The effect of vaccination of gilts with a pre-farrow RNA particle vaccine following pre-breeding natural planned exposure on piglet growth and viral shedding
Authors: Anderson Reever Amanda V. , Pittman Jeremy S. , Kumar Deepak , Marthaler Douglas , Walthart Becca , Karriker Locke A. 
Published: April 2024

The objective of this study was to compare rotavirus shedding and performance of piglets from gilts immunized using natural planned exposure (NPE) or an RNA particle vaccine (RPV) prior to farrowing following initial NPE pre-breeding. A final total of 117 farrowed gilts and their piglets were enrolled into 4 groups. All gilts received two administrations of NPE prior to breeding. Gilts in group 1 were later given three NPE administrations at 5, 4, and 3 weeks prior to farrowing (WPF). Group 2 was dosed with an RNA particle vaccine (RPV) at 5 and 3 WPF and group 3 at 1 WPF only. Group 4 (control group) did not receive any NPE or RPV. Fecal samples from gilts and fecal swabs from their piglets were tested for rotavirus A (RVA), rotavirus B (RVB), and rotavirus C (RVC) by qRT-PCR. The 117 gilt samples were tested individually at 5 sampling points from pre-breeding to entry into the farrowing rooms. Piglet samples were pooled by litter (3 piglets sampled per litter) and tested at 3, 7, 14, and 21 days of age. The clinical and production impact of the treatments were assessed by comparing average adjusted weaning weights, the percentage of piglets weighing in the bottom 10% of study piglets at weaning, the percentage of piglets placed in the nursery, and the percentage of litters with the presence of diarrhea. No statistically significant differences were identified but there appeared to be several interesting numerical trends that are both biologically plausible and consistent among treatment groups. The control group, which received no pre-farrow immunization, had the highest percentage of litters with diarrhea at all time points and overall in the farrowing house. A numerical trend was also observed on average cycle threshold (Ct) values for RVC in piglets. Interestingly, the control group had the lowest numerical average Ct value at 7, 14, and 21 days of age. Ct values for RVA were consistently lower than for RVC in the stock solution and natural planned exposure gruel. This likely correlated to a lower level of RVC exposure in all gilts pre-breeding and group 1 gilts pre-farrow. This may have an inverse correlation to the higher levels of shedding of RVC in piglets. The lack of statistical significance between treatment groups with regard to diarrhea and weights in the piglets may be attributed to sample size, lack of sufficient natural rotavirus challenge, pre-breeding NPE in all gilts, or a variety of other reasons. Replication of this study with a larger sample size, using a challenge model, or relocated to a farm with increased natural rotavirus clinical challenges may yield different results. If beneficial under different conditions, the RPV would eliminate many of the risks associated with NPE administration, including continuous introduction of live virus on-farm, potential spread of other pathogens, difficulty of isolating on-farm strains, and labor and cost of producing NPE material.
Limitations of the original research include the use of quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) to measure the levels of RVA and RVC in the stock solution and gruel and to monitor RV shedding, rather than techniques with the ability to differentiate live and non-viable virus. Additionally, these studies were performed on an all-gilt breeding farm. It may be more challenging to appreciate differences in treatment  groups on a multiparous sow farm due to the higher level of immunity correlated to increasing parity. These immunization protocols were designed to be applicable in a field setting for routine rotavirus immunization, though their feasibility must be carefully considered before implementation on each individual farm.