Porcine Reproductive and Respiratory Syndrome (PRRS) has inhibited pig biological and economic performance as well as pig wellbeing on United States pig farms since the 1980’s. Despite massive resource commitments toward innovations in biosecurity, diagnostics, vaccines, therapeutics, and management strategies, pig farms still suffer greatly from PRRS impacts.
Prevention of PRRS disease remains the gold standard management approach, and modern biosecurity programs have demonstrated the technical ability to prevent wild-type PRRS infection on farms. For boar studs and sow farms, the cost of these biosecurity improvements provides a solid return on investment in many cases. Unfortunately, for growing pig farms biosecurity improvements such as air filtration remain not cost effective, despite the dramatic impact PRRS has on growing pigs.
The saying “We always view the problems of today through the lens of our current technology” is applicable to our current PRRS situation. Some producers feel hopeless in their efforts to mitigate PRRS impact with our current technology, but the reality is technology always improves and new innovations can provide breakthrough results.
In 2016 a team of scientists including a large swine genetics company announced the successful creation of a new generation of CRISPR-edited pigs that are resistant to PRRS virus infection. By modifying the CD163 allele, their efforts produced pigs that showed no signs of infection or viral replication in lung or lymph tissue when challenged with contemporary PRRS virus strains.
This milestone event served as proof of concept that genome editing can be used to generate PRRS-resistant pigs that easily facilitate our gold standard management approach of preventing PRRS disease. While regulatory approval is still pending, producers and allied industry challenged by PRRS should keep a watchful eye on this technology and anticipate how it may impact their businesses should regulatory approval be obtained.
While the development of PRRS-resistant pigs is truly a milestone event, implementation of these genetics into operating pig farms will take a strategic approach. The modifications to the CD163 allele must be homozygous for the pig to be fully resistant – this requires both parents to have the PRRS resistant trait at the time of mating. It is expected that semen from PRRS resistant boars will be the primary way producers can access PRRS resistant genes, and that producers will need to use this semen as the primary mechanism to implement PRRS resistant genes throughout their production system.
For most, that will mean first creating PRRS resistant pure-bred animals at the daughter nucleus or multiplication level. Initially, semen from boars who are homozygous to the PRRS resistant CD163 allele will be used to mate pure bred females who are homozygous to the non-edited CD163 allele. This will create a population of heterozygous pure-bred females who each carry one PRRS resistant CD163 allele and one non-edited CD163 allele.
While these females will still be susceptible to PRRS, when they are mated with homozygous PRRS resistant CD163 boar semen they will produce a piglet population that is 50% homozygous PRRS resistant and 50% heterozygous. Genetic testing will be needed at this step to determine which 50% of the offspring from this generation are the homozygous PRRS resistant females, and these females will be retained in the pure breed breeding flow to make commercial females that are homozygous PRRS resistant to populate our commercial sow farms.
Producers wishing to introduce PRRS resistant genes into their pig flows will have options for how quickly the implementation process can go. There are biologic limitations to our timeline, most notably the time it will take to raise the initial populations of heterozygous and then homozygous generations of mature gilts as well as the gestation length associated with these mating programs.
Some producers may choose to introduce these genes using normal replacement rates and normal pure breeding programs. Under this adoption program, producers should consider it will take ~6-7 years until they are marketing full populations of homozygous PRRS resistant market pigs.
For producers wishing to adopt these genetics more rapidly, opportunities for a “hard roll” approach exist, in which pure mating volumes and breeding farm replacement rates are dramatically increased. This will allow producers to create large populations of heterozygous and then homozygous PRRS resistant females capable of achieving 100% of the farms normal breeding target with each population as it matures to breeding age.
To accomplish this, strategic and staggered changes to the breeding program must occur, and growing gilt space must be acquired to raise these larger than normal populations of gilts. Significant planning and coordination will be needed to accomplish this effort, and producers wishing to rapidly implement PRRS resistant genes should have this plan in place prior to the technology being approved and available to United States producers.