Coping with PRRS Virus:

Treatment, Control and Elimination

 

John Roberts, DVM, PhD

North Carolina State University

College of Veterinary Medicine

Raleigh, North Carolina

 

PRRSv Control and Elimination Account Basic Biological Principals

 

Many swine producers have tolerated the effects of Porcine Reproductive and Respiratory Syndrome Virus (PRRSv) in their herds for years. There is no effective long-term treatment for PRRSv. Affected producers can only consider feasible PRRSv control and elimination strategies to reduce losses. The original source of PRRSv in all stages of production is the sow herd. Control has to be achieved at sow farms before positive control is realized at nurseries or finishers. There are a few specific biological principals concerning sow farm PRRSv that cannot be disregarded when attempting PRRSv control or elimination. Understanding the principals is prerequisite to devising and implementing a PRRSv control or elimination plan.

 

1.      Control is Impossible Without Preventing the Entry of New PRRS Viral Strains

 

PRRS viruses are single-strand RNA viruses (Weensvoort, 1991). Like other single-stranded RNA viruses, PRRS viruses have high mutation rates resulting in great strain diversity (Domingo, 1996; Murtaugh, 1997; Yuan, 1999). PRRSv strains are very different when compared across unassociated sow herds (Yuan, 1999; Lager, 1997). Different PRRS viral strains exhibit marked differences in susceptibility to immunity(Lager, 1997;Mengeling, 1996; Mengeling, 1998). Diversity is advantageous to PRRS viral survival as introduction of an alternative prototype may be an adaptation that avoids host immunity (Domingo, 1989). Mutational variations in closed herds were minor when compared to the genomic strain shifts noted after introductions of infected animals from unassociated herds (Roberts, 1999; Mahlum, 2001).

Swine herds are not islands. They interact with other herds. The other herds may or may not be in the same production pyramid. Commercial sow herds produce pigs weekly through a group process. Each week a group of sows will farrow, other groups lactate, waiting groups gestate, and one group is bred. The cycle of the group arrangement is set into time and can not be stopped. An adequate supply of replacement females is required to properly fill every breeding group. Replacement females are available when a multiplier farm achieves its production goals. The multiplier farm may depend on great grand parent animals from a seedstock company. All sow farms depend on a steady semen supply from one or more boar studs.  The stud probably replaces its boars regularly. It relies on replacement boars flowing from a multiplier or seedstock company. The system is relentless, as flow can not be interrupted without severe consequence. The system is also unforgiving. A new PRRS strain can follow semen or animal flow, reaching far into a production pyramid.

Successful PRRS control does not allow the luxury of approaching PRRSv as a single entity. Success is realized only when all PRRS viral entities in a production system are considered. PRRSv control is futile when management tolerates the introduction of “foreign” PRRS viral strains. Assurance of the following prevents foreign strain entry.

 

·        Replacements:

Do not enter male or female replacements from PRRSv positive herds outside the pyramid. Enter only PRRSv free replacement seedstock into a production pyramid.

 

·        Semen:

Do not use PRRSv positive semen from a stud outside the pyramid. Assure any outside semen is from a stud that is confirmed PRRSv free before entering it into a production pyramid.

 

·        Commercial modified live vaccines:

Live vaccines pose a dilemma as vaccine virus may act as a foreign introduction.

 

Managers find it necessary to know the identities of sow herd PRRS viral strains. DNA finger printing is used to identify the strains associated with individual sow farms. Strains from multiple sow farms are compared on charts to assess differences. Knowledge of resident strains and their locations also make it possible to detect new strain entry and movement (Murtaugh, 2001). Foreign strain entry reflects poorly on the ability of management to achieve discipline in replacement flow. Strain charting describe the PRRSv environment that management encounters while attempting to gain control. Minimal PRRSv strain variability is conducive to control (Mahlum, 2001; Roberts, 1999).

 

2.      Control is Impossible in a Production Pyramids with Many PRRSv Strains

 

Producers with unsuccessful control programs are often overwhelmed by the quantity of different PRRSv strains in their production pyramids. Multiple strains interact with herd immunities to cause problems that are difficult to overcome. There may be PRRSv strains within a pyramid that can cause new infections in some unaffected herds (Mahlum, 2001; Roberts, 1999). Some sow herds may have uncontrollable situations related to multiple strains infecting different sows at once (Dee, 2001b). There is also a situation where an incompatibility exists between the immunity of entering replacement females and the resident PRRSv strain of the herd (Roberts, 1999). Producers dealing with pyramids infected with multiple strains may choose to depopulate their sow herds and repopulate with new PRRSv free stock. There are several schemes that make depopulating and repopulating an affordable option.

 

·        Off-Site Breeding Project:

Additional facilities are acquired where 20 weekly groups of PRRSv free replacement females can be bred and housed. When the farrowing date of the first group of bred replacements approaches, a sow herd is depopulated. The facility is cleaned and the herd is promptly repopulated with PRRSv free replacements.

 

·        Alternative Positive Sow Flow:

Depopulation-repopulation programs often cause the loss of future parities when positive sows are culled. The loss represents much of the cost of the program. The potential farrowing are realized if the sows are kept in the pyramid. The positive sows from a depopulating sow herd can be flowed into other sow herds in the same pyramid. They are removed after weaning and bred with the weekly groups of other herds. The receiving herds develop older parity distributions. However, only the best sows are retained and moved. The flow of old sows permits young gilt replacements to be available to fulfil needs at the off-site breeding project.

 

Depopulation-repopulation programs require a prerequisite availability of two essential resources. The boar stud must contain only PRRSv free boars and a steady supply of PRRSv free replacement females must be established.

 

·        Boar Stud PRRSv Elimination:

Boar studs may be manipulated and “flowed” PRRSv free if adequate separation is available to keep positive and free boars on site simultaneously (Reicks, 2001). Prior testing and removal of infectious boars before entering PRRSv free boars enhances success (Reicks, 2001). Alternatively, a boar stud can be depopulated and repopulated with outside PRRSv clean stock.

 

·        Production of PRRSv Free Females from a PRRSv Positive Multiplier Sow Herd:

Production of PRRSv free offspring from a positive sow herd is an expensive and risky venture. A positive multiplier herd is candidate when no PRRS viral replication is occurring in the herd. The situation is signaled when testing of its nursery pigs late in phase indicate lack of PRRSv exposure. The flow of replacements into the positive multiplier herd is stopped. Sometimes sows are screened by blood testing before farrowing. The offspring of “low-test” sows have improved probability of remaining PRRSv free, especially if not farrowed in the same room as “high-test” litters. Female piglets are weaned early to an isolated site. Every piglet is tested to assure the absence of blood borne PRRSv. The piglets are housed at a low stocking density. Empty pens are sometimes placed between pens of pigs to discourage group spread. Multiple tests of pig groups are accomplished during growth to assure those weaned groups remains negative. Any group of pigs found with PRRSv is removed from the site. The ultimate goal is to produce enough PRRSv free females to depopulate and repopulate the original multiplier and establish a source of PRRSv free offspring. The PRRSv free multiplier supplies replacements to an off-site breeding project to repopulate “downstream” commercial sow herds.

 

·        Purchase PRRSv Free Stock:

When the preservation of genetic lines is not an issue, it is easier to source the repopulation of a multiplier sow herd by purchasing PRRSv free replacement stock from an outside source. However, it may not be less expensive. Once a multiplier is established PRRSv free, its offspring enter “down stream” herds as repopulating replacements.

 

3.      Sow Farm Control is Achievable if All Sows are Exposed at the Same Time

 

PRRSv is controlled in an acute herd by establishing a simultaneous immunity through out the entire herd, sparing no nonimmune animals (Dee, 2000). It is difficult to achieve simultaneous immunity in a large herd (Nodelijk, 2000). As a result, large herds are likely to have continual persistence of recently PRRSv infected animals (Nodelijk, 2000). Acutely infected animals are the reservoir of herd PRRSv. Sometimes voluntary infectious methods have been employed to achieve simultaneous immunity (Baustista L,2000; Geiger, 2001; Dufresne, 2001; Philips, 2001; Sanford, 2001). These infective methods include the following.

 

·        Fevered Sows:

Fevered sows ill with PRRSv can be moved through out sow farm facilities to increase the contact with unaffected sows. Many sows are shuttled to increase transmission.

 

·        Back Injection:

The serum of fevered sows can be injected into susceptible animals in the same herd to assure exposure. The procedure is more efficient than moving sows and transmission is more likely.

 

·        Vaccine:

Killed and modified live vaccine may provide mass exposure when the entire sow herd is vaccinated, especially when repeated within a month. Results are variable as not every PRRS virus is affected by vaccine stimulated immunity. It is best if the commercial vaccine strain chosen has not been previously used in the herd.

 

Simultaneous exposure maximizes the portion of sows infected with PRRSv. It is ideal if 100% of the herd is infected. Simultaneous immunity is beneficial as all susceptible animals are eliminated. The infective sows turn into resistant sows after about 6 to 12 weeks. When all sows are resistant, the herd is noninfective as no acute reservoir animals are possible. Resistance does not last indefinitely.  Sow immunity to PRRSv is shown to be effective about 2 years or less following initial exposure (Lager, 1997). The introduction of susceptible animals during this time degrades the resistant population. It is best to voluntarily infect the replacement females bound for an extensively exposed sow farm. Gilt exposure before entry is a necessity addressed by acclimation.

 

·        Gilt Acclimation:

Every replacement group can be exposed with a voluntary option. This is accomplished in an all-in all-out replacement facility. The replacements remain in the facility for at least an additional 12 weeks after exposure to recover. The resistant replacements are then entered into the sow herd. An acclimatized or pre-exposed replacement gilt is resistant for two to three parities. It is difficult to find a voluntary exposure method does not deplete in the long term. The sources of sow herd virus dissipate as the herd recovers. Even frozen materials have limited shelf life. Some producers have used the continuous flow acclimation units to maintain PRRSv for gilt exposure. The method is not advised as health of the replacements is jeopardized and PRRS viral strain “drift” may be enhanced (Roberts, 2001).

 

·        Gilt Breeding Farms:

Replacement gilt exposure may be maintained using a specialized herd for breeding only gilts. The gilts are usually moved to other herds as open parity one weaned sows. The gilts have an entire gestation period to develop PRRSv immunity before transfer into a sow farm. It is assumed that the pace of animals coming into and leaving the gilt breeding facility is so high that exposure is maintained by continuous flow and gilts are exposed on entry. The assumption may not always be valid and the disadvantages of continuous flow apply.

 

Development of simultaneous immunity is effective, as every animal is immune to a resident PRRSv strain. It is unlikely that the control will be of value should a different PRRSv strain enter the herd (Dee, 2001b; Roberts, 1999). The extent that the entire sow herd was exposed is measured by monitoring with blood tests. Monitoring to assure whole herd infection is important early in a PRRSv control program. The monitor shows if wide spread exposure was accomplished or if the emergence of recent infection is continued. At least two testing methods are available. It is common to use a 40-sow testing scheme.

 

Figure 1.  Forty-Sow Testing Scheme

Sow Group Characteristics
WeeklyGroups	Stage	Activity	Pregnancy	# Test
1 – 5	Wean – Gestation 1	Breed – Heat Check	(5) – 30 Days	10
6 – 9	Gestation 2	Condition	31 – 59 Days	8
10 – 14	Gestation 3	Condition	60 – 94 Days	10
15 – 20	Gestation 4-Farrowing	Prepare-Lactate-Wean	95 – (21) Days	12
			Sample Size =	40

 

4.      PRRSv is Does Not Transmit Well Between Pigs

 

PRRSv is a paradox compared to other swine pathogens. Like most viral agents, PRRSv appears to quickly move through a susceptible nursery group. Yet, it is well known that PRRSv transmits slowly (Benfield, 1999). The probability of PRRSv transfer between pigs is so low that PRRSv may not transfer between an infectious pig and a susceptible penmate (Torremorell, 1997).  Infectious transmissibility (R) is a measure of how effectively infection spreads pig to pig within a population. PRRS infectious transmissibility (R) in a sow herd was estimated as 3.0 (Nodelijk, 2000). The infectious transmissibility of measles in closed human populations was estimated to be about 9.0 (Rothman, 1998). The infectious transmissibility of the Human Immunodeficiency Virus in an open human population was estimated to be about 4.5 (Rothman, 1998). Infectious agents that do not maintain a transmissibility (R) greater than 1.0 will spontaneously “die out.” Low transmissibility is advantageous when PRRSv control techniques are applied.

 

5.      The Quantity of Recently Infected Sows in a Farrowing Group Determines the Rate that Nursery Pigs are Infected

 

PRRSv transmissibility is low enough that a unique relationship between the quantity of cases in a weekly sow group and rate of that disease spreads through nursery offspring. Pigs from PRRS endemic sow farms often experience disease in the nursery (Keffaber, 1992). Maternal protection from sow milk is short lived and fails to protect pigs throughout the nursery phase. Declining maternal protection leaves many pigs susceptible as they near seven weeks of age (Senn, 1998; Sanford, 2001). If infective pigs are present in a weaned group, the susceptible pigs are vulnerable to infection. New cases of acute infection result in increased viral shedding. The quantity of PRRS infected pigs are exponentially multiplied by PRRSv spread until almost all pigs are exposed. An initially infective pig is considered the product of infection prior to birth (Benfield, 1999). A piglet infected the month before birth sheds virus for many weeks (Benfield, 1999). The appearance of PRRSv in nursery pigs is related to viral replication in individual sows. Many infective pigs at weaning increases nursery pig exposure and the rate that the infection spreads. Many infective pigs cause all the pigs to be infected at a young age.

When a weekly sow group has minimal infective sows, nursery disease is less severe and disease spreads slowly. A slow infectious rate results in many pigs avoiding infection until they are older. Nursery disease is less pronounced and appears to happen late. When viral replication in gestating sows is a rare, the age when an entire associated nursery group is infected is extended even more. After time, conversion may occur so slowly that pigs reach the finishing phase before infection in the group is detected (Dee, 2001a). Incomplete conversion of pig groups can appear and susceptible pigs are found with the resistant. Endemic sow herds can produce offspring that remain PRRS naïve, even as adults (Le Potier, 1997).

The relationship of infective sows to nursery illness demonstrates how simultaneous sow exposure eventually benefits nursery performance. Other sow farm management options can further reduce the quantity of infective piglets. Following are examples of methods that reduce the quantity of infective pigs at weaning.

 

·        Limited Cross Fostering:

Limited cross fostering reduces the risk of PRRSv spread between litters. A pig is transferred only in the first 24 hours of life to a sow that farrowed in the last 24 hours. The age limit assures that a transferred piglet will gain the protection afforded by the sow’s first milk. The limit also assures that piglets later infected within a litter are not transferred out to infect other litters. Limited cross fostering is advantageous until PRRSv is controlled. Limited cross fostering is part of McRebel procedures (McCaw, 1995).

 

·        Identifying “Fall Behind” Pigs Placed on Nurse Sows:

It is a common practice to place small withering pigs on an open lactating sow when the pigs are 10 to 14 days old. The commingling of health challenged pigs qualifies these piglets as probable infective cases, especially when the sow herd is exhibiting PRRSv symptoms. These pigs are identified by ear notch and kept in a cull pen after weaning to avoid infecting pigs in the general population.

 

Other nursery and finishing options are employed to reduce the rate of infectious spread through pig groups. The options are intended to reduce pig-to-pig contact or exposure to infective pigs.

 

·        Limited Sizing:

Pigs are initially sized to match pen mates when they enter the nursery. Many nursery and finishing protocols asked for pigs to be moved between pens through out the phase with the hope of reducing competition. Limited pen movement discontinues pig movement between general pens until PRRSv is controlled. Pigs are only removed to cull or hospital pens. Following are examples of limited pen movement.

 

·        Hospital and Cull Pen Isolation:

Pigs entered into a cull or hospital pen are identified and never returned to the general population.

 

·        Pen Integrity Shipments:

Some managers have maintained the occupancy of individual pens with same pen mates in the finisher as the nursery. The practice is maintained until PRRSv is controlled.

 

Nursery pigs can be tested to determine if acute PRRSv replication is sustained on the sow farm where pigs originate (AASP, 1996). Testing nursery and finishing pigs discloses low-level sow farm infections not detectable by sow farm testing. Infective sows are detectable through their offspring. However, testing in a nursery infers only to specific weekly sow groups and not the entire herd. Nursery pigs in the last or second to the last week of the nursery phase are often tested to avoid maternal immunity and maximize the probability of finding exposed pigs. Usually a sample of 10 pigs from a group can detect PRRSv activity.

The ability of nursery testing to detect sow infection is limited when infected pigs are few. When incidence of viral replication on a sow farm is negligible, nursery testing shows varied results. Some tests show PRRSv presence and others do not. Minimized sow infections are not randomly placed through a sow herd. Sites of sow infections are confined to small “pockets” of sows that are proximal to each other (Dee, 2001a). It is also possible that PRRSv replication in a sow group is so minimal that the rate of nursery infections is so slow, that blood samples fail to find any positive pigs. When fewer than 10% of pigs are infected, nursery testing returns some entirely negative tests. Passing time increases the quantity of PRRSv cases in a group. Sampling finishing pigs may detect PRRSv when nursery testing fails to find positive pigs. Eventually, finishing tests will also fail to detect the low PRRSv prevalence when control is effective. 

 

6.      Herds that Achieve PRRSv Control Can Elect More Effective Options

 

Initial PRRSv control culminates when the sow herd contains all resistant PRRSv exposed sows, but no animals supporting PRRS viral infections. The situation is signaled when nursery blood testing consistently returns negative results. This state represents a window of opportunity when management can implement more effective control options. The window is limited as sow immunity is time limited. All the procedures begin by ceasing replacement acclimation. A pool of PRRSv free replacements is allowed to build. At the same time, the sow herd culls inventory and fills with as many acclimated replacement animals as possible. The goal is to pass at least 4 months without entering any replacements (Torremorrell, 2000). Several options can be implemented after the closure to replacements. All options remove sows suspected of harboring persistent infections as PRRSv free replacements are entered. The options vary only in how PRRSv free stock is entered into the herd and the rate that old PRRSv exposed sows are replaced. Following are a few examples.

 

·        Test and Removal (Dee, 2000):

Negative replacements are flowed into the sow herd. PRRS Elisa testing detects exposed animals for about a year after. The sow herd is monitored until less than 15% of animals are PRRS test positive. The test-remove option is implemented by testing the entire herd at once with PRRS Elisa and polymerase chain reaction (PCR) testing. Any sows positive to either test are immediately culled. The ideal outcome produces a PRRSv free sow herd.

 

·        Wean and Removal (Sandri, 2001):

Wean and removal is a modified test and removal program focused on weaned sow groups. Sow groups are tested prior to farrowing with PRRS Elisa and PCR. Any PCR positive animals are removed immediately. These are few. PRRS Elisa positive sows are culled at weaning and replaced with PRRSv free replacements. A wean and removal program proceeds for 20 weeks. It is slow in comparison to the immediate test and removal program. 

 

·        Rolling Repopulation (Torremorrell, 2000; Geiger, 2001):

PRRS free replacements are entered into the herd. Cull rate is initially accelerated to infuse many PRRSv free animals into the population as quickly as possible. PRRSv free replacements are eventually entered with typical replacement rates. Success of the program assumes that replacement rate will discard persistent sows faster than PRRSv transmission can occur. A modification of the method breeds the PRRSv free replacement gilts off-site and enters them into the farrowing portion of the sow herd, avoiding entry into breeding. The procedure permits a 20-week avoidance of replacement entry.

 

Summary: PRRSv Control Decisions Across Time

 

There are several routes of overcoming PRRSv in a sow herd. None are functional unless the production system is protected from the entry of outside PRRSv strains. There are very few options available when the herd is acute and showing symptoms. A uniform and simultaneous exposure is required. When broad exposure is achieved and protected by replacement gilt acclimation, a state of minimal shedding may eventually be reached. Herds that reach this state can choose from several options to reduce or eliminate PRRSv. The options vary in the rate that infected animals are removed. Some producers have PRRS viral environments in their production systems that make initial control impossible. These environments are usually related to the multiplicity of viral strains entered in their systems from outside sources. These producers are better served to consider a depopulation of their system with an associated repopulation with PRRSv free stock.  

 

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