Comparison of Time to PRRSv-Stability and Production Losses

To control PRRSv, herd closure (pig introduction into breeding herds is interrupted to allow the pathogen to die out) combined with whole-herd exposure to a live PRRSv (modified-live virus, MLV vaccine, or live-resident herd virus, LVI) is a commonly attempted strategy to control and eliminate the virus from infected breeding herds.
calendar icon 17 June 2016
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This study compares the use of MLV vs LVI in regard to:

  • the time required to PRRSv-stability (TTS)
  • the time it took for farms to recover the production levels they had prior to PRRSv infection (time to baseline production, TTBP)
  • the magnitude of the production losses following PRRSv infection and adoption of load-close-expose strategy

How is it done?

It was prospective study that enrolled a total of 61 herds that were infected with PRRSv (with information on herd demographics, type of PRRSv strain and PRRSv infection history) were recorded from November 2009 to March 2012.

The inclusion criteria were:

  • stable sow inventory
  • diagnostic evidence of PRRSv
  • intent to eliminate PRRSv
  • no replacement pigs introduced
  • willingness to share production performance data
  • enrollment in the study within 2 months of starting the load-close-expose program
  • enrollment in the study before the PRRSv monitoring phase started.

The exclusion criteria were:

  • if there was a new unrelated PRRSv introduction
  • introduction of replacement animals before TTS was reached
  • the owner opted to withdraw.

TTS and TTBP were compared between treatments using survival analysis.

  • Sampling consisted of 30 due-to-wean piglets on a monthly basis. Serum was tested for PRRSv RNA by RT-PCR. Herds in which PRRSv was not detected over a 90-dayperiod were classified as reaching stability.
  • Productivity was measured as “total number of pigs weaned per 1000 sows” per week.

What are the results?

The median TTS among participating herds was 26.6 weeks (range 12–42 weeks). 85% of herds in the LVI program and 60% of herds in the MLV program reached stability by the end of the observational period. Herds that were exposed with LVI had significantly shorter TTS than herds treated with MLV.

The overall TTBP was 16.5 weeks (range 0-29 weeks). The magnitude of production losses following whole-herd exposure averaged 2217 pigs not weaned/1000 sows and was correlated with TTBP. Herds in the MLV group recovered production sooner (p < 0.0001) and had less total loss (p = 0.0095) than herds in the LVI group

What implications does this paper have?

This study provided new metrics to assist veterinarians to decide between methods of exposure to control and eliminate PRRSv from breeding herds.

Although the use of LVI allowed d shorter time to wean negative piglets (TTS), herds in the MLV group recovered production sooner and had less total loss. This fact should guide future decisions in the face of a PRRS outbreak.

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