31-12-2010
Article by:
Rodney B. Baker,
Introduction
Since 1987, porcine reproductive and respiratory syndrome virus (PRRS) has emerged as a global drain on pig productivity, profitability, and producer morale. The origin of PRRS has been debated for more than 20 years, but its ancestry remains unknown. Apparently only the pig is involved. How it made an almost simultaneous entry into global pig populations exhibiting significant genetic diversity remains one of the many compelling mysteries. One thing that remains certain is PRRS virus continually finds ways to circumvent our best biosecurity and management efforts. PRRS marches on in a ceaseless continuum of antigenic change rendering current vaccines and other control techniques of temporary and marginal value.
After the discovery of the causative agent in 1991, it was hoped that control measures would soon follow, but now, after 18 years of exhaustive research and vaccine use, the industry has gained little management ground over the “Mystery Swine Disease.” Control methodologies of many colors have found favor only to be dispossessed as the next seasonal isolates roared through hog belts. Oftentimes when management strategies fail to contain and control a disease agent, numerous experimental schemes evolve. This has been witnessed repeatedly over the years with attempted PRRS control as the commercial pig industry toyed with various vaccine schemes—commercial and autogenous—serum inoculations, natural exposure, feedback, and a host of genetic introduction vaccination and novel exposure plans. As with many other viral disease agents, there is a natural passage to broad population immunity, which, in the case of PRRS, often eliminates the virus from small, batch-farrowing herds—even when the growing pigs remain on site. This same immunologic progression after a new isolate has entered a farm or system often confuses us into believing that the management interventions were successful when actually the population response to naturally occurring immunity is responsible for the observed improvement. Even as our best management control schemes met with failure, the biosecurity toolbox has steadily improved, and likewise, farm, system, and now area eradication programs have found success. PRRS elimination strategies have evolved along with scientific knowledge and field experience and to date, appear as the best alternative over control.
Many improvements in biosecurity systems have evolved over the past 20 years, and the breeding stock industry has established free herds and proven that they can remain free from PRRS when strict standards are applied and when the farms are isolated from other pig production facilities. The magic separation distance is unknown but more miles between pig sites is advantageous. From documented experience, it appears airborne PRRS introduction may occasionally occur from distances up to 2.5 miles. In areas where pig density is concentrated, this distance may often be exceeded. Certainly large populations of PRRS positive growing pigs are a significant risk to any breeding herd whether that herd is currently PRRS positive or negative. The filtration option has gained ground with many successful testimonies coming from the field and from trial research. Regional eliminations have been successful as well as a number of large- and medium-sized multi-site operations. Many isolated, single site breed-to-wean and continuous flow operations, which have been closed to outside genetic introductions, have remained PRRS negative for years. Studies of feral, wild, and transitional swine indicate that they do not serve as a PRRS reservoir and should not impede national eradication ambitions. The following updated table suggests that the value of eradicating PRRS from the United States and North America has significant value compared to the eradication of pseudorabies virus (PRV) and to the bacteria Mycoplasma hyopneumoniae. The significant difference when comparing the three economically important swine agents is that PRV had widespread support for eradication by the swine commodity organizations, state and local associations, and the federal government. The direct cost to the producer at the time of final eradication was a very small $40 million annually compared to nearly one billion dollars direct producer cost associated with PRRS. PRV vaccines were highly effective, and the allure of significant increases in exports facilitated much of the eradication support. Although vaccination as an eradication tool is not currently feasible for PRRS, economic hardships and a desire for improved global competitiveness, productivity, well-being, and lower production costs has rekindled the interest in PRRS eradication in the United States, Mexico, and Canada.
A comparison of PRRS and Mycoplasma to PRV, exploring potential justification for a national PRRS eradication effort.
To summarize, what is needed for a successful national eradication can be listed within a few bullet points even though the details of a national program will require significant thought and planning.
❙ First and foremost is a majority commitment by producers and their representative organizations. After commitment, they must own and drive the program.
❙ PRRS negative boar/semen and sow replacement stock, which is now widely available.
❙ Development of individual herd and system elimination plans and timelines followed by coordinated area efforts.
❙ An organized national plan and program run by producers, which recognize zones (areas, states, split state, etc.) based on PRRS status and activity. The terminology needed is in the final stages of development.
❙ Communication between the stakeholders.
❙ National recognition of eradication by USDA and eventual program funding.
❙ Eventual pig movement restrictions based on the PRRS status of the states, provinces, and zones.
❙ Continual development of biosecurity barriers, monitoring tools, and perhaps a universal vaccine.
Breeding Herd Elimination Techniques
Elimination of PRRS from multi-site production has become a standard process, especially in the standalone breed-to-wean unit. Farms with same site nurseries or growing pigs have significant track records of elimination when the growing pigs are moved off site. Nonclinical farms that have a good history and diagnostic monitoring can fast track through rollover eliminations. In this situation, the PRRS negative replacement stock can be placed into the site as sentinels after a 60- to 90-day closure to all additions. Once sentinels remain negative for 60 days, the farm is re-opened to normal replacement stocking rates with negative gilts and boars. Other variations of this basic plan have been successful and each depends on specific farm knowledge and producer commitment level.
Farms with active outbreaks must be closed to all introductions for longer periods before successful addition of sentinels. The exact closure time for success is dependent on the population size, piglet fostering strategy, parity distribution, management commitment, and perhaps the nebulous characteristics of the specific virus present. Closure for 200 days from last clinical signs to first sentinel addition has been a good benchmark or target for rollover elimination in active farms. Many variations of the rollover PRRS elimination have been successfully applied and are best contrived by those who know the farm and its limitations.
PRRS virus has been eliminated by any of the following or by a variety of hybridizations of these methods.
1. Depopulation, site cleanup, followed by repopulation with PRRS negative replacements and semen.
2. Partial depopulation in which all growing pigs leave the premise at weaning for the duration of the rollover as described in the examples below.
3. Rollover breeding herd closure for 200 days post last clinical signs before introducing negative gilts.
4. Breeding herd closure for 200 days with an offsite breeding project commencing approximately 85 days after last clinical PRRS signs.
5. Serum inoculation and tissue feedback of all sows during an outbreak followed by one of the herd closure methods commencing after last clinical signs (200-day benchmark).
6. Whole herd and gilt replacement MLV vaccination followed by negative replacements after a period (200 days) of farm closure. This method eliminates vaccine virus introduced by the vaccine or a closely related wild isolate.
7. Continuous inoculation of gilts with the “farm strain” prior to entry. Although most would argue this does not eliminate PRRS, in many cases, the weaned pigs will remain negative to market age providing the gilt replacements are inoculated a minimum of 200 days prior to farrowing. This can easily be transitioned to rollover by negative gilt introduction.
8. Parity segregation can be used to drive multiple farms to negative pig flow status in a relatively short time (20 weeks in some situations). The 200-day benchmark still applies, but this can easily be accomplished in the gilt pool and P1 farm. Pig flow from the P2 and greater sites will achieve negative status quickly.
9. Continuous quarterly or semiannual addition of negative gilts to small herds, especially batch farrowing operations, which tend to go negative without other interventions other than biosecurity.
10. Three-breed rotational herds that add genetic stock only occasionally and especially those that batchfarrow tend to go negative without intervention. This is effective when all outside introductions are negative and providing functional biosecurity efforts aimed at blocking reintroduction are implemented.
Undoubtedly, there are other opportunities to eliminate PRRS, but successful implementation of any of these methods depends on many factors including the ability to monitor, effectively isolate, and apply a functional biosecurity strategy. These are necessities for any hope of lasting success and value creation. Two hundred days is only a benchmark, but it is a useful “rule of thumb” when planning PRRS eliminations. Adhering to this timeline greatly enhances success rates. Starting an elimination project in a herd that already appears stable without a good history may require significant monitoring to arrive at a project start date, but the closure time can be significantly reduced with a good history and visual observations. Monitoring also is recommended when following a 200-day closure. Specific testing protocols are varied but generally focus on statistical evaluation of newborn piglets. Moving negative gilts, especially those from an offsite breeding project, into a farm that is in the early stages of unrecognized PRRS incubation is not the best scenario.
Test-quarantine-removal is a valid method for agents of low prevalence and that are not highly infectious/ contagious and in which carrier or persistent infections may exist. Pseudorabies and brucellosis are good examples in which test and removal eradication programs have been effective. Test and removal is not a valid method of PRRS elimination.
All elimination projects must include certain key ingredients for success. Careful planning, along with accurate assessments of expected costs, cash flow disruption, and value creation potential (payback period), should be developed. Biosecurity is important, but distance to growing pig sites can greatly improve durations of negativity.
Summary
The success of close and rollover PRRS elimination methods is well recognized. Growing pig populations are generally believed to be the primary source of virus change resulting in area spread. The significant movement of weaned pigs and feeder pigs, and proximity of growing pigs to sow production sites has curtailed widespread eradication success. This issue must eventually be addressed if a national eradication attempt is to be successful.
References
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