Dynamics and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 following modified live PRRSV vaccination in a PRRSV-infected herd

The objective of this study was to investigate the dynamics and evolution of porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 following the use of a modified live PRRSV (MLV) vaccine. A PRRSV-positive farm with coexistence of types 1 and 2 and no history of MLV vaccination was investigated. Vaccination with a type 2 MLV (Ingelvac PRRS MLV, Boehringer Ingelheim, USA) was implemented. All sows were vaccinated at monthly intervals for two consecutive months and then every third month. Piglets were vaccinated once at 7-10 days of age and weaned to nursery facilities at 21-23 days of age. Serum samples were collected monthly before and after vaccination from four population groups, including replacement gilts and suckling, nursery and finishing pigs, and assayed by PCR. After a year of blood collection, amplified products were sequenced, resulting in 277 complete ORF5 gene sequences from 145 type 1 and 132 type 2 isolates. Prior to and following vaccination, both type 1 and type 2 PRRSV were isolated and found to coexist in an individual pig. Each genotype evolved separately without influencing the strain development of the other. Although the substitution rates of both genotypes were relatively similar, MLV vaccination appears to increase the heterogenicity of type 2 PRRSV, resulting in the emergence of three novel type 2 PRRSV clusters in the herd, including an MLV-like cluster, which disappeared within the month following whole-herd vaccination. Two additional clusters included one related to the MLV vaccine and one related to the endemic cluster of the herd.