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

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.