Despite decades of intense research and tremendous deployment of resources, porcine reproductive and respiratory syndrome (PRRS) remains the most costly production disease among swine industries throughout the world.

Of all the losses from PRRS, approximately half can be attributed to losses in breeding herds and the other half attributed to losses in growing pigs. Economic losses are significantly greater in herds that are naïve for the PRRS virus (PRRSv) compared to herds that are antibody-positive at the time of PRRSv exposure.²

I often refer to the cost of PRRS as a “tax,” and although not all producers pay the same tax, it’s important they all understand the tax man may come calling at any time! We hire accountants to minimise the amount of income tax we pay, and similar to an accountant, one of my roles is minimising the PRRS tax my producer clients suffer. Like income tax, I can rarely get the PRRS tax to zero, but the impact of PRRS can be greatly minimised, particularly with adequate gilt acclimation.

Elimination versus acclimation

Generally speaking, producers with very occasional PRRS problems often rely on elimination strategies. Producers with frequent PRRS problems more often use acclimation strategies to minimise the clinical impact of the disease. Although PRRSv eradication remains a long-term industry objective, it’s not always a practical option because it can be difficult and costly to implement a biosecurity programme that’s robust enough to prevent new PRRSv introductions.

The greatest impact from PRRS occurs during acute outbreaks, but this is also the time we have the greatest opportunity to mitigate performance and economic losses. In breeding herds, we focus on minimising the death loss in both piglets and growing pigs that are infected during the farrowing period. We also want to minimise the duration of shedding from sow to piglet.

Two metrics have been developed to help us measure the impact of PRRS during an acute outbreak:

• Time to PRRSV stability (TTS):
  • measures: the duration of piglet infection;
  • calculation: number of weeks it takes to obtain four consecutive negative results based on polymerase chain reaction monitoring of due-to-wean piglets.
• Time to baseline performance (TTBP):
  • measures: the volume of piglet mortality;
  • calculation: number of weeks it takes to recover the volume of weaned pigs per week that the herd averaged prior to the PRRS outbreak.

Acclimation goals

PRRSv acclimation is aimed at minimising both the duration of infection and the volume of mortality during an acute outbreak, improving TTS and TTBP and ultimately reducing the economic impact on the producer. PRRSv acclimation strategies are focused on incoming replacement gilts.

Specifically, our objective is to transition naïve gilts to a state of immune competence. While PRRSv acclimation will not prevent a future infection, we know it minimises clinical disease when wild-type PRRSv exposure occurs.

PRRSv-acclimation programmes follow the core principles of all disease acclimation. We want to convert susceptible populations to a state of immune competence that minimises clinical disease and shortens the duration of shedding and pathogen transfer to offspring any time the adult animal encounters exposure to a novel PRRSv.

Important rules of thumb

While acclimation strategies vary among farms and production systems, there are several rules of thumb for producers to understand:

• Exposure to a replicating PRRSv is necessary to build robust immunity. This can be accomplished with modified live vaccination (MLV) or live virus inoculation (LVI).
• LVI provides robust immunity, but the performance cost is extreme compared to MLV, so I currently don’t advise farms to routinely acclimate gilts using LVI.
• Gilts naïve to the PRRSv should be vaccinated with a MLV vaccine as soon as they exit the isolation facility to ensure rapid immunologic development and elimination of infection prior to breeding. If you don’t have an isolation facility, gilts should be exposed immediately upon entry to your farm.
• PRRSv infection can persist for up to 157 days in an individual gilt. If we assume gilts have a breeding age of 210 days and a first-farrow age of 335 days, that means we must expose them no later than 53 days of age to minimise the impact of PRRS on breeding. Exposure must absolutely not occur later than 178 days of age to ensure there’s no active PRRSv infection at the time of farrowing.
• PRRSv MLV vaccines do replicate in gilts post-vaccination, which poses a threat to future gilt introductions that aren’t yet vaccinated. Introducing batches of gilts of different ages at the same time allows for PRRSv exposure and a “cool down” time prior to the next gilt introductions. Ideally, introduction of gilt groups should occur no more often than every four weeks. This will minimise the chances of your most recently vaccinated gilts infecting your newest gilts prior to their vaccination.
• If you expect exposure to a known wild-type virus, use of a killed PRRSv vaccine four weeks after administration of the PRRSv MLV vaccine can further help develop immunity against specific isolates.

Conclusion

The dramatic economic impact of PRRSv makes virus acclimation an attractive way to improve farm performance. Effective implementation of the “rules of thumb” above can provide a level of PRRSv immunity that decreases the impact of future infections, decreasing both the duration of shedding as well as the volume of piglet mortality losses.

The impact on performance can be dramatic when comparing naïve herds to well-acclimated herds. Figure 1 demonstrates this by comparing the wean-pig volume on PRRSv-naïve versus PRRS-acclimated sow farms when infected with a common PRRSv.

Stage 4 (the naïve) sow-farm wean-pig volume following a PRRSv outbreak is represented by the red line, and Stage 2 (acclimated) sow-farm wean-pig volume following the outbreak is represented by the green line. PRRSv-acclimated farms suffered performance losses but were able to avoid the tremendous piglet mortality experienced in the PRRSv-naïve farms.