Principal Investigator: Cassandra Jones
Institution: Kansas State University
Senecavirus A (SVA), previously known as Seneca Valley Virus, is a detrimental pathogen in the United States swine industry. Transmission is not well understood, but its similarity to foot and mouth disease virus (FMDV) suggests direct contact with people or fomites may spread the virus. Once present, viruses in feed, feed ingredients, and feed mills are difficult to mitigate. While contaminated surfaces in a feed mill have been demonstrated as a potential vector for bacterial and viral transmission, there is currently no approved method for its evaluation of viral contamination. Therefore, the objective of this Experiment 1 was to validate standardized swabbing techniques for detection of SVA. A secondary objective was to determine if a freeze/thaw cycle impacted detectable RNA. This experiment included 3 forms (inoculum, feed, or swab), 4 doses of SVA (none, low, medium, or high), and 2 storage methods (analyzed initially vs. after a freeze/thaw cycle). The SVA was added to swine feed, with 1 g reserved, and the remaining spread over a stainless steel coupon. Feed was removed, but residual feed dust remained. Next, surfaces were swabbed and samples split, with one set analyzed initially, and another frozen for 7 days, then thawed and analyzed. Results are reported as the quantity of detectable SVA as determined by threshold cycle (Ct) in qRT-PCR, where the higher the Ct, the less detectable virus was identified. The results demonstrate that sample type impacted the quantity of detectable SVA, where feed samples were approximately 8 Ct higher than the inoculum, and swab samples were approximately 4 Ct higher than feed. A freeze/thaw cycle did not impact detectable SVA compared to samples that were analyzed immediately.
In Experiment 2, the objective was to determine the prevalence and distribution of SVA in United States swine feed mills as an indicator of risk of domestic and foreign animal disease transmission through feed. A total of 375 samples were collected from 11 surfaces + one feed sample collected from 11 different feed mills manufacturing swine feed located in 8 different states. Feed mills include 5 producing both mash and pelleted feed in KS, CO, OK, NC, and IA, and 6 producing only mash feed in KS, NC, MN, IA, IN, and IL. Within a mill, locations included ingredient pit grating, fat intake inlets, exterior of pellet mill (only in feed mills with pelleting capacity), finished product boot bin, load-out auger, finished feed, floor dust in the break/control room, floor dust in receiving, floor dust in the manufacturing area, floor dust in the warehouse, worker shoe bottoms, and broom in the manufacturing area. To account for potential seasonality associated with pathogenic hazards, the same locations in feed mills were swabbed in Late Fall 2016, Winter 2016/17, and Summer 2017. Notably, no mills were manufacturing feed for SVA-positive herds at the time of analysis. Five of 375 samples analyzed positive for SVA, with Ct ranging from 37.4 to 39.9. One positive sample was collected in late Fall, while the other four positive samples were collected in Winter. No positive samples were identified in Summer. Two samples were from load-out augers, and one each from fat intake inlet, floor dust in the receiving area, and worker shoes. A sow farm being fed by the mill with SVA on worker shoes was subsequently diagnosed with SVA after the sample as collected.
These results indicate that an environmental swab can be used to detect SVA in feed, however with approximately 4 Ct less precision than analyzing feed samples directly. Furthermore, the limit of detection of SVA in environmental swabs appears to be near 10^3 TCID50/mL. Samples can be frozen prior to analysis without impacting detectable SVA RNA. Finally, SVA was not widespread throughout the swine feed mills analyzed in this experiment, but its presence in a mill may be indicative of disease risk or entry into pig populations, particularly through worker shoes.