# Detection and Comparison of Sow Serum Samples from Herds Regularly Mass Vaccinated with Porcine Reproductive and Respiratory Syndrome Modified Live Virus Using Four Commercial Enzyme-Linked Immunosorbent Assays and Neutralizing Tests

## TL;DR

This study examines why some vaccinated sows test negative for PRRS antibodies using ELISA tests, suggesting that relying on a single test may not accurately reflect their immune status.

## Contribution

The study identifies discrepancies in ELISA test results for PRRS-vaccinated sows and highlights the limitations of relying solely on N-protein-based ELISA kits.

## Key findings

- ELISA-negative cases were frequently reported in MLV-vaccinated sows, especially in high-parity sows.
- Poor agreement was found among different ELISA kits, with positive rates ranging from 50.0–100%.
- Only 15.9% of ELISA-negative samples tested negative in a viral neutralizing test, indicating potential insensitivity of ELISA kits.

## Abstract

The immune status of porcine reproductive and respiratory syndrome (PRRS), evaluated and monitored using one specific commercial enzyme-linked immunosorbent assay (ELISA) kit, was well received and widely implemented by pig farmers. However, ELISA antibody-negative cases were frequently reported in sow herds regularly mass vaccinated with PRRS virus modified live virus (PRRSV MLV). In this study, a cross-sectional investigation was conducted to assess four main issues: firstly, whether ELISA-negative cases commonly exist in MLV-vaccinated sows; secondly, to assess the correlation between common commercial ELISA kits based on the nucleocapsid (N)-protein and glycoprotein (GP)-protein, including the positive rate, antibody level, coefficient of variation, and agreement rate; thirdly, to investigate if there are disparities within sows at different parities; and lastly, to determine if the sole use of N-protein-based ELISA kits is adequate to evaluate the immune response following vaccination.

Porcine reproductive and respiratory syndrome virus (PRRSV) modified live virus (MLV) vaccination is used to control PRRSV. In China, farms conduct random sampling from sow herds every 4 to 6 months. They use the enzyme-linked immunosorbent assay (ELISA) method to monitor the immune status of the herd by tracking the positive rate or the sample-to-positive ratio. However, in farms that implement mass vaccination and have stable production, the positive rate of ELISA antibodies has decreased, especially in high-parity sows. This poses a considerable challenge to the current monitoring approach of PRRSV immunity. It remains unclear whether this reflects insufficient sensitivity of the kits for these special scenarios or the fact that the sows have truly lost immunity. In this study, 233 samples from four farms (A–D) across different regions of China were acquired. They were tested using four representative ELISA kits, two targeting the nucleocapsid protein (N) and two targeting the glycoprotein (GP) to evaluate PRRS immune status. The respective sample positive rates in A–D were 57.1–100%, 50.9–100%, 50–100%, and 75.7–100% using the kits. The positive rates using the four ELISA kits were 50.0–75.7%, 70.0–75.7%, 82.5–97.1%, and 100%, respectively, with poor agreement among them. The positive rates and humoral antibody levels for parity 1 and 2 sows were significantly lower than those with higher parities (>4). Eighty-eight ELISA-negative samples identified using ELISA kit A were verified using a viral neutralizing test (VNT), with only 15.9% of the samples testing negative. In conclusion, the ELISA antibody negativity issue existed, mostly occurring in specific farms tested using a specific kit. However, the low correlation with the VNT results and the poor agreements among the kits suggest that relying on one ELISA test is insufficient to monitor the immune status of PRRSV MLV-vaccinated herds.