SHIC-Funded Porcine Sapovirus Study Addresses Strain Isolation and Vaccination-Boosted Antibodies in Milk/Colostrum

Porcine sapovirus (PoSaV) is an emerging pathogen that causes diarrhea and weight loss in pigs during weaning and post-weaning. Because isolation of circulating field strains remains a challenge, the Swine Health Information Center funded a study led by Dr. Quihong Wang at The Ohio State University to characterize and isolate contemporary PoSaV strains and to evaluate antibody responses in the colostrum and milk of sows/gilts immunized with a PoSaV vaccine. Results indicate that a new cell line supports PoSaV propagation, a benefit to producers and their veterinarians seeking diagnostics, and confirm that vaccination boosts antibody titers in colostrum and milk, although future vaccination-challenge studies are warranted. 

Read the industry summary of study #23-073 here. 

As an emerging swine industry disease with the potential to affect pigs of all ages, the assessment of vaccine performance against PoSaV is critical to prevent production losses. Eight PoSaV genogroups have been identified, with GIII being the most prevalent worldwide. The isolation of field strains in cell lines remains a significant challenge to enable development of autogenous vaccine candidates for PoSaV disease control and prevention on farms. Therefore, the objectives of this project were to 1) isolate contemporary PoSaV strains in cell culture, 2) evaluate viral neutralizing (VN) antibody responses in the colostrum/milk of gilts/sows immunized with a commercial PoSaV vaccine, and 3) correlate the VN antibody titers with piglet performance such as body weight.

Dr. Wang’s paper on objectives 2 and 3 was recently published in the journal Pathogens. Read the paper here.

To characterize and isolate contemporary US PoSaV strains, forty-two clinically suspicious fecal and intestinal samples were collected from diarrheic pigs in late lactation across five farms for PoSaV PCR testing. Positive samples were then serially passaged in swine testicular (ST) cells, porcine kidney (LLC-PK1) cells, and porcine intestinal epithelial (IPEC-J2) cells. PoSaV replication was confirmed by testing both viral nucleic acids and antigens. Additionally, the genomes of representative positive samples were sequenced.

Overall, PoSaV detection rate was 60% (25/42). Representative GIII PoSaV strains shared 87.5%-87.7% nucleotide identity in the viral capsid protein with the prototype GIII PoSaV Cowden strain detected 40 years ago. The success rates for isolating field PoSaV samples were 8% (2/25) in ST cells, 16% (8/25) in LLC-PK1 cells, and 0% (0/10) in IPEC-J2 cells. Importantly, this represents the first successful isolation of PoSaV in ST cells, creating new opportunities for future viral pathogenesis and vaccine research.

For objectives 2 and 3, a commercial swine farm with approximately 650 sows and gilts operating on a 4-week batch farrowing system was utilized. A total of 30 animals were randomly assigned to two groups: vaccination and non-vaccination, with each group comprising 15 animals (two gilts and 13 sows). For the vaccination group, animals were immunized intramuscularly with a PoSaV vaccine at gestation day (GD) 73 – 75 (about 5 weeks pre-farrowing) and at GD 94 – 96 (about two weeks pre-farrowing). Body weights of individual piglets were obtained at birth and at 3 weeks of age. Colostrum and milk samples collected at parturition, one week, two weeks, and three weeks post-farrowing were tested for VN antibody titers. Rectal swab samples from the sows/gilts and three piglets per sow/gilt at 2 weeks and 3 weeks of age were tested for PoSaV shedding.

All rectal swab samples from sows and piglets in both groups tested negative for PoSaV, indicating no PoSaV was circulating among these pigs during the study period. Both groups of piglets had similar weights at birth and at 3 weeks of age and piglet growth performance did not differ between treatment groups.

Additional results indicated that sows and gilts within the same group had similar levels of VN antibody titers at each time point. Colostrum samples contained high VN antibody titers (> 10^4.5 VNT50/mL), with the vaccinated group having significantly higher antibody titers in colostrum (p < 0.01) compared to the non-vaccinated group. Further, the vaccinated group maintained significantly higher (p < 0.05) VN antibody titers in the milk at 1-week post-farrowing but had no significant differences at 2 and 3 weeks post-farrowing compared to the non-vaccinated group.

These results indicate that sows/gilts had been exposed to PoSaV natural infection prior to vaccination and that vaccination boosted the existing immunity. The high antibody levels in both groups of sows/gilts may have provided full protection for piglets against PoSaV infection at this farm, although the threshold level of VN antibodies needed for PoSaV protection has not been determined. These findings indicate that vaccination effectively boosted pre-existing immunity; however, additional vaccination-challenge studies are needed to determine vaccine efficacy in naïve pigs since no PoSaV shedding was detected.

Overall, this project advances understanding of contemporary PoSaV biology, establishes a novel virus isolation platform, and provides the first evaluation of a prescribed PoSaV vaccine produced under commercial conditions. Expanding the potential for PoSaV isolation in the ST cell line offers a promising avenue for the development of effective diagnostic tools and vaccines against PoSaV. Advancing tools for PoSaV aligns with SHIC’s mission to generate knowledge and mitigate the potential impact of emerging disease threats for the US swine industry.

The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at [email protected] or Dr. Lisa Becton at [email protected].