The porcine reproductive and respiratory syndrome (PRRS) is one of the most persistent and economically impactful swine diseases. Thirty years after discovering the PRRS virus (PRRSV), the industry struggles to control it despite significant efforts, said Lucina Galina, Pig Improvement Company, during the 2023 Leman Swine Conference.

Recent breakthroughs in gene editing have shown animals resistant to infection and, in so doing, created the potential to control epidemics. Using gene editing with CRISPR-cas9, a portion of DNA coding for a protein can be deleted. Unlike genetically modified organisms, in this case, DNA from another organism is not introduced to modify the genetic code, Galina said.

Evidence supports that CD163 is an indispensable monocyte and macrophage receptor, and specifically, its scavenger receptor cysteine Rich 5 (SRCR5) domain is crucial for PRRSV infection.¹ CD163's main function is the clearance of cell-free hemoglobin and participation in inflammatory processes through a domain different than SRCR5, and no specific role has been associated with SRCR5 other than PRRSV infection.² A founder population carrying an SRCR5 414 base pair deletion was used to build a commercial breeding population.³

Since the inheritance of the CD163 edited gene is recessive, homozygous pigs must be derived from homozygous dams and sires to carry the resistant genes.

Galina cited research where homozygous, heterozygous, and null pigs were inoculated with four PRRSV isolates, including type I (SD13-15, SD01-08) and II (144 L1C, NVSL97 L8). The studies were conducted in Biosafety Level 2 (BSL-2) facilities at Midwest Veterinary Services in Oakland, Nebraska, USA.

Veterinarians and caregivers who oversaw the inoculation and sample collection were blinded to the zygosity of the pigs. Weaned pigs arrived at the BSL-2 facilities, acclimated for one week, and were intranasally inoculated with 3 mL of PRRSV (± 10⁴ TCID50). Sera were taken before and at 3, 7, 10, 14 and 21 days post-inoculation. Clinical assessments were obtained, including daily temperatures, demeanor, and respiratory scores (0-3). PRRSV PCR and ELISA tests were conducted at the ISU VDL, and the diagnostician was blinded to the pig identifications.

Overall, the type II isolates were more pathogenic than type I. No pigs showed demeanor or respiratory scores higher than 1. There were some differences, however, in the clinical assessments when comparing homozygous vs. heterozygous and null pigs, which were evident when pathogenic strains were evaluated, she said.

For example, heterozygous and null pigs inoculated with 144 1C had higher average temperatures and clinical scores than homozygous pigs. The homozygous pigs were consistently negative by PCR across the bleeding times, while heterozygous and null pigs were positive. The homozygous pigs did not mount an immune response detectable by ELISA. In contrast, heterozygous and null pigs seroconverted, Galina noted.

Additional studies using flagship isolates from contemporary US dominant lineages L1H, L1A, and L1E have shown consistent results. These outcomes suggest that gene editing may offer an unprecedented opportunity for controlling this challenging virus that has plagued the global swine industry for several decades, she concluded.

¹ Whitworth et al., 2016, Nature Biotechnology.

² Burkard et al., 2017, PLOS Pathogens.

³ Cigan et al., CABI Agriculture and Bioscience, 2022.