The effect of season on PRRS time-to-stability in the Midwestern United States

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

Key Points

  • Seasonal conditions may effect the time to stability of a farm
  • Understanding seasonal effects on time to stability can help producers and veterinarians plan herd closures

This week, we are sharing a report by the MSHMP team in collaboration with Dr. Andreia Arruda from the Ohio State University regarding the impact of seasons on PRRS time-to-stability.

The time needed between an outbreak and consistently weaning porcine reproductive and respiratory (PRRS) virus PCR negative pigs is referred to as time-to-stability (TTS). In this analysis we describe differences in TTS according to the season when the PRRS outbreak occurred in farms located in the Midwestern United States.

161 PRRS outbreaks in 82 sow farms were classified based on the date of the outbreak:

  • March 21st to June 20th: Spring
  • June 21st to September 20th: Summer
  • September 21st to December 20th: Autumn
  • December 21st to March 20th: Winter

TTS was calculated as the time from the reported PRRS outbreak to the time of the last PRRS PCR negative result in wean-age pigs.

A significant difference was detected in TTS among seasons. The median TTS was higher in spring and summer, compared to autumn and winter.

An explanation for the observed TTS difference among seasons may be found in environmental survivability of the virus as for PRRS outbreaks that occur during spring or summer, the last phase of the stability process coincides with the arrival of winter where the reduced ventilation and decreased temperature within the farm may favor PRRS survival resulting on a lower likelihood of elimination during this time.

PRRS time to stability season

Science Page: Within farm PRRS time-to-stability differences in sow farms in the Midwest

This is our Friday rubric: every week a new Science Page from the Bob Morrison’s Swine Health Monitoring Project. The previous editions of the science page are available on our website.

This week, we are sharing a report by the MSHMP team regarding PRRS time-to-stability differences in sow farms.

Keypoints:

  • There is significant within farm PRRS time-to-stability variation.
  • Several factors contribute to PRRS time-to-stability variability; however, there is still a significant amount of unexplained variability.
  • The role of within farm management practices and internal biosecurity measures should be further explored.

Introduction

Porcine reproductive and respiratory syndrome (PRRS) stability is reached when no evidence of infection is observed in wean-age piglets. Sample size to detect PRRS virus in wean-age piglets usually involves blood sampling of 30 piglets, at least four times, 30 days apart (Holtkamp et al., 2011). The cumulative time from the intervention (i.e. whole herd exposure, herd closure) to PRRS stability is usually referred to as time-to-stability (TTS).

Here we summarize differences in TTS in MSHMP participating farms located in the Midwest that have had at least two PRRS outbreaks.

Methods

Six systems that are similar in the way they test to classify a herd as stable were selected for inclusion in the study. PRRS outbreaks reported from 2011 to 2017 were used for analysis.

TTS was defined as the time period from the date of outbreak reporting to the date when PRRS stability was reported (last consecutive negative PCR result). To assess the variability in TTS, only farms that had at least two PRRS outbreaks were selected.

Results

Overall, 133 PRRS outbreaks in 53 farms were recorded withtwo, three, four and five outbreaks in 35, 11, 5, 2 farms, respectively. The median TTS standard deviation of PRRS outbreaks within the same farm was 12 weeks (minimum = 0 weeks, maximum=88 weeks).

After accounting for the effect of the intervention using MLV or FVI, the RFLP pattern of the virus associated with the outbreak and previous PRRS outbreaks in the farm, the PRRS time-to-stability correlation of outbreaks in the same farm and system was only 1.2%.

In other words, TTS of two given outbreaks in the same farm were not correlated indicating that TTS within farm is highly variable.

Conclusion

There is a high TTS variability after a PRRS outbreak within the same farm that is not accounted for by the effect of the intervention used, the virus (i.e RFLP), previous PRRS outbreaks in the farm and system.