Feed As a Vehicle for Prrs Virus Transmission and the Effects of Formaldehyde on Porcine Reproductive and Respiratory Virus in Feed: Proof of Concept

This study was conducted to determine the risk of porcine reproductive and respiratory virus (PRRS) infection through feed using a PRRS modified live virus vaccine (MLV) mixed in feed as a model, and to determine if treating feed with Sal CURB [(SC) Kemin Industries, Des Moines, IA] reduced the risk of PRRS transmission through feed. Pigs (N=24; 21±0.5d old) were used in a 5-d study conducted as a RCBD (block = litter). One day before weaning, 4 pigs of similar BW and the same litter of origin were randomly assigned to one of the following treatments offered on day 1 of study: Trt1) No MLV exposure; Trt2) SC Feed+PRRS: Pigs offered 2 rations of feed paste (300g of sow feed treated with SC at 3.25kg/Metric ton mixed with 150ml MLV (Ingelvac PRRS^® MLV, Boehringer Ingelheim, St. Joseph, MO), 150ml of water, and 150g of nursery feed); Trt3) No-SC Feed+PRRS: as Trt2 but feed on feed paste was not treated with SC; Trt4) Positive Control: 2ml MLV intramuscular injection. At weaning, pigs were transported to a BSL2 facility and placed by treatment in pens of 2 pigs. All pigs from the same treatment being located in one room. After a 5-day acclimation period, blood samples were collected and treatments were applied accordingly. Throughout the study period, all research activities and daily chores were conducted following the same order of treatments (1 to 4). Pigs were fed a common nursery diet throughout the acclimation and study period. On day 5 of study, serum was collected via venipuncture and pigs were humanely euthanized for collection of tonsils. Serum, tonsils, and feed paste samples were sent to Iowa State University Veterinary Diagnostic Lab for quantitative polymerase chain reaction (qPCR) PRRSV analysis and ELISA analysis (blood and tonsils only). PCR and ELISA results for Trt1 were negative (CT≥37). PCR and ELISA results for all pigs fed feed mixed with MLV vaccine (i.e., Trt2 and 3) were negative regardless of SC application. PCR results were as follows (CT values and viral particles, respectively): MLV vaccine = 18.7 and 1.3×10⁷; Feed paste without SC (Trt3) = 24.6 and 4.2×10⁶; Feed paste with SC (Trt2) = 32.2 and 2.6×10⁴. Results suggest that the risk of PRSS contamination through feed stored 12 hours post-exposure with PRRSV is relatively low. Efforts to prevent PRRSV contamination should first be focused on more important potential sources of infection.