery sources were PRRS positive stable sow farms) and/or the impact of sprayed dried plasma cannot be ruled out.

The commercial PRRS OF kit is a highly sensitive test which was able to detect early seroconversion before the modified ON protocol. There was also numerical variation noted between results of the labs performing the ON protocol, which may have been influenced by the non-commercial adaptations to the PRRS X3 kit for OF.

Disclosure of Interest: None Declared.