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PRRS: Time to stability and time to baseline production study

The purpose of this study was to compare efficacy of LCE (load-close-expose) using attenuated PRRSV to that using live-virus exposure on breeding herds acutely infected with PRRSv.

Friday 16 December 2016 (10 months 4 days ago)


Several strategies to manage PRRSv infections in breeding herds have been described (Corzo et al. 2010). Herd closure and/or whole-herd exposure to PRRSv are methods commonly used in North America.

For the purpose of this study, herd closure was defined as temporary interruption of replacement pig introduction into the breeding herd for several weeks and whole-herd exposure is defined as exposing a PRRS immunogen (commercial PRRSV vaccine or live-virus inoculation) to all breeding age females in the herd.

The combined use of herd closure and whole-herd exposure is often called load-close-expose (LCE): load the breeding herd (in site or off site) with extra gilts ⇒ expose all sows and gilts to PRRSv ⇒ introduce unexposed gilts when breeding herd starts producing PRRSv-negative pigs at weaning.

The purpose of this study was to compare efficacy of LCE using modified-live (aka attenuated) PRRSV to that using live-virus exposure (aka serum inoculation) on breeding herds acutely infected with PRRSv.


Study design

We conducted a prospective field study with 61 breeding herds recently infected with PRRSV (within 60 days of virus detection) adopting LCE as method to produce PRRSv-negative pigs at weaning.

Three outcomes were developed measure "efficacy": time to stability (TTS= number of weeks it took to reach 4 consecutive PCR-negative results on monthly monitoring of 30 due to wean piglets); time to baseline production (TTBP = time to recover volume of weaned pigs/week that herd had in the 21 weeks prior to PRRS outbreak as defined by exponential weighted moving average methods); total loss (sum of pigs not weaned following intervention, until reaching TTBP).

We compared TTS, TTBP and total loss between herds using LCE with modified live vaccine (LCE with MLV, n=20 herds) to those using LCE with live-virus inoculation (LCE with LVI, n=41 herds). We also collected demographic information and history of PRRSv infection for risk factor analyses. Survival analysis and regression models were used to compare the outcomes between treatment groups.


Results and Discussion

We found that 77% of herds reached TTS by end of the observational period. The average TTS for all herds was 26.6 weeks, ranging from 12 to 42 weeks. Herds reached TTS significantly sooner if used LVI as exposure method (25 weeks for LVI, 32 for MLV herds), if they had history of PRRSV infection within the previous 3 years or if they were part of a particular production system. Conversely, the production impact was significantly less severe in herds using MLV as part of the LCE program. More specifically, MLV herds reached TTBP 7 weeks sooner and lost 1,300 less pigs than those using LVI. Economic analysis taking into account all outcomes indicated MLV was advantageous over LVI.

In summary, results showed that when TTS is the only desired outcome, using LVI as part of a load-close-expose program was the economically advantageous over the LCE+MLV strategy. This may be the case of genetic multiplier herds. However, when productivity was an outcome of interest, the LCE+MLV was the strategy that resulted in less losses. There were “early TTS herds” in both groups (LCE+LVI and LCE+MLV), which proves the concept that early TTS is possible using both strategies. Further field research is needed to identify underlying causes associated with shorter TTS. Another key parameter that veterinarians use to select the strategy to manage PRRS infection is the success rate to achieve the desired outcome (TTS or TTBP for instance). In this study, there were no statistical difference between treatment groups in the success rate to achieve TTS or TTBP.

Another key finding of the study was the intermittent pattern of PRRSv detection by PCR. Forty percent of herds had at least one monthly negative test followed up by a positive test in the following month. This finding supports that PRRSv monitoring should be done repeatedly over time as the virus can sustain low (but not zero) prevalence for a few months before infection dies out in the population level. We are currently conducting field studies to assess efficacy of different surveillance methods to detect PRRS infection at near zero prevalence with high confidence. Results will be available at the pig 333 portal.



The full peer reviewed manuscript was published in the Preventive Veterinary Medicine Journal by authors Daniel Linhares, Jean Paul Cano, Montserrat Torremorell and Bob Morrison in 2014.

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Article Comments

This area is not intended to be a place to consult authors about their articles, but rather a place for open discussion among pig333.com users.

Neil DeBuse, DVM19-Dec-2016 (10 months 1 days ago)

I question whether the economic value is as omportant as definitively making herd negative (non'shedding) more rapidly.

In PRRS control, we have to recognize that simultaneously circulating 2 viruses within the herd leads to adverse impact - emergence of new strains.

My challenge to this author and others in the field is to research further options with a more clear ethical usefulness which avoids the clear disadvantage of simultaneously making entire or majority of population viremic with 2 strains of PRRS. Several exist, including delayed vaccination with MLV's and use of other non-MLV vaccine technologies (killed, subunit and polyvalent antigen concentrate vaccines).

The only feature of PRRS that allows this potentially dangerous situation is the fact that PRRS is not zoonotic as for example IVA-S.

As a matter of long term control, we, as veterinarians, must take steps to see the future differently in my opinion. I believe better treatments or interventions already exist which are 10+ years ahead of those measured here.

Daniel Linhares02-Jan-2017 (9 months 18 days ago)

Dr DeBuse --

Thank you for the comments.
On the "economic question" - we reported that using MLV as part of load-close-expose was economically advantageous over live-virus (LVI) as the production impact on vaccinated herds was significantly less than that of live-virus-exposed herds (full manuscript with the model is available at http://dx.doi.org/10.1371/journal.pone.0144265).

On the virus vs no-virus exposure, good question. I agree that we need more field studies on PRRS control/elimination. Experimental research (at isolation units, controlled conditions) are great to generate hypothesis, but field studies are crucial to validate those concepts on actual farms (real life). I understand that mixing viruses creates opportunities of creating new variations, but also understand that attenuated (or attenuated-like) viruses are economically preferable to producers, when compared to wild-type virulent viruses. No virus is better than an attenuated virus, but sometimes in high density areas the chances of keeping naive herds is limited - that's where "living with" attenuated virus is an option to consider.

On another observational (field) study (manuscript to be submitted in February 2017) we described the success rate of achieving stability (TTS) was significantly lower in herds with no whole-herd exposure to PRRSV, when compared to herds that did load-close-expose. We shared the findings of that study at Leman conference and at the North American PRRS symposium (Chicago) last month.

Thanks again for the comments and discussion.
Happy new year.

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