Effect on total pigs weaned of herd closure for elimination of porcine reproductive and respiratory syndrome virus

Herd closure has been reported as a method to eliminate porcine reproductive and respiratory syndrome virus (PRRSV) from breeding herds. However, there is concern

01-05-2007


Article by:

Nathan Schaefer; Robert Morrison 

PRRS has plagued the swine industry for the past two decades and is considered to be the most economically devastating disease of modern swine production. The National Pork Board estimates the annual cost of PRRS to be approximately $600 million ($US). Acute outbreaks of PRRS can cause fever, lethargy, and increased late-term abortions and stillbirths in pregnant sows. Young piglets and finishing pigs infected with PRRS virus (PRRSV) experience more respira- tory disease, increased susceptibility to other diseases, and consequently, increased mortality.

Common methods to control PRRSV and eradicate it from swine herds include total herd depopulation and repopulation, par- tial depopulation, segregated early weaning, test-and-removal, and herd closure.3 Herd closure is financially advantageous over depopulation because there is no required downtime, sows are not slaughtered, and there is no clean-up cost. The sow herd is closed to replacement animals for a recom- mended 6 months, but remaining females are continuously bred and sales continue.3 In some cases, closure follows a program of deliberate exposure of existing animals to the PRRSV that is resident within the herd. Herd closure has had a success rate above 85% for farms with segregated pro- duction, and the use of isolated three-site production can minimize the economic costs of closure. 

While herd closure has been reported to be effective, there has been little work done on its financial ramifications. A key measure of sow-herd productivity is the number of pigs weaned per week. By studying the trend in total pigs weaned across sow herds that have been temporarily closed to attempt to eliminate PRRSV, we can measure a major potential effect of herd closure.

Case description
Production records were made available from 15 multiplication sow farms in one production system that had performed herd closure for PRRSV and that had at least 52 weeks of production data before and after closure. The herds ranged in size from approximately 500 to 1200 sows per site, and the entire multiplication system was composed of approximately 10,000 sows.
All farms had good biosecurity programs. Replacement gilts were obtained from a single naive nucleus herd and boars from two company-owned studs that had been PRRSV-naive for the previous 4 years. The nucleus herd was tested weekly and boar studs were tested monthly by PRRS ELISA and pooled polymerase chain reaction (PCR). A minimum of 12 hours without pig contact was required before entering the farm. Washing, disinfection, and assisted drying of all transportation vehicles were mandatory. Each multiplication farm was located in an area of low pig density a minimum of 1.6 km from a commercial swine herd. All farms were located in areas of wooded or mountainous terrain.

During herd closure, blood samples were collected from a convenience sample of 30 to 60 weaned pigs (one pig per litter) from each multiplication herd. Samples were pooled and tested by PCR for PRRSV (five samples per pool). If two consecutive pooled PCR tests from a herd were nega- tive, that herd was declared “stable.” Once each herd had been closed for 6 months and was producing PCR-negative weaned pigs, the farms were eligible for gilt replace- ment deliveries. However, these herds were closed for an additional 3 months because gilt replacements were not available.

Most samples were tested by PCR for PRRSV at Boehringer Ingelheim Vetmed- ica’s Health Management Center in Ames, Iowa. This test was reported to have a specificity of 99.42% (Wayne Chittick, Boeh- ringer Ingelheim employee, written commu- nication, 2006). Additional sampling was performed when apparently false-positive samples were identified, with emphasis on pens surrounding suspect pigs. The dates of herd closure and opening were determined for each herd, and per- formance data from 52 weeks prior to closure through 52 weeks after closure were exported to an Excel spreadsheet (Micro- soft Corporation, Redmond, Washington). For each herd, we calculated the number of pigs weaned for the 52 weeks prior to clo- sure and compared that to the number of pigs weaned for the 52 weeks after closure. The preclosure and postclosure total pigs weaned for the 15 herds were compared by two-sample paired t-test. To view the weekly change in pigs weaned after closure, the number of pigs weaned for each of the 52 weeks after closure was subtracted from the number of pigs weaned for the corresponding week before closure. For example, the number of pigs weaned week 1 after closure was subtracted from the number of pigs weaned week 52 prior to closure. Correlation analysis and stepwise linear regression were performed in Statistix version 8.0 (Analytical Software, Tallahas- see, Florida) to determine the association between four production measures and change in total pigs weaned. The four production measures were difference in total number of services for the 52 weeks before and after closure, and differences in average weekly farrowing rate, liveborn pigs per litter, and preweaning mortality. A probability of 0.2 was used as a cutoff for inclusion in the regression model. A P value of .05 was considered significant for all statistical analyses.

Production data
On average, 686 more pigs were weaned per herd (P < .05) during the 52-week period after closure than during the 52 weeks before closure. The change ranged from 410 fewer to 2222 additional pigs, and total pigs weaned decreased in only two herds (Table 1). No herds had a marked decrease in pigs weaned over the 52 weeks (Figure 1), and those that experienced an increase did so in a relatively constant fashion (data not shown).
Total services (r = 0.77, P < .01),
farrowing rate (r = 0.58, P < .05),
and number of liveborn pigs (r = 0.57, P < .05) were correlated with the change in number of pigs weaned.

Predictor variables in the best-fit regression model were total services and farrowing rate, and these together accounted for 60% of the variability in change in total pigs weaned. Change in total services alone accounted for 57% of the variability.

Discussion
Overall, the results suggest that herd closure can be performed in sow herds similar to the ones in this study with minimal negative consequences on number of pigs weaned. The combined herds had an increase of 10,300 weaned pigs post clo- sure. If the estimated value of one weaned pig is $35, then the system experienced $360,500 in additional annual income (all currency in $US).
Although the recommended minimum closure time is 6 months,3 these herds were closed for approximately 9 months because PRRSV-naive gilts of the desired genetic lines were unavailable. The fact that number of pigs weaned per week was maintained and even increased over this unusually long closure period is encouraging for herds that might be closed for only 6 months. The long closure period may have contributed to the success of the eradication program.
This system’s multiplication farms effec- tively eradicated PRRSV without nega- tively affecting throughput by maintaining the number of services during the period of closure. This was achieved by preloading the sow herd with gilts. Farrowing rate might increase after closure, as apparently occurred in some herds in this study, due to elimination of PRRSV, improved management, or both.



Implications 

  • Under the conditions in this production system, herd closure can eliminate PRRSV.
  • Sow herds of approximately 500 to 1200 sows can be managed to minimize impact on number of pigs weaned per week during a period of herd closure.
  • Maintaining the target number of services during herd closure will help maintain throughput. 
References

1. Kahler SC. JAVMA News. AASV coverage. PRRS: Is elimination attainable? May 1, 2004. Available at: http://www.avma.org/onlnews/ javma/may04/040501g.asp. Accessed 25 Mar 2007. 
 

2. Neumann EJ, Kliebenstein JB, Johnson CD, Mabry JW, Bush EJ, Seitzinger AH, Green AL, Zimmerman JJ. Assessment of the economic impact of porcine reproductive and respiratory syndrome on swine production in the United States. JAVMA. 2005;227:385–392.

3. Zimmerman J, Benfield DA, Murtaugh MP, Osorio F, Stevenson GW, Torremorell M. Porcine reproductive and respiratory syndrome virus (porcine Arterivirus). In: Straw BE, Zimmerman JJ, D’Allaire S, Taylor D, eds. Diseases of Swine. 9th ed. Kansas City, Missouri: Blackwell Publishing; 2006:387–417.

4. Desrosiers R, Boutin M. An attempt to eradicate porcine reproductive and respiratory syndrome virus (PRRSV) after an outbreak in a breeding herd: eradication strategy and persistence of antibody titers in sows. J Swine Health Prod. 2002;10:23–25.

5. Torremorell M, Henry S, Christianson WT. Eradication using herd closure. In: Zimmerman J, Yoon KI, Neumann E, eds. 2003 PRRS Compen- dium. National Pork Board; 2003:111–115.

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